Desal-Wiki (14)
adiabatic, adiabatic change
An adiabatic change is a thermodynamic process, in which a system changes its condition without exchanging heat and/or mass with its environment.
Anion Exchange Membrane AEM
anolyte
anti-corrosives
Chemical agent to reduce the feed water's corrosion potential.
anti-foaming
Chemical agent to reduce the formation of foam or to break a foam already formed.
anti-scalant
Chemical agent to reduce scaling disposition of the feed water.
auxiliary power
Power for auxiliary systems in a desalination plant, for instance electric power supply for control units, circulation pumps, vacuum pumps etc.
batch mode
Recycling of the brine in several cycles in order to achieve higher brine concentrations.
bio-electric potential
brine
Concentrated feed water.
Capacitive Deionisation, Membrane Capacitive Deionisation Electrochemical Demineralisation, Electrosorption CDI, MCDI
Cation Exchange Membrane CEM
catholyte
chemical potential μ
The chemical potential is an intensive thermodynamic state variable describing the change in free enthalpy in thermodynamic systems with variable number of atoms or molecules of the species. Desalination-related parameters like osmotic pressure or vapour pressure are derived from the concept of the chemical potential.
Cleaning-in-Place CIP
concentration ρi, wi, ci, xi
Concentration of a component i (solute) in a solution can be given
• as mass concentration ρi per volume in [kg/m3],
• as mass fraction wi per mass in [-] or in [g/kg],
• as molar concentration ci per volume in [kmol/m3] or
• as molar fraction xi per mol [-] of the solution [xxx_klein].
For standard seawater, the mass fraction of the sum of all solutes is
wSi = 35.00 g/kg [xxx_millero].
For “salt concentration” in seawater, see “salinity”.
Concentration Factor CF
is the relation between the salinity of the feed water and the salinity of the discarded brine< (“brine out”). In other words, CF is the ratio between feed mass flow to mass flow of the discarded brine and therefore directly connected to the Recovery Ratio RR:
contact angle Θ
The contact angle Θ is the angle, where a liquid–gas interface meets a solid surface. Θ is conventionally measured through the liquid. It quantifies the wettability (see hydrophilic and hydrophobic) of a solid surface by a liquid. The cosine of Θ is a function of the interfacial energies of solid-gas and liquid-solid as well as of the surface tension (interfacial energy of liquid-gas).
counter current
Heat transfer between two fluids in a heat exchanger flowing in opposite directions to each other.
cross current
Heat transfer between two fluids in a heat exchanger flowing in a direction of about 90° to each other.
crystallisation limit
Maximum concentration of a solute in a solvent before crystal formation of the solute starts to occur.
demister
Porous structure, mostly a steel wire mesh, to hold back droplets of saline water from the condenser units in thermally driven desalination systems.
dew point
The dew point is the temperature in humid air, where at a given absolute humidity, water vapour in air reaches saturation or, in other terms, starts to condense.
diffusion
Diffusion (lat. diffundere “pour out”, “scatter”, “spread”) is a natural physical process originating from the Brownian Movement of molecules. After a certain time, it leads to a total mixing of two or more substances due to a homogeneous distribution of the involved particles [xxxRömpp]. Diffusion occurs due to an omnidirectional, statistic movement of particles as a result of their thermal energy and a local difference in concentration, which in the end is a difference in chemical potential.
distillation
In distillation, single components of a liquid mixture are separated by selective vaporisation and condensation. The distillation technique makes use of different partial vapour pressures or in other terms, boiling points (temperatures) of the single components in the mixture.
electricigen
Electrodeionisation, Continuous Electrodeionisation EDI, CEDI
Electrodialysis, Electrodialysis Reversal ED, EDR
electrical double layer EDL
Electrosorption
enthalpy
Enthalpy is a thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume. [xxxoxf1] In other words, enthalpy is the sum of the energy required to create the system and the work to be done to displace its environment and establish its volume and pressure. In desalination applications, mostly the specific enthalpy h in [J/kg] is used, which is the absolute enthalpy H [J] per unit mass [kg].
entropy
Entropy is a thermodynamic property of matter. Based on a comparison of entropy, the viability of thermodynamic transformations can be analysed. Furthermore, irreversibilities or in other words internal losses can be quantified [xxxsat]. In most cases, the generation of entropy stands for thermal or performance losses.
evaporation
Evaporation is a phase change from liquid to vapour (gas) at temperatures lower than the boiling temperature at a given pressure. Evaporation occurs always at the phase boundary to the liquid phase, e.g. the water surface and air in a system. The driving force of evaporative processes is a difference between the vapour pressure of a liquid substance (e.g. water) at the phase boundary and, in case of air, the partial vapour pressure of the gaseous substance (e.g. water vapour) in the air.
exoelectrogen
exoelectrogenic
feed water
Feed water is the raw water to be (pre)treated and fed into a desalination system.
fouling
Fouling is the deposit of unwanted material on solid, functional surfaces, e.g. heat exchanger surfaces or membranes. Fouling can both be of organic (biofouling) or inorganic (scaling) nature. Both forms mostly reduce the efficiency of the affected surfaces, for instance heat exchanger efficiency due to lower heat transfer coefficients or permeability of a membrane due to a higher flow resistance.
Gained Output Ratio GOR
The Gained Output Ratio (GOR) is a key figure for the performance of thermally driven desalination systems with superheated steam as main energy source, for instance Multistage Flash (MSF) or Multi-Effect Distillation (MED). GOR denotes the mass flow ratio of produced distillate to consumed heating steam
GOR can be interpreted as quality grade for heat recovery: how often is the heat of evaporation contained in the heating steam reused for vaporizing and condensing (= amount of distillate) pure water. Therefore, GOR values in MSF and MED plants correlate with the number of stages or effects.
heat capacity rate
The heat capacity rate is the product of the mass flow rate of a liquid and its heat capacity at constant pressure. It is a figure for the amount of enthalpy (total heat content) that can be absorbed or released by a fluid per unit temperature change and per unit time. Heat capacity rates are given in [kJ K-1 s-1] and are an important quantity in heat exchanger theory.
heat of condensation, enthalpy of condensation ΔhC
Under equal thermodynamic conditions, the heat of condensation is equal to the heat of evaporation with the opposite sign. By definition, in enthalpy changes of evaporation heat is absorbed by the substance (positive sign). On the other hand, in enthalpy changes of condensation, heat is released by the substance (negative sign).
heat of evaporation, enthalpy of evaporation ΔhV
In order to achieve a phase change of a substance from liquid to gas, a certain amount of energy must be added per quantity of the substance. This amount of energy is the heat or enthalpy of evaporation. The heat of evaporation is a function of the pressure and the temperature at which the phase change takes place.
heat exchanger
A heat exchanger is a device for exchanging heat between two or more fluids that are at different temperatures [xxxInci]. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact [xxxKak]. For a deeper understanding of heat exchanger technology, please refer to Annex E1.
Humidification-Dehumidification, Multi-Effect Humidification HD, HDH, MEH
Humidification-Dehumidification (HD, sometimes also HDH or in case of multiple effects “Multi- Effect Humidification” MEH) uses the principle of evaporation of pure water in a humidifier and subsequent condensation of the water vapour in a dehumidifier. A large majority of HD set-ups uses air as a carrier medium. Thus, HD imitates the natural rain cycle in an artificial environment.
The separation of pure water and salt is due to a substantially lower vapour pressure of salt compared to water. At a given temperature, water will evaporate while salt is remaining dissolved in the liquid brine. The fundamental driver of HD is a net vapour pressure difference between feed water, carrier gas (mostly humid air) and condensate. This vapour pressure difference is induced by temperature differences between feed water and condensate, which are commonly generated by heating the feed water (addition of heat).
HD is a thermal low-temperature (60 °C – 85 °C) process and can therefore be driven with solar energy or waste heat. Another key advantage is its robustness towards the feed water quality and its suitability for Zero Liquid Discharge (ZLD) applications.
hydraulic fracturing, fracking
Hydraulic fracturing or fracking can therefore be classified as a well stimulation technique for the exploitation of crude oil, natural gas or geothermal water deposits, where the required fluids are not automatically flowing towards the wellbore. A so-called “fracking fluid” is injected into the wellbore at high pressure in order to break up the deep-rock formations around the deposit. Through the so-created cracks, the required fluids will flow much easier. When the fracking fluid is removed, it often gained a high salinity and very often contains substances that are harmful for the environment. In this case, desalination techniques may be applied to reduce the amount of waste water and hence costs.
Hydraulic Retention Time HRT
hydrophilic
In desalination applications, the term “hydrophilic molecular entity” mostly stands for the properties of a hydrophilic surface, for instance a Reverse Osmosis (RO) membrane. Hydrophilic substances attract water molecules and have a high solubility in water. Water getting in contact with hydrophilic surfaces will exhibit a low contact angle. The opposite of hydrophilic is hydrophobic. [xxx_chemie, xxx_Arieh]
hydrophobic
Hydrophobicity is the physical property of a molecule that is repelled from water. In desalination, these are mostly molecules of a membrane, for instance a Membrane Distillation (MD) membrane. Hydrophobicity is commonly seen as an absence of attractive forces between water molecules and the membrane. Hydrophobic molecules mostly are nonpolar, while water molecules are polar. Thus, hydrophobes do not have a high solubility in water. Water getting in contact with hydrophobic surfaces will show a high contact angle. [xxx_chemie, xxx_Arieh]
latent heat
is the thermal energy released or absorbed by a substance during a process with constant temperature. The application of latent heat usually involves a phase change (solid ↔ liquid, liquid ↔ gaseous or a change in crystal configuration) of the substance and is therefore connected to the terms “heat of evaporation” and “heat of condensation”.
mass fraction wi
Concentration measure giving the mass of a solute in relation to the mass of the solution in [kg/kg] or consequently dimensionless [–].
Mechanical Vapour Compression MVC
Membrane Distillation MD
Analogous to Humidification-Dehumidification (HD) systems, Membrane Distillation (MD) also uses the principle of evaporation of pure water and subsequent condensation of water vapour in a condenser. The main difference to HD is, that in MD, the evaporation is established across a hydrophobic membrane. This allows for a slightly higher Performance Ratio PR and a much lower need for installation space, whereas HD plants usually have a lower complexity and a higher tolerance to problematic feed waters.
A large majority of MD systems (except Vacuum Membrane Distillation VMD) evaporates water to humid air and therefore is a purely evaporative process. The separation of pure water and salt is due to a substantially lower vapour pressure of salt compared to water. At a given temperature, water will evaporate while salt is remaining dissolved in the liquid brine. Similar to HD, the fundamental driver of MD is a net vapour pressure difference between feed water, humid air and condensate. This vapour pressure difference is induced by temperature differences between feed water and condensate, which are commonly generated by preheating the feed water (addition of heat).
Microbial Desalination Cell MDC
molar
A molar unit relates physical or chemical properties of a substance on its amount or its number of particles. The term “number of particles” stands for the number of atoms and/or molecules contained within the respective amount of substance.
Multi-Stage Flash Evaporation MSF
Non-Condensable Gases NCG
A Non-Condensable Gas (NCG) is a gas that cannot be condensed under nominal process conditions [xxxHelle]. In desalination processes, this is typically air or carbon dioxide and other trace gases dissolved in the feed water.
NCG-related problems mostly occur at condenser surfaces of thermally driven desalination processes. In the vicinity of the cold condensate film (or in some cases the dry heat exchanger surface), a negative concentration and temperature boundary layer is developing. A locally reduced water vapour concentration and temperature both lead to a decrease in the (partial) vapour pressure difference between the water vapour in the gas phase and the condensate film. Thus, the driving force for water mass transfer to the condenser is strongly reduced and condensation rates decrease accordingly.
For more details on concentration and temperature boundary layers in desalination applications please refer to Annex D. For NCG-related problems, see Annex D3.
packing, packing material
Packing material are small pieces of tube or spheres that mostly are optimized to have a high volume-specific surface and high porosity. The high specific surface is to allow an optimal heat and mass exchange between two liquids, in desalination mostly seawater and air. High porosity is needed for minimum pressure losses for instance of air in Humidification-Dehumidification (HD) humidifiers, thus increasing humidifier performance.
partial vapour pressure pV,i
The vapour pressure of a single component i in a mixture. According to Dalton’s law, the total pressure of a mixture is always a sum of the partial pressures of its single components [xxxsat]. Therefore, it is obvious, that the partial vapour pressure of a component is not only a function of temperature, but also of concentration of the respective component in the mixture.
The understanding of thermodynamic concepts around (partial) vapour pressure is crucial for the understanding of thermally driven desalination systems. Therefore, a deeper insight into the theory of vapour pressures is provided in
Annex C.
Performance Ratio PR
The Performance Ratio (PR) as analogue to the Gained Output Ratio (GOR) is a key figure for the performance of thermally driven desalination systems with sensible heat input, like Humidification-Dehumidification (HD) or Membrane Distillation (MD). PR can be directly derived from GOR and denotes the enthalpy ratio of produced condensate to consumed heat
flow Qin:
Note, that for the sake of comparability of plants with different evaporation temperatures and pressures, ΔhV is set to a standard value of 1,000 BTU/lb, which corresponds to 2,326 kJ/kg in SI units.
permeate
In membrane desalination processes, permeate is the sum of substances, mostly pure water and a small amount of salt, that is percolating through a membrane. Note, that the nature of “percolating” in this context depends on the desalination process. In Membrane Distillation (MD), this can mean evaporation through the membrane, in Reverse Osmosis (RO), it can mean diffusion through the active layer of a membrane.
pretreatment
is the treatment of raw or feed water with the goal to protect the desalination plant and to guarantee given standards for the product water (distillate or permeate).
Rapid Spray Evaporation RSE
Recovery, Recovery Ratio, Recovery Rate RR
mostly given as Recovery Ratio RR is the relation between the output of desalinated product water to the input of feed water. RR can be expressed as ratio of mass flows
and is generally 0 < RR < 1.
Reverse Osmosis RO
The main driver of the Reverse Osmosis (RO) process is the application of a pressure difference between feed water and permeate (in desalination: product) across a semi-permeable membrane. This applied pressure difference has a significant influence on the difference in chemical potential between feed and permeate side, which is the universal driving force in RO. Due to this chemical potential difference, pure water flows through a membrane from the feed to the permeate side, while salt is held back at the feed side. The pressure and hence chemical potential difference is applied by a series of electrically driven high-pressure pumps on the feed side.
Due to its versatility and low Specific Energy Consumption (SEC), RO is the most commonly applied technology in the global desalination market [xxxIDA].
salinity, Knudsen Salinity, Practical Salinity Scale, Practical Salinity Unit w, SK, PSS-78, PSU
By standard definition, the term “salinity” stands for the mass fraction wsalt [gsalt/kgwater] of dissolved salt in a body of pure water. Salinity is a thermodynamic state variable that governs physical properties like density, heat capacity and partial vapour pressure or chemical potential of the components.
Quite often, seawater salinity is described by the Knudsen Salinity SK [ppt], which gives the total amount of all dissolved inorganic material. SK can be determined by boiling off the water component. A more recent unit for seawater salinity is the Practical Salinity Scale PSS-78, which is tailored for salinity measurements using the electrical conductivity. PSS-78 can be given in [g/kg], [ppt], but mostly dimensionless [-]. Sometimes, PSS-78 is given as “Practical Salinity Unit” PSU [-]. [xxxSharkawy]
For standard seawater,
w = 35.00 g/kg, SK = 35.00 ppt, PSS-78 = PSU = 35.0.
