Gardening Articles: Care :: Soil, Water, & Fertilizer
Graywater For Gardens (page 2 of 2)
by Robert Kourik
Keep Graywater Underground
The current approach emphasizes no "daylighting," or surface puddling. If the graywater-irrigated soil doesn't get moist on the surface, then nobody can come in contact with pathogens or parasites it may contain. Thus, in a modern graywater system, the graywater goes directly to the roots of trees and shrubs via subsurface distribution, either a shallow leachfield or buried drip irrigation.
A leachfield system utilizes a perforated pipe set 18 inches deep, surrounded by a trench of gravel and covered with nine inches of soil. Each perforated pipe should be set level and be no longer than 10 feet. (By comparison, a septic leachfield is four to six feet deep.) Best suited to watering trees and shrubs, this system features three main benefits: It's simple to operate, doesn't clog and works well if downhill from a gravity-feed system. Limitations include the difficulty of installation among existing plants, relatively inefficient use of water, uneven distribution and unsuitability for upward slopes.
Drip systems have several advantages. They apply graywater close to the soil surface for more aerobic "digestion" of its organic matter and nutrients. They spread the graywater over a large area, and deliver it to the largest number of plants with the greatest amount of control. Finally, a drip system is best if you're designing a system around slopes and grade changes, or simply dealing with heavy clay soil that's slow to accept water.
California's Appendix J requires that the in-line-emitter tubing (black plastic tubing with emitters installed inside the tubing at regular intervals, with an emitter flow of no more than 1/2 gallon per hour) be buried nine inches below the surface to prevent daylighting. The best tubing is made by Geoflow and is called Rootguard Emitter Line. A mail-order source is The Urban Farmer Store, 2833 Vicente St., San Francisco, CA; (800) 753-3747. A 100-foot roll costs $51.
The chief limitations of graywater distribution by drip are that the systems are more costly and complex, and inadequate filtration is a disaster. The lint, dirt and oils in graywater act like large logs, huge boulders and oil slicks in tiny in-line-emitter openings. Some systems filter the graywater with a fine-meshed bag, some with self-cleaning canisters of clean sand and others with paper-cartridge chambers. Most require diligent and frequent cleaning of the filter medium. A failed filter can permanently ruin the entire drip system in a very short period of time. And drip is not well suited to most gravity-feed graywater systems because they need some water pressure, at least 10 pounds per square inch (psi) of water pressure, the equivalent of a 23-foot drop in elevation.