There are several rules for the re-use of grey water and these are:
1. Thou shalt not store grey water: This is the first law and may not be changed: the problem is that grey water has temperature and food value for anaerobic bacteria to breed and produce methane and hydrogen sulphide, the stuff that smells so badly. Any talk of a tank into which grey water is fed and used for any purpose whatsoever is simply not possible. Any tank will build up sludge, and this is quite unacceptable.
2. Do not let grey water pool: sending water to flood irrigate gardens on a regular basis means that bacteria will clog the soil, preventing penetration of water into the soil, that needs to be overcome by some sort of cultivation.
3. Grey water must be sprayed under very low pressure that does not exceed 6 metres head at the sprinkler. This will prevent any atomising of the sprayed water particles. Grey water contains bacteria of many types, and the particles of water must land on the soil, and not float down wind to be breathed in downwind. Spraying means that the likelihood of polluting groundwater is eliminated. Grey water is dirty water and may not be fed into any irrigation system; this includes drip irrigation because all irrigation systems require clean water and high pressure.
4. Perhaps the most important aspect to the re-use of grey water for irrigation purposes is what to use for washing powders. The use of any phosphate rich washing powder will poison your soil over the long term. There are phosphate free washing powders on the market, and these must be used.