Phase Transition

Entrance Desalination technology

Desalination Technology is developing globally very fast these days. More and more desalination technologies getting applied into the conventional water treatment business today. DME does follow and support this development on an international basis starting from the first idea up to commodity products.

In order to give some direction what technology is available today this area will give a basic direction.

Following the navigation of this side you will find fundamentals such as on the right listed.


Classification of Desalination Technologies

Desalination technology is used by humans since thousands of years. Being in the desert surrounded by sand and rocks or being on the sea sailing through salty waters, always fresh water is needed every day to keep the human body in a balanced hydrate mode.

Nature started using desalination processes from the very first moment. Different concentration levels drive everything alive on our planet. Humans started using first evaporation and condensation processes to purify unusable water. In the last century the number of processes to desalinate water and other liquids has gone through a very big development. New processes have been found, existing processes have been improved.

Overviewing all known desalination processes of today we may find a large number in a lot of science areas. In order to give this convolute of technology a systematic structure a larger group of desalination experts and scientists developed a Classification of Desalinaiton Technology (CDT) system.

This Classification of Desalinaiton Technoogy system is based on the natural working principles of chemistry, physics and biology. It starts as low as electrochemistry and nuclear physics. From here on it is building up its structure. Under the five main science fields 20 subordinated science areas have been found describing all natural working principles in desalination systems.

In order to structure all known desalination technologies a general structure was build up to the Classification of Desalinaiton Technology system (CDT):

The first classification level does divide in transient phase and stable phase technologies.
The second level classes represent the separation processes  of desalination technologies.
The third level classes show the main technology, in some cases without further structure.
The fourth level classes are split in advanced sub-desalination technologies.
In total there were 51 desalination technologies found and classified inside CDT.

As of today the used technologies mainly are Multi-Stage Flash and Reverse Osmosis they can be found within the structure at the third level.

Everybody is free to make use of this CDT but is asked to make a reference to DME GmbH on behalf of all people being involved, please.

We will keep developing CDT and you are free to participate in this discussion!

(more details LINK)

Desalination Technology in use

Today three technologies are dominating the international market. These are Multi-Stage Flash (MSF), Multiple-Effect Distillation (also Multi Effect Distillation or MED) and Reverse Osmosis (RO).

Looking into a global installed capacity in 2016 of about 90 mio m³/d app. 90% of this water is produced by these three technologies.

Multi-Stage Flash (MSF) takes app 20%, Multiple-Effect Distillation (also Multi Effect Distillation or MED) app. 5% and Reverse Osmosis (RO) app 65% of this.

The number of installed plants differs very much because a single MSF plant today easily does  produce 60.000 m³/d versus a RO plant very often produces 5.000 m³/d or less. Today more then 50 different desalination technologies are classified.


Future Desalination Technology

Saving material and energy per ton of water produced is the main target of all developments in desalination. Due to this basic attempt every of the today known technology has to prove its benefits against the established ones. Not pointing out every new technology three of them are selected and briefly described at this stage (2017).

Obviously an idea for a technology needs to develop. On the right hand side you can see an overview of different technologies. This sketch does illustrate the state of science knowledge and in the same way the state of Development, Technology and Art.

Legend: State of science knowledge values

0 Very high, 1 High, 2 Medium, 3 Low, 4 Very low

In this context DME does illustrate some of the know how build up and some of the support given so far in this field of activities preforming. In order to classify the development of a technology DME did introduce the

– Desalination Technology Development Benchmark –

also called “DesTeDeBe”. You will find more details “here (LINK)”

Some of the latest developments in technology are now benchmarked here. In order you want to know more, please get in contact with DME.


