Water saving toilet: toilet tank-sink combination (C) Daniel Friedman Greywater or Graywater Toilet Flush Systems
Water Conservation, Re-Use, Graywater Toilet Flushing

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Flush toilets using graywater:

This article describes the use of greywater to flush toilets, reviewing the benefits of water conservation and the risks of soil or nearby water supply contamination, odor complaints, bacterial hazards.

We include citations of authoritative research on gray-water re-use for flush toilets and on graywater disposal into septic systems.

This article series defines and explains the disposition of greywater, a type of wastewater, as a water conservation measure and for re-use in watering plants, crops, or for other applications.

Green links show where you are. © Copyright 2017, All Rights Reserved.

Use of Graywater to Flush Toilets

Water saving toilet: toilet tank-sink combination (C) Daniel Friedman

Our photo shows a toilet flush-tank or cistern top that incorporates a wash basin. This water-saving feature feeds fresh water, though not necessarily potable water, to the wash basin and drains the used water (from washing hands) into the toilet cistern to be used for the next toilet flush.

This toilet is installed at the Nirvana restaurant in Guanajuato, Mexico, near San Miguel de Allende.

Question: We want to use our soapy shower water to flush our toilets

Giesela said:

We want to use our soapy shower water to flush our toilets with, as we are in a drought stricken area. No other water goes into the septic tank.Will the soap have a bad influence on the conditions in the septic tank?

- 2017/08/01, this question and our reply were posted originally at SEPTIC TREATMENT FAQs



Is it OK to use graywater exclusively to flush toilets & to send into the septic tank and drainfield?

For other readers, “Graywater” is defined (for example) by the California Graywater Standards in the California (US) Administrative Code, as “untreated household wastewater that has not come into contact with toilet waste. More practically, graywater is wastewater from bathubs, bathroom lav basins (sinks), showers, and clothes washers.

Watch out: wastewater from kitchen sinks and dishwashers as well as washing machine wastewater from laundering dirty diapers is heavily contaminated with various pathogens and should be sent to the septic system, nolt into the separate graywater system that would handle the graywater sources I listed just above.

For short term situations you can certainly dump by hand soapy water into a toilet tank to flush the toilet. Sending that limited volume of soapy sink water into a toilet tank and thence into a septic system should not make one iota of difference to a conventional septic tank/drainfield in systems where all of the building’s plumbing fixtures already drain into the septic tank and absorption field.

Watch out: longer-term use of un-treated graywater for flushing toilets can lead to odor complaints as well as to bacterial hazards in the toilet itself. (Jeppsen 1996) (Albrechtsen 2002). Use of graywater for toilet flushing might safe as much as 35% of drinking water, but additional treatment of the water for disposal will be needed. (Mourad 2011)

Generally using graywater (sink and tub or shower waste) to flush toilets is a water-conservation method that is used in areas where water conservation is important.

There are in fact toilets that incorporate a hand sink above the toilet tank specifically to aid that purpose. The example of a toilet top lavatory sink (shown above) is discussed at TOILETS, WATERLESS ALTERNATIVES

Also see FILTERS SEPTIC & GREYWATER for graywater filters used to protect septic systems from debris in graywater, the first step in making graywater useful for watering grass, trees, plants, etc.

But there is more we should consider about proper handling of graywater when we start collecting all building graywater for disposal. Where should that wastewater go and how should it be best handled to protect the septic system and the evnironment?

Over a longer term there are concerns with sending all graywater-wastewater that's high in lint (from washing machines) or soap, and un-filtered, un-treated, into a septic system. Both of these ingredients can clog the drainfield (lint and soap film) and also because the chemistry of detergents high in phosphates (for example) may be harmful to local soils or to nearby wells or water sources. For those systems a graywater treatment system is what’s appropriate.

Home & Small Building Greywater Treatment Systems

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Research on Use of Graywater for Flushing Toilets

and Research on impact of soap and graywater on septic systems & on detergents in & escaping the septic system

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Also see


Continue reading at GREYWATER SYSTEMS - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see CISTERN CONSTRUCTION GUIDE a combination of rainwater collection and graywater collection, diversion, re-use on a home built in an area of water scarcity

Or see ALTERNATIVE & WATERLESS TOILETS other graywater & toilet systems



Or see this

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