Why we need our water filtered:
Water treatment plants make water drinkable by filtering out chemical and biological contaminants. Sewage treatment plants attempt to prevent disease-causing wastes from mixing with drinking water. These systems are successful most of the time.
Most wealthy industrialized countries, like the US, have modern water treatment and sewage treatment systems carrying drinkable water many miles from remote reservoirs to those who will be drinking it. Listed below are some of the ways water is purified.
� Sand filters. Sand or other filtration media are used to remove turbidity. However, the location of the fine media on top of the coarse media causes the sand filter to clog quite quickly and the coarseness of sand allows many smaller impurities to pass through.
� Neutralizing filters. Neutralizing filters usually consist of a calcium carbonate calcite medium (crushed limestone or marble) to neutralize low pH water.
� Oxidizing filters. Oxidizing filters use a medium treated with oxides of manganese as a source of oxygen to oxidize and precipitate iron, manganese, hydrogen sulfide, and others.
� Activated carbon filters. Activated carbon (AC) is similar to ion exchange resin in density and porosity. It absorbs low molecular weight organics and reduces chlorine or other halogens from water, but does not remove any salts. These filters must be changed periodically to avoid bacterial growth, but are not easily reactivated in the field. Accumulated solids require frequent backwashing of the filter unless installed after reverse osmosis or ultrafiltration.
� Dual- or multi-media filters. Dual-media filters remove suspended solids to as low as 20 microns in size, but no dissolved solids. The top layer is a coarse anthracite followed by fine sand.
Pre-coat filters
Usually with a media of diatomaceous earth, pre-coat filters remove very small particulate matter, including some bacteria. They are practical only for limited volume applications but are common for swimming pools, beverage plants, and small installations.
Cartridge filters can now be described two general ways: as depth filters or surface filters.
� Depth cartridge filters. In a depth cartridge filter, the water flows through the thick wall of the filter where the particles are trapped throughout the complex openings in the media. The filter may be constructed of cotton, cellulose, synthetic yarns or "blown" microfibers such as polypropylene. The best depth filters have lower density on the outside and progressively higher density toward the inside wall. The effect of this "graded density" is to trap coarser particles toward the outside of the wall and the finer particles toward the inner wall. Depth cartridge filters are usually disposable, cost-effective, and are in the particle range of 1 to 100 microns. Generally, they are not an absolute method of purification since a small amount of particles within the micron range may pass into the filtrate.
� Surface filtration-pleated cartridge filters. Pleated cartridge filters typically act as absolute particle filters, using a flat sheet media, either a membrane or specially treated non-woven material, to trap particles. The media is pleated to increase usable surface area. Pleated membrane filters serve well as sub-micron particle or bacteria filters in the 0.1 to 1.0 micron range. Newer cartridges also perform in the ultrafiltration range: 0.005 to 0.15 micron.
