Irrigating with gray water can be a great option for watering the garden, but keep in mind a few of these safety concerns.
In order to save money and eat better quality food, I grow my own vegetables (as do many people these days). I recycle the “gray water,” used liquid that does not contain human waste, from our washing machine and kitchen sink by irrigating the garden with it. Is this a safe practice? I assume that, with the new “no phosphorus” soaps, this setup won't harm my plants or be dangerous to the environment.
Sounds good to me, but you need to keep a few key points in mind. First, your city, county, or state health department will likely oppose your practice, since they may confuse it with black-human waste-filled-water, which can contain pathogens that cause sickness, or even death. (If small children live nearby, you can surely expect such opposition.) Second, some gray waters do contain pathogens (bacteria and viruses) from washed diapers, underwear, and similar items. So use caution appropriate to the origins and composition of your own kitchen and laundry waters. Third, I strongly recommend against using gray water directly on edible crops, particularly on ones that will be eaten raw or slightly cooked because of the risk of contamination.
On the positive side, using household gray water for corn, grains, fruit trees, and lawns makes a lot of sense in almost any climate. Rather than broadcast or spray the liquid, though, trickle the water directly to the plant roots through a “soaker” or perforated hose.
Since phosphorus is an essential nutrient for vegetation (all commercial fertilizers contain the element), I'd recommend that you go ahead and buy phosphate-based detergents, instead of non phosphate types based on carbonate or-especially-silicate buffers. Read the package to know what you're going to be putting on your lawn and garden.
— David Burmaster, consultant on surface- and ground-water quality and hazardous waste management