Sometimes, the term “Total Dissolved Solids” (TDS) is also used to describe the salinity of water. However, this is not correct, as TDS also contains organic compounds. For the exact composition of seawater, please refer to Annex B – “Thermodynamic Properties of Seawater”.
scaling
The term “scaling” is a sub-category of “fouling” phenomena and refers to the precipitation or crystallization of sparingly dissolved inorganic compounds of the feed water on a solid host material [xxxKhayet]. The host material mostly is a functional surface like a heat exchanger wall or a membrane.
sensible heat
In the context of heat transfer, sensible heat in- or output appears as a temperature change of a medium, which can be detected (= sensed) by a temperature sensor. In sensible heat transfer, no phase change is involved, see also “latent heat”.
Solar Still
Solar Stills follow the same operation and separation principles as Humidification-Dehumidification (HD) installations. The fundamental driver also is similar. The unique features of Solar Stills are
(1st) the necessity to operate them with Solar (Thermal) Energy;
(2nd) the integration of humidifier and dehumidifier in one casing, which additionally serves as solar thermal heater.
Whereas the main advantage compared to other HD systems is the robustness and simplicity of Solar Stills, this goes at the expense of a good heat recovery or in other terms of high Performance Ratios.
Specific Energy Consumption SEC
Consumption of the main driving energy per unit of fresh water (condensate or distillate in thermal units, permeate in membrane systems). SEC is given in [kWh/m3] of distillate. Note that the auxiliary energy consumption mostly is not included!
sponge balls
A sponge ball cleaning system is used to clean the tubes in a tube bundle heat exchanger. The core parts are sponge rubber balls that are oversized in relation to the tubes to be cleaned. By circulating (i.e. through pumping) the sponge rubber balls through the tubes, fouling layers are removed. This results in a permanently good heat transfer. [xxxTapro]
standard seawater
As salinity and chemical composition in the oceans is widely varying, a standard composition of seawater for scientific purposes has been defined. Mostly, the standard salt concentration is given with 35.00 g of salt per kg solution or roughly 35,000 ppm. For a deeper understanding of chemical and thermodynamic properties of seawater please refer to Annex B.
stripping
Stripping is a physical separation process, where specific components of a liquid mixture are removed by a carrier gas. The liquid stream and carrier gas mostly are brought together in a packed column and can have either co-current or counter-current flows. For instance NH3 can be removed from its aqueous solutions by a counter current air flow in a packed column.
Top-Brine-Temperature TBT
Maximum temperature of the feed water in a desalination system, for instance right after the (feed) heater.
Total Dissolved Solids TDS
describes the content of all dissolved substances, organic as well as inorganic, in a liquid solution. The solutes can be contained in the solution in a molecular, ionized or micro- granular form. A micro-granular form stands for colloidal solutions with particles smaller than 2 mm.
Total Suspended Solids TSS
Total Organic Carbon TOC
Vacuum Membrane Distillation VMD, V-MEMD, MEVMD , MSVMD
Different from conventional Membrane Distillation (MD) systems, which have been characterised as diffusion-driven, evaporative processes, Vacuum Membrane Distillation (VMD) is a pure vaporisation process. This is due to the absence of Non-Condensable Gases or, in other terms, of a carrier gas establishing the evaporation process in the system. Thus, VMD is a real distillation process, making use of the vaporisation of pure water and subsequent condensation of water vapour in a condenser. The difference to other thermally driven processes is, that in VMD, vaporisation is established across a hydrophobic membrane, which basically serves as a containment for the feed water film or, in other terms, as vaporisation surface.
In the case of several stages or “effects”, the terms “Vacuum-Multieffect Membrane Distillation” (V-MEMD) [xxx_Zhao], “Multi-Stage Vacuum Membrane Distillation” (MSVMD) [xxx_Chung] or “Multi-Effect Vacuum Membrane Distillation” (MEVMD) [xxx_Kiefer] can be found in literature.
The separation of pure water and salt is due to a substantially lower vapour pressure of salt compared to water. At a given temperature, water will evaporate while salt is remaining dissolved in the liquid brine. Similar to all other thermal desalination processes, the fundamental driver of VMD is a net vapour pressure difference between feed water and condensate. This vapour pressure difference is induced by temperature differences between feed water and condensate, which are commonly generated by heating the feed water (addition of heat).
vaporisation, boiling
Vaporisation is a phase change from liquid to vapour (gas) at temperatures equal to or higher than its boiling temperature at a given pressure. Note that even in the scientific literature, the terms evaporation, boiling and vaporisation are frequently mixed up. In the present overview on desalination systems, the term “evaporation” stands for phase change phenomena at temperatures lower than the boiling point of a substance at a given pressure. Evaporation necessarily occurs in systems of three phases or more (e.g. liquid aqueous solution, water vapour and air) while in vaporisation, only two phases are involved (e.g. liquid aqueous solution and water vapour). In the present desalination compendium, the terms “boiling” and “vaporisation” are used in the same context.
vapour pressure pV
The pressure, at which a fluid starts to vaporise at a given temperature. The understanding of thermodynamic concepts around vapour pressure is crucial for the understanding of thermally driven desalination systems. Therefore, a deeper insight into the theory of vapour pressures is given in Annex C.
wetted area
The area in a humidifier or evaporator that is covered by a liquid film. The wetted area is often given as ratio of wetted to full heat exchanger area.
Zero Liquid Discharge ZLD
Zero Liquid Discharge in a desalination context is a water treatment process in which all fresh water is separated from a feed water stream and the remaining salts are recycled or deposited. Therefore, at the end of the treatment cycle, zero liquid discharge is arising. In fact, all desalination techniques listed in the present DME Technology Map can only achieve salt concentration values near the crystallisation limit. That is why the term “Near Zero Liquid Discharge” (NZLD) has become popular.
References (xxx in alphabetical order)
[1_arieh] Arieh, B.-N., Hydrophobic Interactions, Springer US, 1980, ISBN 978-1-4684- 3545-0.
[2_Chung] Chung, H.W., Swaminathan, J., Warsinger, D.M., Lienhard V, J.H., Multistage Vacuum Membrane Distillation (MSVMD) Systems for High Salinity Applications, Journal of Membrane Science, Vol. 497 (2016), pp. 128–141.
[3_chemie] Chemie.de, entry on “Hydrophilie”, last access 14-03-2019, http://www.chemie.de/lexikon/Hydrophilie.html
[4_Helle] Hellemans, M., The Safety Relief Valve Handbook – Design and Use of Process Safety Valves to ASME and International Codes and Standards, Elsevier Ltd., 2010, ISBN 978-1-85617-712-2.
[5_IDA] IDA Desalination Yearbook 2015 – 2016, ISBN: 978-1-907467-40-0.
[6_Kak] Kakaç, S., Liu, H., Heat Exchangers: Selection, Rating and Thermal Design, 2nd edition 2002, CRC Press, ISBN 0-8493-0902-6.
[7_Khayet] Khayet, M., Fouling and Scaling in Desalination, Desalination, Vol. 393 (2016), pp. 1–196.
[8_Kiefer] Kiefer, F., Spinnler, M., Sattelmayer, T., Multi-Effect Vacuum Membrane Distillation Systems: Model Derivation and Calibration, Desalination, Vol. 438 (2018), pp. 97–111.
[8_klein] Klein, H., Stoffübertragung, material to the lecture “Stoffübertragung”, Institute of Plant and Process Technology, Technical University of Munich, 2011.
[10_Millero] Millero, F.J., Feistel, R., Wright, D.G., McDougall, T.J., The composition of standard seawater and the definition of the reference-composition salinity scale, Deep-Sea Research I, vol. 55 (2008), pp. 50–72.
[xxxoxf1]
[xxx_Römpp]
[11_sat] Sattelmayer, T., Technische Thermodynamik – Energielehre und Stoffverhalten, Scriptum for the Lecture Thermodynamics I, Institute of Thermodynamics, Technical University of Munich, Mai 2012.
[12_Sharkawy] Sharqawy, M.H., Lienhard, J.H., Zubair, S.M., Thermophysical properties of seawater: a review of existing correlations and data, Desalination and Water Treatment, Vol. 16 (2010), pp. 354–380.
[13_Tapro] Taprogge GmbH, entry on “Tube Cleaning”, last access 04-03-2019, http://www.taprogge.com/products-and-services/in-ta-ct/tube-cleaning/index.htm
[14__Zhao] Zhao, K., Heinzl, W., Wenzel, M., Büttner, S., Bollen, F., Lange, G., Heinzl, S., Sarda, N., Experimental study of the memsys vacuum-multi-effect-membrane- distillation (V-MEMD) module, Desalination, Vol. 323 (2013), pp. 150–160.
Microbial Desalination is a technology to remove salt ions from saline water (deionize saline water). Driven by electric current (electric potential difference) generated by the metabolic activity of exoelectrogenic microorganisms, i.e. driven by a bio-electric potential difference, ions are transported through ion selective membranes, thus generating an ion-enriched stream (brine) and an ion-depleted stream (desalinated water). Ion transport is effected in proportion to the electric current being generated by these microorganisms [1,2,3].
1. Technical Description
The Basics – Separation Principle and Operating Mechanism
An exoelectrogenic microorganism is a microorganism that has the ability to transfer electrons extracellularly. Exoelectrogenic microorganisms are also referred to as “exoelectrogens”, “electrochemically active bacteria”, “anode respiring bacteria” and “electricigens” [4]. A known species of exoelectrogens is, e.g., Geobacter sulfurreducens.
In a Microbial Desalination Cell (MDC) exoelectrogenic microorganisms serve to generate electric current (electrons) from oxidation of organic matter present, e.g., in wastewater [3] or in artificially created nutritious solutions. The electric potential difference between anode and cathode generated by the metabolic activity of these microorganisms is the driving force for desalination in an MDC. It is used to induce ion transport through ion exchange membranes [2,5], thus desalinating water.
Therefore, the driving force of desalination in MDC may be – more precisely – referred to as a “bio-electric” potential difference. The fundamental separation principle to separate ions from water in MDC is the capability of ions to move selectively according to their charge.
MDC is in nature similar to Electrodialysis (ED), yet different from ED because it does not require external electricity [1]. Instead, energy is provided by the exoelectrogenic microorganisms, being an integral part of the MDC.
It goes without saying that the particular attraction and benefit of MDC lies in the fact that MDC allows desalination without external electrical energy input and without the need of high feed pressure. In consideration of the relevance of energy cost in desalination, a desalination technology like MDC, which does not consume more energy than generated by itself, must be of particular interest.
A further benefit of Microbial Desalination Technology is that it combines at least two simultaneous functions: electrical energy production and desalination. Depending on design, it may also take up a third function. If, for example, wastewater is used to feed the MDC and the organics contained therein serve as food source for the microorganisms, the MDC can even achieve three functions simultaneously: energy production, desalination and wastewater treatment.
Process Description
The first proof of concept of MDC was demonstrated in 2009 by a 3-chamber configuration [1, 6, 7]. This basic Microbial Desalination Cell resembles a fuel cell, containing an anode and a cathode, between which a pair of ion selective membranes is inserted. Fig. 1 depicts the basic design of a Microbial Desalination Cell (MDC).
Fig. 1: MDC – basic design
It consists of three chambers:
1. the anode chamber with an anion exchange membrane (AEM). Next to the anode a film of exoelectrogenic microorganisms is attached as a kind of biofilm. The chamber is typically filled with a liquid (e.g. wastewater) which contains nutrition for the exoelectrogenic microorganisms. These exoelectrogenic microorganisms oxidise organic matter, and, in doing so, release protons and electrons.
2. the cathode chamber with a cation exchange membrane (CEM) positioned next to the cathode. This chamber, in which the electrons transferred from the anode are consumed by a chemical reduction reaction (as described below), is typically filled with a liquid that enhances cathode performance (e.g. water + air injection).
3. the middle chamber between the two ion exchange membranes (AEM and CEM) is filled with feed water to be desalinated [1].
Process Description MDC
The MDC process (here with an air-cathode) is depicted in the following sequence (Fig. 2 A – C)
Fig. 2 A: MDC process - anode chamber Fig. 2 B: MDC process - cathode chamber Fig. 2 C: DC process - resulting stage
Exoelectrogenic microorganisms on the anode oxidise organic matter fed into the anode chamber. In doing so, they release protons (H+) and electrons (e-) (see Fig. 2 A). The following equation represents the oxidation reaction at the anode [8]:
(CH2O)n + nH2O → nCO2 + 4 ne- + 4nH+
The electrons (e-) leave the anode chamber and are transferred as electric current via external wire to the cathode. Protons (H+) cannot leave the anode chamber, as they are rejected on the anion exchange membrane (AEM). The surplus of positive charge in the anode chamber resulting from protons (H+) produced, however, desires balancing. The only source of balancing charge is drawing negatively charged ions (Cl-) through the AEM from the middle chamber into the anode chamber (see Fig. 2 A).
Simultaneously, in the cathode chamber, electrons (e-) are received as electric current (see Fig. 2 B). There they combine, e.g., with the oxygen provided by an air-cathode or by oxygen injection into the cathode chamber. They combine as well with the protons (H+) drawn from the liquid in the cathode chamber (chemical reduction reaction) to form H2O. The following equation represents the reduction reaction at the cathode [8]:
O2 + 4 ne- + 4nH+ → 2 H2O
The extraction of protons required for the formation of H2O leaves a deficit of positive charge in the cathode chamber which can only be replenished by attracting positively charged ions (Na+) from the middle chamber. As only positively charged ions can pass the cation exchange membrane (CEM), Na+ ions cross the CEM and replenish the protons consumed in the formation of H2O (see Fig. 2 B).
As a result (see Fig. 2 C), the feed water in the middle chamber is depleted of salt ions, i.e. desalted. Thus, the generation of electric current through oxidation activity of exoelectrogenic microorganisms induces an electric potential difference between anode and cathode, which in turn induces a selective ion transfer through the AEM and CEM, thereby depleting the feed water stream from salt ions.
This depletion of ions in the middle chamber results in water desalination without any water pressurisation or use of draw solutions or externally supplied electrical energy. Instead, energy is generated while water is desalinated, and, as the case may be, organic matter is removed from wastewater while being metabolised by the microorganisms ́ activity in the anode chamber [1, 2, 5, 6, 8, 9, 10].
MDC Process Peculiarities
- MDC operation does not require electrical energy input and no elevated pressure level like Reverse Osmosis (RO) [1].
- MDC requires, however, specific exoelectrogenic microorganisms which themselves need a continuous supply of a nutritious substrate for their metabolic work. MDC also requires an aerobic environment on the cathode side, which may be achieved by oxygen injection or otherwise procuring an aerobic water environment in the cathode chamber [6,8].
- The supply of microorganism substrate may be secured by connecting the MDC to a wastewater plant effluent to benefit additionally from wastewater treatment as a co-performing element of the MDC process.
- MDC may constitute a (limited) barrier against bacteria, viruses and other microbes as the pore sizes of ion exchange membranes used will be in the range of 1 nm (about 350 Dalton), i.e. considerably smaller than the size of viruses and bacteria [2]. However, contradicting this opinion, it should be noted that not the complete volume flow to be desalinated needs to pass through the ion exchange membranes, thus limiting the true sanitary impact of MDC technology.
- Due to yet missing field experience, it is not known to what extent MDC modules might need intermittent cleaning during operation or to what extent they might need to be preserved against biological growth in case of shut down for a prolonged period.
For further requirements concerning MDC plant layout, please also refer to Section 2 (Plant Design) hereinafter.
Motivation of Use
In consideration of the remarkable energy cost of other desalination technologies, MDC has been developed to overcome this drawback [1]. The idea to desalinate water without consumption of energy is certainly fascinating. It requires, however, the integration of another process element (energy production by exoelectrogens from digesting organic matter). At this point of time, MDC has a unique potential to emerge as a technology to perform three fundamental tasks simultaneously: (seawater) desalination, electric energy generation and wastewater treatment [8].
