Posted by & filed under Biological Cleaning, Compost, Soil Conservation, Storm Water, Swales, Waste Water, Water Conservation, Water Contaminaton & Loss, Water Harvesting.

by Cam Wilson, Forest Edge Permaculture

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.

You can irrigate a 1/8 acre suburban orchard for under $200, which is pretty good value I reckon compared to the $10,000-$20,000 approved treatment systems.

9 Responses to “Greywater Mulch Pits”

  1. Don

    I’ve installed this idea outside my washing machine and the water is working it’s way down through the garden very nicely.

  2. Mark .R

    Hey Cam this article is very good, Thanks. I am always fascinated by your way of thinking, thanks for the inspiration.

  3. Jo Hoy

    Hi, I am building a simpliarish design but it is to collect Nursery water that will contain organic fertilisers (fish/seaweed etc). In summary my plan is:
    The system has three catchments, First a deep stainless sink with Blue metal, Lomandra Hystrics, this is small to easily clean and its main function is to catch solid sedements such as soil etc. The overflow is collect into an ag pipe that is layed at the bottom of a large cattle trough. On top of this is two bails of Barley straw, 10x potted reeds. A screened hose connected a 1/3 down is covered with soil to prevent light, travels down to an underground 500ltre tank. From here I solar pump up to a 1000ltre header tank. From the header tank I have twin hose one turned on all the time with recycled soaker hose running out to all the beds including raised. The extra hose is turned on in wet times to run out the water, as living in Mullumbimby we can experience 3months of continual rain.
    What do you think?

  4. phantom

    SOunds great yeah,, like you concept and use of sinks,, have you been designing it for long?,, how is it travelling from completion,, would like to know how it runs for you, if good?

    Good stuff keep it up..

    My pit system is collecting lots of water this time of year,, at the moment I have no slotted agi pipe at the bottom of the pits,, I wanted to observe to see how much water I am holding and storing in different sections of the pits.. Its off contour a bit at present the pits, some flat some hilly bits slightly,, am deepening the pits in a few week with a blitz team,will keep you updated.

  5. Stephen Couling

    Cam – when you come here to Milkwood you might like to take a look at the system that was installed here behind the Woolshed – it looks like it was modelled on your scheme – built by Floyd and WOOFERs after Xmas this year. Would like to discuss with you how it is operating.

  6. Mark

    Hi Cam,
    We have just purchased a property with a concrete tank located below ground next to the house which accepts water from washing machine, showers, basins & kitchen sink. I am looking to utilise this water on the grass & nut trees located approx. 60m away. We currently utilise a simple water transfer pump & above ground hose to water grass. This can be quite time consuming & when we have a full house have to do roughly twice a week. As I work away this is left up to my partner to sort out which as you can imagine is not thrilled about. I have read your article & would think this is best way forward. I have just have a few questions,
    1) Can the mulch pits be covered with soil so as to blend in which existing area?
    2) What is the best mulch to use?


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