Grey water compost worm grease trap

Grey water grease trap

Grey water grease trap

Slightly downhill and about 20 metres away from the sink at our student space (which is now our main eating area), our wonderful long term volunteer Dani implemented a worm farm grease trap design. 

Plastic cow trough used as grease trap
Plastic cow trough to be used as grease trap

Tom got a black plastic cow trough, around 1.2m diameter and 0.5m deep. Dani leveled the ground where the grease trap was to be situated and made a circle of bricks to sit it on. He then compacted the earth around the bricks so that toads cannot dig in under the bricks and de-stabilise the grease trap.

Compacted soil under and around grease trap trough
Compacted soil around the grease trap trough, with the trough sitting on bricks

The bottom of the container is level with the top of the swale, and is about 20cm removed from the swale. This way the outlet drainpipe can drain into the swale. The drainpipe has an access box above to check for and clear all blockages, and is perforated at the bottom so water can slowly drip into the swale.

(Perforated) access box to outlet pipe.
Access box to outlet pipe, access from the top, with perforations all around to allow water to flow through.
Access to outlet pipe for grease trap with lid.
Access to outlet pipe with lid on.
Perforated outlet pipe for the grease trap outlet.
Perforated outlet pipe to be connected to the elbow leading up into the trough. Outlet hole is inside the perforated access box.

After the container was sitting level on the pad, it was filled to about 2/3 with gravel and small pebbles, about 1/2cm – 2cm diameter. On top of this a 2cm layer of coarse river sand was laid to create a nice environment for the compost worms that were to be added. Bigger stones were placed around the access box, so that the smaller stones would not block the perforations of the access box. We have since found out that we need to fill the grease trap with more worms and mulch for them to live in, so we will bring the level of gravel down to 120mm. This will provide enough drainage, but leave more space for the worms to do their jobs.

Dani getting creek rock for the grease trap
Dani getting, sifting and washing creek rock to put into the grease trap
Creek rock in the bottom of the grease trap
Creek rock put into the bottom of the grease trap, larger rocks around the access box to prevent blocking the perforations.

The river sand was then followed by 2cm of compost under the inlet area and the remainder filled with thick mulch. The mulch filters the coarse parts and the sand filters the finer parts. The gravel provides an environment for the organisms that have to be present to digest small particles like detergents etc.

River sand laid on top of rock in the grease trap
River sand with the rock underneath in the grease trap

3 Handfuls of compost worms were added. These compost worms can handle being submerged in water for up to 20 minutes and are thus ideal for this application. The compost worms eat all the kitchen waste that comes down the sink. Only 3 handfuls were added at this time. If too many worms are added in the beginning there would not be enough food for them so they would starve. This way worms (and bacteria) will take a little time to naturally build up to optimum amounts.

Grease trap compost worms
Compost worms are added to the grease trap to digest kitchen waste

A metal plate was put directly under the inlet so that the grey water can be dispersed over a larger area, rather than just drip in one spot.

Metal plate under inlet of grease trap to disperse grey water
A metal plate is placed under the grease trap inlet to disperse the grey water

The inlet also has access to clear any blockages, there has been no need for that as yet, since shower water tends to flush out any blockages at this stage. The top is covered with a piece of reo mesh and a tarp, so the worms are kept in darkness, otherwise they would come to the surface.

Grey water inlet pipe with access
The grey water inlet pipe with access to clear blockages
A tarp covers the grey water grease trap
The covered grey water grease trap in day to day operations

The swale has reeds in it to filter the water even more, this reed bed will be further planted out to ensure an optimal filtering process.

©2013 Permaculture Research Institute Sunshine Coast – Compost worm grey water grease trap on our Permaculture Demonstration Site.


  1. Thank you, I have been designing a shower grey water system I want to implement at home and this has further inspired me and aided in my design process

  2. How do the worms cope in this system with hot water? I am assuming the grey water is still warm when it comes from showers or kitchen? Or do you have a way of cooling it down before it gets to the grease trap?

    1. Hi Nicky, the pipe that delivers the water to the grease trap is in the ground and around 20 meters long which will cool the water down. If there has been a long enough period of water flow to build up the temperature I’m sure by then the very active compost worms will move away to a cooler spot. Thanks for your comment, Tom

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