Technical Variations
MDC Design Variation and Optimisation
The basic MDC cell, incorporating three chambers forming one stack, is illustrated in Fig. 1 above.
While this type of model will be indispensable for lab purposes, multiple stack designs are desirable to increase desalination volume and to decrease manufacturing cost. Multiple stack structuring is achieved by repetitive addition of AEM and CEM membrane pairs to the module as illustrated in Fig. 3 below, thus creating repeating desalting and concentration cells [7].
Fig. 3: Stack structure of MDC
Stacking increases the magnitude of ionic separation. In a stacked MDC, a single electron transfer from anode to cathode can separate as many ion pairs as the number of repeated AEM and CEM membrane pairs. For instance, in a 3 cell pair stack, a single electron transfer separates 3 pairs or mono-valent cations and anions by use of an (ideal perm-selective) ion exchange membranes [2,8].
Other basic design considerations in MDC technology relate to:
a) sizing of the anode surface in relation to cathode surface, whereby the cathode appears to require larger surfaces or a boost in speed of performance due to the relatively low speed of chemical reduction performance of the cathode;
b) sizing of the anode chamber volume versus the desalination chamber and cathode chamber volume. It appears that anode chambers require a significantly larger volume than the other chambers;
c) choosing optimum electrode materials, particularly for the cathode due to its slow chemical conversion characteristics. In this context, improved material choice for the purpose of elevated current densities per m2 of electrode surface is of particular interest [11];
d) choosing optimal electrode materials to improve electrode stability and lifetime.
MDC Operational Variations & Performance Optimisation
As MDC works with organic matter and may fulfil three tasks at once, there are numerous parameters to work on simultaneously. Research is on its way to study variations of parameters and analyse patterns of dependence. Yet, as MDC technology is relatively young, most current knowledge is based on lab tests. To the best of the author ́s knowledge, MDC field experience is still missing. Therefore, the following aspects of operation are of indicative nature and observations may yet not all be fully studied. Particular complexities are apparent when applying MDC:
- A particular optimisation complexity of MDC is to understand the convergence or divergence of achieving three optimisation targets simultaneously, i.e. maximum electrical power generation, highest desalination volume or highest degree of desalination and highest reduction of organic matter.
- A further particular complexity of MDC is to work with a multitude of parameters that may be altered and to understand their relative alteration contribution on the desired performance targets, both technically and commercially.
Some operational aspects, however, have been researched already in some detail and appear to have influence on MDC performance and its optimisation:
- pH value variation between anode and cathode chamber: As the desalination cycles proceed, the pH value in the anode chamber decreases due to proton accumulation by microbial respiration and increases in the cathode chamber due to hydroxide accumulation by oxygen reduction [8]. Therefore, decrease in pH value is a major challenge for MDC operation [2]. It adversely affects both, the microbial activity and growth in the anode chamber and the chemical reduction efficiency of electron acceptors (O2) in the cathode chamber. Such pH imbalance may be alleviated by increasing the anodic chamber volume or by adding acid or base. The addition of acid or base as well as the addition of a phosphorous buffer solution do not appear feasible with respect to cost and environmental concerns. Furthermore, frequent exchange of the anolyte solution, i.e. the fluid contained in the anode chamber, has proven beneficial [6].
A recirculation of a fraction of the anolyte to the cathode chamber and from there back to the anode chamber, a so-called recirculation microbial cell (rMDC) design has been investigated to overcome the pH problem. As this solution generates new issues (e.g. transfer of O2 to the anode side), an economic and sustainable method of controlling the pH remains an urgent development need in MDC technology [6, 8].
- Impact of Internal (Ohmic) Resistance: A decrease in conductivity of the saline feed water increases the internal resistance (IR), i.e. the resistance within the MDC cell and thus decreases the overall electric cell potential of the MDC. As a countermeasure, membrane distance may be increased to decrease IR [1, 6].
Also, stacking of MDC improves the desalination rate, but also leads to a higher electric potential loss due to an increase in IR from increased stacking [2, 6].
MDCs operated with lower Hydraulic Retention Time (HRT), i.e. the period until exchange of the anolyte, shows lower IR [6].
- Impact of External (Ohmic) Resistance:
The external resistance (ER), i.e. resistance in the wire between anode and cathode, is of importance for the desalination efficiency. At lower ER values (e.g. 10 Ohm instead of 100 Ohm), the electrical current density (in theory) is higher and thus increases the rate of desalination. In other words, more Total Dissolved Solids (TDS) is removed with lower ER (i.e. higher electrical current density) [6].
- Batch vs. continuous MDC operation:
In batch mode, the performance of MDCs decreases with time as the internal resistance (IR) increases due to the lowering of electrolyte conductivity in the desalination chamber. Cyclic batch mode, or even better, continuous mode operation can be the optimum choice for practical operations because it reduces total capital cost and facilitates control of decisive parameters such as pH and substrate concentration [6].
- Variation of Hydraulic Retention Time (HRT):
MDCs operated with lower HRT of anolyte solution exhibit lower Internal Resistance (IR) [6, 12].
- Scaling of CEM membranes and fouling of AEM membranes:
Depending on the salinity of the water to be desalinated, CEM scaling is of significant importance for MDC performance [6, 7]. A complete prevention of membrane scaling appears impossible; however, use of an oxygen cathode with acidified water may alleviate scaling [7]. Membrane fouling may be observed on AEM due to biofilm growth [5, 6]. Chosen ion exchange membranes also differ considerably in pore size, degree of organic matter.
- Choice of exoelectronic microorganisms:
It will be obvious that the metabolism of microorganisms differs from species to species. Therefore, study of MDC performance in terms of electron generation capabilities and degradation potential of organic matter are relevant areas of study [13].
Differences to and Similarities with other Relevant Technologies
The MDC process has great similarity with Electrodialysis (ED), but in the case of ED external electrical energy is used to provide energy for the separation of ionic species.
With respect to Reverse Osmosis (RO), MDC does not need pressure elevation to high levels.
However, different to all other desalination technologies, MDC integrates bio-electrochemical energy production with desalination and water treatment, i.e. performs multiple functions. Due to this specialty, a one to one comparison of MDC to other (pure) desalination technologies is not advisable, also because the technologies have different outputs.
A further comparison of benefits and drawbacks vis à vis other desalination technologies appears premature due to the fact that MDC does not yet have practical field experience for realistic comparison.
2. Plant Design
Process Requirements
A full plant arrangement may - apart from the core desalination unit (MDC) - also require the following:
- A feed pump for supplying pretreatment and desalination unit with feed water is needed.
- Appropriate pretreatment to limit scaling and fouling of MDC desalination chambers with elevated scaling risk seems to be necessary.
- Appropriate process treatment to reduce scaling potential, particularly on CEM membranes, appears necessary.
- Depending on finally preferred channel width of the desalination chamber (e.g. 0.3 – 1.5 cm) and the preferential widths of anode and cathode chambers, the MDC cell requires some specific degree of protection against blocking by particles which may differ for the three chambers into which fluids are fed. Particle filters in the range of a few microns could serve as the last step of pretreatment for the feed water to be desalinated, for the influent water from waste water treatment process to be fed to the anode chamber as well as for the liquid in the cathode chamber [6].
- Depending on application, the feed water streams may need further individual pretreatment to protect the micron filtration systems suggested beforehand (e.g. further pre-filtration step at larger mesh size).
- Depending on Total Suspended Solids and Total Organic Carbon (TSS/TOC) loading of the feed water, further pretreatment to reduce TSS/TOC volume may be required for the feed water to be desalinated; also, the TSS/TOC levels of the anolyte and catholyte need separate specification.
- In applications with elevated scaling risk, appropriate process treatment to reduce scaling potential, particularly on CEM membranes, appears necessary.
- Compressed air injection may be required for optimised cathode performance.
- Cleaning-in-Place (CIP) unit (possibly with neutralisation) for cleaning of MDC modules may be a requirement.
- A wastewater disposal facility to allow for occasional CIP wastewater discharge (if in fact required), intermittent filter backwash water discharge and sludge discharge for used up anolyte discharge may be needed.
- Depending on application, posttreatment of the desalinated water may be required.
MDC Plant Layout
Due to the advantage of being able to build MDC units module based, MDC plants are scalable in size [3]. Fig. 4 illustrates an exemplary MDC system arrangement.
Fig. 4: Exemplary MDC system arrangement (in conjunction with wastewater treatment plant)
3. Feed Water
1. Different from other desalination technologies, three feed streams need to be considered: the feed flow of the water to be desalinated, the wastewater treatment flow connecting to the anode chamber and the water flow feeding the cathode chamber.
2. Without any field experience so far, it is premature to take a judgement on the potential techno-commercial limitations of MDC due to the level of feed water salinity of the water to be desalinated.
3. As far as other pretreatment requirements are concerned, please refer to the points mentioned hereinabove.
4. Temperatures may be limited to AEM and CEM operational limits, possibly also to anode and cathode temperature limitations.
5. For general guidance purpose, the following Table 1 characterises general MDC feed water limits for the water to be desalinated.
Feed Water Salinity | Technically feasible | Now in Use for Desalination | Potentially for techno-commercial Viability for the Future |
> 100,000 ppm |
|||
50,000 – 10,0000 ppm |
|||
10,000 – 50,000 ppm |
X | X | |
1,000 – 10,000 ppm |
X | X | |
< 1,000 ppm |
X | X |
Table 1: Stage of development of MDC technology in relation to water salinity [2, 6]
Because of the comparatively low desalination efficiency for high-salinity water, one potential application of MDC technology could be brackish water desalination [6].
4. Desalinated Water & Brine
Without any field experience so far, it is premature to characterise desalinated water and discharge flows in volume or quality other than recognising their existence as depicted in Fig. 4 above.
5. Data and Information
In the absence of field test results so far, it does not appear appropriate to take assumptions on data and information under this chapter.
6. Preferred Use
The techno-commercial feasibility of MDC has not yet been demonstrated in the field. Therefore, the preferred uses listed below are of indicative nature only:
1. Seawater desalination
Lab tests vary widely < [1,9] but show up to 99 % salinity removal depending on Hydraulic Retention Time (HRT) [6].
2. Brackish water desalination
An obstacle of low salinity desalination by MDC is that the low conductivity increases the internal resistance on the MDC. This disadvantage may be overcome by adding an ion exchange resin to the desalination chamber. Also, salt removal from the desalination channel increases the salinity of the anolyte which may negatively affect the activity level of the microbial community in the anode chamber [6].
3. Water softening
Up to 90 % hardness removal has been demonstrated in lab tests [6].
4. Ground water remediation (in particular nitrate removal) may be a feasible application [6].
5. Simultaneous desalination, wastewater treatment and hydrogen production (while adding energy to an MDC process in a modified “Microbial Electrolysis and Desalination Cell”) may be a desired application [6].
7. Environmental Impact
With respect to energy consumption the MDC may be considered a sustainable process, as it is based on a renewable primary energy source. However, research has not yet demonstrated dependable field test results to judge environmental impacts of MDC professionally.
Also, and as mentioned above, it appears likely from lab test demonstrations that MDC will not be able to survive without countering measures of scaling and fouling. In this respect dosing of chemical additives is very likely to be required to maintain MDC performance.
8. Stage of Maturity
Table 2 summarises MDC technology with respect to the technology ́s stage of maturity.
1 | 2 | 3 | 4 | 5 |
Basic R&D |
Prototyping/ |
Field Tests/ Demonstration |
Commercialization (up to 7 years) | Use/Established (more than 7 years) |
X |
Table 2: Stage of maturity of MDC technology
9. Further Developments
These are the challenges for improvement in MDC:
1. Primary target for MDC will be to reach field test level to measure and benchmark real performance and to generate cost reduction from exploiting scale effects [14].
2. Scaling up of MDC devices will be a further target. While primary tests have been performed with desalination chambers of 3 ml, larger desalination chambers (15 litres) are subject of more recent investigation. Simultaneously the anode chamber has been increased from 27 ml to 60 litres. The largest MDC (total volume of 105 litres) has achieved an electricity generation of 2,000 mA and a salt removal rate of 9.2 kg/m3/d [6, 14].
3. For developing more breadth and depth in applications, it is envisioned that MDC may be used as pretreatment of RO systems to reduce the overall cost of desalination [6]. In particular, the energy produced in MDC operation could be used to run downstream RO or Electrodeionisation (EDI) plants.
4. Major challenges for the development of MDC as a desalination technology are:
- managing pH variations;
- managing scaling and fouling, in particular cathode scaling;
- controlling internal and external resistance;
- improving anode and cathode materials and longevity;
- improving exoelectrogenic bacteria performance.
10. References
[1] Cao, Xiaoxin; Huang, Xia; Liang, Peng; Xiao, Kang; Zhou, Yingjun; Zhang, Xiaoyuan; Logan, Bruce E.: A New Method of Water Desalination Using Microbial Desalination Cells. Environmental Science & Technology 43 (2009), pp. 7148-7152.
[2] Kim, Younggy; Logan, Bruce E.: Microbial desalination cells for energy production and desalination. Desalination 308 (2013), pp. 122-130.
[3] Subramani, Arun; Jacangelo, Koseph G.: Emerging desalination technologies for water treatment: A critical review. Water Research 75 (2015), pp. 164-187.
[4] Logan, Bruce: Exoelectrogenic bacteria that power fuel cells. Nature Reviews Microbiology 7 (2009), pp. 375-383.
[5] Wang, Heming; Ren, Zhiong Jason: A comprehensive review of microbial electrochemical systems as a platform technology. Biotechnology Advancs 31 (2013), pp. 1796-1807.
[6] Sevda, Surajbhan; Yuan, Heyang; He, Zhen; Abu-Reesh, Ibrahim M: Microbial desalination cells as a versatile technology: Functions, optimization and prospective. Desalination 371 (2015), pp. 9-17.
[7] Barastad, Kristen S.; He, Zhen: Water softening using microbial desalination cell technology. Desalination 309 (2013), pp. 32-37.
[8] Saeed, Henna M., Husseini, Ghaleb A.; Yousef, Sharifeh; Saif, Jawaria; Al- Asheh, Sameer; Abu Fara, Abdullah; Azzam, Sara; Khawaga, Rehab; Aidan, Ahmed. Microbial desalination cell technology: A review and a case study. Desalination 359 (2015), pp. 1-13.
[9] Luo, Haiping; Xu, Pei; Roane, Timberley M., Jenkins, Peter E., Ren, Zhiyong: Microbial desalination cells for improved performance in wastewater treatment, electricity production, and desalination. Biosource Technology 105 (2012), pp. 60-66.
[10] Asadi-Ghalhari; Mehrdadi, Nassar; Nabi-Bidhendi, Gholamreza: Simultaneous Desalination of Was water and Electricity Production with New Membrane Technolgoy, Air-Cathode Microbial Desalination Cells. Current World Environment, Vol. 10(1) (2015), pp. 115-120.
[11] Sharma, Mohita; Bajracharya, Suman; Gildemyn, Sylvia; Patil, Sunil A.; Alvarez-Gallego, Yolanda; Pant, Deepak; Rabaey, Korneel; Dominguez-Benetton, Xochtil: A critical revisit of the key parameters used to describe microbial electrochemical systems. Electrochimica Acta 140 (2014), pp. 191- 208.
[12] Qu, Youpeng; Feng, Yujie; Liu, Jia; He, Weihua; Shi, Xinxin; Yang, Qiao; Lv, Yiangwei; Logan, Bruce: Salt removal using multiple microbial desalination cells under continuous flow conditions. Desalination 317 (2013), pp. 17-22.
[13] Logan, Bruce: Bioenergy production using microbial fuel cells. Presentation posted on YouTube Jan. 6, 2016. URL: https://youtube/x-iN-O89TNA (Stand 21.08.2017)
[14] Zhang, Fei; He, Zhen: Scaling up microbial desalination cell system with a post-aerobic process for simultaneous wastewater treatment and seawater desalination. Desalination 360 (2015), pp. 28-34.
