(Copied from the Permaculture Research Institute)
Greywater mulch-pits provide an excellent solution when re-using greywater on your garden – they are cheap to construct, they improve the quality of water entering your soil and after some time provide you with valuable compost. They’re very easy to construct too. You basically just dig a hole, wack in some 100mm ag-pipe and then fill it up with nice chunky mulch.
Where possible a number of pits should be constructed around the garden. This enables you to rotate your greywater around and prevent the inevitable waterlogging that occurs if you leave your hose in one spot too long. For flat ground it’s great to create round pits, with each one midway between a few fruit trees. If on a slope, they will be on contour and can double as a swale.
The volume of each pit should be about 4 times the peak flow that leaves your house at any one time. For example if your washing machine pumps out 100 litres, the size of the hole needs to be 400litres (as a guide, 1m3 = 1,000litres). This is to allow for the space taken up by the woody mulch (about 2/3 of the volume) plus a bit extra. 40 cm is plenty deep enough, or else you’ll start to send most of the water down below the main feeder/drinker roots of your trees.
If you have very sandy soils in which most water just disappears straight down, it can pay to line the inside of your pit with plastic. A few punctured holes here and there allow you to infiltrate the water in the direction(s) of your choice. It also gives the critters more time to clean up the water.
With the huge increase in the use of greywater on Australian gardens, particularly here in Victoria where we’ve been on restrictions for a number of years now, there is concern about the effect it will have on soils in the long term. Even if using liquid detergents, which are much lower in sodium and phosphorus than powders (see lanfaxlabs for more info), the alkaline nature of soaps will affect soil pH. Fats and oils from our bodies can also clog up soil pores and make them hydrophobic and any bleaches or harsh cleaners will of course have a huge impact on soil life.
By filling these pits with chunky mulch, this acts to filter and clean the water, resulting in better quality irrigation for your valuable fruit trees. It’s not the mulch that does the filtering but rather the tiny soil critters that will colonise its surface and just like in a reedbed system, they greedily grab onto any nutrient that passes by. Inevitably, this mulch will be broken down into compost, at which time you should say “Awesome!” and fork it out of your pit straight onto the fruit trees beside. Then, give your local tree lopper a call and get a free/very cheap load of mulch delivered and refill them. (By the way, this is so much easier than cleaning out a clogged up reedbed, plus you get the compost out of it instead of a mess of aggregate you don’t know what to do with.)
The simplest way to get water to each pit is by extending the washing machine outlet hose. You can rotate this hose once a week or so. A few tips to prevent your washing machine’s engine from burning out: 1. Utilise gravity as much as possible; 2. Over 10m+, ensure the extension hose is at least 50mm to reduce strain on the pump; and 3. Don’t pump uphill (if you do need to, you’ll have to get a pump built for this purpose).
If you include an appropriate length of 100mm ag-pipe inside each pit, with one end just slightly sticking out, this means that you can poke your washing machine hose down inside so that the water infiltrates sub-surface as regulations rightly demand (stops kids and pets getting sick from the pretty nasty pathogens that greywater can contain).
If you want to utilise your bath and shower water also, by law you’re supposed to get a plumber in to divert the water. From here, a more permanent option is to construct branched drains which evenly distribute the water around the garden. Detailed design and installation instructions are available for this method in Art Ludwig’s book The New Create an Oasis Using Greywater.