Back of envelop gray-water calculations

Source

Gray-water is once-used, non-drinking-quality that is not sewage-water.

While gray-water has bacterial loading, the bacteria are primarily organisms that feed on dead organic materials (like skin-dander and proteins from sweat) and there are relatively few disease-causing bacteria.

"...relatively few..." is not "zero" and running it through sprinklers, for instance, is not wise. Nor is it wise to use it on lettuce or vegetables that are exposed to rain-splash and are typically eaten raw.

According to Art Ludwig at Oasis Design, the optimal use for gray-water is to run it to swales or mulch-filled basis where it can percolate into the ground beneath the surface of the mulch. That ground serves as a "battery" or a reservoir that fruit trees (for instance) can pull on when natural precipitation during the growing season is inadequate.

Mr. Ludwig offers several recommendations (paraphrased for compactness):

• Use at least 3/4" diameter, poly tubing. Laundry water has fibers in it. Water from sinks has food particles and water from bathing has hair, skin-dander and soap-scum. Large diameter tubing reduces clogging.
• Use GRAVITY. Make sure hose completely drains. Trapped water breeds bacteria and can burst tubing when it freezes.
• DO NOT use reservoirs or filters or depend on motors. Reservoirs jack up bacterial counts and turn gray-water to sewage water. Filters and motors fail.
• Run the ends of tubing into sumps sunk into mulch-filled basins. Sumps reduce the likelihood of roots growing into the end of the tubing and plugging it.
  
• Plant fruit or nut trees in basins. Tree fruits are held well above the soil which will distance food from potential bacteria issues
• Either move the end of the hose from basin-to-basin or install "Y" connectors and 1/4 turn valves to split flow. Do not attempt to modulate flow because partially closed valves will clog. Either all-on or all-off.
• Do not attempt more than four valves in series
  
• SHowers and laundry are the big gray water generators and offer the biggest bang-for-buck.
• In Australia, water from wash basin is directed to toilet tanks for re-use.

How much gray-water and what is its potential?

The bathtubs and washing machine generate about 30 gallons of gray-water per day, per occupant in the United States. That number varies widely. Some people run four loads of laundry a day (colors, whites, permanent press, delicates). Others run two loads a week. Some people run the shower 10 minutes before stepping into it and then spend another 30 minutes getting a hydraulic massage.

Sticking with the 30 gallon* number and an average of 0.15" of potential evaporation per day through the growing season, that amount of gray-water is sufficient to keep 6 trees/bushes with crowns of 8' diameter (each) growing PER PERSON even without natural rain.

"OK, fine for Michigan, but what about Texas?" Consider a line running from Laredo on the Rio Grande to Childress on the Red River. Nearly all of the Texas population lives east of that line. Cities along that line get about 20" (on average) of precipitation a year and have the evaporation potential of 90" a year for a deficit of 70". That shrinks the number of trees/bushes to about 3.

The numbers, 3 small fruit trees (or large fig bushes) for places like San Angelo, Tx and 6 for Eaton County, Mi double to 6 and 12 respectively if there is sufficient soil beneath the swales to absorb gray-water year-round. The water is banked during the dormant season and can be accessed by the plant's roots during the growing season.

In very-dry places, it is important to funnel natural rainfall into the swales/basins. Gray-water contains various salts that build up in the soil unless an occasional flushing happens that carries the salts out of the root-zone.

Alternatives to fruit trees in places where salt-buildup is a possibility include asparagus (10 feet of row replacing each fruit-tree) or alfalfa to feed meat rabbits. Both asparagus and alfalfa are considered salt-tolerant crops. The caveat of not eating the asparagus raw applies.

*30 gallons * 231 cubic inches per gallon/144 square inches per square-foot yields 48 "inch-feet" per day. If you need 0.15" per day, divide 48 by 0.15" and that will tell you the number of square-feet 30 gallons can replenish. If you need 0.35 like San Angelo in July, then divide by 0.35.