What Is Greywater?

Greywater is the wastewater generated by showers, bathtubs, bathroom sinks, and clothes washing machines, that is, all household or building wastewater except the toilet and kitchen streams that make up blackwater. Within wastewater treatment it is treated as a distinct category because its contaminant profile is different: it carries soaps, surfactants, skin cells, hair, lint, and low concentrations of fecal indicator organisms, but it lacks the high nitrogen, suspended solids, and pathogen loading of toilet waste. That difference makes it the most readily reusable stream in a building, since a lower treatment burden separates it from potable-quality discharge. Greywater typically accounts for between 50 and 80 percent of the wastewater a residence produces.

Interest in the stream comes from water scarcity rather than water quality alone. Diverting it for irrigation or toilet flushing reduces both potable demand and the hydraulic load on municipal sewers and treatment plants. The principal constraints are the cost of treatment and dual plumbing relative to the price of potable water, the need for ongoing maintenance of filters and disinfection equipment, and the risk of soil sodium accumulation where detergent selection is not managed. Regulatory treatment varies by jurisdiction, and the US Environmental Protection Agency maintains a compilation of state rules through its water reuse program, which organizes guidance by source water and end use.

Composition and Treatment

Untreated greywater has a chemical oxygen demand typically in the range of 100 to 700 milligrams per liter, a pH raised by detergents, and elevated sodium, boron, and phosphorus from cleaning products, all of which matter when the effluent is applied to soil. Because it is warm and nutrient-bearing, it degrades quickly, so storage beyond about 24 hours without treatment produces odor and bacterial regrowth. Treatment trains scale with the intended use. Simple laundry-to-landscape systems use a diverter valve, a lint filter, and subsurface drip or mulch basins with no storage. Intermediate systems add sedimentation, coarse filtration, and disinfection. Higher-grade reuse for toilet flushing or surface irrigation generally uses biological treatment, most often a membrane bioreactor, a rotating biological contactor, or a constructed wetland, followed by filtration and disinfection by chlorine or ultraviolet light. Design attention centers on hydraulic surge from a washing machine discharge, on biofilm control in the distribution piping, and on avoiding cross-connection with potable supply.

Standards and Regulation

Reuse systems are governed by plumbing codes and public health rules rather than by discharge permits. The Uniform Plumbing Code and the International Plumbing Code both contain greywater provisions covering pipe marking, backflow prevention, minimum burial depth for irrigation lines, and setbacks from wells and property lines. Performance of packaged treatment units is judged against NSF/ANSI 350, the standard for onsite residential and commercial water reuse treatment systems, which sets effluent criteria for biochemical oxygen demand, turbidity, and E. coli in two classes, one for single-family residential use and one for multi-family and commercial installations. State practice differs widely on what is permitted without a permit; EPA's summary of California's onsite non-potable reuse rules illustrates a common structure, in which small subsurface irrigation systems are exempt from permitting while treated reuse for toilet flushing requires a permitted system and monitoring.

Applications

Greywater collection and reuse have applications in a range of settings, including:

• Residential landscape irrigation through subsurface drip and mulch basins
• Toilet and urinal flushing in offices, hotels, and multi-family buildings
• Cooling tower makeup water in commercial and institutional facilities
• Water-scarce and drought-prone regions seeking to defer supply expansion
• Green building certification, where reuse contributes to water efficiency credits
• Off-grid dwellings, remote camps, and disaster relief installations
• Reducing inflow to overloaded sewers and onsite septic drain fields