Tapping into the gray water well

Gray water system codes are still a gray area. Here are some tips for engineering a nonpotable water system.


According to the U.S. Geological Survey, less than 1% of the Earth’s water is ever suitable for direct human consumption. Like most of the Earth’s natural resources, the current water supply is not increasing at the same rate as the world’s population. So, consumers are turning to alternative solutions like the use of gray or nonpotable water to maximize the existing freshwater supply.

By definition, gray water is unfiltered wastewater generated from the use of bathroom wash basins, bathtubs, showers, or washing machines, as opposed to recycled or black water, which is wastewater generated from the use of urinals, toilets, or kitchen sinks, treated to remove solid human waste and chemicals before reuse.

Of utmost concern for any engineer designing a gray water system is to protect the health of the building’s inhabitants. Because gray water is not necessarily filtered before it is reused, its systems must be designed so that the gray water has no contact with humans or animals and therefore is principally used in subsurface landscape irrigation. (Note: Treated gray water with the approval of the authority having jurisdiction (AHJ) can be used for flushing toilets and urinals as long as regular testing and inspections are performed.)

Before designing a gray water system, the design engineer must know all local codes, standards and variances required. He or she must also understand some of the challenges of gray water systems, both in general and in each local municipality, in order to design a system that will meet the goals of the building owner, facility operator, and local health officials.

System setup

Because there are a limited number of commercially available gray water systems that can be bought off-the-shelf, system design largely rests on the project engineer. Required elements include: a dual waste piping system, storage tank, electrical pump to remove gray water from the system as needed, and the required maintenance, which includes weekly testing and accurate record keeping.

Showers are often a significant source for collecting gray water, so gymnasiums, hotels, and apartment complexes where hundreds of gallons of water are generated per hour are ideal for gray water system installation, more so than commercial office buildings where sink hand washing for 5 to 10 sec may produce only the equivalent of a cup and a half of water per wash.

Know the codes and standards

While Europe and Australia have been repurposing water for centuries, gray water reuse is fairly new to the United States where anyone delivering water publically, whether to a commercial building or residential home, must follow strict codes and standards that often go beyond those required overseas.

The National Sanitation Foundation (NSF) sets standards for water delivery in the U.S. and NSF/ANSI Standard 350: Onsite Residential and Commercial Water Reuse Treatment Systems sets clear guidelines for domestic water reuse treatment systems. Focusing on public health and appropriate water quality criteria for reuse applications, NSF/ANSI 350 specifies that gray water is not allowed to run off the site or property from which it is generated and cannot be exported to another property, must be prohibited from potable water contact sources, including lakes and streams or well water, and can only be distributed by subsurface irrigation or drip tubes below the soil.

While codes and standards for gray water systems are still emerging, most of the existing laws that govern its use are established at the state, county, and municipal level. As many as 30 to 40 states have adopted the Uniform Plumbing Code (UPC), published by the International Association of Plumbing and Mechanical Officials (IAPMO). Chapter 16 of the UPC provides gray water recycling guidelines for estimating gray water discharge, producing drawings and specifications, and obtaining system permits, while it also covers gray water system inspection and testing requirements.

For example, the UPC requires that the pipe distribution system for all gray water be labeled “CAUTION: NONPOTABLE WATER, DO NOT DRINK.” In this way, cross-connection control is strictly enforced. In one UPC illustration (Table 16-1), the minimum horizontal distance and clearance requirements for gray water from buildings, streams, and lakes and water supply wells are illustrated. Even the type of soil slated for gray water discharge has composition and design criteria. The UPC includes several piping diagrams and illustrations for the use and location of a gray water system.

A number of states, like New York, have their own codes, while other groups of states have adopted additional standards including the International Plumbing Code, whose Appendix C also provides procedures for estimating gray water discharge and how to discharge the gray water for use.

Because gray water codes and standards aren’t one-size-fits-all, it is crucial to research and consult with all state, county, and city AHJ before design begins to determine which codes apply to a potential project. For example, the California Plumbing Code, which includes modifications to the UPC, currently says that gray water systems are applicable only to residential homes where the gray water generators are also its end users, while the Los Angeles city code says that as the size of the home increases, so do its gray water regulations, and that in order to specify a gray water system for a commercial office building, a code variance is necessary.

Beyond required codes and standards, gray water systems are often considered and employed in buildings seeking sustainable certification, both voluntarily for private sector buildings and per city code for new government buildings. The City of Los Angeles, for example, now requires any building more than 50,000 sq ft to be a minimum of LEED Silver certified. In this way, developers are required to conserve materials and are looking beyond just high-efficiency, low-flow bathroom products. They also are encouraged to incorporate gray water systems, rainwater harvesting, and reuse systems, and even black water treatment and reuse systems in their projects to further improve water efficiency and reuse.

Beyond design

Beyond the design of gray water systems, their implementation and maintenance needs are important to consider up front. The specifying engineer must work together with the project’s electrical engineer to assure that the system, including alarms and system monitors, is installed to perform as designed in order to assure the building owner that it has a safe system.

Securing a long-term maintenance agreement for the upkeep of this unique system is also crucial to the gray water systems’ success. For example, keeping weekly records of collection quantities, taking samples for testing quality and odor, and regular evaluation of the system’s effective use of gray water to validate the engineer’s assumptions of the end users’ typical fixture usages of water quantities are also important in maintaining a healthy gray water systems.

Finally, while environmentally responsible, gray water systems aren’t fiscally appropriate for every application. The amount of water that will be garnished from the gray water system must be weighed against the cost of a second internal plumbing infrastructure to determine if the additional first costs, environmental stewardship, and anticipated ROI will come together to meet the project’s goals.

However, when designed with local codes and standards and project goals in mind, a gray water system can provide a number of benefits to a building owner and operator as well as the local municipality. They include, but are not limited to, reduced water usage by reusing existing water for landscape irrigation, environmental stewardship earned by actively engaging in preserving a crucial world resource, and minimizing the burden of local sewage treatment facilities by decreasing the amount of wastewater municipalities must treat.

Nishida is an associate partner at Syska Hennessy Group and is the firm’s lead plumbing and fire protection engineer. With more than 40 years of experience, he has planned and designed high-profile projects in multiple markets including aquariums, healthcare, hospitality, aviation, and public assembly.




The University of California Los Angeles Institute of the Environment published an article in the Fall of 2009 on “Graywater – A potential source of water.”