How much graywater is needed for irrigation?
The water demand of your landscape/garden will vary with temperature, rainfall, soil type, and soil drainage. The water demand is also dependent on the type of soil and how it drains. A general rule of thumb during peak irrigation season in Marin is ½ gallon per week for each square foot of footprint. (The footprint is the diameter of the branches and leaves at its widest point). When calculating, consider microclimates, sun, wind, type of vegetation, season, etc. The San Francisco Graywater Design Manual may be helpful in calculating the water demand.
How much graywater does your household generate?
Consider supply and demand when sizing the graywater system for your home. The San Francisco Graywater Design Manual has detailed instructions and calculation worksheets to use when calculating the volume of graywater generated in your home.
Remember to consider periods when graywater will not be generated or needed such as vacations, rainy seasons, and cold weather when plants need less water. Also, future volume may change if the number of occupants or water-use habits in the house change. Consider when you produce atypical amounts of graywater (e.g., if you sometimes do five loads of laundry in one day rather than spread them out over the week) and consider this when you design and operate your system.
The following are basic calculations for a Laundry to Landscape system:
- Washing machines (weekly flow):
___ gallons/load (your machine’s rating) X ___ loads per week = ___ gallons per week.
- Washing machines (daily flow):
___ gallons/load (your machine’s rating) X ___ loads per typical laundry day =
___ gallons per typical laundry day.
___ gallons per minute (your showerhead flow rate ) X ___ minutes you shower X
___ showers per day X ___ actual number of home occupants = ___ gallons per day.