Plants can absorb graywater through their roots and then transpire the moisture into the air. A graywater system that relies on such transpiration is called an Evapotranspiration System. Such a system may consist of a tank to settle out the solids, with the effluent draining or being pumped into a shallow sand or gravel bed covered with vegetation. Canna lilies, iris, elephant ears, cattails, ginger lily, and umbrella tree, among others, have been used with these systems. An average two-bedroom house may require an evapotranspiration trench that is three feet wide and 70 feet long. One style of evapotranspiration system consists of a shallow trench lined with clay or other waterproof lining (such as plastic), filled with an inch or two of standard gravel, and six inches of pea gravel. Plants are planted in the
gravel, and no soil is used. A "mother-in-law friendly" evapotranspiration system is the Watson Wick (Figure 9.3).
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