Greywater harvesting is the use of water which has been recovered from sinks, baths etc.  Kitchen water (i.e. dishwashers and washing machines) cannot often be used as these will have high levels of detergents within the water.  

Grey water is most commonly used for garden watering and toilet flushing.  Unfortunately large quantities of grey water are not advised to be used regularly on the garden as the high level of detergents found within this type of water can often contain faecal coliforms which can cause illness. 

It is also advisable to use grey water within a day or two as the compounds within the water can start to cause an unpleasant smell.

Grey water is often stored in a separate tank that has a fine filter to sort particles, it will also need separate water pipes for the potable supply, which will need to be clearly identified under legislation required by law.