Desalination

Reverse osmosis (RO) involves pushing water under pressure through a membrane which retains the salt and other impurities. Membrane separation processes are used for:

• Desalination of seawater.
• Production of process and potable water.
• Polishing and demineralization of boiler feedwater and pharmaceutical water.
• Rehabilitation and recycling of process and municipal effluent.
• As part of an integrated zero liquid discharge process.
   

Thermal desalination processes are best applied to water and process streams with a high dissolved salt content, where the applied pressure required for an equivalent membrane system would be excessive. 
The use of a low-cost energy source, such as the one available from cogeneration power plants, is advantageous with respect to reducing energy cost, if available. Another option is to use low-temperature, high-efficiency vacuum evaporation systems that can be operated on waste heat and a low electrical energy demand.

Hybrid systems combine both thermal and reverse osmosis technologies which, in some situations, can allow an optimization of water product costs.

According to the International Desalination Association, the cumulative contracted desalination capacity has now surpassed 100 million m3/d. Roughly 60% of desalination is devoted to human consumption. It is estimated that some 300 million people rely on this process for their daily water usage. 
Kuwait for instance has an average rainfall of 110mm per year and produces almost 100% of its fresh water use through desalination.

The sun has done it forever ... by evaporation to provide rain. 
Steam-powered ships were first equipped with evaporators for the production of boiler makeup water in the early 1900’s. Desalination units were first used on board ships and were then widely developed with the growth of OPEC countries (1970). The capacity of the plants has grown dramatically over that period, from a few hundreds cubic meters per day in the 1960's to hundreds of thousands of cubic meters per day for the largest desalination plants nowadays.

Distillation processes are not sensitive to seawater quality. A 0.5mm filtration, chlorination and a suitable anti-scale and anti-foam treatment will be sufficient to guarantee a steady water quality versus time, particularly with the low temperature Multiple Effect Distillation (MED) process. Bacteria and pathogenic organisms are generally destroyed by these processes. Distilling pollutants are the only ones to create a problem when mixing with condensed vapor, if a seawater screening solution is not set up in due time.
Membrane processes are more sensitive to seawater quality. Depending on seasonal conditions, seawater quality may be affected by the weather or by biological activity. In such cases, pretreatment systems must be designed to withstand such conditions and prevent  bio-fouling which is the clogging of membranes by bacteria. 

Desalination plants reject slightly concentrated seawater called brine to the sea. The salt content of the brine is roughly 60g/l for seawater at 40g/l. The temperature of the brine reject is slightly higher than that of the seawater - a few degrees Celsius. This excess salt concentration and temperature quickly vanish in the vicinity of the rejection point.
Seawater is chlorinated at the plant inlet to avoid the growth of seaweed and seashells inside the piping. Chlorine is accurately dosed so as to avoid active chlorine remaining at the outlet of the plant.
Anti-scale and anti-foam additives are also dosed in seawater entering the plant, dose levels being in the range of a few grams per ton. Once they have reacted inside the plant and due to their huge dilution, they present no risk of pollution at the plant outlet.

As such, a desalination plant has no other reject to the environment. However it can be coupled with a power plant. The rejects of such plants are controlled by environmental standards. These combined plants provide the best efficiency and minimise the overall impact of power and water production on the environment.