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
Future desalination Technology
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 –
Some of the latest developments in technology are now benchmarked here. In order you want to know more, please get in contact with DME.
Looking into what is available in the market today three desalination technologies just coming up are described. These abstracts are basically giving a glance of the technology not reviewing specific developments of researchers and manufacturers.
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.
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.
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.