This project at our Tempe, Arizona, home is intended to capture graywater from our washing machine for the purpose of supporting several small fruit trees. (This requires special laundry detergent so as to not inundate your vegetation with sodium and other deleterious chemicals.) Additionally project design objectives include rainwater capture, if it were to ever rain here.
For us the project had the following phases:
- Design – sizing and where to put the graywater ditch and trees
- Dig – manually remove dirt currently occupying the hole
- Fill – the ditch with mulch to hold the graywater
- Plumb – the means to get the washing machine water to the ditch
- Plant – selected fruit trees (dwarf Mexican lime and Mid-Pride Peach)
- Planter-handle extra soil
Along the back of the house runs a porch with a roof overhang and a concrete floor. Beyond the porch, and parallel to it, we would place the greywater capture ditch. It would be nominally 7 feet wide with a profile as shown on the right.
It would have a maximum depth of two feet and from the max depth slope up toward the house over a three foot run, and slope up away from the house over a five foot run. The shorter run by the house would make it easier to get rainwater into the capture ditch and the gentler slope heading away, would allow a suitable hole for the trees to be planted. The capture ditch would displace 84 cubic feet of dirt – slightly more than 3 yards.
Essentially it is a trapezoid, wide base at the top, narrow base at the bottom, and sides slanted inward.
How big it should be would be determined by how much water gets placed in the ditch, how fast it percolates into the soil, and probably how fast the mulch decomposes (and must be removed). All of these were unknowns to me, but in the spirit of experimentation, this was my first guess.
The results from our first day of washing machine use – 5 loads – was that the percolation rate was slow in that water from a single wash would take 45 minutes to an hour to percolate into the ground. Still, the high water mark was at least a foot below the top of the top of the capture ditch. This is enough excess capacity that I would recommend only digging to a max depth of 18 inches. During the first substantial rain we had (one-eighth of an inch), the ditch filled to within 8 inches of the top, so 18 inch depth would have handled that. But it is probably good to have a bit of excess capacity.