Branched Drain Grey Water Diversion

After deciding I wanted to keep my grey water onsite but finding the commercial products a little on the expensive side I went looking for cheaper alternatives.

My first attempt was a 200L drum outside my bathroom connected to the waste outlet from my bath, lined with some rudimentary filters and draining to the garden via 19mm poly pipe. This was a failed experiment with a positive outcome. I quickly discovered where grey water gets it’s name from – when stored for any length of time this is the colour it quickly turns.

My filters clogged up as did the relatively small diameter outlet pipe and the all of the grey water made its way to the sewer via the overflows. The commercial solutions use higher quality filters and most often pumps to ensure the grey water is not stored more than 24 hours.

From this I found Oasis Designs branched drain approach. A very simple solution using larger diameter (40-50mm) pipes with no storage. It relies on having a sufficient fall available but is elegant in it’s simplicity. No storage and no filters means very little maintenance. The large diameter pipes mean much less opportunity for contaminants to cause blockages.

Whilst it’s ok to use grey water on areas you grow food it should never come in contact with it. Irrigating fruit trees is probably the simplest and safest use.

Dealing with an established garden integrating a branched drain design wasn’t going to be so easy and I already had other food plants close to my fruit trees. As much as I wanted to re-use the water on-site I mostly wanted to avoid putting it in the sewer. My solution was to connect the bath waste via 2 inch poly pipe directly to a single large mulch pit.

This has worked happily for 3 years now with zero maintenance. No blockages, no cleaning. The mulch pit is topped up periodically with leaves and the soil happily absorbs all the water.

Given the opportunity I would make more use of the water though. Some thoughts on setting up a branched drain system for your grey water in Australia:


  • Start by reading the excellent Grey Water book by Oasis Designs.
  • Purchase the required number of flow splitters from them. They’re a modified double ell fitting allowing for inspection and access. I can’t locate these from Australian vendors.
    • They’re sold in 1.5″ and 2″ variants.
  • The fitting is ABS which is available but not commonly used in Australia.
  • For the pipes, you may be able to find ABS pipe from specialist plumbing suppliers or alternately use PVC-U pressure pipe.
    • ABS fittings have internal diameters of 48.514mm and 60.579mm for 1.5 inch and 2 inch respectively.
    • PVC-U Pressure pipe use the same outside diameter in 40 and 50mm nominal bore pipes which are readily available locally.

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