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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






Chemical agent to reduce the feed water's corrosion potential.



Chemical agent to reduce the formation of foam or to break a foam already formed.



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



Concentrated feed water.



Capacitive Deionisation, Membrane Capacitive Deionisation Electrochemical Demineralisation, Electrosorption CDI, MCDI



capacitive ion storage



Cation Exchange Membrane CEM






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.




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 (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.




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.






Electrodeionisation, Continuous Electrodeionisation EDI, CEDI



Electrodialysis, Electrodialysis Reversal ED, EDR



electrical double layer EDL






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 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 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.









feed water
Feed water is the raw water to be (pre)treated and fed into a desalination system.



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


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]



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


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.



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.




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”.




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 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.
[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.

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