If you intend to use untreated greywater in your garden then it is essential to reduce the amount of fertilizer you use. Soaps and washing powders contain nutrients such as nitrogen and phosphorous, which should be removed if you intend to use large quantities of grey water in the garden.
Grey water watering systems should be installed under the ground (sub surface irrigation - the most efficient way to water plants) since greywater can contain disease-causing pathogens which you don’t want to spray around your garden.
You should avoid using grey water on your vegetable patch if you intend to eat any of your crop raw or lightly cooked (including herbs and fruits) unless you first filter the water well using a DIY sand filter or commercial alternative.
Greywater tends to be alkaline, whereas collected rain water is acid. Therefore it is necessary to understand which plants thrive in which conditions and to organise your garden and watering accordingly.
According to: http://www.reuk.co.uk/Greywater.htm