Thermal processes
This method evaporates seawater by applying a heat source and then condenses it. Repeat the operation several times, in some cases adding elements to the process that help capture any substance present in the impure water that you want to extract. The heat source is applied in each of the phases.[5].
This method is similar to the previous one, but the primary heat source is applied only to the first stage. For the next stage, the heat from the steam generated in the previous stage is used.
It is an energy recovery process where energy is added to low pressure vapor (usually water vapor) by compressing it. The result is a smaller volume of steam at a higher temperature and pressure, which can be used to do useful work. Typically, compressed steam can be used to heat the mother liquor to produce low pressure steam.
This method uses a closed space exposed to sunlight inside which the water evaporates and then condenses upon contact with the colder surface that separates it from the outside. The drops are carried down a slope until they gather at the edge of space. Its production is 1-4 l/day/m².[5].
Desalination with nuclear reactors.
Nuclear reactors can also be used in desalination processes, either in dual configurations producing electricity and process heat, or dedicated solely to supplying thermal energy. This technology is considered a low-carbon option for freshwater production in water-limited regions.[6].
Membrane technologies
Electrodialysis was first proposed in 1890 by Maigrot and Sabates, who built the first device for syrup desalination. This device used carbon as an electrode and permanganate paper as a membrane.[7].
The electrodialysis process used for desalination consists of passing salt water through charged membranes that trap the ions dissolved in the salt water with the aim of extracting and producing fresh water.[8].
In summary, when a direct electrical current is applied, the membranes selectively allow only sodium ions (Na+) or chloride ions (Cl-) to pass through. This causes the ions to move and become more concentrated as they are retained in these membranes, thus obtaining fresh water. The efficiency of this process depends on several factors, such as the intensity of the current and the permeability of the membranes used. On the other hand, the cost of the process depends on the concentration of salt in the water, since the more ions that need to be removed and depending on their electrochemical properties, the more electrical energy is required.[9].
In 1954, the first electrodialysis desalination plant was built for the Aramco company (Saudi Arabia). Starting this year, many other desalination plants were built that operate using this technology.[7].
Reverse Osmosis (RO) is a process in which fresh water is obtained from salt water. Natural osmosis is a phenomenon that consists of, if there is a semipermeable membrane separating two solutions with the same solvent, the solvent passes through it, but not the dissolved salts, from the side where the concentration of salts is lowest to the highest, until on both sides of the membrane the solutions have the same concentration. This process is carried out without external energy input, and is generated by what is called osmotic pressure.
Reverse osmosis consists of passing the solvent (in this case water) through the semipermeable membrane from the side where the most concentrated solution is (sea water, with dissolved salts), to the opposite side, without the salts passing through. In this case, energy is required, in the form of pressure, which will be slightly higher than the osmotic pressure that would make the low concentration solvent pass to the high concentration side. The pressure necessary to achieve reverse osmosis depends on the amount of dissolved salts and the degree of desalination that is desired. An increase in entropy results from the use of energy in the process.
The sea is a virtually unlimited source of salt water. A reverse osmosis plant needs to process a volume of seawater up to three times greater than the total amount of desalinated water that will be obtained in the end. Therefore, the design of the wells or collection system must consider this factor for their capacity.
The use of a graphene sheet with 1.8 nm pores to replace membranes in the reverse osmosis process for water desalination is in the research phase (TRL2). According to current research, much higher efficiencies would be obtained than with current membranes, and there would be lower energy requirements. In the current state, the drawback is the cost of graphene membranes, but it is expected that in the future these costs can be reduced.[10].
Generally, a large tank or pond is used that is filled by gravity at sea level, after coarse filtering. The water from the pond is transported by feed pumps to the desalination system. A supplement of chemical products arrives at the inlet of the feed pumps by means of metering pumps. This is how the water is prepared to pass four types of filters that retain particles larger than four microns. The main step in water production is the separation of H O from the mixture of salts and minerals present in sea water. This step is carried out in the reverse osmosis stage, ensuring that the salts do not cross the membranes of the RO modules. Previously, it must be ensured that the diatom and microalgae particles do not reach the membranes and for this there are three previous sand filtration steps before the last microfiltration step using synthetic fiber cartridges. Filtration success also depends on the proper introduction of coagulants. Depending on the filtration quality, the membrane change cycle is generated between 2 and 5 years. Chemical dispersants introduced before microfiltration prevent the precipitation of minerals within the membranes.
Since all aspects of the process are automated, the operators' job is supervision and maintenance.
The rejected brine is 55% of the raw water (although it depends on the desalination technology used). While 45% of the water obtained leaves at atmospheric pressure, a regulated backpressure must be ensured in the rejection flow. This reject flow always contains something like 55% (100% - % gain) of the pressure energy provided by the pumps and it is highly desirable to recover this energy for higher performance. Some of the recovered energy can return to the same desalination and recovery cycle more than once.
While the plant is in production mode, the outlet pressure is controlled by a regulating valve. 'Pressure Exchanger' converters are used and with them in the pressure exchange up to 95% of the energy of the rejection flow can be recovered directly by means of pumping using positive displacement. This energy recovery pump increases the flow of more raw water to the entrance of the membranes. The plant uses 'Pressure Exchanger' units near each set of reverse osmosis element tubes.
The osmosis water or the permeate from the reverse osmosis modules must be conditioned to meet certain high quality characteristics, since the water produced has an acidic pH and a low carbonate content, which makes it a highly corrosive product. This requires its preparation before distribution and consumption. The pH is adjusted with calcium carbonate to a value of 7.7. Additionally, if required by municipal regulations for the use of drinking water, sodium fluoride and hypochlorite are also added.