If the contaminant is a hazard only if ingested, water used for cooking and drinking needs treatment. A small unit that supplies a separate faucet at each location for water consumption is sufficient. A large home unit or small commercial unit is necessary. Most major manufacturers produce units in a wide range of sizes. The price and complexity of management increase with size.
Small units have a throwaway membrane and filter cartridges.
Larger units often have cleanable filters. Because they are too expensive to discard when partially plugged, they usually are cleaned several times before replacement. The larger units usually last longer, however, and produce higher quality water at a lower cost per unit. Reverse osmosis membranes are manufactured of organic chemicals that degrade with age, may be attacked by some chemicals and are often plugged by some minerals. Selecting the appropriate type, pretreating to remove problem chemicals or minerals and minimizing the shelf storage before use all help increase the life of RO membranes.