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.
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.
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.
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.
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.
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.
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.
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 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.