- Technical Description
- Plant Design
- Feed Water
- Desalinated Water & Brine
- Data and Information
- Preferred Use
- Environmental Impact
- Stage of Maturity
- Further Developments
- References
1. Technical Description
Adsorption is a technology to remove ions from water by applying an electrical voltage difference between two porous carbon electrodes, in which ions will be temporarily stored.
CDI is a technology also known as Electrochemical Demineralization 〈1〉 or as Electrosorption 〈2〉. CDI's preferred use is in low salinity desalination applications (in principle below 10,000 ppm (1), in practice below 4,000 ppm (4)), where the benefit is lower energy consumption and higher recovery rates in comparison to Reverse Osmosis (RO) or distillation as alternative methods of brackish water desalination. This is mainly because CDI removes only salt ions, i.e. a tiny mass, from the salty feed water, whereas in RO or distillation a massively larger mass of water is removed from the salty feed water (1).
2. Plant design
The humidification dehumidification (HD) process uses the temperature dependence of the vapour pressure of water in air. The vapour pressure of water in air at saturation rises exponentially with the temperature. The dependence can be approximated e.g. by the Antoine equation:
- Technical Description
- Plant Design
- Feed Water
- Desalinated Water & Brine
- Data and Information
- Preferred Use
- Environmental Impact
- Stage of Maturity
- Further Developments
- References
1. Technical Description
Electrosorption is a technology to remove ions from water by applying an electrical voltage difference between two porous carbon electrodes, in which ions will be temporarily stored.
CDI is a technology also known as Electrochemical Demineralization 〈1〉 or as Electrosorption 〈2〉. CDI's preferred use is in low salinity desalination applications (in principle below 10,000 ppm (1), in practice below 4,000 ppm (4)), where the benefit is lower energy consumption and higher recovery rates in comparison to Reverse Osmosis (RO) or distillation as alternative methods of brackish water desalination. This is mainly because CDI removes only salt ions, i.e. a tiny mass, from the salty feed water, whereas in RO or distillation a massively larger mass of water is removed from the salty feed water (1).
2. Plant design
The humidification dehumidification (HD) process uses the temperature dependence of the vapour pressure of water in air. The vapour pressure of water in air at saturation rises exponentially with the temperature. The dependence can be approximated e.g. by the Antoine equation:
- Technical Description
- Plant Design
- Feed Water
- Desalinated Water & Brine
- Data and Information
- Preferred Use
- Environmental Impact
- Stage of Maturity
- Further Developments
- References
1. Technical Description
Desorption is a technology to remove ions from water by applying an electrical voltage difference between two porous carbon electrodes, in which ions will be temporarily stored.
CDI is a technology also known as Electrochemical Demineralization 〈1〉 or as Electrosorption 〈2〉. CDI's preferred use is in low salinity desalination applications (in principle below 10,000 ppm (1), in practice below 4,000 ppm (4)), where the benefit is lower energy consumption and higher recovery rates in comparison to Reverse Osmosis (RO) or distillation as alternative methods of brackish water desalination. This is mainly because CDI removes only salt ions, i.e. a tiny mass, from the salty feed water, whereas in RO or distillation a massively larger mass of water is removed from the salty feed water (1).
2. Plant design
The humidification dehumidification (HD) process uses the temperature dependence of the vapour pressure of water in air. The vapour pressure of water in air at saturation rises exponentially with the temperature. The dependence can be approximated e.g. by the Antoine equation:
- Technical Description
- Plant Design
- Feed Water
- Desalinated Water & Brine
- Data and Information
- Preferred Use
- Environmental Impact
- Stage of Maturity
- Further Developments
- References
1. Technical Description
Sorption is a technology to remove ions from water by applying an electrical voltage difference between two porous carbon electrodes, in which ions will be temporarily stored.
CDI is a technology also known as Electrochemical Demineralization 〈1〉 or as Electrosorption 〈2〉. CDI's preferred use is in low salinity desalination applications (in principle below 10,000 ppm (1), in practice below 4,000 ppm (4)), where the benefit is lower energy consumption and higher recovery rates in comparison to Reverse Osmosis (RO) or distillation as alternative methods of brackish water desalination. This is mainly because CDI removes only salt ions, i.e. a tiny mass, from the salty feed water, whereas in RO or distillation a massively larger mass of water is removed from the salty feed water (1).
2. Plant design
The humidification dehumidification (HD) process uses the temperature dependence of the vapour pressure of water in air. The vapour pressure of water in air at saturation rises exponentially with the temperature. The dependence can be approximated e.g. by the Antoine equation:
- Technical Description
- Plant Design
- Feed Water
- Desalinated Water & Brine
- Data and Information
- Preferred Use
- Environmental Impact
- Stage of Maturity
- Further Developments
- References
1. Technical Description
Electrical Double Layer (EDL) is a technology to remove ions from water by applying an electrical voltage difference between two porous carbon electrodes, in which ions will be temporarily stored.
CDI is a technology also known as Electrochemical Demineralization 〈1〉 or as Electrosorption 〈2〉. CDI's preferred use is in low salinity desalination applications (in principle below 10,000 ppm (1), in practice below 4,000 ppm (4)), where the benefit is lower energy consumption and higher recovery rates in comparison to Reverse Osmosis (RO) or distillation as alternative methods of brackish water desalination. This is mainly because CDI removes only salt ions, i.e. a tiny mass, from the salty feed water, whereas in RO or distillation a massively larger mass of water is removed from the salty feed water (1).
2. Plant design
The humidification dehumidification (HD) process uses the temperature dependence of the vapour pressure of water in air. The vapour pressure of water in air at saturation rises exponentially with the temperature. The dependence can be approximated e.g. by the Antoine equation:
- Technical Description
- Plant Design
- Feed Water
- Desalinated Water & Brine
- Data and Information
- Preferred Use
- Environmental Impact
- Stage of Maturity
- Further Developments
- References
1. Technical Description
Feed Water is a technology to remove ions from water by applying an electrical voltage difference between two porous carbon electrodes, in which ions will be temporarily stored.
CDI is a technology also known as Electrochemical Demineralization 〈1〉 or as Electrosorption 〈2〉. CDI's preferred use is in low salinity desalination applications (in principle below 10,000 ppm (1), in practice below 4,000 ppm (4)), where the benefit is lower energy consumption and higher recovery rates in comparison to Reverse Osmosis (RO) or distillation as alternative methods of brackish water desalination. This is mainly because CDI removes only salt ions, i.e. a tiny mass, from the salty feed water, whereas in RO or distillation a massively larger mass of water is removed from the salty feed water (1).
2. Plant design
The humidification dehumidification (HD) process uses the temperature dependence of the vapour pressure of water in air. The vapour pressure of water in air at saturation rises exponentially with the temperature. The dependence can be approximated e.g. by the Antoine equation:
- Technical Description
- Plant Design
- Feed Water
- Desalinated Water & Brine
- Data and Information
- Preferred Use
- Environmental Impact
- Stage of Maturity
- Further Developments
- References
1. Technical Description
Brackish Water is a technology to remove ions from water by applying an electrical voltage difference between two porous carbon electrodes, in which ions will be temporarily stored.
CDI is a technology also known as Electrochemical Demineralization 〈1〉 or as Electrosorption 〈2〉. CDI's preferred use is in low salinity desalination applications (in principle below 10,000 ppm (1), in practice below 4,000 ppm (4)), where the benefit is lower energy consumption and higher recovery rates in comparison to Reverse Osmosis (RO) or distillation as alternative methods of brackish water desalination. This is mainly because CDI removes only salt ions, i.e. a tiny mass, from the salty feed water, whereas in RO or distillation a massively larger mass of water is removed from the salty feed water (1).
2. Plant design
The humidification dehumidification (HD) process uses the temperature dependence of the vapour pressure of water in air. The vapour pressure of water in air at saturation rises exponentially with the temperature. The dependence can be approximated e.g. by the Antoine equation:
More...
- Technical Description
- Plant Design
- Feed Water
- Desalinated Water & Brine
- Data and Information
- Preferred Use
- Environmental Impact
- Stage of Maturity
- Further Developments
- References
1. Technical Description
Reverse Osmosis (RO) is a technology to remove ions from water by applying an electrical voltage difference between two porous carbon electrodes, in which ions will be temporarily stored.
CDI is a technology also known as Electrochemical Demineralization 〈1〉 or as Electrosorption 〈2〉. CDI's preferred use is in low salinity desalination applications (in principle below 10,000 ppm (1), in practice below 4,000 ppm (4)), where the benefit is lower energy consumption and higher recovery rates in comparison to Reverse Osmosis (RO) or distillation as alternative methods of brackish water desalination. This is mainly because CDI removes only salt ions, i.e. a tiny mass, from the salty feed water, whereas in RO or distillation a massively larger mass of water is removed from the salty feed water (1).
2. Plant design
The humidification dehumidification (HD) process uses the temperature dependence of the vapour pressure of water in air. The vapour pressure of water in air at saturation rises exponentially with the temperature. The dependence can be approximated e.g. by the Antoine equation:
Capacitive Deionisation (CDI) & Membrane Capacitive Deionisation (MCDI) also known as “Electrochemical Demineralisation“ or as “Electrosorption“
Written by Detlef TaproggeCapacitive Deionisation (CDI) is a technology for removing salt ions from water by applying an electrical voltage difference between two porous carbon electrodes in which the ions will be temporarily stored [1].
The driving force of the process is the application of an electrical voltage difference [2]; the separation principle of salts from water is based on the capability of ions to move selectively according to their charge; in different words: positively and negatively charged salt ions in aqueous solutions migrate towards electrodes of opposite charge.
1. Technical Description
The Basics – Operating Mechanism
CDI´s preferred use is in low-salinity desalination applications (in principle below 10,000 ppm [1], in practice below 4,000 ppm [3]), where the benefit is lower energy consumption and higher recovery rates in comparison to Reverse Osmosis (RO) or distillation as alternative methods of brackish water desalination. This is mainly because CDI removes only salt ions, i.e. a tiny mass, from the saline feed water, whereas in RO or distillation a considerably larger mass of water is removed from the saline feed water [1].
Literature has traditionally classified CDI by its nature as an electrochemical technology, however, recent research [1] suggests that the most fundamental underlying principle of CDI is capacitive ion storage, a phenomenon based on the formation of a so called electrical double layer (EDL). In simple words: upon applying an electrical charge, ions are captured electrostatically and stored capacitively (analogous to electrical charge being stored in an electrical condenser/“capacitor“) in a diffuse layer formed next to the carbon surface. Ideally, pure physical accumulation of ions (sorption) dominates the process, not chemical (faradaic) reactions. The process is therefore easily reversible. Congruently, CDI may also be viewed as a process of ion sorption and desorption on electrically charged surfaces; CDI is, therefore, also named Electrosorption [1].
The formation of a capacitive EDL has today been identified as the actually dominant underlying principle for ion removal and is, therefore, at the heart of in-depth understanding and optimising the CDI process and its performance [1].
Apart from the aforementioned “capacitive ion storage“, a number of further electrochemical mechanisms are believed to accompany the “capacitive ion storage” phenomenon in a CDI cell which may influence CDI performance. Abstracting from the presence of such parallel electrochemical side effects, the CDI process – in its ideal form - is of purely physical nature and potentially enables CDI devices to have a long service life and low maintenance [1].
In practice, however, some electrochemical impacts are undoubtedly active, typically leading to some degree of electrochemically induced deviation from ideal performance of CDI cells over time (mainly faradaic reactions) [1].
Following the above explanations, CDI is a desalination technology that operates by adsorbing ions in an EDL formed in the proximity of electrodes by the application of an electrical potential difference. These electrodes are made of porous carbons optimised for high (ion) salt storage capacity and for good ion and electron transport [1].
In a nutshell, two fundamental technical phenomena need to be mastered and controlled in CDI systems [1]:
- “capacitive ion storage“ (capturing ions electrostatically and storing them capacitively)
- “ion kinetics“ (the differences in mobility of the various ions on EDL formation).
While scientists may have today opposing convictions about the most dominantly underlying principle of CDI being of physical or chemical nature, categorisation also appears to depend on the breadth of definition of the term “electrochemistry”. It will be an interesting debate of whether or not it is preferable to hold on to historic practice of categorising CDI methodologically under the class of electrochemical desalination technologies or differently.
Process Description CDI
The basic mechanism underlying CDI is schematically shown in Fig. 1. It depicts a single CDI cell. In practice, multiple CDI cells are stacked on top of each other to form a stack; several stacks are packed together to form a CDI module. The process of CDI comprises three steps:
Step 1 – Purification (Ion Electrosorption) (see Fig. 1 A)
Upon applying a direct current (DC) voltage difference (typically between 1.2 and 1.5 V) between two porous carbon electrodes (typically attached to metallic current collectors) ions are attracted to the respective electrodes, i.e. cations (ions with positive charge, here, for example, Na+) to the negatively charged electrode, and anions (ions with negative charge, here, for example, Cl-) to the positively charged electrode. During this first “ion electrosorption” step, ions are stored in each of the two porous carbon electrodes. As a result, the saline feed water gets more and more deprived of the salt ions and, thus, is desalinated [1].
Fig. 1 A: CDI purification step Fig. 1 B: CDI regeneration step Fig. 1 C: CDI flushing step
Step 2 – Regeneration Step (Ion Desorption) (see Fig. 1 B)
As the electrodes become saturated with ions electrosorbed into the porous carbon, the electrodes require regeneration after some time. For regeneration purpose, the potential difference is reduced to zero (or reversed) and, in consequence, ions are desorbed from the electrodes and released back into the water stream [1] for the purpose of subsequent flushing them out with the bulk fluid as brine.
Step 3 – Flushing Step (see Fig. 1 C)
Finally, the desorbed ions are released into the water stream and flushed out of the CDI cell as brine. A further desalination cycle can restart hereafter [3].
To separate desalinated water from intermittent brine flushing, corresponding flow separation needs to be procured in the system´s piping arrangement.
A disadvantage of this CDI design is that – when reversing the electrical potential difference – ions may be driven to the electrode on the opposite side of the feed water channel. Once the electric potential difference is reversed, the adsorbed ions will be desorbed; at the same time, however, oppositely charged ions from the bulk fluid will be attracted and adsorbed. Therefore, ion desorption and adsorption occur simultaneously during this step. Consequently, some typical drawbacks will need to be taken into consideration, namely:
– incomplete regeneration of the electrodes leading to a deterioration in their adsorption capacity,
– longer regeneration times and
– residual ion accumulation blocking the way of other ions during the next purification step [4].
To avoid this drawback, a modification of CDI exists using specific membranes which is generally referred to as “Membrane Capacitive Deionisation” (MCDI).
Process Desciption MCDI
To enhance desalination performance, traditional CDI cells are modified by integration of ion selective membranes, as depicted in Fig. 2 A. This design is known as Membrane Capacitive Deionisation (MCDI) (see Fig. 2 B).
Fig. 2 A: CDI basic design Fig. 2 B: MCDI basic design
In MCDI, ion selective membranes (ion exchange membranes) are applied as thin films (typically 50 – 200 μm thickness) or coated (typically 20 μm thickness) directly to each of the two carbon electrodes, i.e. a cation exchange membrane to the cathode and an anion exchange membrane to the anode [1]. Cations can pass cation exchange membranes freely, anions are repelled; vice versa, anions can move freely through anion exchange membranes, cations cannot.
The MCDI process works similarly to the CDI process, as shown in Fig. 3.
Fig. 3 A: MCDI purification step Fig. 3 B: MCDI regeneration step Fig. 3 C: MCDI flushing Step
As a particular advantage of MCDI, the ion selective membranes attached to the respective electrodes hinder salt ions to get electrosorbed by the opposite electrode when switching the potential difference during the regeneration step [4].
