The methods for producing earthworm castings vary, depending on the producer. Here are a few different strategies that are used.
Composting Bins...You can call them "Garbage Worms"
One of the more common methods of producing worm castings is with a composting bin. With this strategy, composting materials are placed in a bin. The type of bin can vary, depending on the space available and level of investment. Often, large barrels are used, while others use wooden or metal rectangular bins.
Composting materials such as table scraps, coffee grounds, leaves, grass clippings, garden waste, mulch, and other items are placed into the bin. From there, composting worms such as red wigglers are also placed into the mixture within the bin.
After a certain amount of time, the mixture is then removed from the bins and used as compost. In some cases, the worms are filtered out using screens and only the compost remains. Depending on the situation, there could be a lot of inconsistency in the final product because of the variation of the items put in initially.
That means that you might have some materials that are fully broken down and some that aren't. If the compost isn't thoroughly screened, you may also find some rocks, hard clumps of dirt, and other things that you don't really want in the final product.