Greywater Irrigation Efficiency (overview)

The most important concept of this guide is water re-use efficiency.

Diverting washing machine and bath/shower water into a few mulch basins in the garden does save water, but is only about 20% efficient. If you would like to save up to 90% of the water, this guide is for you.

  • If you put 5 gallons of water in one 4 foot diameter mulch basin every day (150 gallons per month), you have not saved 5 gallons per day
  • In Tucson, Arizona, that basin only needs 12 gallons for the whole of July for low to medium water-use plants & trees
  • So while you have irrigated 150 gallons for the month the actual amount saved is 12 gallons. The irrigation efficiency is 12/150 or 8%. The rest of the water simply drained into the subsoil, wasted
  • Instead, the 5 gallons per day could be irrigating 90 square feet of garden
  • A family of four, easily generating 100 gallons of graywater per day, could be irrigating 1,700 square feet in Tucson, or over 2,400 square feet in San Diego
Each of the common irrigation methods will be explained in the following pages of this section, however, for now, I simply present an introductory summary of efficiencies and costs.
Irrigation Method 
Buckets
Branched Drain
Laundry to Landscape
Gravity Dripperline
Pumped Dripperline
 Efficiency
50%
20%
30%
90%
90%
The efficiency rates listed for branched drain and Laundry to Landscape methods are based on typical installation parameters and can vary by +/- 10% subject to the design and installation experience.

Cost / Benefit Analysis of the different methods.

After applying the irrigation efficiencies of the methods, and determining how much potable water is no longer needed to irrigate the garden, these charts can now be presented.

If you would like the math behind the following charts, please contact us.

Cost vs. Gallons of Potable Water saved over 5 years

Assumptions:

Paid labor has been used for the installation of all methods except for bucketing of water. A small amount has been added to the bucketing methods, for 1 chiropractic visit (this isn’t a joke, it’s an ongoing issue in Australia).

While voluntary labor (you) can be used to reduce the cost, the efficiency of Branched Drain and Laundry to Landscape methods will typically halve. Those interested in installing their own Branched Drain network should read our detailed section on installing this style of system.

Cost per gallons of water saved over 5 years use

This chart illustrates how Dripperline irrigation is far more cost effective over 5 years (at about 0.5 cents per gallon) than even Laundry to Landscape at about 2.5 cents per gallon.

It all comes back to how much potable water (irrigation water out of the tap) is saved by each of the methods of diverting graywater.

Leave a Reply

Your email address will not be published. Required fields are marked *