CDI Process Peculiarities
- Sequential production of desalinated water is a peculiarity of (M)CDI operation. In some typical CDI cell geometries (M)CDI desalination water production is intermittently interrupted by the need to flush the (M)CDI cell. Precise valve switching for the separation of brine from desalinated water flow is, therefore, essential.
- Energy requirements of (M)CDI strongly correlate with the level of feed water salinity and to the level of ions removed from the feed water [2].
- (M)CDI does not provide a barrier against bacteria, viruses and other microbes [5].
- (M)CDI requires prevention against scaling and fouling [6].
- Pressurised air is required when (M)CDI modules use air scouring [7]. The latter is a method to remove sediments and to avoid biofilm build-up. In air scouring, clean (compressed) air is injected into the feed flow of the (M)CDI cell, thereby generating a series of air slugs. The slugs scour away the sediments while the water flow expels the undesired waste to an outlet to be discharged.
- (M)CDI systems are module based; (M)CDI plants are, therefore, scalable in size.
- (M)CDI modules need to be preserved against biological growth by a biocide conservation solution in case of shut-down for a prolonged period, e.g. a week or more [8].
Requirements pertaining to the layout of an (M)CDI plant are covered under section “Plant Design“ hereinafter.
Motivation of Use
(M)CDI has been developed as a non-polluting, energy-efficient and cost-effective alternative to desalination technologies such as RO and Electrodialysis (ED) [9].
Technical Variations
- Basic CDI cell design (CDI or MCDI)
As mentioned in section 1, CDI cells are used in the traditional CDI design or in membrane enhanced MCDI design. MCDI design has operational advantages. - Basic CDI cell geometries
CDI cells may be built in different geometries. However, the so-called “flow-by” mode in which the water channel lies in-between the electrode pair, as illustrated in Fig. 1, represents the very typical cell geometry today. The pathway of the feed water flows between the electrodes can be designed as an open channel (about 1 mm thickness) or may be constructed from spacer material, then about 100 - 300 μm in thickness. Flow may be one-dimensional from an entry point in a straight line to the exit point, as depicted in Fig. 1 hereinabove or e.g. in a 2-dimensional plane from corner to corner or from a central input with radial flow [1]. - Increased independence from feed water salinity fluctuation
For making the CDI process less sensitive to variations in the feed water salinity, a recycle line with reservoir to take up desalinated water being recycled may be worthwhile considering [2]. (→ see Fig. 4). - Electrical operation mode of CDI plant
a) Constant voltage operation
In constant voltage operation, the effluent salt concentration increases first, but then decreases over time, as electrodes will get saturated with ions over time. In other words: in the beginning, the EDLs are still uncharged and, hence, the potential difference between electrodes (the driving force of ion electrosorption) is high. Over time, with more and more ions becoming electrosorbed in the EDLs, the EDL potential (charge) increases and the remaining potential difference between the electrode pair, which drives the ion transport, decreases. Because of the decreasing ion removal rate over time, the effluent concentration increases again [1].
b) Constant current operation (MCDI design)
As the ionic charge transported into the porous electrodes is equal to the applied electric current, applying a constant current allows a better control of the effluent salt concentration compared to constant voltage operation. For stable effluent salt concentration, an MCDI design (i.e. incorporation of ion selective membranes into the CDI cell) is preferable [1]
Differences to and Similarities with Other Relevant Technologies
(M)CDI appears to compare favorably to RO on operational cost [1] and has come closer in recent years to get to parity with comparable RO investment amounts [10]. CDI compares favorably to RO on energy cost and environmental burden [10]. In particular the higher recovery rates of CDI compared to RO decrease water intake and disposal volume and add to the sustainability benefit of CDI versus RO [11]. However, unlike RO, (M)CDI does not provide a barrier against microbes.
Compared to Ion Exchange (IX) in e.g. softening applications, (M)CDI, like RO and other electrochemical desalination technologies, is not competitive on the amount of investment, but extremely competitive on operational and environmental cost [7].
Compared to electrochemical desalination technologies such as Electrodialysis/Electrodialysis Reversal (EDR) [11] and Electrodeionisation (EDI) and Continous Electrodeionisation (CEDI) [12], relative differences are less clearly documented in literature and may need to be evaluated case by case. All such technologies, however, operate at high recovery rates, whereby EDI/CEDI appears to be limited even to brackish total dissolved solids (TDS) levels [2], but then again can produce water with extremely low salinity (e.g. boiler feed water or high purity water), which is not the domain of CDI.
2. Plant design
Process Requirements
Besides the core desalination system (M)CDI, a full plant arrangement reguires the following components:
- feed pump (> 3 bar head pressure) for conveying the feed water through pretreatment and desalination unit
- appropriate pretreatment, particularly in applications with elevated scaling risk, to avoid scaling and fouling of (M)CDI cells, e.g. pH adjustment, anti-scalant dosing etc. [6]
- a protection for the (M)CDI cell against blocking by particles given the narrow spacer channels of the (M)CDI cell; typically, a 5 μm particle filter will do as the last step of pretreatment [7]
- further pretreatment to protect the 5 μm filtration unit (e.g. further pre-filtration step at larger mesh size) depending on application
- further pretreatment to reduce the volume of total suspended solids (TSS) and total organic carbon (TOC) depending on TSS/TOC loading of the feed water
- pressurized air (6 bar), if air scouring of (M)CDI units is desired for enhanced performance [7]
- a cleaning in place (CIP) unit (possibly with neutralisation) for intermittent acidic cleaning of CDI modules [10]
- a waste water disposal facility to allow for occasional CIP waste water discharge and intermittent flushing water discharge
- no particular post-treatment other than mandated by the application of the desalination system
- voltage, electrical current and TDS control to monitor and control system operation; in particular, precision of valve switching to separate concentrate flow from desalinated water flow for a high recovery rate
(M)CDI Plant Layout
Due to (M)CDI being module-based, (M)CDI plants are scalable in size [7]. Fig. 4 shows a generic (M)CDI plant layout in batch mode operation, and, alternatively, in continuous mode operation (dotted line with recycle loop).
In batch mode operation the product water from a first pass through the (M)CDI desalination unit is collected and may then reinjected again and again as required for a desired level of desalination. As such batch operation is tedious and costly, a continuous mode of operation is generally preferred. This is achieved by recycling the product water via a recycle loop. In continuous mode operation the degree of desalination can be easily pre-set and variations in feed water salinity are more easily controllable.
3. Feed Water
- Different from RO and distillation technologies, energy consumption of (M)CDI is strongly related to salinity [5].
- (M)CDI performs well in brackish water applications, preferably at a feed water salinity below 4,000 ppm TDS [7].
- To avoid blockage of the spacer channel in CDI modules, particles in the feed water stream need to be controlled. Typically, a 5 μm pre-filtration will be appropriate [8/6].
- Temperatures may be slightly more elevated than in RO (< 60° C) [6].
- Feed water scaling potential needs to be evaluated [6].
- For general guidance, Table 1 characterises general (M)CDI feed water limits [6].
Parameter | Unit | Range | Intermittent |
Total Dissolved Solids | ppm | 0 – 4,000 | |
Total Organic Carbon | ppm | > 15 | |
Chemical Oxygen Demand | ppm | > 50 | > 100 |
Turbidity | NTU | > 4 | > 100 |
Fats, Oils, Greases | ppm | > 0,5 | |
Total Suspended Solids | ppm | > 4 | > 20 |
Free Chlorine | ppm | > 1 | > 25 |
pH | – | 2 – 10 | 1 – 12 |
Iron Total | ppm | > 0,5 | |
Total Hardness (as CaCo3) | ppm | > 1,000* | |
M Alkalinity (as CaCO3) | ppm | > 1,000* | |
Pre-filtration | μm | 5 | |
Temperature | °C | 5 – 60 | |
°F | 40 – 140 |
* Limits depend on set TDS reduction and water recovery
Table 1: Typical feed water composition for (M)CDI [6]
Table 2 illustrates existing and future applicability of (M)CDI technology in relation to the water salinity in which the technology is applicable. While literature sees the brackish water salinity segment (1,000 – 10,000 ppm salinity) as the core applicational segment for (M)CDI technology today [3, 6, 7, 8], there are meanwhile also claims from (M)CDI suppliers to have supplied first (M)CDI systems to the salinity segment above 10,000 ppm, even to applications above 100,000 ppm [13]. However, these applications have not been typical seawater desalination applications, but mainly were used to concentrate produced water in the oil & gas industry. Results on techno-commercial feasibility are not yet in the public domain.
Feed Water Salinity | Technically feasible | Now in Use for Desalination | Potentially for techno-commercial Viability for the Future |
> 100,000 ppm |
X | ||
50,000 – 10,0000 ppm |
X | ||
10,000 – 50,000 ppm |
X | ||
1,000 – 10,000 ppm |
X | ||
< 1,000 ppm |
Table 2: Stage of development of (M)CDI technology in relation to water salinity [3, 6, 7, 8]
4. Desalinated Water & Brine
The level of remaining salts in deionised water from a (M)CDI desalination process is highly dependent on the recovery rate applied and the energy input. The level of remaining ions in the desalinated water is – in limits – adjustable in (M)CDI operation [14].
Typically, (M)CDI produces deionised water at high recovery rates compared to RO [11]. As a result, the volume of brine in (M)CDI systems is generally lower than in RO systems. Depending on application (e.g. antiscalant dosing, pH adjustment), the brine of (M)CDI systems may or may not be contaminated by chemical additives.
Depending on application, waste streams will come from disposing intermittent CIP (cleaning-in-place). Due to (M)CDI´s capability of deionising at comparatively high recovery rates, feed water volume and brine disposal volume will usually be lower than in RO operation [11].
5. Data and Information [6]
- Typical field of economical desalination with regard to energy consumption:
Brackish water < 4,000 ppm TDS - Feed water temperature 5 – 60 °C
- Typically, higher recovery rate than RO (up to 95 %)
- Low pressure application, e.g. feed pressure < 3 bar [4]
- Compressed air requirement (6 bar) for air scouring
- Typical CDI cell voltage 1.2 – 1.5 V
- CDI specific energy consumption strongly depends on feed water salinity, as shown in Fig. 5.
- The specific energy consumption of CDI for feed water below TDS 4,000 ppm is significantly lower than that of RO, as depicted in Fig. 5.
- Exemplary module and system data [4]
• 1 module contains 12 CDI stacks
• 1 stack contains 23 CDI cells (flow between 2 electrodes)
• smallest system: 2 modules, total flow rate: 0.2 – 1.8 m3/h -
• system at industrial scale: 12 modules, total flow rate 1.4 – 10.8 m3/h
• typical module flow rate: 0.4 m3/h at 80 % recovery rate - CDI is operated fully automatically (low personnel cost for operation)
- Module replacement (no reliable data available yet)
6. Preferred Use
- Brackish water desalination (energy efficient below 4,000 ppm feed water TDS) [7]
- Water softening
- Commercial laundry waste water deionisation (waters with temperatures too high for RO, but manageable for CDI)
- Treatment of cooling tower make-up water (side stream filtration; IX replacement)
- Waste water / RO brine recovery
- Horticulture irrigation (maintaining of tuneable ion levels in the desalinated water) [14]
7. Environmental Impact
With energy consumption typically below RO in comparable brackish water applications [10], CDI ranks on a privileged level of sustainability in brackish water desalination.
Like RO systems, also (M)CDI systems are prone to inorganic and organic fouling in practical applications, even though it has been shown that switching the potentials of electrodes, as is standard in CDI operation, may drastically reduce fouling [2]. Also, (M)CDI is a low pressure operation compared to RO, which in principle should reduce fouling tendency.
Dosing of chemical additives in CDI operation is typically restricted to intermittent CIP (cleaning-in-place) with acids, and – as may be required – to antiscalant and/or pH adjustment in case of elevated scaling potential [6].
While (M)CDI systems appear to be affected by fouling in practical operation, these fouling problems may in tendency be viewed as somewhat less severe compared to RO [2].
Depending on application, (M)CDI brines will be or will not be contaminated by chemical additives. Depending on the application, CIP waste water requires neutralisation and disposal. Due to (M)CDI´s capability of deionising at comparatively high recovery rates, feed water utilisation and brine disposal will be lower than in RO operation
[2].
In comparison to RO, (M)CDI may in total be ranked preferable in environmental sustainability.
8. Stage of Maturity
Table 3 summarises (M)CDI technology with respect to the technology´s stage of maturity.
1 | 2 | 3 | 4 | 5 |
Basic R&D |
Prototyping/ |
Field Tests/ Demonstration |
Commercialization (up to 7 years) | Use/Established (more than 7 years) |
X | X |
Table 3: Stage of maturity of (M)CDI technology
9. Further Developments
Challenges for improvement in CDI are:
- To develop sufficient application breadth and depth for CDI, so as to lower CDI investment from experience and volume effects (economies of scale), particularly on module assembly cost. While RO has already travelled along the experience curve for a few decades, economies of scale improvement are yet to be experienced by CDI technology [1].
- As highest specific surface area at low resistivity is a key contributor to increased CDI performance, improvements in the choice of suitable electrode materials are of key importance. Key requirements for CDI electrode materials and associated CDI performance are:
- large ion-accessible specific surface areas (accessibility of electrode surface to ions)
- high electrochemical stability over pH and voltage range, to avoid oxidation and to ensure system longevity
- fast ion mobility within the micro- and macro-pores of the carbon electrode (e.g. inter-particle distance, intra-particle pore sizes, pore structure and thickness)
- high electronic conductivity of electrodes (metal like electronic conductivity to ensure that the entire electrode is charged without major voltage gradients and to avoid larger energy dissipation and heating up of the electrode)
- low contact resistance between the porous electrode and the current collector
- good wetting behaviour (good hydrophilicity ensures participation of the entire pore volume in the CDI process)
- low material cost
- ease and scalability of material processing for higher volumes
- abundant availability of the material and low CO2 footprint for material purchase
- high bio-inertness to avoid biofouling of CDI cell [1] - A particular challenge is the combination of high specific surface area with high ion mobility. The ultimate limit to the pore size is the bare ions size, e.g. 1.16 Angstrom for Na+ and 1.67 Angstrom for the Cl-, or, more precisely, to 3.58 Angstrom for sodium and 3.31 Angstrom for the chloride as solvated ions [1]. Commonly the pore sizes for most common carbons will be significantly larger (in the small nm area).
