Graywater Introduction

Graywater, (sometimes spelled gray water, grey water or greywater) is untreated wastewater resulting from lavatory wash basins, laundry and bathing.  It never contains wastewater from toilets, urinals or any industrial process.  Wastewater from kitchen sinks is also often excluded because of the high food and grease content.   In most cases, graywater is recovered and used at single-family homes; although green building industry is promoting graywater in multi-family, commercial and institutional settings as well.   In its most common usage, graywater is often coarsely filtered to remove suspended solids, but not treated with sanitizing chemicals and biocides such as chlorine.  The lack of biocides dictates the water must be used immediately, and subsurface irrigation is the predominate use for this water.

Health Safety

Food particles and other contaminates in graywater present an attractive environment for bacteria and pathogens to thrive.  The graywater becomes septic within days if not utilized.  Graywater should always be used within 24 hours of collection, or dumped into the municipal sewer system.   The pathogens are quickly neutralized when applied to soil; thus, there is little danger when used in subsurface irrigation systems.  The graywater should never be sprayed over the top of vegetation using conventional irrigation spray heads for two reasons: a) the pathogens become airborne and could be inhaled by someone nearby; and, b) the suspended solids in the graywater will quickly clog the irrigation pipes and sprayheads.  Subsurface irrigation is suitable for providing water to trees, shrubs and flower gardens; and is generally considered unsuitable to irrigate turf grasses or plants for human consumption (fruits and vegetables).  It is important to note that, despite our cautions of health risks of improper handling of graywater, there is no known case of any person becoming ill due to exposure to graywater irrigation.

Graywater Capture and Use

More than half of the water used inside the home is potential graywater as it drains into the wastewater system.   A typical home with older fixtures could generate 35,000 gallons (132.5 m3) of graywater per year while a newer more efficient home could generate 25,000 gallons (94.6 m3) of graywater per year (Aquacraft, Inc. 1999, 2004, 2008).  The key to using this resource is separating graywater from the blackwater (water flushed down toilets and urinal) and using the water in a safe and productive manner.

Capturing the water requires separate drainlines for the fixtures and appliances generating the graywater.  A system to temporarily store the graywater is necessary, along with a pump to convey the graywater to the desired location.  Separating the drainlines is relatively easy during new construction, while retrofitting an existing home can be very expensive.

 In many retrofit projects, homeowners have chosen to only capture the wastewater from the clothes washer, because the clothes washer drain line is not permanently connected to the home sewer system, and the washer already has an intergrated pump to convey the graywater to an alternate storage vessel.   An older top loading clothes washer alone can generate more than 14,000 gallons (53 m3) of effluent (graywater) per year; adequate for most residential subsurface irrigation needs.    This is enough water to irrigate more than 800 square feet (74.3 square meters) of non-turf plants (trees, shrubs, flower gardens, etc) in most climates. 

Some local ordinances and statutes prohibit the use of graywater, though many jurisdictions are relaxing restrictions in response to a better understanding of the very limited health risks when graywater is properly handled.  Even where graywater use is allowed, local plumbing codes often dictate the design, hardware and installation of the: collection process, storage system and irrigation system.   In many cases, a plumbing permit is required by the local building code enforcement agency before the system is constructed and operated.  

Other Source of Information

Sheikh, Bauman (2010) White Paper on Graywater 

Package Graywater Recovery and Treatment Systems

Alternative Water Sources Introduction 

RO Discharge Water Introduction 

ACT (2007) Greywater Guidelines Residential Canberra Australia 

New Mexico State University

Oasis Design Group