A different approach to grey water

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I’m now four weeks into the part-time Winter Permaculture Design Certificate course being run in Sydney by Milkwood Permaculture. In addition to the course material itself, Nick and Kristen bring along a great library of books to browse through during the breaks.

One that caught my eye was Create an Oasis with Grey Water by Art Ludwig.This is perfectly timed, as we’re half way through the design process for our house extension, and grey water was definitely on the list.

“Standard” grey water systems

I’d been looking into “standard” approaches to grey water, and wasn’t very impressed.

The basic design is to connect up the washing machine to a small-ish under-floor holding tank. A pump then extracts the water and sends it out to drip feeds throughout the garden. Health department regulations prevent the water from being held for more than 24 hours, adding complexity to the system. With tanks and pumps, there needs to be filters, maintenance and careful use.

These systems also aren’t cheap. Having a casual chat to my friendly water tank folks, they quoted a figure of $3000-4000 for an installed system, including all the extra plumbing.

The biggest issue is their limited use. Standard approaches involve connecting up perhaps just one appliance to the system:

  • Washing machine: yes, assuming that suitable washing powders are used.
  • Hand basins: yes, but why bother, doesn’t produce much water.
  • Shower: NO, produces too much water, would overwhelm the drip feeds.
  • Dishwasher: NO, too much solid matter.
  • Kitchen sink: NO, too much solid matter.

So that’s $4000 for a system that diverts perhaps 200L of washing machine water into the garden once a week. Hardly seems worth the effort!

Branched drains

The grey water book listed earlier outlines a very different approach. The big advantage we have is that our back garden has a fairly substantial slope, meaning that gravity can do all the work.

This allows the installation of a “branched drain” approach, work works as follows:

  • All relevant grey water sources (washing machine, dishwasher, shower, sinks) are piped to a single location under the house.
  • The water enters into a gravity-fed system of 1-1.5″ pipes which head into the garden. (The pipes self-clean, preventing any blockages from solid matter.)
  • The water is carefully split into as many feeds as required.
  • Instead of using drip feeds (which block and only support low flows), the water is directed into mulch-filled basins.
  • These basins feed trees, which can soak up a lot of moisture.
  • The rest is absorbed into the ground, for the benefit of the rest of the garden over time.

The big benefit of this approach is its simplicity. From what I’ve read, it takes a lot of careful up-front design, but after installation it runs itself. No pumps, filters or maintenance. It will cope with large volumes of water, even when the ground is already wet.

The starting point for the design is to work out two things:

  • grey water produced by the house
  • “perk value” of the soil (how quickly water is absorbed)

I’ll work these out over the next few weeks, and will blog the results. Then I’ll start liaising with the plumber and architect to work out how to factor it into the building designs.

In the meantime, the Laundry to Landscape© Grey Water System page outlines a simple approach that can be taken by almost anyone to make use of the washing machine water with little or no effort or cost.


2 thoughts on “A different approach to grey water

    biggreencoat said:
    June 22, 2010 at 6:36 pm

    I’m sceptical about the “self-cleaning pipes”. What’s the theory there?

      James responded:
      June 29, 2010 at 5:50 pm

      You’re right to be skeptical of such a claim, but this is one of the best thought-out aspects of “branched drain” approach.

      There are three reasons why the pipes don’t clog:

      • Slope. All the pipes should have a slope of 2-8%. Less than that, water can pool. More than that, the water rushes ahead of any solids, potentially leaving them behind to clog the pipes.
      • Pipe size. Counterintuitively, smaller pipes help to stop blockages. Too small, the system can’t cope with the water flow. Too large, and the water can flow around any blockages. Just right (1.5″ diameter), and the water flushes everything through.
      • “Double-el” connections. Instead of normal T joints, this approach uses connections that look like two elbow joints connected back-to-back. Experience has shown that this helps to regulate flows, and prevents blockages.

      At least that’s the theory! 🙂

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