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Introduction To Greywater

What is greywater?

The average family of four uses 400 gallons of water every day.  Of those 400, 280 gallons are used indoors.  Some of this water is considered blackwater, heavily polluted with biological contaminants (think toilet flushing).  Then there’s the water you use for various types of washing – clothes washing, dish washing, hand washing, and body washing.  What is that considered?


According to some definitions, greywater is all the water used in a home except by toilets. This definition makes sense in buildings that use composting toilets.


Most definitions of greywater, however, including those in the new California regulations, are less inclusive. By these definitions, greywater does not include wastes from kitchen sinks, in-sink garbage disposals, and dishwashers; it is limited to washwater from showers and baths, bathroom sinks, and clothes washers.


What are the concerns about greywater?

  • Given that greywater has been used before it heads down the drain, soaps, detergents, oils, and flecks of dead skin can be present. 
  • Microorganisms and pathogens are living organisms that can cause illness.  They are primarily found in blackwater however they can be found in greywater too.  
  • Biochemical oxygen demand (BOD) is a measure of the concentration of decomposable organic matter in water.  BOD is based on the quantity of oxygen that would be used up as the organic matter is decomposed by microorganisms.  If there are high levels of BOD present and if greywater is allowed to sit, it can become smelly.  


What does this mean?

  • We can conclude that there are some health concerns with greywater.  That’s why before you consider a greywater system, you must look into the health codes in your state.
  • Wastes (such as that from the garbage disposal) with high levels of BOD should be kept out of greywater supplies. 


How can greywater be used?

Light greywater systems use very selective sources of greywater.  For example, water from the bathroom sink is primarily used for hand washing and tooth brushing.  This water is much less “dirty” than other sources of greywater and can be used for flushing toilets and for landscape irrigation.

            Other sources of greywater can also be used for landscape irrigation.  However, given the health concerns previously mentioned, several principles should be kept in mind when using greywater.

  “1) get the graywater into the soil as quickly as possible instead of storing it;

2) irrigate with graywater below the surface of the ground only;

3) try to deliver the graywater to biologically active soil where the organic matter will quickly be broken down (in general, the closer to the surface, the more soil bacteria are present);

4) design the graywater irrigation field in a manner that will prevent graywater from surfacing—which entails careful sizing of the field based on soil type and expected flow; and

5) design flexibility into the graywater system so that the graywater can be channeled elsewhere (into a different drainage area or into the conventional sewage system) if soils become saturated or other conditions make using graywater unwise—see below on managing graywater systems.” (Environmental Building News March/April 1995)


What are the benefits of using greywater?

  • Decreased water demands for toilet flushing and irrigation
  • Ability to irrigate during drought conditions
  • Increased lifespan for in-ground septic systems



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Green Options › Articles › Introduction To Greywater