Ask Mr. Smarty Plants

Greywater (or grey water) is slightly used household water—any water other than that from toilet flushing (called black water). This would include water from the dishwasher and kitchen drains, the clothes washer, showers, tubs, and lavatories. Kitchen water is sometimes put into the black water category because it contains a high percentage of oils, fats and food particles. Swimming pool drainage water is not recommended for greywater use because of its high salt concentration and stabilized chlorine or bromine which would be detrimental to most plants.

Greywater is used mainly for irrigation of plants but it can also be used for flushing toilets. Both uses contribute to the conservation of drinkable water that would otherwise be used for these purposes.

Storing greywater is not recommended unless it treated for bacterial growth; thus, greywater must be used when it is created. The water is an obvious benefit to plants, but it must be applied judiciously since oversaturation of the soil would not be beneficial to the roots of most plants. Other potential problems for plants depends on the level of the various chemicals included in the greywater. For instance, some plants will be more tolerant of greywater with substantial salinity; whereas, others may be sensitive. The chemical makeup of greywater varies from household to household depending on the personal habits of the members of the household and their choice in soaps and other cleaning solutions. Lanfax Laboratories in Australia has an excellent paper, Domestic Greywater, from their Laundry Products Research that gives data for the pH, the salinity, the sodium content, and the phosphorous content of greywater containing various clothes washing products. In general, greywater is alkaline because of the cleaning products it contains and, as such, should not be used on acid-loving plants (e.g., Rhododendrons). Unless greywater is treated, it is recommended that it not be used to water plants with edible roots and only surface watering should be used to protect edible leaves or fruits from possible bacterial exposure.

Several studies have investigated the effect of greywater on soil composition. A study by M. Travis, N. Weisbrod and A. Gross ("Accumulation of oil and grease in soils irrigated with greywater and their potential role in soil water repellency". Science of the Total Environment 394 (2008) pp. 68-74) suggests that oil and grease from greywater can accumulate in soils and affect the ability of the soil to absorb water—essentially making it water repellent. Another study by A. Gross et al. ("Environmental impact and health risks associated with greywater irrigation: a case study" Water Science & Technology Vol 52, no. 8 pp. 161-169) found evidence that: "Long term irrigation of arid loess soil with greywater may result in accumulation of salts, surfactants and boron in the soil, causing changes in soil properties and toxicity to plants."

For more technical articles, you could visit a university library and search their bibliographic databases. Also, there is a great deal of information available on the internet about greywater and its uses. Here are a few links:

Safe Use of Household Grey Water from New Mexico State University.

Greywater: what it is...how to treat it...how to use it from Greywater.com.

Grey Water Central on the Oasis Design page.

Introduction to Greywater Management from EcoSanRes.