For reference: 1 Angstrom = 0.1 nm
For further reference: IUPAC Classification of pore size:
1. Macropores: > 50 nm
2. Mesopores: 2 – 50 nm
3. Micropores: < 2 nm [1]
Carbon materials of relevance for ion adsorption in porous electrodes are amongst others:
• activated carbon (100 – 3,500 m2/g)
• ordered mesoporous carbon (OMC – typically 750 – 1,500 m2/g)
• carbon aerogels (400 – 1,100 m2/g)
• carbide-derived carbons (1,200 – 2,000 m2/g)
• carbon nanotubes and graphene (fluctuating largely from 50 – 500 m2/g)
• carbon black (< 120 m2/g) [1] - Improvement in operational modes to increase yield in deionised water (e.g. move to continuous/semi-continuous operation; recycle-loop layout) [1]
- Improvement in cell unit design (e.g. wire type anodes) [15]
- Further reduction of energy cost by harvesting the energy generated in the ion adsorbing (charging) CDI module
10. References
[1] Porada, S.; Zhao, R.; van der Wal, A.; Biesheuvel, P.M.: Review of the science and technology of water desalination by capacitive deionization. Progress in Material Science 58, (2013), pp. 1388-1442
[2] Anderson, M. A.; Cudero, A. K.; Palma, J.: Capacitive deionization as an electrochemical means of saving energy and delivering clean water. Comparison to present desalination practice: Will it compete? Electrochimica Acta 55 (2010), pp. 3845-3856
[3] Voltea: Voltea´s Technical Bulletin – Desalination Cooling Water with CapDI, URL: http://www.aquafit4use.eu/userdata/file/Mid-term%20Conference/Session%203%20Advanced%20Treatments/Voltea%92s%20desalination%20technology%20applied%20on%20cooling%20water%20B%26J.pdf (accessed 16.01.2017)
[4] Al Marzooqi, F. A.; Al Ghaferi, A. A.; Saadat, I.; Hilal, N.: Application of Capacitive Deionisation in water desalination: a review. Desalination 342 (2014), pp. 3-15
[5] Oren, Y.: Capacitive deionization (CDI) for desalination and water treatment – past, present and future (a review). Desalination 228 (2008), pp. 10-29
[6] Voltea: Voltea´s Technical Bulletin – Feed Water Guidelines, URL: http://voltea.com/wp-content/uploads/2016/03/402D021_Rev02-Technical-Bulletin_Water-Quality-Criteria.pdf (last accessed on 16.01.2017)
[7] Voltea: Technical Specification Industrial Series – IS 2 to IS 48 Systems, URL: http://voltea.com/wp-content/uploads/2016/03/402D026EN_Rev06-IS-Technical-Specifications.pdf (last accessed on 16.01.2017)
[8] Voltea: Voltea´s Technical Bulletin – Preservation Handling Guidelines, URL: http://voltea.com/wp-content/uploads/2016/03/402D004_Rev01_Tech-Bulletin_Preservation-Handling-Guidelines-1.pdf (last accessed on 16.01.2017)
[9] Welgemoed, T. J.: Capacitive Desalination Technology: Development and Evaluation of an Industrial Prototype System, University of Pretoria; Dissertation (2005)
[10] Voltea: Technical Comparison CapDI vs. RO, URL: http://voltea.com/wp-content/uploads/2016/05/CapDI-vs-RO.pdf (last accessed on 16.01.2017)
[11] Voltea: Voltea´s Technical Bulletin – Technology Comparison, URL: http://www.voltea.com/wp-content/uploads/2015/12/402D002_Rev01_Tech-Bulletin_Technology-Comparison-1.pdf (last accessed on 16.01.2017)
[12] Subramani, A.; Jacangelo, J. G.: Emerging desalination technologies for water treatment: A critical review. Water Research 75 (2015), pp. 164-187
[13] Atlantis Technologies Website, URL: http://www.atlantis-water.com/value-proposition/ (last accessed on 29.04.2018)
[14] Voltea Website, URL: http://voltea.com/technology-4/ (last accessed on 16.01.2017)
[15] Porada, S., Sales, B. B., Hamelers, H. V. M., Biesheuvel, M.: Water Desalination with Wires. Journal of Physical Chemistry Letter 3 (2012), pp. 1613-1618
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Humidification-Dehumidification (HD or HDH) & Solar Stills & Multi-Effect Humidification (MEH)
Written by Dr.-Ing. Markus SpinnlerHumidification-Dehumidification (HD), sometimes also named HDH or in case of multiple effects “Multi-Effect Humidification” (MEH) imitates the natural rain cycle in an artificial environment. In the natural rain cycle, only pure water evaporates from the surface of the oceans, while the salt ions remain in the seawater.
HD systems also use the principle of evaporation of pure water in a humidifier and subsequent condensation of the pure water vapour in a dehumidifier [28, 29]. The separation of pure water and salt is due to a substantially lower vapour pressure of salt compared to water. Thus, at a given temperature, water will evaporate while salt is remaining dissolved in the liquid brine. Consequently, the vapour phase consists only of pure water. Different from vaporisation systems, a large majority of HD set- ups uses air as a carrier medium. Therefore, HD can be classified as diffusion-driven, evaporative process. The fundamental driver of HD systems is a net vapour pressure difference between feed water, carrier gas (mostly humid air) and condensate. This vapour pressure difference is induced by temperature differences between feed water and condensate, which are commonly generated by heating the feed water in an external heater (addition of heat).
1. Technical Description
The Basics – Operating Mechanism
A major advantage of the HD process compared to other phase-change technologies is that it needs thermal energy at a low temperature level (60 °C – 85 °C). Thus, one particular benefit is the possibility to drive HD processes with solar irradiation or waste heat. Another key advantage is its robustness towards the feed water quality and its suitability for Zero Liquid Discharge (ZLD) applications.
Operating Mechanism/Process Description
First of all, it is important to know that the vapour pressure of water is exponentially rising with increasing temperature. HD systems make use of this strong temperature dependence of both vapour pressure of water and partial vapour pressure of water in humid air.
Fig. 1: Principle of water vapour mass transfer in HD systems with air as carrier medium – Assumption of uniform temperature T in feed and condensate film as well as in the air stream, where TFeed > TAir > TCondensate. Boundary layer phenomena are neglected.
Let us consider a small section of an HD system with air as carrier medium (see Fig. 1). In an HD system the humidifier (sometimes “evaporator”) or the feed water itself is heated in order to raise the heat of evaporation. The humidifier releases water molecules from the feed water to the air, where they appear as pure water vapour (evaporation), whereas the salt ions remain in the feed water. A dehumidifier (often “condenser”) captures the water vapour molecules in a condensate film and releases the heat of condensation.
It is important to realise that the mass flow of pure water molecules is always following the gradient from high to low (partial) vapour pressure.
We have already learned that the vapour pressure of water pV,W is an exponential function of its temperature T. In fact, in can be described by the Antoine equation log10 pV,W = A – B/(C + T) with empirical constants A, B, and C. Note also, that the total pressure of a mixture is always a sum of the partial pressures of its single components (Dalton’s law). Therefore, it is obvious that the partial vapour pressure of a component is not only a function of temperature, but also of concentration of the respective component in the mixture:
pV,i = ƒ(wi, T),
where pV,i is the partial pressure of component i, w is the mass fraction or in other words concentration of component i and T is the temperature of the mixture. It is important to note that pV,i is rising with both increasing temperature and concentration!
Applying a temperature difference between feed water, humid air and condensate, which is mostly done by just heating the feed, will create
(1st) a difference between the partial vapour pressures of the water in the feed pV,Feed and of the water vapour in the humid air pV,Air;
(2nd) a difference between the partial vapour pressure of water vapour in air pV,Air to the saturation vapour pressure in the condensate pV,Condensate.
See Fig. 1, where TFeed > TAir > TCondensate. Therefore, pV,Feed > pV,Air > pV,Condensate, driving the net water flow from feed (humidifier) to humid air and from humid air to condensate (dehumidifier).
As was already pointed out, the partial vapour pressure of water in aqueous solutions is not only a function of temperature but also of salinity. Increasing salinity means decreasing the concentration of water in the solution. Decreasing the water concentration means decreasing the partial vapour pressure of water. Especially in applications with high salinity, for example in ZLD, the temperature-induced vapour pressure increase has to overcome the salinity-induced vapour pressure reduction, which demands high feed water operation temperatures. Thus, Specific Energy Consumption (SEC) will increase with increasing salinity. For details on temperature- and salinity-dependent water vapour pressures and partial pressures of water vapour in air, please refer to Annex C.
It has to be observed that the term distillation, which is frequently used in both HD and Membrane Distillation (MD) literature, is somewhat misleading, as it strictly denotes a selective vaporisation process. Mostly, HD devices work with air at ambient pressure [30] as a carrier medium, clearly indicating the evaporative nature of the HD process. The diction of “Solar Still” as one very simplified variant of HD, “Membrane Distillation” (except Vacuum MD) as well as using the term “evaporation” for vaporisation processes, like in “Multi-Stage Flash Evaporation“, is common practice but imprecise from a thermodynamic point of view.
Operating Mechanism
Fig. 2. Operating mechanism of a single-stage, closed air, water heated HD system
In an HD system, feed water (i.e. seawater) is heated to Top Brine Temperatures (TBT) of ≈ 85 °C. In most cases, according to the available heating power and in order to maintain a constant humidifier feed inlet temperature, the feed water mass flow is adjusted with a controllable pump. The warm feed water is sprayed into the so-called humidifier through a distribution system. To enhance the heat and mass exchange area at minimum feed mass flow, packing material is installed in the humidifier, where the feed water is trickling down forming a water film. Dry and relatively cold air gets into contact with the warm feed water film in the humidifier.
Due to the difference in partial water vapour pressures between feed water film and air, evaporation and sensible heat transfer take place. Both temperature and moisture of the air increase. Therefore, its density decreases. Thanks to the so created buoyant forces (free convection) or, in most plant configurations assisted by a fan (forced convection), the air is rising and entering the dehumidifier [37].
The dehumidifier is mostly realised as heat exchanger, in which the warm, humid air flow is cooled down to temperatures lower than the dew point. Air moisture is condensing outside the heat exchanger coils, is trickling down and collected as condensate. In potable water production, condensate is the product water. As coolant inside the heat exchanger coils, fresh feed water is used. During the condensation process, the heat of condensation is released on the air side. Thus, it can be used to preheat the fresh feed water.
Note that with this method, it is possible to recover the huge amount of heat of evaporation quite effectively. Except some heat losses in the containment, entropy generation in both humidifier and dehumidifier and varying material properties, the heat of evaporation can be recovered as heat of condensation. The quality of heat recovery can be described by the Performance Ratio PR, which, similar to the Gained Output Ratio GOR for steam-driven plants, is a direct indicator for the Specific Energy Consumption SEC.
In the heater, feed water is heated from dehumidifier outlet temperature to TBT, commonly ≈ 85 °C. This is on one hand done to raise the operation temperatures of the HD system in order to increase the driving vapour pressure differences between feed, air and condensate. On the other hand, the heater provides the large amount of heat of evaporation, which is needed to keep the evaporation process running.
Process Requirements/Pecularities
HD is a relatively robust technique, where any kind of aqueous solution with vapour pressure of the components lower than that of pure water can be treated. Feed water needs to be coarsely filtered and volatile components, such as oil, solvents, alcohol, compounds of NH3 etc., in short everything with a vapour pressure higher than that of pure water, needs to be separated. In some applications however, these components can be seen as product. For instance, in liquid manure processing, NH3 – water solutions can be used for the production of fertilisers. [40, 44]
Depending on the humidifier packing, scaling in the humidifier to a certain extent does not harm the evaporative processes. In contrary, it but might even be advantageous due to an increase of the wetted area [37]. Scaling phenomena inside the dehumidifier tubes, however, need to be prevented by adding anti-scalants to the feed water or by applying conventional cleaning methods, i.e. sponge balls, etc. [40, 41].
Due to the relatively high feed water inlet temperatures of ≈ 85 °C, microbiological contamination of the feed water doesn’t affect the condensate quality. Nevertheless, as the HD containment might be contaminated, a condensate disinfection or biological post-treatment might be required [40]. HD processes run with low-temperature heat and thus can be used in a great variety of applications. Solar heat makes HD suitable for solar desalination applications, whereas the use of waste heat is reasonable for a great variety of industrial or agricultural water treatment tasks.
HD is one of the few desalination processes suitable for Zero Liquid Discharge (ZLD) applications, even though the Performance Ratio PR tremendously suffers from rising salt concentrations in the feed water [25]. The reason for that is the decrease in water vapour pressure with increasing salt concentration, as was already explained beforehand [15] and in more detail in Annex C. However, as in other thermal evaporation and vaporisation processes, condensate quality does not depend on feed water salinity.
The plant sizes of HD plants are relatively small in the range of 0.01 to 50 m3/day [26,27]. Depending on the plant configuration, Performance Ratios lie between 4.7 and 1.2, which corresponds to a Specific Energy Consumption of 140 kWh/m3 to 550 kWh/m3 [29] for standard seawater (approximately 35,000 ppm salinity, see Annex B1/B2) as feed. Thus, compared to other thermal processes, HD belongs to the group of high-energy-consuming desalination techniques. However, in locations where sufficient waste heat e.g. from a diesel generator set is available, energy consumption cost can be reduced to almost zero.
Compared to the process of Membrane Distillation (MD), HD is based on the same operational principles; hence it offers the same energetic output figures in terms of production rate, performance ratio and required feed water inlet temperatures [20]. Due to the application of a structured and highly porous packing in the HD humidifier, the heat and mass transfer area is maximized. However, this makes the construction volume in HD usually much higher than in MD. Due to a generally lower local temperature difference between feed and condensate over the membrane, resulting in lower heat exchanger losses in MD (see chapter “Improved concepts”). and Annex E1 performance ratios are slightly higher than in HD. However, as no membrane with all its disadvantages in terms of fouling and scaling issues is involved, HD still is the more robust technique.
Note, that the Performance Ratio is strongly dependent on the operational parameters of a specific plant and behaves inverse to the condensate output. Thus, PR can strongly vary even in the same plant configuration [36, 37].
In this context, it is important to mention the large discrepancy between the sensible heat capacity of water and the heat of evaporation. If we cool down the brine in the humidifier by 40 K, which is a reasonable assumption for a 1-stage once-through plant as shown in Fig. 2, we can invest only 170 kJ/kg for evaporation, which is only 7 % of the required heat of evaporation of roughly ΔhV = 2,326 kJ/kg (standardised value equivalent 1,000 BTU/lb [39]). In other words, only very low Recovery Ratios (RR) can be realised in a single stage. With this background, it can easily be understood that a higher condensate output would mean a higher feed mass flow leading to a lower brine temperature difference over the humidifier and thus finally to a lower PR.
Motivation of Use
Even though energy consumption is quite high, due to its robustness and usually small plant sizes, there is a broad variety of suitable HD applications:
- Solar Seawater Desalination in remote areas and developing countries with no or limited access to skilled operation staff [26]. In autonomous systems, HD plants might be operated with waste heat of a diesel generator set.
- Concentration of liquid waste waters in many industrial and agricultural applications, such as industrial water treatment, treatment of liquid manure from biogas plants, etc. [40, 41].
- Zero Liquid Discharge (ZLD) applications, where feed water is concentrated up to the crystallisation limits. ZLD applications might be
– salt production or production of other raw materials from seawater [42];
– regeneration of liquid desiccant solutions like LiBr, LiCl, CaCl2, ... [19-20];
– treatment of highly concentrated salt solutions in industrial and mining applications [34, 45] i.e. treatment of water from hydraulic fracturing.
2. Plant Design
Pretreatment
Due to the robustness of the HD process, feed water needs only to be coarsely filtered to remove particles larger than 1 mm [13]. If pure water is the product, any volatile components with a vapour pressure higher than water, such as some oils, solvents, alcohol, compounds of NH3 etc. needs to be removed in order to obtain a high product quality [44].
As HD is a low-temperature process, most of its components are made of polymers. Thus, anti-corrosives are seldom required. Anti-foaming agents might be required depending on feed water quality, in order to avoid blocking the humidifier. As HD is also quite robust to scaling on the humidifier side, only the dehumidifier coils in a sensible heat exchanger configuration need to be protected. Here, some slight use of anti-scalant might prove useful. In some configurations, sponge balls are used for cleaning the dehumidifier [13, 40, 41].
HD System Configurations
In HD, a large variety of system configurations has been investigated and demonstrated. A classification of HD systems can be developed according to power supply, arrangement of flow schemes or economic considerations [39]. In the following, a coarse overview on the variety of configurations will be given.
