I strongly recommend against using gray water directly on edible crops, particularly on ones that will be eaten raw or slightly cooked. It's fine, however, for corn, grains, fruit trees, and lawns.
I need help in locating manufacturers of what I refer to as “gray-water purifiers,” devices that process gray water (some even process black water, I understand) to a drinkable condition, thus enabling the water to be used over and over again.
As one who's concerned with the possibility of polluted groundwater (in addition to the high cost of drilling a well), the gray-water purifying system seems like a logical alternative. The possibility of combining this system with a composting toilet is also an appealing option.
I'd appreciate any information you have concerning this topic.
Recycling Water Using Gray-Water Purifiers
I know of no such equipment on the commercial market, and if it were available for purchase, I'd recommend against it in the strongest possible terms when talking about recycling water. Why? Let's start with black water, or any household water that includes toilet waste. First, human feces always contain “ordinary” pathogenic bacteria and viruses that cause gastrointestinal distress, and, depending on the circumstances, they may contain life-threatening bacteria and viruses, such as those responsible for typhoid, diphtheria, polio, or worse. Second, we humans have a habit of flushing down the toilet various materials and fluids that contain toxic substances, such as toxic heavy metals present in photographic developing solutions and toxic organic compounds in cleaning formulas, pharmaceuticals and pesticides. Even if you don't dispose of materials in this way, a visitor or friend may do so.
In any event, using ordinary methods and care, no one can safely and routinely disinfect and detoxify black water for consumption. The theoretical methods that come to mind as possibilities in extreme situations involve:
 Large amounts of energy to distill the water multiple times
 Large amounts of chemicals, especially chlorine
 Large amounts of land for ponds or soil filtration
 A laboratory for running chemical and bacteriological tests every day.
Now, gray water — or any household water that doesn't include toilet wastes — contains all the same pathogenic and toxic components as black water, only in lower concentrations. For example, both bath and laundry water can contain pathogens washed from a body or clothing.
Having disappointed you, let me close with two thoughts about recycling water. First, with great care, you may be able to use gray water — but never black water — on your lawn or garden. As I pointed out in an earlier Ask Our Experts, I strongly recommend against using gray water directly on edible crops, particularly on ones that will be eaten raw or slightly cooked. It's fine, however, for corn, grains, fruit trees, and lawns. Second, never try to store gray water inside or outside your home for even an hour; use it promptly in the summer and empty it into your septic tank or sewer in the winter.
— David Burmaster, consultant on surface- and groundwater quality and hazardous-waste management