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About Graywater

The average person uses over 45 gallons per day for simple tasks such as bathing or showering, and using the washing machine.

This doesn't sound like much, but over one month this amounts to over 1350 gallons per person

In a new house, with new appliances and plumbing fixtures, this figure drops to about 25 gallons per day, or 750 gallons per month.

Graywater is household waste water that includes the following:

  • Shower Water
  • Bath Water
  • Lavatory (basin) Water
  • Laundry Water
  • Untreated Spa Water


Typically, two thirds of the water used in a house results in graywater (excluding water used for the garden and lawns).

Blackwater is household waste water that includes:

  • Toilet Water
  • Laundry Water, if the waste water is from washing diapers, or other materials containing feces


Darkwater is household waste water from the kitchen. It may contain food contaminants, oils and powerful detergents (especially dishwashers).

Although many jurisdictions ban the re-use of kitchen water (darkwater), in some situations this water may be used for garden irrigation IF a grease trap is installed between the kitchen waste outlet and a graywater pumping system.

The above definitions apply generally throughout the world, although exact definitions vary from country to country, and in the US from state to state.


Practical Graywater Re-use

In an urban situation, with a utility provided sewage system, we recommend only re-using shower, bath, spa bath and laundry water.

Kitchen and lavatory water is best left for the blackwater waste pipe network.  Lavatory (basin) water is in most cases does not provide enough water to justify the connection cost, and in any case provides additional water flow to help flush the blackwater pipe system.

In a rural situation (i.e. a septic system is used), kitchen water can be used (subject to local regulations), IF the following occurs:

A grease trap is installed between the kitchen sink and the graywater system. This ensures food scraps, fats and oils are not irrigated in the garden. In addition to containing high bacteria levels, the significant quantity of fats can create an impervious barrier within the top soil.

The dishwasher does not empty into the kitchen sink waste, because the detergent is too caustic.

Graywater Names

Graywater is called Graywater, Gray Water, Greywater & Greywater. Although debate is still underway (and will probably never cease), the likely outcome is Graywater in the US, and Greywater in other countries.
Fortunately, most search engines now search for all of the above terms if only one is searched.

Graywater contains contaminants such as dirt, skin cells, body oils, hair and lint. It also contains bacteria which may present a health risk if not used properly.


Untreated Graywater

Untreated graywater must be used within 24 hours, and in most jurisdictions can only be used  to irrigate gardens. Beyond 24 hours, bacteria will multiply within the graywater and present an unacceptable health risk.

Although the 24 hour rule is applied almost universally throughout the world, graywater can develop offensive odors within several hours, and we recommend not storing graywater at all. This is particularly the case with shower water. The body oils, soap and bacteria interact rapidly, creating a very noticeable odor quickly.

Washing Machine graywater is less susceptible to developing odors within 24 hours.

Untreated graywater should not in any circumstance be re-used within the house (eg. bucketing shower water back into the toilet cistern).  This creates significant health risks.


Treated Graywater

Graywater that has either been chemically or UV treated can be re-used within the house, but only in the laundry and for toilet cistern refilling. It can NOT be used for human / animal consumption, nor can it be used to supply water for swimming pools.

While graywater treatment systems can be viable for commercial applications, the high supply and installation cost (typically over $10,000 + annual maintenance costs) in most cases rules out this technology for residential use.