Greywater, (also called grey water, graywater and gray water) refers to water that has been “gently used” in sinks, showers, baths and light industrial applications and has not yet been treated. It is clearly distinguished from water from toilets, referred to as “black water”. Properly filtered and stored, greywater can be a valuable source of water to flush toilets and urinals or irrigate landscaping. Toilet flushes can account for 25-65% or more of the total water use in a commercial building, even when low-flush fixtures are used.

An efficient greywater system first requires a steady source of greywater. The most abundant source is showers in buildings with full time residents – apartments, dormitories, hotels, schools, etc. Some manufacturing facilities can harvest water used in their manufacturing processes if it is not heavily loaded with contaminants. Office buildings generally do not produce enough usable greywater to warrant the cost of a system. When there is an abundant supply of greywater, it can be a more reliable source of water for flushing toilets than rainwater. The amount of shower and sink usage generally ties to the amount of toilet use in a building, so there is almost always a balance in supply and demand for greywater.

There are significant implications for the use of greywater. Greywater harvesting requires additional treatment versus rainwater and is always an active system versus a passive if it is to be used inside a building or for spray irrigation. The additional filtering and sterilization requirements increase the cost of a system vs. rainwater harvesting systems. However, because daily supply and demand are in balance with a greywater system, the cost of storage tanks for a greywater harvesting system can be significantly lower than for a rainwater harvesting system, bringing the total system cost into the price range of a large rainwater system.

As a relatively new form of water conservation, many communities have not yet set standards for greywater processing and storing. Local codes often restrict the use and storage of greywater but the codes are meant to protect public safety from the contaminants in untreated greywater. Once the greywater has been treated, its definition becomes “onsite treated non-potable water” and falls under a different set of usage and storage guidelines. Water Harvesting Solutions has extensive experience in developing greywater systems for commercial properties. Our greywater system has third-party certification for both IAPMO 324 and NSF/ANSI 350. Read more about Greywater Harvesting Systems.