Costly. But how effective?

Costly. But how effective?

A water purifier has to deal with physical, chemical and biological adulterants including dissolved solids, heavy metals and pesticides. Besides, all contaminants have different characteristics like molecular weight and size, water solubility, volatility and ability to react with oxygen. So, water treatment is a multi-pronged process, and the contaminant challenge has a big say in selecting appropriate technology.

Treatment processes either remove contaminants or destroy them. Removal technologies include ion exchange, adsorption and filteration (see flow diagram Water treatment). Then there are technologies that decompose chemicals and pesticides by oxidation or hydrolysis. Chlorination, for example, is also known to oxidize and remove many pollutants. Disinfection is a similar technology.

Let us now see how most common filters work

Activated carbon filter
These filters follow a two-pronged approach.Firstly, some contaminants are blocked by pores--filters with small pores are more effective. Secondly, these purifiers have carbon filters which draw molecules from dissolved contaminants--this process is called adsorption. Characteristics of the carbon material--particle and pore size, surface area, surface chemistry, density and hardness--influence the process's efficiency.

In many household filters, carbon's adsorbing powers are enhanced by adding a slightly-positive electrical charge. As water passes over the positively charged carbon, the negative ions of the contaminants are drawn to the surface of the carbon granules and are removed.

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.

Water purifiers must address the contaminant challenge

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 (see diagram How reverse osmosis refines water at a micro level)

Enlarge view

ro purification units involve two processes. The unit consists of two filters a carbon or sediment filter, also called pre-filter 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.

RO filters waste 60-75 per cent water

Complicated mechanism
A lot of systems combine a variety of filters
Source www.espwaterproducts.com as viewed on February 28, 2008

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 (also see box Think before you use ro).

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 taste 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, manyorganic chemicals and chlorine.

The best technology
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.

However, the third edition of who's guidelines for drinking water quality point out that more conventional technologies like activated carbon and ozonation are most suitable for removing pesticides and other organic chemicals.

The guidelines show that activated carbon is more effective for removing most pesticides (see table Cheap technologies do well). who also has a system to rank water purifiers according to their complexity and economics. Activated carbon and ion exchange are ranked the highest, while membrane treatment is deemed the most complex and costliest.

Cheap technologies do well
In removing pesticides
Note + stands for limited removal of the contaminant; + + for 50 per cent removal and triple plus for 80 per cent removal Source Anon 2003, Guidelines for Drinking Water Quality, 3rd Edition, World Health Organisation, Geneva, October

Integrating technologies
Integration of different technologies is more efficient in dealing with a complex pollutant matrix. And in fact, lot of modern day filters integrate different processes.

A typical system is shown in the diagram it is basically a reverse osmosis system, which employs pre-filter mechanism to remove odour and taste impurities. At this stage, some chemicals--up to about 0.2 micron--are also removed.

The water is then passed through ro membranes. At this stage concentrated waste is discharged and fresh water is passed through another filter to remove any contaminant--chemical or microbial--that would have escaped the ro membrane's pores.

Some systems employ uv light at the end of the treatment chain to get rid of bacterial contaminants.
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