The Use of Filters to Purify Drinking Water

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Water filters play an important role in everyday life. Sediment, bacteria, cysts, active chlorine and organics are the most common water contaminates found in drinking water. There are various types of filters available to purify drinking water.

Sediment Filters

Sediment consists of various suspended particles such as sand, silt and clay. These particles are usually introduced into the water through erosion.In order to remove sediment, water must pass through sediment filters. Sediment filters are classified into two systems. The absolute pore size rating system specifies the size particle that will be retained with 100% efficiency. The more common of the two is the nominal pore size rating which expresses the size at which the majority of particles will be retained. Typical nominal rates for sediment filters are 1, 5, 10, 20, 25, 30, 50 and 100 microns.

Sediment filters use one of two main mechanisms to filter water: surface filtration and depth filtration. Surface filters, also called pleated sediment filters, have a larger surface area and thinner filtration media (1-2mm vs. 20-30 mm). These filters generally have a higher flow rate and capacity. Pleated sediment filters are made from micro-fiber sheet media, such as cellulose or polyester fibers, that are pleated around a polypropylene core. Some models of pleated filters can be cleaned and reused. Depth filters allow the particles to penetrate into the filter media where they are captured in the “depth” of the cartridge. Most depth filters are made from twisted yarn or extruded melt blown fibers. Typical materials of depth filters consist of polypropylene, nylon and polyester fibers. They will not impart taste, odor or color of the water. The most common sizes for sediment filters are 2.5” or 4.5” in diameter and 10″ or 20” in length.

Activated Carbon Filters

Carbon filters are used to remove active chlorine and organic contaminates, and to improve the taste and smell of the water. Because activated carbon is highly porous, only 1 ounce of the media has a surface area of 150 to 400 thousand square feet, depending upon manufacturing technology. Activated carbon works in two ways: absorption (to remove organics) and catalytic reduction (to remove active chlorine, chlorine dioxide, chloramines and more). Absorption of organics requires a lower flow rate and longer contact time with the contaminated water and activated carbon media. The catalytic reduction of active chlorine molecules into chloride ions is a faster process and can be achieved at flow rates 10 times higher than those required for absorption.

Granulated Activated Carbon (GAC) and Carbon Blocks are the two main types of activated carbon. In a GAC filter, water goes through the bottom of the plastic case, passes through the GAC media bed, then leaves the filter through the top of the cartridge. Carbon block filters are made from powdered activated carbon extruded into hollow, cylindrical blocks. Carbon block filters have a higher porosity than GAC filters. Water goes through the filter similar to sediment filters – from the outside of the filter to the center core.

Activated carbon is carbon produced from materials high in carbon such as coconut shells, wood, or coal. They are burnt into ashes in the absence of oxygen (pyrolyzed) and then activated by oxidation in high temperatures.To reduce the amount of leachable substances such as phosphate, activated carbon is washed in deionized water or acids such as hydrochloric or sulfuric. The four most common carbon filter sizes are: 2.5” and 4.5” in diameter and 10” and 20” in length.