A flow-through worm bin is a larger container with a metal grid at the bottom. When the castings dry, a bar runs along the bottom of the bin, causing them to fall to the ground or onto a conveyor belt. These castings then fall into a catch-all pan. The bin does not have a lid, so the worms naturally move to the best food sources. A flow-through bin needs ample space; is key to a happy and healthy worm population.