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Rising water costs, water restrictions, and stricter pollution standards drive more homeowners to grow away water that was used for washing.

In a bathroom, we wash our hands or take a shower, then watch a lot of warm water with a small volume of particles, soap, and skin oils—and usually little else—drain away.

We flush the toilet, and see a concentrated effluent—with high nitrogen, suspended solids, and fecal coliform—swirl down the pipe. 

The two effluents meet in a common pipe draining to an onsite treatment system or sewer.

But increasingly, the public is spotting this missed opportunity to keep the two separate: diverting the cleaner effluents to irrigate plants, flush toilets, or recharge groundwater with lower treatment costs.

Graywater-use systems—from drip irrigation to ponds to even disinfection for toilet flushing—have entered the popular imagination. The drivers: rising water and wastewater treatment costs, forecasted water shortages, stricter wastewater regulations, and the growing market for “green” building features. Regulators and markets will have to catch up.

Graywater is water used for washing. It drains from sinks, showers, tubs, and washing machines. Blackwater is everything that leaves toilets. In many states, blackwater can also refer to effluent from kitchen sinks when blood, solids, and greases are present.

Toilet flushes and kitchen sink drains account for about 40% of an average American home’s wastewater discharge. That means a little over one half of household effluent is graywater, with an average BOD of 530 mg/l and low TSS, nitrogen, and pathogen content when compared to blackwater (see Table 1).

With hundreds of gallons of potable water from wells and municipal supplies used by the average home to irrigate its landscape, this is a missed opportunity.

More Than Soapy Water
Graywater’s relative clarity prompts many property owners to assume it is benign and to drain it directly (and illegally) from their washing machines to their backyards. Plants will appreciate this steady pulse of warm water, but, since the BOD and particles decompose slowly, odors and fiber mats can build up.

Graywater has treatment challenges: Skin, hair, oils, salts, fibers, and other organics (BOD) can clog drip-emitters and soils. BOD can cause odors when graywater is in open containers or applied to a surface. Also, graywater can contain fecal coliform, especially if it has been used to wash bodies or diapers, for example. Although the health risk is usually low, it does warrant prudence and treatment.

Graywater can be slightly alkaline depending on the acidity of tap water, cleaners and other substances used.

To a lesser extent, salty soaps, high-strength cleansers, disinfectants, and chlorine bleach can pose challenges.

All of this points to graywater treatment: Filter, float and settle out particles and fibers, knock down BOD, and deactivate pathogens. Clarification and disinfection is only required for surface applications.

General categories of graywater-use systems are: shallow leachfields, drip irrigation, planted evapotranspiration systems, branched drain system, filtration and disinfection, and various versions of wetlands and ponds.

There are four general approaches to treating and using graywater.

Common features of most graywater-irrigation systems are filters to remove particles, subsurface irrigation components (no graywater should be exposed), chunky growing media or aerators to promote fast-acting aerobic biological activity, and hearty plants. The plants work to use up nutrients, evapotranspire the water, and provide root systems to support facultative bacteria.

Start With ...
Those seeking to install graywater systems commonly complain about regulations requiring installation of a full-size or smaller septic tank for the system, even if a full-size tank is already in place for a full combined wastewater system. A septic tank is best replaced with the following for better performance and safety:

• Filtration: It is best to first prefilter and pretreat graywater. Many simple systems suggest nylon stocking filters to filter out particles. This will be sufficient for 240-micron fibers; however, many cloth fibers are about 40 microns in size, including Polar Fleece and other nondegradable fabrics. The Septic Protector and other filters offer a more controlled filtration with 40-micron cloth filters encased in cartridges. These should be rinsed off outside. For best results, use a 120-micron followed by a 30-micron filter.
• Grease interceptors, or “grease traps,” are key in kitchen sinks where greasy pans are rinsed. Greasy sludges are even found in bathroom sinks. Resembling miniature septic tanks with baffles to hold back floatables, their drawback is owners’ unwillingness to maintain them by removing the top and skimming out the floating scum layer. In the same vein, Clivus Multrum’s graywater filter is a box with baffles that hold back fibers and solids.
• Surge tank:  A surge tank—ranging from a septic tank to a 55-gallon drum—helps equalize flow to the system and avoid drowned system and breakout. It also helps cool the water, and it can serve as a grease trap if the scum layer is periodically skimmed. Pumps and filters are often installed in the surge tank.
• Diverter valve: Another key component for peace of mind is a three-way diverter valve. This allows diversion of graywater to a septic system or sewer if the system is overloaded with graywater (or rainwater) or if the content of the graywater is a concern, such as when toxic cleansers are used.

The Systems
Subsurface irrigation often provides the best cost-benefit ratio for graywater use.

Incresingly, the public is spotting the opportuinity for diverting cleaner effluents to irrigate plants.

