لخّصلي

More than 11,000 desalination plants are in operation throughout the world, producing more than 20 million cubic meters (m3)--roughly six billion gallons--of water per day.Also, some microorganisms unique to saline waters may not be removed by the desalination process or post disinfection.Most desalination plants use seawater and/or brackish water as their sources for drinking water production, however; membrane technologies are also used for wastewater treatment and for removing salts from processed wastewaters for recycling applications including prior-to-aquifer recharge.Due to rapid advances in technology and improved efficiencies, the cost of producing desalinated water is now approaching $0.50 USD per cubic meter in large plants ($1.89 per thousand gallons or Kgal), so it is becoming much more accessible in areas where alternative fresh water supplies are not available.WHOGDWQ cover a broad spectrum of contaminants from inorganic and synthetic organic chemicals, disinfection byproducts, microbial indicators and radionuclides.Health risks can be imparted from consumption of reconstituted or restabilized water from general reduced or selective mineralization, or from reduced intake of specific minerals like calcium and magnesium.Some issues that distinguish desalination processes from typical drinking water operations include:

Source water quality (see Table 1)

Total Dissolved Solids (TDS) in the range of 40,000 ppm for seawater and about 10,000 ppm for brackish water.High levels of metal salts including sodium, calcium, magnesium, bromides, iodides, sulfates and chlorides.Because desalination is applied to non-typical source waters and often uses non-typical technologies, the existing WHO GDWQ may not fully cover the unique circumstances that can be encountered during production and distribution of desalinated drinking water.The environmental impacts of desalination facility siting, operations and brine disposal can be significant.Treatment

Reverse osmosis membranes and thermal distillation.Pretreatment and antifouling additives.

More than 11,000 desalination plants are in operation throughout the world, producing more than 20 million cubic meters (m3)—roughly six billion gallons—of water per day. About 60 percent of the capacity exists in West Asia and the Middle East. North America has about 11 percent and North Africa and Europe account for about seven percent each. South and Central America together account for about four percent of desalination capacity. Desalination is also a significant and growing source of fresh water in the Caribbean Islands. Plant sizes and designs range from more than 500,000 m3/day (13,200,000 gallons per day or 13.2 mgd) down to 20 to 100 m3/day (5,280 to 26,400 gallons per day). Due to rapid advances in technology and improved efficiencies, the cost of producing desalinated water is now approaching $0.50 USD per cubic meter in large plants ($1.89 per thousand gallons or Kgal), so it is becoming much more accessible in areas where alternative fresh water supplies are not available.

Most desalination plants use seawater and/or brackish water as their sources for drinking water production, however; membrane technologies are also used for wastewater treatment and for removing salts from processed wastewaters for recycling applications including prior-to-aquifer recharge. Performance, operating and product quality specifications have evolved virtually on a site-by-site basis relative to source and the specific end product water use. Most drinking water applications outside of North America use World Health Organization Drinking Water Quality Guidelines (WHO GDWQ) as water quality specifications. WHOGDWQ cover a broad spectrum of contaminants from inorganic and synthetic organic chemicals, disinfection byproducts, microbial indicators and radionuclides. They are aimed at typical drinking water sources and technologies. Because desalination is applied to non-typical source waters and often uses non-typical technologies, the existing WHO GDWQ may not fully cover the unique circumstances that can be encountered during production and distribution of desalinated drinking water.

Drinking water production
Drinking water production chains can be divided into three broad categories, each of which will impact the quality of the finished water received by the consumer: source water quality, treatment processes and distribution. Some issues that distinguish desalination processes from typical drinking water operations include:

Source water quality (see Table 1)

Total Dissolved Solids (TDS) in the range of 40,000 ppm for seawater and about 10,000 ppm for brackish water.
High levels of metal salts including sodium, calcium, magnesium, bromides, iodides, sulfates and chlorides.
Total Organic Carbon (TOC) type.
Petroleum contamination potential.
Microbial contaminants and other organisms.
Treatment

Reverse osmosis membranes and thermal distillation.
Leachates from system components.
Pretreatment and antifouling additives.
Possible unique disinfection byprod-ucts.
Post treatment blending with source waters.
Distribution

Corrosion control additives and corrosion products.
Bacterial regrowth in distribution systems.
There are other issues of interest and/or concern as well. System components that can contribute chemicals to the water as direct additives or indirectly from surface contact. Health risks can be imparted from consumption of reconstituted or restabilized water from general reduced or selective mineralization, or from reduced intake of specific minerals like calcium and magnesium. The environmental impacts of desalination facility siting, operations and brine disposal can be significant. Also, some microorganisms unique to saline waters may not be removed by the desalination process or post disinfection. Thus, monitoring of source water, process performance, finished water and distributed water must be rigorous to assure consistent quality at the consumer’s tap. Moreover, additional water quality or process guidelines specific to desalination are needed to assure water quality, safety and environmental protection.