In a flow-through worm bin food is added to the top of the farm and castings are removed from the bottom. The worms process the food at the top of the bin and turn it into castings. As the completed castings are removed from the bottom the casting/food mixture on top moves down to fill the space.
The height of the worm bin is important. It should be deep enough to allow the worms to process all the food into castings and not so deep that the castings at the bottom become compacted and hard to remove. A distance of one to 2 metres is usually used. An obvious sign that the height of the farm is not adequate is that it still contains worms and cocoons.
Most flow through worm bins have a grate at the bottom of the farm. As worm castings have a clay like texture they do not easily fall through the grate and have to be raked out.
Large commercial flow-through worm bins have an automatic harvesting system underneath. In these systems the grate is moved hydraulically or mechanically forcing the castings out of the bin.
A flow-through worm bin is a lot more sophisticated than other designs and can be harder to run. For this reason we recommend that flow-through systems be left to users who have experience in running worm farms or for commercial users who will benefit from a more efficient system. Beginners in worm composting should stick to either a simple worm bin, horizontal worm bin or vertical worm bin system.