These systems provide some treatment of the graywater via soils or substrate while distributing graywater to plant roots. This prevents contact by humans, animals and other vectors. It also distributes water closer to plant roots, and minimizes evaporation and temperature losses.

Small shallow leachfields, such as one of California’s two state-approved designs, are usually a series of gravel-filled trenches in which perforated pipe distributes graywater. California specifies a one-foot depth to allow root access and evaporation.

Planted evapotranspiration systems, such as the Washwater Garden, are typically used where all graywater must be used up, such as on an environmentally sensitive site. Much like a leachfield trench or bed, these are often lined and feature very careful selection of aggregate and plants to use up the graywater through evaporation and transpiration.

Subsurface wetlands and plant-rock filters are saturated systems and must be planted with varieties that like their roots wet. When used for graywater alone, they might fail due to a nitrogen deficit. One solution is to add urine, leachate or liquid nitrogen fertilizer.

Using drip irrigation to disperse graywater to an entire lawn is the ideal of many property owners who seek to use every drop of graywater to avoid using any tap water at all. This approach requires filtration and reduction of the BOD to avoid clogging the emitters. One way to do that is to filter the graywater through a sand filter, such as a swimming pool filter, or aerate the graywater to reduce BOD.

Mulch-basin node systems pipe graywater to basins of mulch, ideally next to trees and other plans that can use it. The advantage is that no filtration is required as long as the basins are monitored and periodically replaced. Two such systems have been developed and chronicled in books by graywater-use advocates Anna Edy and Art Ludwig (see sidebar).

Systems that discharge to the surface will require disinfection, such as with an ultraviolet unit or reverse osmosis, in all states except New Mexico (up to 250 gallons), to receive local approvals.

This is due to the possibility of contact with humans, animals and insects.

Surface-flow wetlands and ponds are usually chosen where storage of graywater for irrigation or a pond feature is desired. 

Surface drip and spray irrigation will always require disinfection, and pretreatment is critical to prevent clogging of emitters and the receiving soils. Disinfection allays concerns of liability but does not treat organics, and those who discharge graywater to the surface often experience odors on the ground, as organics decompose slowly.

Recycling for toilet flushing and outdoor washing is the ideal of many, especially as environmentally lower-impact building rating systems, such as the LEED (Leadership in Energy and Environmental Design), gain favor. This requires full water treatment with filtration and disinfection, and therefore can be onerous as far as permitting. Toilets flushed with unfiltered graywater have clogged with accumulations of soaps and oils. Even when filtered with high-end reverse osmosis systems, graywater often retains a yellowish color. Recycling graywater for this purpose will require some special permitting in all of the United States, making this a cost-effective option mostly for those seeking to demonstrate green building practices and for severely water-restricted sites.

Complete recycling of graywater to potable quality is the dream of many, but the treatment systems for this are usually expensive and elaborate enough to discourage this in all but the most idealistic and water-scarce places. Disinfection kills most potential pathogens of concern, but water will still be cloudy with TSS and BOD.

State Regulations
Few states specifically address graywater systems beyond specifying a full- or reduced-size leachfield for its dispersal, often with a full-size septic tank.

Driven by intermittent droughts, California was the first state to broadly allow two irrigation treatment options for graywater: a reduced-size shallow leachfield and a drip-irrigation system. A sizing formula is keyed to flow, although the state’s climates and geology vary widely. And although these designs are state approved, final approval rests with local health agents, and not every property owner is allowed to install these systems. Preliminary soils and depth-to-groundwater tests are required.

A common complaint with this state’s rules is that they are too conservative, essentially requiring construction of a duplicate leachfield, the costs of which aren’t immediately recovered with avoided water costs.

New Mexico recently adopted a graduated standard that allows irrigation of graywater of up to 250 gallons per day without a permit if general guidelines are followed, including an overflow to a septic system or sewer. (There is no program in place to enforce this.) This is more than most households will produce. After that, the graywater must be disinfected, such as with an ultraviolet unit or reverse osmosis. Water used for washing anything soiled with excrement is categorized as blackwater.

Arid Arizona’s Type 1 Reclaimed Water General Permit allows private residential direct reuse of graywater for a flow of less than 400 gallons per day if discharge is at least 5 feet above the groundwater table. The law calls for prefiltration but does not specify the means.

Water-rich Massachusetts has specified guidelines for graywater use in the past few years mostly as a means to promote better discharge and system opportunities. Graywater must be dispersed under 9 inches of soil and should be fronted by a septic tank or a graywater filter approved by the state’s plumbing board. No surge capacity is specified. Commercial and public facilities get a 30% reduction in tank size for a graywater-only system. 

Carol Steinfeld is a freelance writer and projects director for Ecowaters projects.

OW - November/December 2006

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