Seawater Desalination

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China

Seawater, brackish water, and fresh water have different levels of salinity, which is often expressed by the total dissolved solids ( TDS) concentration. Seawater has a TDS concentration of about 35, 000 mg/ L, and brackish water has a TDS concentration of 1000– 10, 000 mg/ L. Water is considered fresh when its TDS concentration is below 500 mg/ L, which is the secondary ( voluntary) drinking water standard for the United States. Salinity is also expressed by the water' s chlorine concentration, which is about half of its TDS concentration.

As natural fresh water resources are limited; sea water plays an important part as a source for drinking water as well. In order to use this water, it has to be desalinated. So, seawater desalination is a real challenge for western civilization.

Water desalination processes separate feed water into two streams: a fresh-water stream with a TDS concentration much less than that of the feed water, and a brine stream with a TDS concentration higher than that of the feed water.

º The base method for seawater desalination is distillation. Approximately 620 kWh of evaporation enthalpy is necessary to obtain 1 t of drinking water.

º In technical installations, a multiple stage flash ( MSF) evaporator is used requiring approximately 100 kWH/ t ( hot steam at 2 bar) and 3.5 kWh/ t of electric energy for pumps.

º In contrast, the energy demand of reverse osmosis ( RO) can be brought down to 7 kWh ( large plants with pump pressure recycling and low water recovery rate) of electric energy for pump operation to produce 1 t of drinking water with a salt content of below 500 ppm from sea water ( total salt content of 3.7 percent) . In small plants, however, pump pressure recycling is not applicable. In this case, energy requirements rise to 30-40 kWh / t of water.

º In electrodialysis by means of ion exchanges membranes, however, the driving force is the electric power removing the ions from the water which is to be desalinated. As the amount of electric power needed is proportional to the salt content of the water, it is impossible to give a specific amount of electric energy required. Therefore, this method is much more favorable in case of low salt contents, such as brackish water or water extracted from briny sources ( heavily exploited fresh water sources near the sea) with an energy consumption of 3-8 kWh/ t, than in case of high salt concentrations ( energy required up to 15-25 kWh/ t) .

Feiyang provides customized solutions to local authorities and industry in all of our businesses, from seawater, to brackish water, ultra pure water, pure water, soft water, river purification, lake purification, and irrigation water. To promote ever more environmentally friendly economic development, our research and development division strives to develop novel, practical, efficient solutions, using highly innovative means.

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