The public water supply in India is so unreliable that the urban middle class, with its increased spending power, prefers shelling out an extra buck on one of the numerous water purifiers available in the market, rather than risk health- Suresh Babu SV Activated carbon filters typically use granular carbon or blocks of powdered carbon. The latter remove more contaminants, including volatile organic chemicals (VOCs), pesticides and herbicides, chlorine, and other chemicals found in tap water. It also removes unpleasant taste and odour. However, some heavy materials and bacteria can slip through the filter cartridge if the carbon is not densely compacted. Carbon filters need recharging. Ion-exchange filters Ion-exchange is mostly used to remove hardness in water caused by carbonates and bicarbonates of calcium and magnesium. These filters have a resin bed, which draws out calcium ions and magnesium ions in water and replaces them with sodium ions. Ion-exchange filters can remove iron, lead, copper, barium, radium and nitrates. Some such filters can also remove arsenic and selenium. Suraksha, one of India's earliest tap filters, used the ion-exchange process. Now, most filter manufacturers prefer integrating ion exchange into a system that has several water purifying technologies. Reverse osmosis filter RO uses pressure to force water through a membrane that retains the contaminants on one side and allows the pure water to pass to the other side. RO purification units involve two processes. The unit consists of two filters: a carbon or sediment filter, also called prefilter and a semi permeable membrane. The pre-filter removes large particles from the water before it passes through the semi permeable membrane. The membrane allows water to pass through, but traps pollutants and bacteria in its porous surface. Some RO systems also have a UV disinfectant which rids water of microbiological contaminants. The flow rates of these household filters vary from 8 to 72 litres per hour. Today, reverse osmosis has become very popular and its proponents claim that RO can remove virtually every contaminant. But that's a myth. WHO says that RO systems cannot completely remove some pesticides like lindane, solvents and other VOCs. The cost of household RO filters range from Rs 9,000-Rs 46,000 and they are heavy on maintenance. The pre filters and the RO membranes must be changed according to the manufacturer's recommendation. The storage tank must be cleaned periodically. Damaged membranes are not easily detected, so it is hard to tell if the system is functioning normally and safely. There are other negatives. As the water free of contaminants passes through the membrane, highly concentrated wastewater is discharged. 60-75 per cent of the water is rejected in an RO system. The amount of water rejected by the system depends on the quality of raw water, particularly its dissolved solid content. Ultra Violet light disinfection systems In such filters, water is passed through a clear chamber where it is exposed to UV light, which destroys bacteria and viruses. UV lamps found in filters in the Indian market last 5,000 to 10,000 hours of operation. The system's efficacy depends on the resistance of an organism. If the energy dose is not high enough, an organism's genetic material might only be damaged-and not disrupted. This means the organism would remain and can regenerate in suitable conditions. UV cannot be a stand alone treatment process because water has multiple contaminants. A water purification system must have a high quality carbon filter or a reverse osmosis system if it has to contend with heavy metals and chemicals. UV filters are employed as the final purifier in some systems. Some advantages of UV filters are: * It does not introduce any toxic or significant nontoxic byproducts * Does not alter smell or taste of treated water. In fact, it actually improves the taste of water by destroying organic contaminants * Does not affect minerals in water * Cleanses in seconds (chemical disinfection takes several minutes). The system's disadvantages are: * It is not suitable for turbid water with high levels of suspended solid or soluble organic matter. This can react with UV radiation, reducing the quality of disinfection * UV light is also not effective against soluble salts, pesticides, heavy metals, many organic chemicals and chlorine. Marketing gimmicks have created the impression that RO is the most effective technology to remove pesticides and other toxins from water. Kent RO systems, for example, has come out with a purifier which claims to adjust dissolved solid levels and minerals in water by a mere turn of a knob. Down To Earth Feature Service