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How Water Filters Work

Filters are composed of a substance which traps, adsorbs, or modifies pollutants in the water which flows through them. This substance is called medium. There are many different types of filter media. Some mechanically trap pollutants with an ultrafine sieve or strainer, while others use a process called absorption in which contaminants are retained within the microscopic pores of the medium.

The rating of a water filter or purifier tells you what size particles it will and wont remove. Filters are rated in micro metres or microns. A micron is one millionth of a metre. A human hair is 70 microns in diameter, a cryptosporidium oocyst 4-6 microns and a Giardia cyst 8 - 12 microns.

There are two types of filter ratings: nominal and absolute. A nominal rating indicates the smallest particle size that the filter should remove or reduce, in accoradnce with its design criteria. It is an estimated value, not a precise one. A 5 Micron nominal filter, for example should trap 95 per cent of all particles 5 microns or larger.

On the other hand, an abslute filter rating refers to a certified reduction rate, usually 99.9 per cent. Therefore a 5 Micron absolute filter will remove 99.9 per cent of particles 5 microns or more in diameter.

Sediment Filters

Sediment or particulate filters are fine sieves which reduce dirt and other particles. Using one as a pre-filter will protect a water purifier from damage and exten its life, because it will take longer to become clogged with unwanted media.

Sediment filters range from coarse to fine, so they are rated accordingly.

Sediment filters can be made from wound string, rigid foam (Polyspun) or pleated film. They are usually mounted under the sink. The life of a sediment filter depends on the rubbish in the water - six to twelve months is average.

Activated Carbon Filters

Activated carbon filters are particuularly effective at removing pollutants which create unpleasant taste, colour, and odour in water. These fast-acting filters can eliminate or reduce the levels of chlorine by-products, pesticides, herbicides, and other organic and industrial chemicals.

Activated carbon is made from a variety of organic materials such as coal,coconut, lignite, and wood.

When these are activated by exposure to high tempratures in the absense of oxygen, the result is a substance with millions of microscopic pores and a vast surface area; half a kilo of activated carbon provides more than 50 hectares of surface with the capacity to cling to or absorb smaller organic molecules.

There are two forms of carbon in general use: granular and block. Carbon granules are about the size of coarse sand while carbon block is finely powdered carbon compressed into a solid mass.

To get the most of a carbon filter, it should be kept free of sediment and heavy organic impurities by the incorporation of a sediment filter as an integral part of the system design. It is imperative that filter cartridges be replaced regularly before they reach their expiry date, rather than after.

Ceramic Filters

These are effective against bacteria, parasites, and sediments. Some models can filter down to .9 of a micron absolute. 

The filter has a hollow core of ceramic which can be scrubbed with a soft bush or scothbrite when cleaning becomes necessary. This type of ceramic filter can be used as a sediment pre filter in replacement of a standard string wound, foam (poly spun) or pleated filter.
Some ceramic filters are fitted with an additional activated carbon block core to increase their taste and odour reduction efficiency.

Reverse Osmosis Purifiers

Osmosis is a process which occurs when two solutions of different concentrations are seperated by a semi-permeable membrane. Reverse osmosis water purification works by forcing the water under pressure against an ultrafine semi-permeable membrane designed to allow single water molecules to permeate through, while at the same time rejecting most contaminants. The membrane acts as a mechanical filter, straining out particulate matter, micro-organisms, asbestos, even single molecules of heavier organic compounds.

A typical RO purifier consists of four filters in series plus a storage tank. The fisrt is a sediment filter, the second a carbon block, the third a membrane and the fourth a activated carbon block to remove any remaining chlorine by-products.
Such a system removes a wide spectrum of impurities from water; the only energy required is that of mains-water pressure.

Reverse osmosis effectively removes turbidity, sediment, collodial matter, total dissolved solids, toxic metals, radioactive elements, pesticides, and herbicides. This can have significant health benefits.

A typical system produces water at a slow rate - almost drop by drop - so most under sink systems have a pressurised storage tank and a seperate dedicated faucet or all in one three way mixer installed on the sink. Water drawn from the faucet or mixer comes from the storage tank.

A counter top system works the same way except without a pressurised tank. Instead these systems attach directly to an existing faucet and used to fill a bottle or glass directly from the system. These systems are suited for use when renting, unable to plumb a system in, or for travel.

The average system produces about 200 liters per day*, more than enough for  an everage family.
(* Depending on inlet water pressure and membrane capacity.)

The average domestic RO will use about 40 litres per day to flush contaminants - average household consumption is around 1000 litres per day.
Unlike filters, RO membranes dont accumulate pollutants but the membranes themselves gradually degrade with use. While the sediment and carbon filters will probably need replacement every 6 - 12 months, membranes should be changed every 3 - 5 years or as specified by the manafacturers.

John Archer (1998). Sydney On Tap. Australia: Pure Water Press. p74-80.