Error: Use of grey water for irrigating lawns
The only proven safe and reliable way of irrigating lawns with grey water is through underground drip tubing supplied by a backwashing sand filter type system; far beyond what most residences are likely to install. Unfortunately, turf accounts for the bulk of the irrigation need in the typical landscape, and lawn grey watering is by far the most prevalent violation of common sense grey water safety rules.
This is awkward to write. Do I criminalize thousands of grey water users who see no harm in what they are doing, or do I condone a marginal activity?
If the lawn receives traffic, by applying grey water to the surface you are short circuiting the all-important purification step (see health rulespages 4, 8), inviting direct contact with untreated grey water and the possibility of transmitting pathogens. The likelihood of transmitting disease is small (it would be laughed off in most developing countries) but it exists. The nightmare scenario: the day care center that "saves money and the environment" by irrigating the lawn with diaper wash water, which a dozen toddlers from other families then play in (I know you think I'm making this up, but I saw it at my daughter's very highly regarded day care; they were just trying to do the right thing and spaced out a bit about the context).
If the lawn doesn't receive traffic, then it is less risky to irrigate it with grey water but it shouldn't be a lawn in the first place; the only legitimate reason to have one of these resource hogs is that they are more fun to play on than, say, a gravel and cactus garden. A better solution would be to replace the un-trafficked lawn with something else and irrigate that with grey water, if it needs irrigation at all.
Besides the health issue, grey watering a lawn is a pain in the rear. The system almost universally used is a hose from the washing machine or house plumbing which is moved around. Since the water has to be applied within the root system to benefit the plant, you have to move this hose to numerous locations in a very small grid, as compared to say, a large fruit tree, which would benefit from water left to dump anywhere within an area of hundreds of square feet.
Perforated pipe under the lawn will have efficiency in the single digits, and leave some areas completely dry.
We suggest that you replace most of your turf with something else, replace what's left with a water-conserving grass such as Tall Fescue, watered with the freshwater you save from using grey water elsewhere, or just let your lawn go dormant when there's not enough rain to sustain it.
Lawns can be irrigated well and safely through subsurface drip ($1500 on up. 300 gpd grey water generation/irrigation need is the break even point where such a system starts to make sense).