Compost factory: A keyhole garden for composting with worms

or how to make your worms survive danish cold winter outdoors

When we started participating in the urban farm of Prags Have, our first project was to design and build a worm compost bin that can protect them from winter cold.

Why a Compost Factory in an urban farming garden ?

The kitchen of the garden produced a lot of vegetable scraps that was thrown, and the participants were not aware of its value when turned into compost. However, gardening organic waste was sometimes piled in a large wooden box, but that was not appropriate for composting, nor for worms. Each year, the garden has to buy compost or manure to fertilize plantations. We decided to build the ‘compost factory’, with 100% recycled materials, to encouraged people to compost more and recycle using worms to produce one of the richest natural solid and liquid fertilizers.

Compost Factory design

Composting with worms is the more efficient method to recycle kitchen waste, but how would the worms survive outdoors during the cold days of the year below freezing and under the snow ? For its design, we found inspiration in the African keyhole gardens. These gardens are used originally to conserve humidity and temperature when there are large differences between winter and summer. Keyhole shaped, a compost bin in the center and the vegetables growing around can easily be accessed. The stone construction help regulate humidity and temperature, essentials for compost worms.

Compost Materials
  • Compost Worms Eisenia foetida or Red Wigglers : price 389DKK (sometimes found at recycle center in Copenhagen)
  • Vegetable scraps, organic waste, straw, woodchips, paper and cardboard

Construction Materials

  • Stones, Gravel, found in the site
  • Metallic trash bin
  • Tarpaulin (waterproof plastic sheet) and geotextile
  • Plastic tube and pot
  • Soil, poor in nutriments

How we built it

  • 1 – We flattened and compacted the soil, and we marked two circles, 22,5cm for the bin (1), and 90cm for the whole construction (2).
  • 2 – We constructed the wall around 50cm height in a keyhole shape.
  • 3 – We ajusted the level of the ground in order to allow a one way runoff of the water.
  • 4 – We posed the tarpaulin (4) and tested the runoff. We conected the tube at the lowest point and opened a hole to the tapaulin.
Finished construction, before filling with soil Step 3 - 6. Plastic at the bottom to collect worm tea
Step 2: Keyhole wall Step 4: Plastic at the bottom to collect compost tea
  • 5 – We tested the runoff of the water towards the exterior of the keyhole in a pot (see blue arrows in figure).
  • 6 – We filled 2-3cm with gravel and flattened (3).
  • 7 – We posed the geotextile and filled with the soil. We added often some gravel in the periphery to fill the holes of the wall.
  • 8 – We installed the worms over a bed of paper and leaves.
  • 9 – We watered the whole bed and bin untill water come out to the bucket
  • 10 – Done regularly: add vegetable scraps, cardboard, paper and plant the keyhole garden!
Step 7: Filling the walls with soil. Yellow bucket is there to collect worm tea Step 2: Keyhole wall
Step 7: Filling the walls with soil. Yellow bucket is there to collect compost tea Finished construction

The test of time

IMG_0390.JPGIn september, we constructed the compost factory and installed the worms. From january to march, the weather was really , with -8°C outside temperature and two weeks of comtinuous snow. The surface layers of compost and soil were frozen (thikness of ice around 10cm) but temperature was positive (around 1°C) deeper inside the compost bin. In early March, we lost all hope, but surprise, we found many adult worms in the compost bin, deep inside, that have survived the danish winter !

Participants : Lina, Philippo, Steen, Sofien

Reflection of Compost Factory design

Although, we reached our goal, our design of Compost Factory has a limitation, that mature compost is very hard to get, as it goes deep in the bin. A small door at the bottom would have been very practical for compost harvesting. The straw, we added by chance, finally played a important role in insulating the top of the compost bin. Before frost in cold weather, the compost should be covered with a thick (30-50cm) of straw or woodchips.


Inspiration videos

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