Multiple-Effect Distillation (MED)

Multiple-Effect Distillation (also Multi Effect Distillation or MED) is a thermal distillation technology based on evaporation and condensation processes in multiple stages, called effects, that is mainly used for desalination. In most MED plants, the seawater enters all the effects in parallel and is raised to the boiling point after being preheated on tubes. The seawater is either sprayed or otherwise distributed onto the surface of the evaporator tubes in a thin film to further rapid boiling and evaporation. The tubes are heated by steam from a boiler or some other source, which is condensed on the opposite side of the tubes (inside). The condensate from the boiler steam is recycled to the boiler for reuse. Only a portion of the seawater applied to the tubes in the effects evaporates. The remaining feed water is collected and fed to the last effect, from where it is removed by a brine pump. The tubes in the various effects are heated in turn by the vapours arose from the previous effect. This vapour is condensed to a fresh water product, while giving off heat to evaporate a portion of the seawater feed in the effects. This continues for several effects, with 4 or 16 effects being found in a typical large plant. The remaining seawater of each effect flows to the next effect through pipes by gravity. Generally, these plants are powered by low temperature heat leading to Top Brine Temperatures (TBT) of 55-70°C and are combined with mechanical vapour compressors or thermal vapour compressors. The number of effects directly correlating with the Performance Ratio (PR). In contrast to this, the PR is not significantly influenced by TBT. Depending on the TBT, pre-treatment of the feed and usage of anti-scalants is necessary. Furthermore, corrosion problems limit the usage of cheap construction materials.


Multi-Stage Flash (MSF)

Multi-Stage Flash (MSF) evaporation desalination plants are based on flash evaporation in a cascade of stages (effects) at different pressure levels. In the brine-recirculation mode, the brine is recirculated through the system.

In the last stage of an MSF plant, the heated feed water is led into the pressure vessel where it partially evaporates in a flashing process due to the pressure decline. The vapour is used to preheat the feed water. The part of the brine that does not evaporate is sucked into the next stage which has a lower pressure. There, the same flashing and evaporation procedure takes place again, but at lower pressure and thus lower saturation temperature.

In this way, the feed that is used to condense the steam at every stage from the lowest pressure at the first stage to the highest pressure at the last stage is heated up continuously. The result is a low mean temperature difference between the feed water and the condensing steam, although the heat transfer is of type latent-sensible. Thus, the exergy losses are low.

The circulation of the brine mainly reduces costs for pre-treatment, but also reaches higher operation flexibility and better thermal efficiency. At the same time complexity and costs for components, construction, and maintenance rise.


Membrane Distillation (MD)

Membrane Distillation (MD) is a thermal desalination process in which water vapour is transported through a hydrophobic porous membrane. Due to a partial pressure difference of vapour between a liquid surface and an air or gas stream, water is vaporised although the liquid phase is much below boiling temperature. A hydrophobic membrane can separate the two phases – liquid water and either vapour or vapour-gas mixture – in order to avoid the transfer of liquid or allow compact stream arrangements. Thus, MD is a non-isothermal membrane separation process with combined heat and mass transfer over the membrane. Parallel arrangement of the membranes in flat or spiral wound configuration allows generating multi-effect setups. The steam, generated over a first membrane layer can be condensed in the permeate channel using the latent heat of condensation for heating an adjacent feed channel. This way, a diversity of possible stream arrangements and system configurations arises. The main distinguishing feature is the configuration of the permeate channel. In order to minimize sensible heat transfer from the hot feed channel to the next feed channel, as it is the main drawback of direct contact configurations (Direct Contact MD (DCMD), DCMD with Liquid Gap (LGDCMD),s. Fig. (a)), either air or any other sweep gas can be filled in the permeate channel. The Air Gap (AGMD) or Sweep Gas Membrane Distillation (SGMD, s. Fig.(c)) minimize the heat transfer over the permeate channel by dividing the condensate from the membrane. The main drawback of these configurations is the higher diffusion resistance over the gas and the worse condensation due to the presence of non-condensable gases. This problem can be handled by running the system under vacuum for removing the non-condensable gases (Vacuum Membrane distillation (VMD), s. Fig. (b)). (Khayet Souhaimi & Matsuura, 2011) Besides the application of flat membranes, systems based on hollow fibre membranes are researched. The main advantage of Membrane Distillation systems is the low temperature heat (below 100 °C) at low pressure for running the system. For that cheap construction materials (plastics) can be used. As described before, the usage of membranes allows compact system setups which reduces the footprint of the plant.