• Solar Stills as a Basic HD System
Astonishing as it may be on a first glance, the various forms of Solar Stills represent HD in its historically oldest and most simple form: pure water is evaporated from a feed water basin and condensed on a cooler glass pane, which additionally serves as solar collector to provide the necessary heat of evaporation. Again, air is the carrier medium. In many classifications of desalination systems, Solar Stills are an independent technology, however in the present overview, HD is seen to be an enhancement of Solar Stills, all working on the same principle. The limitations of Solar Stills can be found (1st) in the missing possibility of heat recovery, (2nd) low condensation rate due to high temperatures on the dehumidifier-glass pane by reason of the released heat of condensation and absorption of solar irradiation, (3rd) in limitations in the heating of the feed basin due to a reduced transmissivity of the mist- and droplets-covered glass pane. These issues have led to the separation of humidifier, dehumidifier and heater as separate functional units [39]. For more details on Solar Stills, refer for example to [16].
• Closed-Air vs. Open-Air
In the closed-air cycle, the air constantly circulates between humidifier and dehumidifier. The advantage of the closed-air cycle lies in a lower enthalpy loss compared to an open air cycle at the cost of a slightly lower output, as air entry temperatures to the humidifier are slightly higher and the incoming air is already saturated with humidity. Anyway, as the water vapour saturation of cold air is very limited, this gives only minor disadvantages. Therefore, the closed-air cycle is implemented more frequently [28, 29] (see Fig. 2), while possibilities to additionally heat the air flow are investigated [37].
• Forced Convection (Fan) vs. Natural Convection (Buoyancy)
The advantages of a forced convection air flow are smaller sizes of humidifier and dehumidifier for the same condensate output due to higher heat and mass transfer coefficients. This leads to lower investment costs – especially for the dehumidifier – and a smaller plant volume [37]. The disadvantage of forced convection is the necessity of electrical energy for fan operation and active control of air-to-water mass flow ratio, which is mostly about 0.2 – 0.3 [44].
A natural convection system is more robust compared to forced convection systems. If the process performs at high temperatures, natural convection is advantageous for the performance, while at low temperatures, forced convection yields a better performance [3]. Note again that PR is working opposed to output, thus an economic optimum has to be found.
• Water and/or Air Heating
Depending on the energy source, there are different methods to supply the process with thermal energy. Depending on the heat source (solar thermal collectors, waste heat, etc.), either feed water or air or both can be heated. Corrosion might be a problem in the solar collectors or in other heat exchangers. This problem can be avoided with an air heated system or using seawater proof materials [22]. On the other hand, heat transfer coefficients and thus efficiencies of air-heated systems are commonly lower.
Generally, air heated systems have a higher energy consumption than water heated configurations because the energy transferred to the feed water in the humidifier cannot be recovered in the dehumidifier [28]. A combined water and air heating system is indeed not cost competitive due to the two necessary heat exchangers and/or solar collectors [1].
• Operation Mode: Batch vs. Continuous Mode
As the Recovery Ratio of a single-stage once through HD system is usually lower than 10 % [44] (see also explanation in chapter 1 – “Process requirements”), HD plants in concentration or ZLD applications normally are driven in batch mode. The advantage of dual-phase (thermal) in comparison with single-phase membrane processes like Reverse Osmosis (RO) is the ability to deal with highly concentrated feed waters. This is due to the fact that the water vapour pressure as driving force of HD does more strongly depend on the temperature of the respective media (feed water and air) than on the feed water salinity. Compared to HD, SEC of Reverse Osmosis (RO) systems linearly depends on salinity [38].
• Energy Supply
A major advantage of the HD process is the possibility to drive it with thermal energy at low temperature level of usually 60 °C to 85 °C. Therefore, solar irradiation or waste heat is commonly used for HD-processes [27]. In case of solar irradiation, the available thermal power is transient, while a constant thermal power supply is possible using waste heat. Thus, both control issues and part load capability are crucial aspects in the design of solar driven HD plants.
Component Design
Major components in any thermally driven desalination system are heat exchangers in their various forms. In HD, these are for example the humidifier and dehumidifier and sometimes also sensible heat exchangers for feed preheating from warm brine etc. Optimisation of HD plants aims also at heat exchanger optimisation, as will for instance be explained in in the following section “Improved Concepts – Multi-Stage HD”. For a deeper understanding of heat exchanger theory please refer to Annex E1.
– Humidifier (see Fig. 2)
In the humidifier, a large boundary between the liquid feed water and the gas phase (humid air) is desired. Additionally, the contact time between water and air should be high in order to achieve a water vapour content near saturation in the gas phase at the humidifier outlet. Gas and liquid phases are led either in cross current or counter current flows. Different humidifier types have been investigated or are in commercial application:
- Spray Humidifier
The spray humidifier is a simple humidifier form. At the top of a vessel, water is sprayed into the humidifier and trickles down driven by gravity. In the falling time period, water droplets get into contact with the surrounding air and evaporate at the phase boundary [4]. A demister is necessary at the humidifier outlet in order to prevent water droplets of being dragged along to the dehumidifier side. Spray humidifiers generally have a large capacity but low humidification efficiency [28, 29]. A direct application of spray humidifiers can be found in the Rapid Spray Evaporation (RSE) technique, which has similarities to an air-heated, open-air HD cycle. - Bubble Columns
A bubble column represents the opposite approach to a spray humidifier. In this case, air bubbles – often air heated – are injected at the bottom of a water column and rise to the top. While both phases are in contact, water molecules evaporate at the phase boundary into the air bubbles [9, 17]. The advantage of this type is the lack of corrosive components, a disadvantage are problems with heat recovery [28, 29], as the heat capacity of humid air is very much lower than the required heat of evaporation. - Packed Tower
Packed towers are the most common type of HD humidifier and represent a species of spray humidifiers that are filled with either structured or random packing material. Structured packing is mainly made of plastics while for random packing different materials e.g. ceramics [18], plastics [10] or metals are used. Experiments with structured, wooden humidifiers were conducted by [10] and [28]. The surface of the packing material is partly coated with water, leading to a bigger phase boundary area, longer time scales and hence a better humidifying efficiency compared to simple spray humidifiers [28]. Hassabou [12] showed that for partially wetted packings in HD an optimized thermal conductivity of the packing material is beneficial. - Fleeces and Tissues
In this type of humidifier, fleeces, tissues or similar constructions are hanging vertically in a tower and are passed by dry air, similar to the drying process of clothes on a washing line. There are also arrangements with fleeces that provide a type of wick at its surface to use capillary effects. These arrangements lead to a high humidification efficiency of nearly 100 % [27, 32].
– Dehumidifier (see Fig. 2)
The dehumidifier is a crucial part of an HD device. In most dehumidifier configurations, the moist air is cooled by the feed water in a heat exchanger. The heat of condensation is used to preheat the feed water. Therefore, the material of the heat exchanger needs to be seawater resistant. The following heat exchanger types are frequently used as dehumidifiers:
- Tube Bundle Heat Exchanger
Flowing inside the tubes of the tube bundle heat exchanger, the fresh feed water that preheated by both the latent heat of condensation and the sensible heat of the air flow, which is passing outside the tubes. As tube material, PVC was used in the case of Seifert et al. [36], while any other seawater proof material is possible. - Finned-tube Heat Exchanger
The surface of finned tube heat exchangers is enlarged by fins, that are welded or soldered to the tube either in a longitudinal [31] or transversal [7] way. As materials for the fins aluminium or copper is frequently used since the air flow at the outside of the tube is much less corrosive than the seawater flow inside the tube. For the tubes, seawater proof material or liners have to be used. - Flat-plate Heat Exchanger
A flat-plate heat exchanger is for instance used by Müller-Holst et al. [26]. They apply double-webbed polypropylene slabs as heat exchanger material. - Direct contact Condenser
Due to the high amount of non-condensable gases in the gas phase, the heat and mass transfer coefficient on the gas side is low. Thus, a big heat exchanger surface is necessary, which leads to significant costs of the dehumidifier. This drawback is addressed with a direct contact condenser. Here, the moist air is cooled within a pure water flow, which mostly is directly sprayed into the dehumidifier compartment. The released heat of condensation is directly transferred to the pure water. This additional heat can be recovered in a liquid- liquid heat exchanger, where the pure water preheats the feed water [21, 43]. However, as the driving temperature differences in the heat exchanger are low, PR suffers.
– Carrier Medium (see Fig. 2)
As was already mentioned in the introduction, the large majority of HD systems uses air as a carrier medium. However, considerable pressure drops and low heat transfer coefficients in the gas side of the dehumidifier are limiting factors for HD efficiency. Narayan et al. [30] investigate helium as an alternative and find improved heat transfer coefficients and decreased pressure drops compared to air. Even though the PR keeps largely unaffected, helium has the potential to reduce auxiliary power consumption by a factor of 5 to 8. [11, 30]
Note that in all evaporative HD systems, a huge problem lies in the presence of Non-Condensable Gases (NCG), which raise severe limitations especially of the dehumidifier performance. For details on NCG, see Annex D3. A solution to that problem would be to evacuate the HD containment, as it is done for example in Vacuum Membrane Distillation (VMD). However, as HD is supposed to be a very simple and therefore robust technique, the majority of HD systems still run with air as carrier medium.
Improved Concepts – Multi-Stage HD
Fig. 3: Process scheme of HD plant with water (brine) extraction, adapted from [6].
Water Recirculation
The aim of an improved system with water recirculation is to minimise the thermal losses of a continuously operating HD plant. With a water recirculation system, the necessary heat as well as the temperature difference between the brine and the dry air at the humidifier entry can be reduced. Thus, entropy generation, or in simpler words further losses can be minimised [6]. Fig. 3 shows a sketch of the process.
In an ideally operated heat exchanger, the heat capacity rates on both sides of the heat exchanger should be equal and constant [28]. The heat capacity rate is the product of the mass flow rate and the heat capacity and therefore a key figure for the transported enthalpy of a medium in a heat exchanger. This is not the case in a single-stage humidifier and dehumidifier. First of all, for both air and water flow, there is a weak dependency of the heat capacity cp on the local temperature at a certain position in the humidifier or dehumidifier. However, this effect is only of minor significance. More important is the fact, that due to the on-going evaporation and condensation of pure water in the humidifier/dehumidifier, the mass flow rate of both air and water (feed or condensate) is strongly varying from in- to outlet. To take the humidifier as an example, the feed water flow is “losing” up to 7 % of its mass flow rate from humidifier inlet to outlet (see also chapter 1 – “Process Requirements/Peculiarities”). On the other hand, the air mass flow rate is enriched by the same amount of pure water vapour [46]. According to heat exchanger theory, see Annex E1, this leads to large temperature differences between humid air and feed water over the length of the heat exchanger. Large temperature differences inevitably result in entropy generation, which means a systematic reduction of efficiency in any heat exchanger [6, 28].
An efficient way to overcome this problem is to split up both humidifier and dehumidifier in several stages and to adjust either the feed water (“water recirculation”) or the humid air mass flow (“air recirculation”) in every stage to its optimum. In this context, the term “stages” stands for several “effects” and describes the core of a Multi-Effect Humidification (MEH) system.
Fig. 4 shows temperature profiles of water and humid air in the humidifier (H) and dehumidifier (condenser, (C)). On the top, temperature profiles over heat exchanger length in a single-stage setup as shown in Fig. 2 are given. On the bottom, temperature profiles of a three-stage setup as shown in Fig. 3 and 5 are outlined. It becomes obvious that with controlled heat capacity rate in every stage, the temperature differences between water and humid air can be minimized. It is interesting to see how the required heat input (red arrows in Fig. 4) for heating the feed water to TBT can be reduced by minimising the temperature differences between water and humid air. To reduce the heat input means at the same time to enhance PR.
Fig. 4: Temperatures (T) of air and feed in an HD-plant without water recirculation (top) and with water recirculation (bottom), adapted from [6]. (H) shows a single-stage humidifier, H1, H2, H3 a three-stage humidifier from top (1) to bottom (2). (D) analogously for the dehumidifier with (1) bottom and (3) top. T0 would be the theoretical temperature profile without mass transfer. T∞ is the ambient temperature. Note the lower heating power requirements in the staged MEH configuration.
With water recirculation as shown in Fig. 3, the heat capacity flow of the feed water is adjusted to the heat capacity flow of the carrier medium humid air by means of adjusting the feed mass flow. The brine that is not necessary to heat up the air in the humidifier is used to preheat the feed water in the dehumidifier [6, 29]. However, Narayan et al. [29] found, that balancing of the dehumidifier leads to better process efficiencies than balancing the humidifier.
Air Recirculation
The aim of air recirculation is similar to the aim of water recirculation. The heat capacity flows of brine and air should be equal along the height of the humidifier [45]. The disadvantage of air recirculation is to handle the control of the air flow properly. In any case, this cannot be achieved by natural convection [6]. Additionally, the absolute humidity of the air in the bypasses is lower than the humidity in the air that passed the complete humidifier. This leads to a lower condensate output [6]. A sketch of the process is shown in Fig. 5. A demonstration plant was realised in the late 1960ies by Hodges et al [14].
Fig. 5: Process scheme of an HD plant with air recirculation, adapted from [6, 14]
Multi-Stage Evaporation
A multi-stage evaporation strategy is especially suitable for air heated systems in order to increase the moisture of the air entering the dehumidifier by several subsequent heating and adiabatic humidification steps. The aim of this multi staging is to reduce the necessary air flow by optimizing its humidity. In this kind of multi- stage evaporation, several humidifiers are switched in series and supplied by one single feed mass flow at TBT. The process is analogue to advanced solar drying processes, e.g. in large solar tunnel dryers.
Similar to evaporative cooling, in adiabatic humidification, the temperature of the air decreases due to the necessary heat of evaporation. A further increase of the humidity seems not to be possible in the first step, since the relative humidity of the air tends towards saturation. In order to increase the absolute humidity of the air, it is therefore re-heated and passed to a subsequent following humidifier [6, 12]. However, multi-staging does not improve the performance of an air heated cycle significantly [28].
3. Feed Water
As already stated in chapter 1, HD is a relatively robust technique, where any kind of aqueous solution with vapour pressure of the components lower than that of pure water can be treated. Feed water needs to be coarsely filtered (particles larger than 1 mm diameter removed), and volatile components, such as some oils, solvents, alcohol, compounds of NH3 etc. need to be separated, which mostly means stripping before entering the actual HD system. Scaling plays no role in the humidifier, but has to be considered in dehumidifier, pipes and the distribution system in the humidifier. Thus, a minor amount of anti-scalants may be applied. As in HD most components can be built in plastic materials, corrosion normally is not an issue [5, 13, 37].
Concerning salinity of the solutions, no restrictions do exist, unless the crystallization limits are reached. Thus, ZLD applications are possible. Exceeding the crystallization concentration limits may result in severe blocking of the humidifier outlet components [19]. However, due to the decrease in partial vapour pressure, note that the energetic performance of HD is dramatically reduced with increasing salinity [38].
The variety of feed waters treated in HD plants ranges from normal seawater, brackish water, industrial and organic waste water to very problematic substances like liquid manure from biogas plants [40, 44].
As energy input, low temperature heat is used from solar collectors, solar ponds, waste heat from industrial processes or generator sets. Furthermore, biomass or fossil fuel firing is possible. A minor amount of auxiliary electrical power is needed for running pumps, fans and control devices.
Feed Water Salinity | Technically feasible | Now in Use for Desalination | Potentially for techno-commercial Viability for the Future |
> 100,000 ppm |
X | ||
50,000 – 10,0000 ppm |
X | ||
10,000 – 50,000 ppm |
X | ||
1,000 – 10,000 ppm |
X | ||
< 1,000 ppm |
Table 1: Stage of development of HD technology in relation to water salinity
4. Desalinated Water & Brine
As long as no compounds with vapour pressure lower than water are contained in the feed water, the product water is of highest quality distilled water. Minor contaminants may be induced to the condensate via the air flow, but this depends largely on the quality of the feed water [13].
As already mentioned, the product water is condensate, hence distilled water quality. Thus, it has to be remineralised and sterilised in order to provide it as potable water.
