Greywater

Greywater is the waste water from clothes washing, showering, and hand washing, that instead of being sent to sewer, is kept separate from toilet waste (blackwater) and is typically used for garden watering in lieu of mainswater.

Unlike rainwater which is sporadic, and somewhat seasonal, greywater is produced daily so you have consistency of supply.  That's the upside.  The downside is that because of the high nutrient content in the soapy water, greywater will go septic if stored for more than 24 hours.  While purification and storage is possible, it currently takes significant infrastructure and energy, as well as having regulated ongoing maintenance regimes.

Greywater however can be used untreated by diverting directly to the garden (by way of subsurface irrigation).  This saves the fresh water, or rainwater, that would have been used for the garden, to be used elsewhere.  As 1/3 of the water use in the average Melbourne home goes to garden watering (~70,000 litres), the water savings are considerable by incoorporating greywater.

The secret to successful grey water use is:

• Try to make sure it all flows by gravity wherever possible.  This avoids pits and pumps and means the pipes will dry out between greywater events. Pits and pumps inevitably go septic and require more frequent cleaning, which is not a fun job.
• Divert greywater to subsoil watering.  Greywater has the potential to carry bacteria, so the EPA require it not to be diverted to the surface where it can come in contact with hands, kids, pets, etc. 
• For guidelines regarding how to set up a sub-ground plumbing distribution system, the old Nylex diverter manual is a good practical source.
• Choose a Greywater diverting unit that can switch between sewer and garden, and that has an automatic overflow to sewer if it gets blocked with gunk. 
• Be very careful what soaps you use when diverting greywater to garden.  Too much salt can damage the soil stucture and kill plants, so choose low salt content detergents.  Here is a list of laundry detergents ranked by salt content.  www.lanfaxlabs.com.au/laundry-detergents-brochure-2009.pdf  (Note, also, the Lanfax brochure also details Phosphorus levels.  In general Phosphorus is a nutrient, and will benefit plants, unless they are Australian natives, which are used to low levels of Phosphorus.  However, if you are sending greywater to sewer, Phosphorus should be avoided, as it leads to nutrient build up at the sewer outfalls, and can lead to algal blooms, which in turn stop oxygen mixing to the water depths and kill the sealife near such blooms.
• Divert greywater to sewer over the winter (or whenever your rainy season) and let the rain wash through and 'clean' the soil.
• Greywater should not be used to water vegies where the fruit/produce grows in contact with the greywatered soil.  Fruit trees are generally ok, as the tree itself will likely filter out any nasties, and the fruit is not touching the soil.
• And here are some other rules to remember...Greywater rules