Sea water desalination factory in Carlsbad
Over 400 years ago, people were seeking to transform the sea water to drinkable water. The realsea water desalination started from 1950s. Generally there are four methods for desalination: evaporation, reverse osmosis (RO), electrodialysis and sea water frozen. RO and evaporation are mature technologies which are most commonly used. Other innovative approaches were proposed but they are still in laboratory. RO is to use a force to push sea water pass through a reverse osmosis membrane while the salts are retained. The removal rate could be up to 90%-97%. Evaporation is letting the sea water evaporate and condense to pure water because salts have a much higher boiling point comparing to water.
Desalinated sea water provided portable water for more than 100 million populations worldwide. It has been widely applied in Mid East, South United States and Asian. So should sea water desalination be a perfect way to solve many problems related to water? It seemsyes. However after going deep inside the technology, you might not give the positive answer.
The first problem associated to sea water desalination is equipment O & M and complex operation handlings. Especially for RO, the sea water needs high standard pre-treatment to protect the RO. And RO needs frequent backwash to prevent fouling and clogging.
Secondly, desalination needs a great amount of energy put-in. For RO operation, pumps have to provide pressure to drive the sea water passing through membrane. The required pressure and energy cost is several times higher than micro filtration (MF) and ultra-filtration (UF) in conventional treatment plant. For evaporation, heat energy or low pressure are the driving forces to make water evaporate. Some desalination factories even need an affiliated power station for energy generation. But this situation is improved by using the solar thermal energy.
Furthermore, the cost of desalination is great. Even in some cheap-labor areas like China, the cost of desalinate one ton water is nearly $0.8. The cost comes from the energy consumption and equipment maintenance. In some water treatment plant, RO needs to be changed every 6 months. If RO is used for such high salt concentration water, it should be changed more frequently. This equipment usage levels up cost. As well the investment for researches and to build plant is significant.
Additionally, the desalination would bring serious environmental issues. Ocean water has salt concentration of 35000 mg/L. The produced water must meet EPA’s standard that is 500 ppm of dissolved solids. Hence the retained waste stream from desalination has a dramatically increased salt concentration. How to disposal the retained water and salt is a big problem. Landfill could cause erosion. Dumpling into ocean might destroy the whole ecosystem in ocean. The only way might be consumed by industry. However the demand is far less than the waste produced.
In a word, sea water desalination is a good way to solve water resources shortage. But it’s NOT a good way to solve all water problems. Singapore always suffers water shortage but they seldom establish the desalination plant. The reason might be related to the cost and energy. California has the State Water Project which supplies sufficient raw water. So the author of this blog suggests not building many desalination plants. The best way to reduce water consumption and reduce treatment footprint might always be traditional methods like reclaimed water the grey water reuse and water efficient equipment.
Lastly the author wants to say is, I really tried to drink the desalinated water in Tianjin City, China. It taste not as good as normal portable water and it’s not good for drinking after training. Therefore think twice before choosing desalinated water, no matter you are government officer, engineer, investor or regular consumer. Don’t let new problems to solve existing problems.
