Graywater Collection and Use

Graywater is wastewater that has been used in clothes washers, showers, bathtubs, and lavatory sinks. In some parts of the country graywater may be collected using separate drainage pipes, then filtered and temporarily stored (without treatment) before being distributed in subsurface outdoor irrigation. There are also systems that direct lavatory washwater to an adjacent toilet tank to be used (after limited treatment with disinfectant) for toilet flushing. Blackwater describes wastewater from toilets, urinals, kitchen sinks, and dishwashers, which may contain food and human waste and is not as easily reused.

Graywater is typically stored inside a building rather than discharged immediately, because enough volume needs to be available to distribute it evenly through irrigation pipes. A significant amount of organic matter, including fats from soaps, is present in graywater. Aerobic bacteria decompose this organic matter, robbing the water of oxygen. Once the oxygen is depleted, anaerobic bacteria will take over, producing methane and foul-smelling hydrogen sulfide. To prevent this, storage time should be short before the tank is emptied into the subsurface irrigation system.