Desalination

Desalinisation refers to processes that remove salt and other minerals from sea water. Sea Water Reverse Osmosis (SWRO) is a reverse osmosis desalination membrane process that has been commercially used since the early 1970s. Because no heating or phase changes are needed, energy requirements are low in comparison to other processes of desalination. 


Pre-treatment

Pre-treatment is important when working with SWRO, it has four major components:
  • Screening of solids: Solids within the water must be removed and the water treated to prevent fouling of the membranes by fine particle or biological growth, and reduce the risk of damage to high-pressure pump components.
  • Cartridge filtration - Generally string-wound polypropylene filters that remove between 1 - 5 micron sized particles.
  • Dosing of oxidizing biocides such as chlorine to kill bacteria followed by bisulfite dosing to deactivate the chlorine which can destroy a thin-film composite membrane. There are also bio-fouling inhibitors which do not kill bacteria but simply prevent them from growing slime on the membrane surface.
  • Pre-filtration pH adjustment: If the pH, hardness and the alkalinity in the feed water result in a scaling tendency when they are concentrated in the reject stream, acid is dosed to maintain carbonates in their soluble carbonic acid form.
  • Pre-filtration Antiscalants: Prevent formation of all scales compared to acid which can only prevent formation of calcium carbonate and calcium phosphate scales. In addition to inhibiting carbonate and phosphate scales, antiscalants inhibit sulfate and fluoride scales, disperse colloids and metal oxides and specialty products exist to inhibit silica formation.

High pressure pump

The pump supplies the pressure needed to push water through the membrane, even as the membrane rejects the passage of salt through it. Typical pressures for brackish water range from 225 to 375 psi (1.6 to 2.6 MPa). In the case of seawater, they range from 800 to 1,180 psi (6 to 8 MPa).

pH adjustment

The desalinated water is very corrosive and as to protect downstream pipelines and storage tanks, lime or caustic are dose to prevent corrosion of concrete or cement lined surfaces. Liming material is used in order to adjust pH at 6.8 to 8.1 to meet the potable water specifications, primarily for effective disinfection and for corrosion control. 

Disinfection

Post-treatment consists of stabilizing the water and preparing for distribution. Desalination processes are very effective barriers to pathogenic organism; however disinfection is used to ensure a "safe" water supply. Disinfection (sometimes called germicidal or bactericidal) is employed to kill any bacteria protozoa and virus that have bypassed the desalination process into the product water. Disinfection may be by means of ultraviolet radiation, using UV lamps directly on the product, or by chlorination or chloramination (chlorine and ammonia). In many countries either chlorination or chloramination is used to provide a "residual" disinfection agent in the water supply system to protect against infection of the water supply by contamination entering the system.


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