Desalination file, Article 2/3
As discussed in the previous article "Solar desalination, a solution to a growing global problem", desalination seems to be the most appropriate technical way to solve the global freshwater shortage. First of all, it would provide another supply source, especially in countries close to the ocean that experience substantial hydric stress. Secondly, treating wastewater from various global industries using the same techniques would certainly enable us to reduce our global consumption of natural freshwater.
Solar desalination: a very promising market
Therefore, these desalination techniques, which can also be applied to wastewater treatment, represent a future way to deal with the fresh water shortage, which will only become more critical in time. The demand for this technology is also growing very rapidly and will become an indispensable resource for the 21st century. Globally, it is expected that the cumulative investment in desalination will reach 88 billion US dollars in 2016. (1) Given that fossil fuels increasingly have more and more disadvantages in terms of their environmental and economic costs, solar desalination is becoming a promising market.
The main desalination techniques used in industry
The salt content of seawater is approximately 35 g per liter (35 000 ppm) and it can reach concentrations of 39 g per liter in certain seas in the world. To make fresh water, desalination techniques must reduce the salt concentration in the water to less than 1 g per liter (1,000 ppm). We have been desalting since the beginning of the century, but desalination and water treatment technologies have evolved in recent years and some have become very effective. We will describe the three main techniques used in the current desalination market: reverse osmosis (RO), multi-flash distillation (MFS), and multiple effect distillation (MED).
- Reverse osmosis (reverse osmosis, RO)
Reverse osmosis is a process for separating water and dissolved salts by means of semipermeable membranes, and this is achieved mainly through the use of high pressure (54 to 80 bars for the treatment of sea water). This method works at room temperature and does not involve a phase change. The polymeric membranes used let through water molecules, but not the larger particles: such as dissolved salts, viruses and organic molecules. Various filters are also used to remove larger particles before the water passes through the aforementioned membrane.
This technique produces about 50% of fresh water and 50% of brackish water and it requires a lot of electricity in order to operate the pumps. But the biggest drawback is the fact that it is a solution that requires a lot of maintenance with qualified personnel. At the end of the process, the salt content of RO water is approximately 500 ppm.
- Multi-stage flash distillation process (Multi-stage flash distillation, MSF)
This method known as "Flash" consists of maintaining the water under pressure for the duration of the heating and when it reaches a temperature of about 120 ° C, it is introduced into a chamber (or stage) in which there is a reduced pressure. This process results in an instantaneous vaporization by expansion called Flash. There are several decompression chambers with even lower pressures (up to 40 Flash decompression chambers in large installations). The evaporated fresh water is then recovered.
This technique requires electric power for pumps and a lot of thermal energy in order to heat the sea water.
The main advantage of MSF is that the evaporation of the sea water does not occur around a surface since the liquid "flashes". This reduces the risk of scaling.
The salt content of the produced fresh water is less than 10 ppm.
- The multiple effect distillation process (Multi-Effect distillation MED)
This technique also uses the principle of distillation, except that the evaporation takes place in contact with a hot surface. This method is based on the principle of evaporation under reduced pressure of the heated sea water to a temperature which varies between 70 and 80 ° C. Evaporation of water takes place on an exchange surface, unlike in the case of MSF. This causes scaling problems of the area over time and depending on conditions. The temperature must be limited to 70 ° C to avoid scaling problems. The multiple-effect evaporators with horizontal tubes are currently the most used devices. In these devices, the heating fluid flows in the horizontal tubes while sea water flows as a uniform film on the outside of the tubes. The water evaporates and fresh water is then extracted from the tubes, while releasing its heat through condensation. A portion of the heating energy is thus recovered. The salt content of the produced fresh water is less than 10 ppm.
All these technologies use a lot of energy and renewable energy sources are increasingly profitable and suitable for this type of installation. Solar concentration is a particularly promising renewable energy for this market, especially when there is a heat requirement to distil the water.
The challenges of desalination
On the other hand, regardless of the technology used: the main waste industry: brackish water with a very high concentration of salt, poses serious environmental challenges. Currently, brackish water is sometimes released into the marine environment having been diluted beforehand, but studies tend to prove that the salt content of the surrounding seas is affected by such practices with dramatic consequences for marine life. In addition, substantial pollutants such as copper and chlorine are released with the brackish water, requiring measures to control the environmental impact of such facilities.
In continental areas, we sometimes dispose of this residue by injecting saline water from deep wells (with the long-term risk of contaminating groundwater), or by creating evaporation ponds. One of the possible solutions for this problem would be to achieve maximum condensation of the salt residue and revalorize them, either for consumption or for the chemical industry.
1. IEA-ETSAP and IRENA Technology Brief I12 - March 2012. IEA-2012.