August 29, 2009

1 - Consider safety when installing greywater systems.

Yard and Garden August 29, 2009


For several years I have thought about having a greywater system installed. I worry about safety and effectiveness, but think it is the right thing to do. Do you have any information about safety or advice about greywater for landscapes?

- Alice M.


Water reuse is an important concept to consider in New Mexico's landscapes, but the health issues must be considered. Also, consider which detergents and other chemicals in your greywater stream may affect your plants' growth.

Craig Runyan, NMSU Water Quality Specialist, has written the following brief article about greywater that addresses some of your concerns. Waste management and water reuse are not contemporary concepts. A writ from the King to the Mayor and Bailiffs of Dublin, Ireland posted in 1489 stated "The fear of pestilence prevents the coming thither of bards, ecclesiastics and lawyers..." and "the King commands the removal of all swine, and to have the streets and lanes freed from ordure, so as to prevent the loss of life from pestilential exaltations". While the possibility of keeping lawyers from "coming thither" may be inviting to some, the practice of using grey water for reuse in lawn and garden applications is not without other risks.

The New Mexico Environment Department has been careful to define greywater as "untreated household wastewater that has not come in contact with toilet waste and includes wastewater from bathtubs, showers, washbasins, clothes washing machines and laundry tubs, but does not include wastewater from kitchen sinks or dishwashers or laundry water from the washing of material soiled with human excreta, such as diapers." The obvious concerns with using greywater for lawn and garden irrigation involve human health issues. Bacterial and viral pathogens populate waste water of even the cleanest of homes. In addition, non-pathogens such as heavy metals, pharmaceuticals, and hormone active substances are common constituents of household waste water. Regardless of the point from which the waste water is drained from the house, extra care should be taken to protect human health when irrigating with greywater.

There are several factors to consider when designing and managing a greywater system for irrigation. First, and perhaps foremost, any hoses, pipes or other plumbing which carry greywater should be clearly marked as non-potable. Industry 'purple pipe' practices are strictly followed to designate waste water plumbing. For do-it-yourself designs, steps should be taken to insure that any plumbing conveying wastewater is not inadvertently used for primary (drinking) or even secondary (contact) uses.

When designing the layout for the system always observe setback distances from wells and water table. The adjacent area should be protected by zero runoff and no ponding should be observed. Also in accordance with environmental regulations, cross contamination with potable or black (toilet) water should be avoided in any event. Greywater irrigation should be discharged directly to the soil. Sprinklers or bubblers are not appropriate for greywater application. Ideally, the discharge will occur below surface where contact with children, pets or other critters will not occur. This principal also applies to the vegetation which is being irrigated. Fruits or vegetables which can or may be eaten raw should not be considered for greywater irrigation.

Household and family situations can also dictate when greywater irrigation is inappropriate. Wastewater from households with illnesses, including resident incontinence should be diverted to septic or sewer drains. It is also wise to divert wastewater to treatment systems during times of high visitor traffic to a home. From the discussion above, it should be clear that greywater reuse has risks and requires active management. Homeowners should weigh the benefits and risks before investing the time and money required for a good greywater irrigation system. It is not simple, or cheap and it could lead to unwanted consequences.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email:, office: 505-865-7340, ext. 113.


For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms.

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