Gray water is water that drains from sinks, baths, showers and dishwashers. It is considered relatively clean in that it has not come into contact with human waste.
Most people use more than 25 gallons of water each day for showering, bathing and washing clothes. This gray water is safe to recycle for watering plants or other purposes. Black water, on the other hand, is household-waste water from toilets, washing diapers, and cooking and cleaning up the kitchen. While the first two sources often contain feces and urine, or traces of each, the third is likely to contain strong detergents and contaminants.