There are many types of filters available in the market place today. I will try to group them by the method they use to filter water. Almost everyone has seen the ads for the filter that fits on the end of your kitchen sink or bathroom spigot. These filters usually use two basic types of filtration: a filter 'pad' catches the large (usually over 25 micron in size) particles or 'chunks' , and a small amount of carbon to adsorb organics and/or chlorine. The main problem here is the flow rates at which they are expected to work at. The consumer expects to turn the tap on as normal and draw "filtered" water. To remove free chlorine, for instance, standard engineering practices set the maximum flow rate at 10 gallons per minute per square foot (144 square inches) of surface area of the carbon, *if* you are using a standard 30" bed depth. To remove chloramines or organics, the maximum flow rate is set at 5 gallons per minute per square foot of surface area. If your spigot will provide a flow of 1.5 gallons per minute, what size filter do you need hanging on the end of that spigot to insure that the chlorine and organics will not be swept past through the filter, into your glass? If you purchase this type of filter, make sure it has a way of limiting the rate at which water passes through it.
Next comes the cartridge type filter. Most common are the 10 1/2 or 20 inch long filters. This type filter will usually have a removable housing, into which different types of "elements" can be placed. A sediment filter cartridge element can be manufactured to remove certain size particles and larger. Most elements for home use will indicate 30 or 50 micron and larger removal. More expensive elements, usually for industrial use, may indicate a particle size (in microns) and add the words "Absolute" after it. No, it isn't Vodka, it simply means that if it says 5 micron absolute, it means it! Very few particles larger than 5 microns will pass through the filter. The regular filter may say 25 microns, meaning that *most* of the particles 25 microns and larger will be caught by the filter. Remember, there filters actually get better, or more effective, as they are used. The 'junk' in the water collects on the surface of the filter and becomes a part of the filter as well. As it builds up, progressively smaller and smaller particles are trapped, and the flow rate through the filter slowly diminishes. This slowing of the flow rate can be a source of problems to water using appliances in your home. If you use such a filter, regular changing of the filter element is very important. Elements for these filters can also be carbon (block or granular, or powdered), can be manufactured for use in hot water, can be ceramic, pleated as well as many other configurations. Some manufacturers are mixing a small amount of silver into the carbon to help prevent any bacteria growth in them. This has yet to be a proven methodology. In fact, make sure that such a filter doesn't give off more silver than is allowed, if not rinsed thoroughly prior to use, especially after a prolonged period of non-use. Remember, all filters, carbon especially, trap organics that bacteria feed on, and as the water sits without moving, they can multiply rapidly. Always change the elements on a regular, frequent basis.