Graywater reuse refers to collecting, filtering, storing and reusing household water that drains from bathtubs, showers, bathroom sinks and washing machines. Graywater contains a minimum amount of contamination and can safely be reused for subsurface irrigation of lawns, flowers, trees and shrubs, but it shouldn't be used to irrigate vegetable gardens.
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Don't confuse graywater with reclaimed treated wastewater, which can be used in ways that aren't permitted with untreated graywater, such as toilet flushing and above-ground irrigation. Graywater also shouldn't be confused with blackwater, which drains from toilets, kitchen sinks and dishwashers. Blackwater has high concentrations of organic waste and can't be reused safely. Similarly, garden and greenhouse sinks, water-softener backflush, floor drains and swimming-pool water shouldn't be included with graywater reuse.
Advantages and Drawbacks
The most obvious advantage of graywater reuse for irrigation is that it can replace potable water, saving homeowners money. Also, using graywater may actually be more beneficial to plants than potable water because graywater often contains nutrients such as nitrogen or phosphorus. When irrigating with graywater, however, it's important to control what cleaning and washing chemicals are used in the water so as not to damage the plants. Avoid powdered detergents because they tend to be high in sodium and salts, avoid boron because it can be toxic to some plants, and avoid chlorine bleach, caustic drain cleaners, petroleum distillates and other chemicals that have unknown effects on plants.
Graywater use also can reduce the pressure on limited potable-water resources in some communities. It may offer financial savings to overburdened sewage-treatment facilities by diminishing sewer flows and thereby lessening the need to expand these facilities. Increased graywater use may have some downsides, though: Diminished sewer flows mean less water to carry waste to the treatment plant. Also, less water will be available for treatment, resulting in less reclaimed water for municipal and other uses.
Separating graywater from blackwater and reusing it isn't allowed in many areas, so be sure to check local building codes before moving ahead with a graywater-reuse system. For new construction in areas that currently prohibit graywater reuse, it's relatively simple to install the home's plumbing so that a graywater-reuse system can be added later.
System Components and Maintenance
If you're planning to collect and reuse graywater, run the drain lines for the bathroom sinks, tubs, showers and the washing machine to a centrally located holding tank with a filter. Crawlspaces and basements are an ideal location for the holding tank. Water can remain in the tank for up to 24 hours. Don't store graywater in the tank for extended periods, because the decomposing organic matter in the water will begin to smell foul. From the tank, water can drain or be pumped directly into the subsurface irrigation lines. Locate the irrigation lines from nine to 12 inches below the ground surface. This is the biologically active portion of the soil and is optimal for breaking down the graywater's organic matter.
Include an overflow valve in the graywater collection system that feeds water directly into the sewer line in case the filter clogs. In addition, include a controllable valve to redirect graywater into the sewer line in case the areas being irrigated become too wet. As for the filter, it should be easily accessible and easy to clean. Improper filter maintenance may make the water unsuitable for reuse.
Regular maintenance is necessary to keeping a graywater-reuse system operating safely and effectively. A basic maintenance plan should include inspecting the system for leaks and blockages, cleaning and replacing the filter as recommended, flushing the entire system periodically, and regularly inspecting the irrigated areas to make sure the amount of water being pumped in is sufficient.