How Greywater Systems Work
Wherever greywater is used to irrigate the grounds of homes or commercial buildings, it is almost invariably because of the demands already imposed on fresh water by a growing society. This is mainly a western state phenomenon, and the majority of those states adopt the UPC as their model code. Therefore, most of this article will refer to the UPC's greywater code, titled Appendix G, and a subsequent improvement, the California Plumbing Code's (CPC) greywater amendment, also titled Appendix G. The IPC and most state codes include provisions for special systems, so if you are in one of those jurisdictions, please keep that in mind as you read.
According to numerous studies, including most recently a study by the city of Los Angeles in 1991-1992, one from Brisbane in 1993, and a 1996 study out of the University of California-Berkeley Environmental Engineering Department, a greywater irrigation system reuses about 50 percent of waste water from a residence. The California Department of Water Resources documented that the average amount of greywater produced is 40 gallons per day per person, IAPMO concurs, and the U.S. Bureau of Reclamation uses 40 gpdpp for municipal water conservation funding purposes.
When we consider the number of houses and other residences being built each year in the West, (about 500,000 projected in 1998), the potential for greywater irrigation in the West is equal to a medium-sized dam costing tens of millions of dollars each year. The obvious advantage to a greywater irrigation system is that the water is already on-site and paid for. The less obvious but equally as important advantage is that such systems are more efficient than traditional irrigation systems, for a number of reasons. Greywater irrigation is here to stay.
What is greywater?In UCP terms, greywater is "untreated household waste water which has not come into contact with toilet waste. Greywater includes used water from bathtubs, showers, bathroom wash basins, and water from clothes washing machines and laundry tubs. It does not include waste water from kitchen sinks or dishwashers."
In CPC terms, greywater is "untreated waste water that has not come into contact with toilet waste. Greywater includes waste water from bathtubs, showers, bathroom wash basins, clothes washing machines, and laundry tubs, or an equivalent discharge as approved by the Administrative Authority. It does not include waste water from kitchen sinks, photo lab sinks, dishwashers or laundry from soiled diapers."
By not restricting greywater use to "household" sources as found in the UPC, the CPC allows a wider range of site possibilities. These are usually multifamily residential units as well as hotels, gymnasiums, etc. This definition also opens up the possibility of reusing "equivalent discharges," perhaps silicon chip rinse water and other relatively clean waters.
In IPC terms, greywater is the "discharge from bathtubs, showers, lavatories, clothes washers and laundry sinks." Toilet and urinal flushing is routinely allowed if the greywater has been treated with chlorine, ozone or something similar, and it can be used in irrigation with permission. Both the UPC and CPC exclude indoor use of greywater because no matter what treatment method is used, there is no guarantee it is safe for children to play in. (It happens.) And on the practical side, even after treatment, it will probably stink.
Old Appendix G and new Appendix GTwo appendices with the same name can be confusing, but it is easy to remember the difference. The UPC's Appendix G, found in 1994 and later editions, only allows greywater use in "mini-leach fields" similar to but smaller than private sewage disposal leach fields. Leach fields are a good method of greywater disposal, but they are not a good method of irrigation. You'd be hard pressed to find an inspector who has actually had a request for a greywater disposal field.
Because California went through a six-year drought that destroyed everybody's previous beliefs about the adequacy of the state's water supply, it wanted a greywater code that would result in the actual reuse of greywater. Using the UPC's Appendix G as a starting point, the state embarked on a five-year greywater code creation process. Participating in that process were not only IAPMO representatives, but environmental health, building and safety, water supply, urban management, agriculture and other officials concerned about whether the final code would actually result in the "maximum safe use of greywater" as directed by the legislature.
Out of that process came the California Plumbing Code's (CPC) Appendix G, which allows greywater use in mini-leach fields and in subsurface drip irrigation systems. With either type of distribution method, collection methods are the same as those in the UPC, meaning, fall, venting, type of pipe, etc., remain the same. (This raises the specter of requiring cast iron plumbing for a greywater system if it is collecting water from a three-story building, but that is an unnecessary cost requirement that IAPMO will supposedly remedy someday, right?)