As the amount of chemicals needed for the operation of standard HD processes is very low, no contamination of the brine has to be considered. The concentration of the brine depends on the operation mode (batch or continuous) and the Recovery Ratio RR. In continuous operation mode, the concentration is about 5 – 10 % higher than in the feed water.
5. Data and Information
Recovery Rate and Concentration Factor
The Recovery Rate (RR) depends on the operation strategy of the plant. If the brine is recirculated, RR is higher than in once-through operation. Li et al. [24] report an RR of 8 % while in a commercial ZLD plant of Terrawater GmbH an RR of 86% is reached in a batch process [42].
Generally, RR of HD-systems operated in continuous mode is much lower than for conventional systems, see chapter 1 “Process requirements”, what may lead to less effort in brine disposal [28].
The Concentration Factor (CF) consequently is a function of RR and can be described by
where mFeed is the feed water mass flow rate and mbrine,out is the brine (= concentrate) mass flow rate leaving the system.
Specific Energy Consumption SEC (Energy per m3 Output)
The Specific Energy Consumption (SEC) depends mainly on the plant configuration (see chapter 2), the operation mode (Performance Ratio PR vs. output), the recovery of the latent heat of evaporation and the vapour pressure difference between air and water phase in the humidifier. The PR varies between 4.5 and 1.2 for HD-plants, what is equivalent to a Specific Energy Consumption of 140 – 550 kWh/m3 [28, 37]. Auxiliary power requirements are very low.
Chemical Additive Consumption
Chemical additive consumption is very low. A small amount of anti-scalants may prove useful, while practically no anti-corrosives, anti-fouling, etc. are needed [13].
Personnel Intensity
The personnel intensity is largely depending on the plant configuration. For single-stage once-through HD, the plants are self-content with only a very small amount of maintenance. They can be handled by shortly introduced but otherwise unskilled staff. [13]
However, a higher grade of complexity is reached with multi-stage plants (brine/air recirculation or multi-staging, see Fig. 3 to 5). Due to their higher control effort, they include more complex components. Hence, these configurations need to be run and maintained by technically trained staff.
Replacements
Again, it is important to point out the robustness of HD systems, which means that the need for replacements is comparatively low. Note that the requirements depend on the quality of the feed. With problematic feed water, there is certainly a need for regularly replacing the packing material. If sponge balls are applied for cleaning the dehumidifier tubes, they also have to be replaced periodically [13].
6. Preferred Use
As in HD, the requirements for feed water quality are rather low, the preferred fields of use are manifold. From the classic applications in the production of potable water in the public or private sector, further fields of use lie in the treatment of industrial waste water (whenever waste heat is available) and agricultural applications like the treatment of liquid manure [40, 41, 42].
As already described, the classical purpose of HD is the production of potable water. Note that HD (along with Mechanical Vapour Compression MVC and (Vacuum) Membrane Distillation (V)MD) is one of the few desalination technologies capable for ZLD applications. Accepting a higher SEC at increased salinities, HD can easily go up to the crystallization limits of seawater [19]. One application could be a higher concentration of brine from a Reverse Osmosis (RO) plant in a ZLD configuration. With such “hybrid” RO-HD systems, the amount of brine to be disposed to the environment can be reduced while the product water output can be increased.
However, in many industrial and agricultural applications, a major goal is to reduce the amount of waste water and thus reduce costs for their disposal. Here, the actual product is not the condensate, which in most cases can be recycled to the production process, but the concentrated brine. One example is the treatment of liquid manure from biogas plants, where concentration factors of up to a factor of 10 are possible. In this case, a welcome by-product would be the condensate containing a high amount of NH3, which can further be used for the production of fertilisers [40, 44].
Another field of application that is becoming more and more important is the mining industry. Here, the treatment of waste water from hydraulic fracturing processes seems to be an attractive application [11].
It has to be said, that for standard seawater desalination in drinking water purposes, HD rarely is economically viable. The main reason for that is its high energy consumption compared to Reverse Osmosis (RO) and its large installation space requirements. Therefore, the application of HD is mostly restricted to special industrial and agricultural water treatment tasks. However, its application for potable water production can be economically viable in remote areas with low infrastructure and no skilled staff available [13, 26, 37]. In the last case, especially the application of Solar Stills (see chapter 2 – “HD System Configurations”) as a special form of HD systems might also prove feasible.
7. Einvironmental Impact
When driven with fossil fuels, which is the case only for a few special applications, HD has a high environmental impact due to its high Specific Energy Consumption SEC. The impact due to brine disposal depends on the application and the Recovery Ratio RR. It goes without saying, that brine disposal becomes more difficult with high concentration rates. Anyway, this is the same issue as in any other thermal desalination system.
Generally, it can be said that compared to other thermal processes, brine disposal is less problematic, as the use of chemicals is very limited. Furthermore, hardly any metal heat exchangers are applied, so the output of heavy metals to the marine ecosystems is quasi nil. Chlorination of the feed or balancing of the pH-value is not required in HD. For the whole complex of environmental problems, see [23].
8. Stage of Maturity
1 | 2 | 3 | 4 | 5 |
Basic R&D |
Prototyping/ |
Field Tests/ Demonstration |
Commercialization (up to seven years) | Use/Established (more than 7 years) |
X |
Table 2: Stage of maturity of MDC technology
9. Further Developments
As HD plants for laboratory purposes can be built up quite easily and with a comparatively small budget, a huge amount of experimental and numerical university-based research was already performed and is still going on. Especially desalination groups in developing countries are interested in the technology and try to achieve either cost reduction or efficiency improvements. A special focus lies on the application of solar thermal collectors to provide the necessary energy requirements. In this context, the Solar Still is one of the most popular topics in low- budget desalination research with roughly 150,000 publications during the last ten years.
Challenges for Improvement
Challenges for improvement certainly lie in the development of multi-stage plants, where the main goal is a reduction of the Specific Energy Consumption SEC. Especially in systems with air or brine recirculation, control problems are a main issue [2]. Currently, much work is done on the improvement of the humidifier. For very problematic feed water, technologies like bubble columns are coming up, especially in the field of treating water from hydraulic fracturing [11, 17, 37].
What is being worked/researched on?
In the HD area, research is still done on the issues mentioned above: multi-staging, control problems in air- or brine recirculation and optimisation of bubble columns. [15, 34, 37]. More work is done on optimised operation strategies in special applications like ZLD, dealing with very high concentration rates [19]. In the field of concentrating liquid manure, work is done on the optimal extraction of NH3 to the condensate [44]. A special focus in the solar field is still lying on the improvement of Solar Stills [16].
10. References
[1] Abdelmoez, W., M. S. Mahmoud, M. S., Farrag, T. E., Water desalination using humidification/dehumidification (HDH) technique powered by solar energy: a detailed review, Journal of Desalination and Water Treatment, 2014, pp. 4622-4640.
[2] Al-Enezi, G., Ettouney, H., Fawzy., N., Low temperature humidification dehumidification desalination process, Journal of Energy Conversion and Management, 2006, pp. 470-484.
[3] Al-Hallaj, S., Farid, M. M., Tamimi, A. R., Solar desalination with a humidification-dehumidification cycle: performance of the unit, Desalination, 1998, Volume 120, pp. 273-280.
[4] Ben Amara, M., Houcine, I., Guizani, A., Maalej, M., Experimental study of a multiple-effect humidification solar desalination technique, Desalination, 2004, Volume 170, pp. 209-221.
[5] Bourouni, K., R. Martin, Tradist, L., Chaibi, M. T., Heat transfer and evaporation in geothermal desalination units, Journal of Applied Energy, 1999, pp. 129-147.
[6] Brendel, Th., Solare Meerwasserentsalzungsanlagen mit mehrstufiger Verdunstung, Dissertation, Ruhr-Universität Bochum, 2003.
[7] Chafik, E., A new type of seawater desalination plants using solar energy, Desalination, 2003, Volume 156, pp. 333-348.
[8] Chafik, E., Design of plants for solar desalination using the multi-stage heating/humidifying technique, Desalination, 2004, Volume 168, pp. 55-71.
[9] El-Agouz, S. A., Abugderah, M., Experimental analysis of humidification process by air passing through seawater, Journal of Energy Conversion and Management, 2008, pp. 3698-3703.
[10] Farid, M. M., Al-Hajaj, A. W., Solar desalination with a humidification-dehumidification cycle, Desalination, 1996, Volume 106, pp. 427-429.
[11] Gradiant Corp., Technology Sheet - Carrier Gas Extraction, 17.11.2017, gradiant.com/technology/carrier-gas-extraction
[12] Hassabou, A. H., Spinnler, M., Polifke, W., The role of conductive packing in direct contact regenerators within humidification-dehumidification cycles–part II: Experimental analysis, International Journal of Advanced Research in Engineering and Technology (IJARET), vol. 5(12) (2014), pp.97-106.
[13] Heyn, N., Terrawater GmbH, phone conversation on November 13th, 2017
[14] Hodges, C. N., Thompson, T. L. , Groh, J. E., Sellers, W. D., The Utilization of Solar Energy in a Multiple-Effect Desalinization System, Journal of Applied Science 3 (1964), pp. 505–512.
[15] Joffe, J., Boiling-point elevation,Journal of Chemical Education, 1945, pp. 270 ff.
[16] Kaushal, A., Solar stills: A review, Renewable and Sustainable Energy Reviews, Volume 14, Issue 1, January 2010, Pages 446-453.
[17] Khalil, A., El-Agouz, S. A., El-Samadony, Y. A. F., Abdo, A., Solar water desalination using an air bubble column humidifier, Desalination, 2015, Volume 372, pp. 7-16
[18] Khedr, M., Techno-economic investigation of an air humidification-dehumidification desalination process, Journal of Chemical Engineering Technology, 1993, pp. 270-274
[19] Kiefer, F., Schummer, F., Präbst, A., Spinnler, M., Sattelmayer, T., "Optimization of a multi-effect vacuum membrane distillation system for highly concentrated aqueous electrolyte solutions in liquid desiccant air conditioning and zero liquid discharge, Desalination for the Environment: Clean Water and Energy, Rome 2015
[20] Kiefer, F., Spinnler, M., Sattelmayer, T., Experimentelle und theoretische Untersuchungen zu einem hocheffizienten solarbetriebenen Klimatisierungsverfahren auf Basis flüssiger Sorbenzien – Solar Powered Air Conditioning Efficiency (SPACE), Final Project Report for Project Nr. 01DH13019 funded by the German Federal Ministry of Education and Research, Munich, 2017
[21] Klausner, J. F., Mai, R., Li, Y., Innovative freshwater production process for fossil fuel plants, U.S. DOE - Energy Information Administration annual report, 2003.
[22] Kroiss, A., Praebst, A., Hamberger, S., Spinnler, M., Tripanagnostopoulos, Y., Sattelmayer, T., Development of a seawater-proof hybrid photovoltaic/thermal solar collector, International Conference on Alternative Energy in Developing Countries and Emerging Economies, 2013.
[23] Lattemann, S., Development of an environmental impact assessment and decision support system for seawater desalination plants, Dissertation, Delft University of Technology, Delft, 2010.
[24] Li. Y., Klausner, F., Mai, R., Performance characteristics of the diffusion driven desalination process, Desalination, 2006, Volume 196, pp. 188-209.
[25] Minier-Matar, J., Sharma, R., Hussain, A., Janson, A., Adham, S., Field evaluation of membrane distillation followed by humidification/dehumidification crystallizer for inland desalination of saline groundwater, Desalination, 2016, Volume 398, pp. 12-21.
[26] Müller-Holst, H., M. Engelhardt, M. Herve, und W. Schölkopf. „Solarthermal seawater desalination systems for decentralised use.“ Renewable Energy, 1998: 311-318.
[27] Müller-Holst, H., Schölkopf, W., Thermally Driven Sea Water Desalination using the multi effect humidification dehumidification method, Proceedings of the ISES Solar World Congress, 2001, pp. 883-891.
[28] Narayan, G. P., Sharqawy, M. H., Lienhard, J. H., Zubair, S. M., Thermodynamic analysis of humidification dehumidification desalination cycles, Desalination and Water Treatment, 2010, pp. 339-353.
[29] Narayan, G. P., Summers, E. K., Zubair, S. M., Antar, M. A., Lienhard, J. H., Sharqawy, M. H., The potential of solar-driven humidification-dehumidification desalination for small-scale decentralized water production, Renewable and Sustainable Energy Reviews 14.4, 2010, pp. 1187-1201.
[30] Narayan, G. P., McGovern, R. K., Lienhard, J. H., Zubair, S. M.,Helium as a carrier gas in humidification dehumidification desalination systems, Proceedings of ASME 2011 International Mechanical Engineering Congress & Exposition IMECE 2011 November 2011, Denver, USA.
[31] Nawayseh, N. K., Farid, M. M., Al-Hallaj, S., Al-Timimi, A. R., Solar desalination based on humidification process - I. Evaluation the heat and mass transfer coefficients, Energy Conversion & Management, 1999: 1423-1439.
[32] Orfi, J., Laplante, M., Marmouch, H., Galanis, N., Benhamou, B., Nasrallah, S. B., Experimental and theoretical study of a humidification dehumidification water desalination system using solar energy, Desalination, 2004, Volume 168, pp. 151-159.
[33] Polifke,W., Kopitz, J.,Wärmeübertragung: Grundlagen, analytische und numerische Methoden, Pearson Studium, 2nd edition 2009, ISBN-13: 978-3827373496.
[34] Prakash, S., Shannon, M., Bellman, K., Water Desalination: Emerging and existing Technologies, in: Aquananotechnology: Global Prospects by Reisner, D.E., Pradeep, T., CRC Press, 2014, pp. 533-562.
[35] Reid, R. C., Prausnitz, J. M., Poling, B. E., The properties of gases and liquids, McGraw-Hill, Boston, 2001.
[36] Seifert, B., Schaufuss, P., Spinnler, M., Sattelmayer, Th., CFD-simulation of a humidification-dehumidification desalination plant for transient solar irradiation, IDA World Congress, Dubai, 2009.
[37] Seifert, B.,P erformance improvements of humidification-dehumidification desalination systems with natural convection, Dissertation, Institute of Thermodynamics, Technical University of Munich, 2017.
[38] Sharqawy, M. H., Lienhard, J. H., Zubair, S. M., The thermophysical properties of seawater, Journal of Desalination and Water Treatment, 2010, pp. 354-380.
[39] Spinnler, M., Seifert, B., Kroiss, A., Kiefer, F., Desalination, material to the lecture “Desalination”, Institute of Thermodynamics, Technical University of Munich, 2017
[40] Terrawater GmbH, terraorganic – Technical Description, last access: 28.02.2019, www.terrawater.de/terra-organic-beschreibung
[41] Terrawater GmbH, TerraPreCon – Technical Description,last access: 28.02.2019, http://www.terrawater.de/terraprecon-beschreibung
[42] Terrawater GmbH,TerraSaline 3.5/7.0 Datasheet, Kiel, 2015.
[43] Tow, E. W., Lienhard, J. H., Heat flux and effectiveness in bubble column dehumidifiers for HDH desalination, World Congress on Desalination and Water Reuse, Tianjin, 2013.
[44] Ugresic, V., Spinnler M., Development of a liquid manure processing plant on the basis of the humidification dehumidification process, Final Report to Project Nr. KF 060810 RH8 in the PRO INNO II program of the German Federal Ministry for Economic Affairs and Energy, Munich, 2009.
[45] Whitefield, S., Treatment of highly concentrated salt solutions in industrial and mining applications, Society of Petroleum Engineers, 2015, last access 22.08.2016.
[46] Younis, M. A., Darwish, M. A., Juwayhel, F., Experimental and theoretical study of a humidification-dehumidification desalting system, Desalination, Volume 94, 1993, pp. 11-24.