Using gray water is certainly a good idea, but it also has its drawbacks. While gray water is a lot less harmful to people than the wastewater that leaves your toilet -- called black water -- it's still technically sewage. Dishwater contains particulate food matter, which may be rotten. Gray water from your washing machine may contain bleach, which is a hazardous chemical. And bath water may contain fecal matter and dead skin that sloughs off while you bathe. The amounts of these wastes are small enough to prevent gray water from going through the treatment process that black water does, but you still shouldn't drink it. Your plants, however, will love your bath water.
As a matter of fact, irrigation for lawns and gardens is the only advisable application for using gray water. The United States Environmental Protection Agency says that, in 2007, water used for irrigation constituted 15 percent of total water consumption in the United States [source: EPA]. And this was first-run water -- the same stuff you drink.
Your plants don't necessarily need fresh water, and what's more, some additives in gray water can actually help them grow. Cleaning agents found in laundry detergent -- stuff like phosphorus and nitrogen -- are actually used in many plant fertilizers sold on the market. Some plants, however, like gray water more than others. Since gray water is alkaline-rich, it's not suitable for use in watering acid-loving plants like azaleas and rhododendrons.
Be careful watering any plants with gray water without knowing what's in it first. In addition to nitrogen and phosphate, laundry detergents also contain sodium salts. These salts can build up over time and become toxic to plants, essentially poisoning the soil. To avoid this, avoid using products that contain softening agents -- which are generally high levels of salts. You can also cut down on the salt build-up in the soil by alternating your watering applications with gray water and fresh water.
Since the soap you use in the shower is generally less harmful than laundry detergent (it's mild enough to use on your skin), bath water is the most prized gray water for your plants. But again, there's the fecal matter. For this reason, some states require that gray water systems remain entirely underground and irrigate plants directly at the roots, using drip irrigation systems. Even gray water proponents who irrigate above ground recommend that the gray water be delivered using the flood method or with drip irrigation. Either way, spraying or misting plants with gray water generally can be a threat to your health. You should also irrigate with gray water on flat ground and avoid allowing it to run off into other yards.
Also because of that pesky fecal matter often found in bath water, gray water proponents advise using gray water to irrigate only your ornamental plants and lawn. It shouldn't be used for plants that bear fruit or vegetables that you'll eat -- like tomato plants. And under no circumstances should gray water be used for edible root plants, like carrots or potatoes. The roots absorb all of the harmful stuff in gray water.
So now that you know about gray water you may be ready to start using it at your house. But how? Read about some of the methods to get a gray water plan started.