Prior to the diagram, greywater would be collected from the legal fixtures and tied into a drainage line separate from the building's main waste water line. Obviously, this separation makes some retrofits impossible, some highly impractical and some possible. In new construction, this process does not add significant cost to the plumbing bid. A good cost-effectiveness rule for a residential system is to separate only the regularly used showers and clothes washer water, as that will result in capturing 95 percent to 97 percent of all greywater anyway.
For Commercial-Industrial-Institutional systems, capture only enough greywater to meet the peak irrigation demand in the hottest summer month. The separated greywater then flows to a diverter valve, which is used to bypass the surge tank and send the greywater directly to the sewer if needed. This might be if repairs are made at some future date. In a normal mode, greywater flows into the tank, which is vented, and which has an overflow line to the sewer, with a backflow prevention valve on that line. If the distribution portion of the system is going to be pressurized, as allowed in both the UPC and CPC, the pump could be placed inside, or somewhere outside and connected through a bulkhead adapter, with a float switch or other water level switches to control its start and stop.
If the system uses the drip irrigation method of dispersal, as allowed by the CPC, an extensive filtration system and special drip emitters are required. Such emitters are designed to work underground with greywater, and they must be proven to keep roots from intruding. The world-renowned Center for Irrigation Technology (CIT) at California State University, Fresno, is the approved testing facility for greywater emitters, and every other type of irrigation device for that matter. It is an IAPMO-approved facility. CIT approval depends on their theoretical understanding and their excavation and analysis of emitters that have been in use for years.
Hand-operated valves in the landscape are allowed, but most of the time an electronic controller will be used on a drip irrigation system. This is usually provided by a manufacturer, and they open and close the required 24VAC anti-siphon irrigation valves. Controller-operated systems can also automatically backwash their own filter and automatically supplement the irrigation program with fresh water if there isn't enough greywater. People buy such systems as their primary irrigation systems.
What are the safeguards?Both the UPC and the CPC require virtually the same safeguards. First, they require a permit. While this seems hardly worthy of mention, consider that before 1994 and legalization, there were thousands of greywater systems in place and practically none of them had been permitted. Who knows what kind of nuisance they are creating? Permitting is a huge start toward eliminating dysfunctional systems.
During the permit process, a plot plan is required. This can consist of a scale drawing showing lot lines and structures, direction and approximate slope, location of retaining walls, drainage channels, water supply lines, wells, paved areas, number of bedrooms and number of plumbing fixtures, location of any private sewage disposal system and expansion area, and the location of the greywater system and its proposed irrigation area.
The permitting authority may also require details of the greywater system's construction to ensure compliance with the code. This is typically found in the manufacturer-supplied owner's manual, but in one-up systems, it may have to be supplied by the designer. All of these components come from existing industries-water filtration, waste water, pool, irrigation-and they all come with their independent lab product certifications. Be sure the components you choose have the appropriate certifications for whatever it is you are planning to do with them or else the permit process could be a nightmare.
The authority may also request a log of site soil formations and water tables. A list of acceptable soil types is provided by the code. If the soil type at the site is unknown, a percolation report may be requested. If the site already has a percolation report, as would be the case if the building was on a private sewage disposal system, it may be used if the area to be greywater-irrigated is basically the same as that described in the report. What the authority is supposedly trying to do with all this information is ascertain if the site has enough of the right type of soil to accept all the water you intend to put on it.
The code includes a table of required setbacks to keep the collection/filtration system and the irrigation lines appropriate distances from property lines, structures, water sources and other concerns. Read the footnotes carefully, as they can make a significant difference to a setback.
Out in the landscape, under the UPC, the mini-leach field method of distribution is required to have triple a certain amount of the classical square trench area to insure water absorption, along with the classical drain rock/perforated pipe/drain rock/filter material/soil design requirement. The CPC only requires single absorption because a system isn't a mandated disposal method.
A drip irrigation system is required to have a certain number of emitters for the anticipated amount of water coming from the building.
These emitters are to be buried 9 inches deep, spaced no less than 14 inches apart. Because this is the depth where most plant roots acquire water, and there is no water lost to evaporation, overspray or run-off, this happens to be a highly effective method of irrigation.
A Reverse Pressure Principle Device on the greywater system's water supply pipe is what most jurisdictions require to protect the municipal water source. Fresh water is required for fully automatic systems to clean themselves and to keep everything irrigated even when there is little or no greywater present.