Ion exchange systems have also been developed for removing nonhardness chemical ions, such as nitrate and metals. Recharging the resins, in these cases, can only be done at special facilities, making this form of treatment expensive. RO membranes can remove all major types of contaminants including bacteria, metals, nitrate, and organics. However, there are some organic chemicals that may not be removed very efficiently. A RO system works by applying a high pressure to the contaminated water, forcing pure water through the mem- brane.
Pure water collects on one side of the membrane while pollutants accumulate on the other side.
The pollutants are periodically flushed to the sewer or septic system. RO membranes should last at least one year before replacement is needed. RO units are generally located at the point of use because of the small volume of water treated. They are typically located under the sink or on the countertop. Pretreatment with water softeners is often recommended for extremely hard water.