Greywater or Rain water harvesting?

Since I have the duty (privilege?) of re plumbing the Herrera house, I can consider springing for the extra expense of separating grey water from sewage. Or I could decide to collect all the rainwater from the roofs. Probably I won't do both, since both require extra expense.

Advantages to rainwater harvesting:

• Rainwater can be legally stored in underground tanks, then you can choose where (and when) to use it. Because untreated grey water has bacteria in it, New Mexico requires you to dump it immediately on the soil.
  
• Using grey water to irrigate means that you have to be rather careful what kinds of soaps (dish, laundry, shampoo, toothpaste) go into your water. The rainwater is actually cleaner than our tap water (which in Socorro has high levels of arsenic). Our attempt to live a greener life involves compromises -- we hang our our laundry to dry, but put liquid fabric softener in the washing machine so the clothes aren't scratchy. Combining xeric plants with low watering (so contaminates build up rather than flush out) and soap residue might be a recipe for a dead garden.
  
• It might be possible in the future to treat and drink rainwater.
  
  

Which will make more water available for the garden?

From WaterCasa (in Arizona) a family of four creates 70 gallons of gray water per day. That's 26,000 gallons a year. Yikes.

The New Mexico Office of the State Engineer has a website about Roof Reliant Landscaping. In it they have a worksheet to calculate how much rainwater you can collect. Because of limited access and and half the property being downhill from the cistern site, I think I can only collect rainwater from half of my roofs -- that's 1100 square feet. Using their average rainfall in Albuquerque (not sure if I can find more specific numbers for Socorro) of 8.66 inches per year, we could harvest about 5800 gallons a year. (For cistern purposes we only calcuate needing 1200 gallon storage, probably a 2000 gallon tank. Which, they point out, is the size of a minivan.)


So we could have five times as much gray water annually.

I think that in fact we will go the gray water route, for several reasons:

• Digging the hole and installing rainwater cistern has to be the first thing we do -- before we grade the property to protect the adobe walls from rain and watering, before we can do any landscaping. It will be a little grim to sink $20,000 into the ground and have nothing but a large place for rainwater to show for it.
  
• Gray water is going to provide a lot more water for us to use in the yard.
  
• I think that plumbing for gray water is going to be much cheaper than harvesting rainwater.
  

Information on laws on grey water in New Mexico http://www.nmenv.state.nm.us/fod/LiquidWaste/graywater.html