- Reverse osmosis water purification systems utilize a filter membrane to remove foreign substances from domestic water supplies. These include dissolved solids such as nitrates, sodium and sulfates, according to the North Dakota State University Extension Service. The systems are sometimes supplemented with carbon filters that reduce or eliminate soluble organic compounds including petrochemicals and pesticides. Understanding the impurities in the target water supply is the first step to selecting the proper filters for the reverse osmosis system.
- Reverse osmosis systems include a pre-filter or sediment filter. This coarse filter removes larger suspended materials that may damage the reverse osmosis membrane. Depending on the raw water-quality problems and the reverse osmosis filter to be used, the pre-filter may be either carbon or fiber. Common pre-filters, according to the Web retailer E.S.P. Water Products, have 5-micron openings.
- The cellulose tri-acetate filters (CTA) offer the advantage of chlorine tolerance. The material is wound into a tube filter for installation into the system. A membrane is rated by its gallons-per-day (gpd) processing capacity. Domestic membrane capacities range from 10 to 16 gpd for household use. Only fiber pre-filters are used with CTA membranes.
- Thin-film composite membranes are also sometimes referred to as Thin-film material membranes and can be abbreviated as either TFC or TFM. Damages occur to these membranes if exposed to chlorine. They need a carbon pre-filter to remove chlorine from the water before it reaches the membrane and causes damage. TFC membranes generally operate at a higher capacity than CTA filters and are rated from 18 to 150 gpd.
- A post-filter, a carbon filter mounted after the storage tank that holds the water after it has been processed through the reverse osmosis system but before it is dispensed from the faucet or tap, improves the taste of the water under some circumstances. Post-filtration is not generally used if carbon pre-filtration is used.