Overview | Water Savings | Applications | Additional Benefits | Resources | Case Studies
Overview
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.
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.
Applications
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.
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.
Resources
The California Department of Housing and Community Development Division of Codes & Standards
The Department is holding public consultation and has already made serious draft revisions to existing standards for use of gray water in the state. To keep current with a lively discussion among gray water aficionados, gray water gorillas, and newly minted gray water officials in the California state government, join the discussion group through this website.
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.
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.
American Water Works Association Gray Water Study
Several years ago, AWWA commissioned a long-term version of the aforementioned City of Los Angeles gray water study, the results of which are expected to be published in a few years. This site will be one of the first to announce the availability of the study report and its findings.
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 and the new Arizona Rules for using graywater safely are located here.
