Gray Water Use


Gray water is household or commercial wastewater excluding toilet, urinal, bidet, and kitchen waste. In most cases, gray water comes primarily from the clothes washing machines, showers, baths, lavatories, and other non-sanitary wastewaters. In order to be able to use gray water for irrigation, the household drains must be reconfigured such that black water (from toilets and the kitchen) go directly to the sewer and gray water sources are collected separately and brought to a central location for temporary storage and use in the field. The organic matter content in gray water is actually beneficial for soil structure and most of the microorganisms (viruses, bacteria, protozoans) in gray water are gradually overcome by the vibrant soil microorganism population, although eggs of helminthes (such as tapeworms) tend to survive in soil for a long time. Therefore, it is advisable to minimize direct human exposure to gray water. This can be accomplished with subsurface irrigation, drip irrigation under mulch, or irrigation in areas fenced off from access, especially by children.

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Water Savings

In many households, gray water can be as high as 80 percent of the total wastewater generated. If all of that water can be beneficially used for irrigation, and if the irrigated area does not need supplemental potable water, then significant water savings is accomplished. Thus, it is a combination of the two factors (percent of gray water captured and percentage of irrigated area served) that determines actual water savings at any given location where a gray water system is installed. One extreme is a multiplex, where the gray water generated by a given household is far more than what that household’s landscape (or vegetable garden) can possibly use. Another extreme is an estate mansion with a huge garden that needs far more irrigation water than the few residents of the mansion can possibly generate. In between these extremes, there are many situations where the supply of gray water is close to the demand for irrigation water, and that is where irrigation water savings can approach 100 percent. Used in conjunction with rainwater harvesting techniques, the savings can be taken even further.

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Gray water applicability, especially for agricultural irrigation, is limited. A typical farm house’s gray water supply can only irrigate a small fraction of the farm’s overall area. Small-scale farms, urban farms and gardens, and nurseries may meet with some significant success. California regulations for household use of gray water are in the process of revision and they are expected to become much more permissive and user-friendly, especially for washing machine and single-source residential gray water systems, which will not even require a permit for installation.

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Additional Benefits

In addition to water savings, gray water systems can reduce the load on septic systems and on the community’s sewerage systems. Gray water can contain small concentrations of organic matter and some nutrients, beneficial for soils and crops grown with it. The soil’s upper (aerobic) layers can decompose and deactivate most of the microbes (and any pathogens) that might be in gray water. Also, the embedded energy in gray water is far less than that in potable water, making its use highly environmentally friendly and sustainable.

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Roofwater Harvesting in California

The WATER Institute released a roofwater report in June 2011 that supports statewide adoption of this valuable water conservation strategy to increase community water security and stream flows for salmonid recovery. This report was a result of a meeting in 2010 and offers a broad overview of the status of roofwater harvesting in California, investigates obstacles, proposes solutions, offers recommendations for next steps and provides resources for further research.

State of Arizona

The State of Arizona provides an excellent model for encouraging safe use of gray water. It appears likely that California regulators will also be moving in that general direction over the coming year. The Water Conservation Alliance of Southern Arizona is a great resource for reference.

Low-Cost Agricultural Roofwater System

This is the latest do-it-yourself booklet from the Occidental Arts and Ecology Center (OAEC) WATER Institute. This booklet visually and verbally debuts the “Wonder Gutter” design which provides plenty of clean drinking water to OAEC hens without the use of any electricity via gravity. The booklet can be accessed here.

Art Ludwig’s Oasis Design

This website offers a wealth of information, opinion, and guidance regarding gray water and its benefits. It also provides links to many other gray water resources and case studies.

Food from Dryland Gardens

Food From Dryland Gardens encourages garden that serve local needs, that are based on both local knowledge and scientific knowledge, and that conserve natural resources and the biological diversity of traditional crops. It was written for field workers, extension agents, students, project workers, and program planners. Both a beginner’s guide and a reference for those with more experience, this book or CD helps the reader observe and with local people to ask appropriate questions about the community, the environment, and the potential for gardens to improve nutritional, economic, and social well-being.

NRCS Technical Guide

NRCS provides a set of key technical resources to guide on-farm water (and other resource) management practices. These include information and recommendations about specific practices related to gray water use as they pertain to local areas. Visit the onlineField Office Technical Guide (eFOTG) and click through to the map to your county for details. Once there, you can search through practices listed in Section IV of the pull-down menu in the left-hand column of the page. Here, you may also find information about financial support that may be available for implementing these practices. In addition to practice-specific assistance, the eFOTG provides key data to help growers in resource management decision-making, including natural resource information (Section II in the pull-down menu) about local soil (e.g. web soil survey), water, air, plant and animal resources; planning tools for developing resource management systems (Section III); and other useful tools and information.

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Case Studies

City of Los Angeles, California

The City of Los Angeles Office of Water Reclamation conducted a one-year detailed study of gray water systems set up at eight residences. Soil and water samples from areas irrigated with gray water and areas irrigated with potable water were collected monthly from each of these eight sites. The samples were analyzed at a certified laboratory for indicator microorganisms, pathogenic organisms, and a number of chemicals of interest to agronomists. Based on analysis of laboratory data, including baseline data, it is clear that backyard soils were contaminated, whether they were in the control areas or in the gray-water-irrigated areas. If these findings can be generalized, the implication is that gray water irrigation—below the surface of the soil—does not by itself elevate the health risks from handling the garden soil.

Indicator bacteria (total coliform) in the soil seem to increase with gray water application. However, the soil is already so heavily contaminated with animal fecal matter that the additional contribution of gray water may be irrelevant. Disease organisms, normally capable of surviving in the soil for a few days, were not present in gray-water-irrigated areas. Nor were these organisms detected in gray water in storage. This may indicate either an entirely healthy test population (highly unlikely), or a mechanism for deactivation of pathogens. Either way, the results indicate that there may be minimal additional risk of exposure from use of gray water for irrigation of residential landscaping.

Planting Justice

This case study explores various water-efficient practices associated with urban agriculture, and highlights Planting Justice, an organization that incorporates principles of small-scale, sustainable food production with food justice and economic justice efforts.

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Page Credit

Content for this page was originally developed by Dr. Bahman Sheikh, a Water Recycling Consultant. Various others have since contributed content.