| Popis: |
The field of desalination is becoming increasingly more complex and interesting. The original concept of the production of potable water from seawater and brackish water now describes only one area in which desalination technology may be applied to the supply of safe drinking water. Reverse osmosis membrane manufacturers are becoming adept at tailor-making membranes to fill other water treatment applications. Ultra-low pressure nanofiltration is a new tool for removal of hazardous organic contaminants in preference to low molecular weight salts. Demands for water quality are becoming increasingly more stringent. In 1986, the USEPA targeted 83 contaminants for which regulations will be developed. Maximum contamination levels MCL's) have already been set for certain constituents which have been identified as possible carcinogens, such as trihalomethanes (THMs). THMs are reaction products from chlorination of water containing natural substances such as humic and fulvic acids, called THM Precursors (THMPs). MCLs for total trihalomethanes (TTHMs) have been set at 100 ppb and will probably be lowered to 50 ppb in the near future. Because of the number of components which are being considered for regulation and because of the diversity of their physical and chemical properties, their removal presents a formidable challenge to the water treatment/desalination engineer. Because of their ability to reject high percentages of many dissolved components, nanofiltration membranes offer a single treatment alternative for removing many of these contaminants. Recent developments in membrane technology have created a new generation of membranes which are much more cost-competitive for treating lower TDS waters for removal of specific contaminants. These new membranes are classified as nanofiltration or ultra-low pressure membranes, typically providing 90–95% rejection of THMPs, 85 to 95% rejection of hardness and over 70% rejection of monovalent ions when operated at only 70 to 100 psi driving pressure. Field testing is now underway in Florida, where substantial THMPS are found in the shallower aquifers. This new technology should be of particular interest where water resource conservation procedures involve blending of desalted water product with untreated groundwater, surface water and/or catchment storage, followed by chlorine disinfection. |
