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 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 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.

Access box to outlet pipe, access from the top, with perforations all around to allow water to flow through.

Access to outlet pipe with lid on.

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, sifting and washing creek rock to put into 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 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.

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.

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.

The grey water inlet pipe with access to clear blockages

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.

Author: Permeco Inc.

Not for profit organisation and charity based in Queensland, Australia, focusing on permaculture education and environmental research. Please note that all articles and recipes written under this label on this website prior to 2025 are written and copyrighted by Willa Sevenstern (previously known as Zaia Kendall).