Agua Latinoamerica

On Tap: Graywater: Reuse and Risks Revisited

By Kelly A. Reynolds, MSPH, PhD

It’s that time of year again, when we pledge to be better people, better friends and better environmental stewards. Continuing to gain momentum is the movement toward residential graywater reuse. With prevailing drought conditions in the US Southwest and charges to reduce potable water consumption, evaluation of graywater pros, cons and additional needs endures.

A variety of systems are available for localized collection and treatment of graywater ranging in cost from a few hundred to thousands of dollars and more. Simple systems may be just a collection barrel with a hose bib for funneling graywater to outside plants. More expensive systems involve plumbed and differential sources throughout the home, including washing machines, showers, bathtubs and bathroom sinks and pumped or manufactured systems with on-site sand filtration treatment. Water from toilets and kitchen sinks—or from processes where harmful chemicals or increased levels of microbes (i.e., washing diapers)—is considered black water and not suitable for household reuse.

Expanded graywater use is one way residents are responding to re- quests to reduce potable water consumption by 20 percent in light of persistent drought conditions in the Southwest(1). From April 2014 through June 2015, California’s Santa Clara Valley Water District is offering a $200 (USD) Graywater Laundry-to-Landscape rebate per residential site to incentivize proper graywater system installation and reuse.2 With an average water use of 80-100 gallons (302-378 liters) per person per day in the US, differentiating our needs for potable water versus graywater is essential for sustainable supplies.3 An estimated 70 percent of water produced in the home is actually graywater. Outdoor irrigation consumes large amounts of household water supplies and is the most popular application for graywater reuse (see Table 1).

Graywater policies are set at the state and local government levels. Driven by the obvious need to conserve water, the state of Arizona has some of the most widely referenced and imitated guidelines on residential water reuse in the nation. Beginning in 2008, the arid southwest city of Tucson has required all newly built residences to incorporate graywater systems for outdoor irrigation. Up to a $1,000 rebate is currently offered to encourage homeowners to install permanent systems.

California led the nation in developing the first graywater regulations, circa 1989. In the beginning, the Golden State’s compliance and permitting process was reportedly too costly and restrictive, resulting in an estimated thousands of residents skirt- ing the process by installing unpermitted systems. Addressing the issue, California modified the graywater code in 2009 to allow the installation of low-cost, ‘laundry-to-landscape’ household reuse systems that did not even require a permit, provided (among other criteria) the source water was from washing machines only.4 More complex systems may require laboratory tests for soil profiling or on-site drainage tests.

Critic’s concerns

Not everyone is shouting the praises of graywater reuse. Some experts warn that graywater is really just dilute sewage, potentially containing all of the contaminants as blackwater but at lower levels.5 Contaminants in laundry water may include nitrates, grease, arsenic and fecal pathogens. The primary concern, therefore, is the potential impact of these con- taminants on drinking water quality, given the risk that non-potable gray- water could seep into groundwater sources as well as plumbing cross- connections. Further, unpredictable and changing environmental condi- tions, such as rainfall, can increase the fate and transport of graywater contaminants. Regrowth of microbes, particularly during periods of warm weather, can increase odors and pos- sible exposure risks.

A review of the scientific litera- ture presents little evidence of micro- bial pathogens present in graywater. Not surprisingly, studies conducted in Arizona have shown high concentrations of fecal and skin bacteria, both of which can be pathogenic but are common environmental contaminants. If an individual in the home is ill, however, the likelihood of a disease-causing organism being present is almost certain as individuals readily shed pathogens during showering and hand-washing and from soiled laundry. One study looked at the needs for graywater treatment to achieve an acceptable risk limit if the fecal contaminant was a frank mi- crobial pathogen (i.e., Salmonella, Giardia, rotavirus) and if direct contact occurred. Conditions of regrowth for certain bacteria and the ability to survive for days despite typical household graywater treatments could lead to unacceptably high risks.6 More research is needed to determine risk management decisions directly related to graywater contact.

Protecting human health

Arizona continues to lead the way in researching and implementing gray- water reuse guidelines. The Arizona Department of Environmental Quality has published a brochure listing 13 best management practices to minimize risks to human health and drinking water qual- ity (see Table 2).7

While there is a plethora of informa- tion on how to install, maintain and use graywater in residential settings, compli- ance may be another issue. Changing conditions of temperature, rainfall and the health status of residents may dramati- cally affect graywater quality. POU treat- ment (i.e., filters, UV light, disinfectants, etc.) are available to treat graywater at the point of discharge. In the absence of direct contact or use on food crops, however, exposure risks are greatest via contami- nated drinking water sources. Therefore, an additional best practice would be the application of a broad spectrum POU drinking water treatment system. Such practice would avoid some of the criti- cism and uncertainty about the safety of increased graywater reuse.



1. Peterson, M. “Here & Now,” Southern Califor- nia Public Radio, 14 April 2014. Online. [Avail- able: recycling-gray-water. Accessed 16 December 2014].
2. Santa Clara Valley Water District, Graywater Laundry to Landscape Rebate Program, Santa Clara Valley Water District, 2014. Online. [Available: Accessed 16 December 2014].
3. United States Geological Survey, “Water Questions and Answers: How much water does the average person use at home per day?” USGS, 23 October 2014. Online. [Available: html. Accessed 16 December 2014].
4. California Department of Housing and Community Development, California Residen- tial Graywater Code: California Plumbing Code,California Code of California Department of Housing and Community Development, 10 February 2010. Online. [Available: Accessed 16 December 2014].
5. Christova-Boal, D.; Eden R.E. and McFarlane, S. “An investigation into greywater reuse for urban residential properties,” Desalination, vol. 106, no. 1-3, pp. 391-397, 1996.
6. Ottoson, J. and Stenstrom, T.A. “Fecal contamination of greywater and associated microbial risks,” Water Research, vol. 37, pp. 645-655, 2003.
7. Arizona Department of Environmental Quality, Using Gray Water at Home, February 2011. Online. [Available: Accessed 16 December 2014].

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