Gray Water Reuse

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Gray water is water that has been used for showering, faucet uses, and clothes washing, and it usually disappears down drains as a wasted resource. Sustainable wastewater treatment involves the recapture of gray water for additional uses, a process better known as water reclamation. These recovery systems do not collect kitchen sink water or toilet water because these can be health hazards due to the presence of disease-causing microbes.

Sustainable houses collect gray water in pipes that run separate from toilets and the kitchen sink. After collection, building designers have options regarding how and where the gray water will be best used. The three main options for gray water use are: (1) as irrigation water; (2) for flushing toilets; or (3) in the home’s fire suppression system.

Two new technologies may soon join the three main uses for gray water. The first involves the routing of gray water into wetlands constructed near the building to naturally degrade liquid wastes. These constructed wetlands work similarly to natural wetlands in which wastes move very slowly through the site so that plants and microorganisms have time to degrade the organic matter. Lori Ryker explained in her 2005 book Off the Grid, “The goal of a man-made wetland is to replicate a natural wetland’s ability to clean and filter water. A constructed wetland system’s selected plants filter gray water in a specific order, and then return the water to the earth, eliminating the need for a fully developed septic system.” The second technology uses very efficient filtration systems to make the water suitable for drinking, known as potable water. The following filtration technologies may soon produce drinking water for direct reuse as potable water: ultrafiltration in which filters contain extremely small pores to capture all contaminants; nanotechnology in which nanoscale materials act to clean or even sterilize water; and reverse osmosis in which water is forced through a fine membrane to clean out all impurities.

A sustainable home manages three different water sources: (1) rainwater, which is usually captured in a cistern such as the 60-gallon (227-l) size shown here; (2) gray water, which is wastewater from showers, sinks, and washers that is not expected to contain diseasecausing microbes; and (3) blackwater, which is wastewater from toilets, sinks, or any other source expected to be contaminated with disease-causing microbes. (Natural Rainwater.com)

Most houses that recover gray water today send the water to a holding tank that degrades waste in a manner similar to a septic tank. The cleaned water moves out of the tank by passive energy; it is pushed along by the force of new gray water entering the system. The cleaned water flows through a short series of baffles that prevent most of the dirty gray water from mixing with the cleaner water, and then the cleaned water goes through the outflow pipe. After completing this cleanup process, the two main current uses for gray water are garden irrigation or flushing toilets. Gray water reclamation does not need to be confined to houses; these closed-loop systems can work in large office buildings, schools, and manufacturing plants.