The home water treatment systems that consume water in the treatment process are reverse osmosis systems and water softeners. Point-of-use water filters that attach to the kitchen sink faucet do not consume any water.
The amount of water consumed by a home water treatment system will vary depending upon the type and size of the system. The manufacturer should be able to provide water consumption values.
In a recent study of residential water use conducted for the American Water Works Association Research Foundation, 8% of the study homes were equipped with a home water treatment system that had some consumptive use. There was tremendous variability in these treatment systems. Consumptive use ranged between 1.0 and 74.0 gallons of water per day. The average water use for homes equipped with a home water treatment system was 15.8 gallons per household per day (Mayer, et. al. 1999). This provides a range of water use that might be expected from a home water treatment system.