Reduce Water Use by Irrigating with Gray Water
Bird's eye view
Water savings, if local inspectors allow
Gray water is what comes from sink and shower drains and the washing machine. While it's not exactly potable, gray water has many fewer bacterial contaminants than toilet waste, or black waterPotentially contaminated wastewater from the toilet, kitchen sink, or other sources. Black water should not be reused without going through a complete treatment system. , and can be diverted for irrigation rather than sent to an overloaded municipal waste-water plant. The result is a substantial water savings.
But make sure to check with local building inspectors before installing a gray-water collection system. Not all of them will approve it.
Keep the equipment simple
A variety of equipment has been devised to recycle gray water. Simple systems are always best. Gray-water systems with complicated mechanical filtration components are often not worth the investment, requiring far more money and energy than sending gray water to the municipal sewage plant.
Kitchen gray water is handled differently
Because it contains organic matter and grease, kitchen waste water must be treated differently than gray water from other sources. With additional filtering, however, it can be reused. The most common design requires a filtered, three-chamber holding tank, similar to a septic tank, that allows the grease to settle out.
Afterward the water passes through a sand filter before treatment in a planter bed or discharge to the biologically active layer of soil.
Washing machine rinse water can be reused
If occupants are willing to monitor their clothes washer operation, it may be possible to recycle gray water used for washing clothes. This method involves switching the drain hose from the drainpipe to a collection container during the final rinse. Tubing at the bottom of the container connects to the washer supply line, and the water is used during the next wash cycle.
It's usually not possible to use gray water to flush toilets without expensive treatment. If untreated, the gray water cannot sit in the tank without fouling the system.
Gas-tight, corrosion-resistant tank required
The International Residential Code includes a section (Appendix O) governing gray water recycling systems. Among the provisions is section AO101.11, which requires that "Gray water shall be collected in an approved reservoir constructed of durable, nonabsorbent and corrosion-resistant materials. The reservoir shall be a closed and gas-tight vessel. Access openings shall be provided to allow inspection and cleaning of the reservoir interior."
ABOUT GRAY WATER
Reuse water, don't waste it
To collect and use gray water for irrigation, drain lines from bathroom sinks, tubs, showers, and the washing machine empty to a centrally located holding tank, typically in the basement or crawl space. A filter at the holding tank screens out particles. Water is either drained or pumped from the tank to the irrigation lines.
A properly designed system has overflow protection for both the holding tank and the irrigation lines. The overflow valve for the holding tank feeds water directly to the sewer line if the filter clogs. Overflow protection for the irrigation lines can take two forms: a diverter valve directs water to secondary irrigation loops or, if the irrigated area becomes saturated, to the sewer line.
Plumbing codes vary. Options for reusing gray water vary according to local plumbing codes. If the building inspector allows, you can decide between systems that capture all of a house’s gray water or just the drain water from specific sources.
Although the bacteria in gray water are generally aerobic, gray water can’t be stored for longer than 24 hours without using up all of the oxygen in the water and encouraging the growth of smelly anaerobic bacteria.
That's why the easiest and least expensive use for gray water is irrigation. The irrigation lines should be in the biologically active portion of the soil — no more than 9 in. to 12 in. below the surface.
MORE ABOUT GRAY WATER
GRAY WATER IN THE GARDEN
Gray water must pass slowly through healthy topsoil for natural purification to occur. Although it doesn’t need extensive chemical or biological treatment before using in the garden, it often contains grease, hair, detergent, cosmetics, dead skin, food particles, even fecal matter and should be used with care.
Use only on well-established ornamental plants, shrubs, and trees, never on seedlings. Since it’s alkaline, never use it on acid-loving plants, such as rhododendrons and azaleas. If irrigating food plants, fresh water is always preferable. Restrict application to soil around vegetables of which only the above-ground part is eaten, never on leafy vegetables or root crops.
Apply gray water directly to the soil over a broad area. Avoid use on slopes or concentration in one area. Don’t use overhead sprinklers or allow gray water to splash and contact the above-ground portion of plants. Don’t use gray water in a drip irrigation system, since it can easily clog the pipe’s emitters. The simplest method is by hauling it in a bucket from your source. Rotate gray water with fresh water use to help leach contaminants. Thick compost mulches help speed the natural decomposition of waste residues.
A square foot of well-drained, loamy soil can handle about a half-gallon of gray water per week. So, a 500-square-foot garden can tolerate up to 250 gallons of gray water per week. Given a choice, use shower and bathtub water first, followed in decreasing order of desirability by water from the bathroom sink, utility sink, washing machine, kitchen sink and dishwasher.
PLANTER BOX WATER FILTERS
Instead of irrigation lines, a planter box can be used to filter and clean gray water.
Filtered gray water is pumped into the top layer of the planter box, which contains at least 2 ft. of humus-rich topsoil. Beneath the topsoil are two layers of sand: common cement-mix sand and, below that, coarse sand. A filter keeps the sand from clogging the pea gravel that makes up the planter’s bottom layer.
Outdoor planters can leach treated water directly into the ground, or the water can be piped to a leach field. A planter can even be used indoors as long as it's plumbed to carry off the treated water.
- Daniel Morrison
- Dan Thornton
Aug 29, 2010 5:37 PM ET
Sep 19, 2009 9:20 PM ET
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