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Getting the most out of gray (grey) water and rainwater harvesting

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Understanding Irrigation Efficiency

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Introduction:

It is tempting to believe that whichever way you have decided to reuse rain or graywater, that you are saving at least as much water as you are placing in the garden.

Over the years, several methods of reusing graywater, based on mulch basins, have become popular. The sad fact is that simply placing graywater into a mulch basin surrounding a tree (a mulch basin is basically a big hole around the tree covered in mulch) isnt very efficient at all.

If you put 10 gallons of water into this mulch basin, it will have the same effect as using 5 gallons of tap water into the hole at a more measured pace. Yes you have saved 5 gallons of tap water (because you ddn’t need to supplement with tap water), but you could have saved 15 gallons of tap water by using the 10 gallons of graywater to greatest effect.

The following discussion is focussed on graywater reuse, however the same principles apply for rainwater reuse.

Detail:

These are the commonly accepted methods of reusing untreated graywater:

  • Buckets
  • Branched Drain (or similar)
  • Laundry to Landscape
  • Gravity Dripperline
  • Pumped Dripperline

Before going in to the pro’s and con’s of each method, the concept of how much water is actually saved needs to be understood.

If you put 10 gallons of water in one 4′ hole in the ground every day, you have not saved 10 gallons per day. In Tucson, Arizona, that hole only needs 12 gallons for the whole of July for medium water use plants.

If this was done daily, then 300 gallons has been irrigated over the month, instead of the required 12 gallons.

The actual amount saved is 12 gallons. The irrigation efficiency is 12/300 or 4%.

This may sound extreme, but I have seen quite a few branch drain systems set up this way (the rest of the water goes down into the subsoil and is wasted, unless trapped by clay in which case the roots of the plants may rot).

So while on the surface some methods may appear to be very inexpensive for the amount of water diverted, the amount of water actually saved needs to be reviewed.

Irrigation Efficiency Rates for Different Methods:

Irrigation Method Efficiency
Buckets 50%
Branched Drain 30%
Laundry to Landscape 40%
Gravity Dripperline 90%
Pumped Dripperline 90%

The efficiency rates listed for branched drain and Laundry to Landscape methods are extremely generous, based on absolute best practice installation methods by highly experienced installers.

Cost / Benefit Analysis of the different methods.

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

If you would like the maths behind the following charts, please contact us via the contacts page for more information.

cost_v_saving_water_reuse.jpg

Cost vs Gallons of Potable Water saved over 5 years

Assumptions:

Paid labour has been used for installation of all methods except for bucketing of water. A small amount has been added to the bucketing methods, for 1 chiropractic visit (this isnt a joke, it’s an onging 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 go to our videos page and review the branched drain installation video (8 minutes)

cents_per_gallon_water_reuse.jpg

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.

For more information about the pro’s and con’s of different graywater re-use methods, go to part 2 of this article http://www.besthomewatersavers.com/pages/Different-ways-to-reuse-graywater-%28part-2%29.html