Graywater systems range from simple off-the-shelf products for the home, which recycle sink wastewater for use in toilet flushing, to complex, custom-engineered systems for large commercial projects. All rely on some method of filtration or treatment. Below, you will find a summary of the pros and cons of several common methods, as described by the Pacific Institute, a nonprofit environmental research group in Oakland, Calif.:
• Activated carbon filters installed in plumbing outlets can simply and effectively remove many organic chemicals and inorganic compounds, such as chlorine, from wastewater. But these filters are costly and don’t remove all of the impurities. These shortcomings also apply to sand filters and bark and mulch beds, all of which are also susceptible to clogging and flooding.
• Aerobic biological treatment involves pumping oxygen into wastewater while it is in storage tanks in order to feed beneficial bacteria, which in turn digest the organic contaminants. Although its design is more complicated, this system allows on-site storage of treated graywater.
• Membrane bioreactors combine biological treatment—using contaminant-consuming microorganisms—with filtration to remove contaminants and pathogens. In general, these are highly effective.
• Chemicals such as ozone and chlorine also can be used to disinfect bacteria, organic matter, detergent residue, and other contaminants for long-term graywater storage. Because of the toxic byproducts of these chemicals—albeit minimal in quantity—this option is less environmentally friendly, so ultraviolet (UV) light has gained popularity for use as a disinfectant. UV light works by disrupting the pathogen’s DNA, which renders it harmless.
Most graywater treatment systems that are implemented in a building design use a combination of two or more of the methods mentioned above.
Transporting graywater to its ultimate destination for reuse—such as a landscape or toilet tank—as quickly as possible is imperative, says Pete Muñoz, a senior engineer in the Santa Fe, N.M., office of Biohabitats, a conservation and ecological restoration design/build firm. “The trick with graywater is to get it out into the environment as fast as you can—within 12 to 24 hours—and not let it sit and break down” inside the storage tank or plumbing system. The problem, he says, is that many state plumbing codes specify that septic tanks—which may be required for on-site graywater treatment systems—must be sized for two-day retention. If graywater is stored in such a similarly sized vessel for such a prolonged time period, it will clog the drain field when it is released.
For the laundry-to-landscape treatment system installed in the mixed-use development at Barrett, the Honors College at Arizona State University, Muñoz and Biohabitats’ subsidiary Natural Systems International designed a reclamation system with a capacity of 10,000 gallons per day, which processes water from the showers and sinks in the 1,700-bed residence and channels it to the surrounding landscape. The water is first screened to remove hair and debris particles, and then continually recirculated through a simple sand filtration system for use outdoors as subsurface irrigation. Because the water is constantly recirculating—at a rate of about three to four times per day—it doesn’t develop filamentous bacteria; clean water is always available for immediate use.