DIY flow-through worm bin (FTWB): how it works and how to build one

1. Frame: build an open box on legs of wood or use a large plastic drum with the bottom cut out. Leave 30–50 cm of space underneath for harvesting.
2. Floor: about 20–30 cm above the bottom, fix metal bars or rods about 5 cm apart, or strong mesh with openings of 1–2 cm.
3. Starter layer: lay a few sheets of newspaper or cardboard on the floor, then 10–15 cm of moist bedding.
4. Worms and food: add the worms, then feed thin layers on top, each time with a little fresh bedding.
5. Cover: a lid or sack keeps moisture in and light out.

Wait until the bin is well filled, usually after a few months. Then scrape along the bars or mesh from underneath with a rake or a flat bar, and the bottom layer of castings falls through. Scrape only a few centimetres each time. The worms stay up in the fresh food. Mark’s version used a chain with small scoops, driven by a drill, that pulls the castings out the front instead.

• Too dry: flow-through bins are open at the bottom and dry out faster. Check moisture often.
• Too much food at once: thick layers heat up. Feed thin layers.
• Harvesting too early brings out worms and unfinished material.
• Cold: Compost worms cope with about 5–29 °C (41–84.2 °F), and they do best at 15–25 °C (59–77 °F). In winter, insulate the sides or keep the bin in a shed.

If you have a small household, a 2-bucket bin or a 3-tier bin is easier. A flow-through bin pays off when you have lots of manure or garden waste, or several bins to harvest.