Capacitive Deionization (CDI)

In Capacitive Deionization (CDI), charged ionic species are removed from aqueous solutions. The ions are adsorbed onto the surface of a pair of electrically charged electrodes, usually composed of highly porous carbon materials, upon applying an electrical voltage difference. Upon charging the electrodes with a voltage difference of typically 1-1.4 V, the salt ions present in the feed migrate to the electrode of opposite charge, cations to the cathode and anions to the anode, and form electrical double layers (EDLs) along the pore surface. Thus, the water flowing through the CDI cell is partially desalinated. In a discharging step, where either the applied voltage is shorted or the polarity reversed, the salt ions are released in a brine stream. The system architecture can be in flow-by or flow-through mode with the feed either streaming past the electrodes in parallel direction or streaming vertically through the electrode. New developments propose floating electrodes suspended in the aqueous solution that enable continuous operation. Various porous carbon materials have been suggested as electrode material, such as carbon aerogels, activated carbon and carbon nanotubes. An important factor is the sorption performance. The technology has been previously applied to brackish and seawater desalination, wastewater remediation and water softening, but has proven to be highly effective for solutions with low molar concentration such as brackish water. CDI does not require high pressure or temperature, as in membrane or thermal desalination, making the technology more energy efficient in comparison. It also has a higher accuracy in removing only particular salts and ions that enables the recovery of valuable compounds such as lithium among others. However, problems may arise in the regeneration phase as during reversed-polarity, repelled ions might be attracted to the oppositely charged electrode and by electrical shorting the only driving force is diffusion, which is slow and inefficient. The special applicability to brackish water offers a great potential for development, as the demand for desalination of brackish water is increasing due to salt intrusions into the groundwater in many regions worldwide. Nonetheless, many basic settings have not been uniquely defined until now and have to be further optimized.


Multiple-Effect Humidification (MEH)

The Multiple-Effect Humidification (MEH) process for the extraction of water from salt-, brackish and well water represents a thermal method that is based on the well-known principle of evaporation (humidifier) and subsequent condensation (dehumidifier). The performance of this natural process was improved in the MEH process such that a major portion of the used evaporation energy remains within the process, permitting the low-temperature extraction of drinking water effectively and reasonably, without much waste heat, even in smaller units. The technical design of the process describes the natural water cycle, from evaporation (sea surface to atmosphere) to condensation (warm air in contact with cold air = rain). Thus, in a closed module, air is forced into circulation by a ventilator at atmospheric pressure. During its cycle inside the module, the air passes through the two main chambers of the system, the humidifier and the condenser, and it transports purest water (in gaseous form) from the humidifier to the condenser where it is retrieved as drinking water in liquid form. Through its various energy use options, the MEH is unique in terms of environmental sustainability. Due to the comparatively low operating temperatures, the energy required for such heating may be obtained as waste heat from operational processes or renewable energies, such as solar or geothermal power.


The hidden talent of mushrooms for solar steam generation Read more: The hidden talent of mushrooms for solar steam generation


In this work, for the first time, researchers utilize living organisms – mushrooms – to generate steam under sunlight. It turns out that the micro- and macrostructures of mushrooms possess all the needed characteristics for a good solar steam-generation device: high solar absorption; efficient water supply and vapor escape; and good thermal management. Interestingly, a mushroom is an unlikely candidate as it typically lives in the shadow, i.e. it doesn’t get to see sunlight that much.
The mushroom maintains its hydrophilicity before and after carbonization because of its components, which include carbohydrates and proteins; the nitrogen functional groups exist even after carbonization.
The scientists attributed mushrooms’ capability of high-efficiency solar steam generation to their unique natural structures, including their umbrella-shaped black pileus, porous context, and fibrous stipe with a small cross section.
First, the umbrella-shaped black pileus can absorb a huge amount of solar energy. Second, the hydrophilic fibrous stipe working as efficient water supply path can pump water into the mushroom context by capillary force. Third, the porous context not only acts as a bridge to pump the water further into the top pileus but also provides sufficient vapor channels.
“What’s more” as Zhu points out, “the geometry of mushrooms is naturally

Read more: The hidden talent of mushrooms for solar steam generation…