The article Vermicomposting gives an overview of the vermicomposting process and a few of its important fundamentals.
There are three main methods of vermicomposting — container, flow-through, and windrow. Vermicomposting is normally started with one pound of worms per sq ft regardless of the method. Because of the swift and bountiful reproductive cycle of composting type earthworms, there is usually not a reason to start out with more than just a few pounds of worms, even if you are starting a worm farm.
If you are starting out with 5 or 10 lbs of worms, you might as well just use a container in the beginning. There will not really be much of a labor saving advantage to using other methods at that point. If you plan on letting the herd grow, it won’t take long though before flow-through worm bins or windrows become handy. Worm farms usually use flow-through worm bins or windrows or both.
Use a container 12-24 inches deep, start it half full of bedding, and feed it until it fills up. Most of the worms will be near the top. Take at least 9" of material off the top and call the rest vermicompost. The worm population will have doubled by then, so you could put the worms back in two containers the same size or one with twice the area.
When I started out with 5 lbs of worms, I put them in a 2' x 3' wooden box 1' deep. I put 6" of bedding in the box, and dumped the worms on top. It took about 2 months to fill up. Then I used a fork to remove the top 9", putting it in two boxes. Because the material crumbled apart and fluffed up a little, those 2 boxes started half full. Two months later, I had 4 boxes, then 8 boxes. Then I built strange-looking double-tiered bunk beds. About 16 months after dumping five pounds of worms in a small box, I had 800 lbs of worms in 400 sq ft of 1' deep worm beds packed into a 600 sq ft outbuilding.
At that point, I moved the worms out of that nice insulated outbuilding into an 8000 sq ft barn without doors. I eventually weatherized the barn, but insanely kept using the container method of digging worms off the top, moving them, and then digging out the vermicompost. I am only telling you all this to show what can be done if someone is crazy enough.
I increased the depth of the concrete feeder boxes in the barn to 2' by setting a row of concrete blocks on the top of them. I made another bed by stacking concrete blocks 3 courses high a few feet from one of the barn’s concrete stem walls. There was no need to cement them together. I made another bed along the opposite stem wall by bracing 2' strips of plywood in a vertical position a few feet from the wall. And I made another bed down the center of the barn near the feeder box by bracing parallel rows of 2' plywood strips in a vertical position.
I was able to feed all the beds with a (Bobcat) skid-steer loader. The hard part came when the beds filled up. I had 2300 sq ft of beds, and kept 300 sq ft of that space empty. I started by forking material off the top by hand into the Bobcat bucket, and dumping it into the empty space. After dumping a couple buckets, I started digging vermicompost out of the bottom, and moving it to another barn to partially dry. When switching between moving worms and moving vermicompost, I wiped out the inside of the bucket to keep from contaminating the finished product. After dumping vermicompost, a huge amount of that extremely sticky material would stick in the inside corners of the bucket and have to be removed with a shovel.
If a guy (or gal) really wanted to, he (or she) could obtain more labor, and scale the container method to any size operation. It works, but it’s a lot of work.
Flow-through worm bins are generally thought of as worm bins in which a mechanical operation causes worm castings to come out the bottom. Aliases for these machines include continuous flow worm bins, continuous flow digesters, continuous flow reactors, flow through reactors, vermicomposting reactors, and vermicomposting digesters. The vermicomposting area is normally 2-3' deep with a screened bottom. The usual mechanism for removing vermicompost is a flat bar pulled the length of the bin by a winch, either sliding on top of the screen or slightly above it. This action breaks up a layer of vermicompost, causing it to fall through the screen. The remaining material settles as one large mass, and bridges on the screen. Screen openings (in inches) are usually either slightly smaller than 2 x 2 or slightly smaller than 2 x 4.
Sometimes parallel pipes are used instead of a screen. These worm bins can also use a moving bar to remove vermicompost. Or they can rely on the use of a rake (usually a potato rake) to scratch vermicompost out from between the pipes. Sometimes wires or even rope are used instead of pipes. Some people have made small flow-though worm bins out of 55-gallon poly drums by drilling holes about 2' from the top, running pipes through, and cutting an access hole from just below the pipes to the bottom of the drum.
At one point, I considered using pipes instead of a screen to make a less expensive worm bin. I would have had pipe caps screwed onto the pipes to seal them and hold them in place. The guy at the store said I would have to show identification and sign for them. I imagine it really throws up a huge red flag when someone wants to buy a large quantity of pipe caps and nipples (short threaded pipes) at a ratio of 2:1. “You’re building a worm what?”
Flow-through worm bins have incredible aeration which helps produce vermicompost with a high percentage of worm castings. Because less than an inch of the very oldest material comes out at a time, there is no guesswork about when the vermicompost is finished. Contamination of vermicompost with feedstocks also becomes less of a concern.
Because of the lack of standard terminology, some people consider any type of container system which does not require digging to be a flow-through system. The Worm Factory and Can-O-Worms use perforated stackable trays in which the bottom tray is emptied and moved to the top whenever the top tray fills up. Those systems can work well for someone who needs to process just a small amount of kitchen scraps. And then there is the Worm Swag, a funnel-shaped bag which is untied and retied at the bottom when you want to let some product out.
There are several variations of windrow vermicomposting. One method involves spreading out a layer of worms and bedding on the floor or ground to start. It can be any combination of dimensions. But to help visualize an example, let’s say 6' wide, 100' long, and 8" thick. That row is fed until it reaches the triangular shape of a windrow in which it cannot be fed anymore without the food staying on top. At that point, it will be 6' wide, 100' long, and about 3' tall. Now the material on top is removed to start a new row, and the material on the bottom is vermicompost. The new row can be started in a new location. Or it can be moved longitudinally by about 20' by dumping the worm inhabited material past the end of the row, digging out some vermicompost, and then shuffling some more worms over.
One of the problems with this method is that it requires some digging by hand. If the windrow is wide enough, it might be possible to drive into the side of it with a loader to remove some material off the top. But the rest of it will have to be forked off by hand because the loader will merely push it over and off the opposite edge of the row. No matter how careful you are, material will roll down the sides when digging off the top, and it takes some care to make sure that material does not end up getting mixed in with the finished product. And just as in the example above from my experience with the container method; if you are moving the windrow longitudinally, you should probably wipe the bucket out each time you switch from moving the worm material to moving the finished product. Some regulatory people are fanatical about elimination of E. Coli in vermicompost.
An improvement on the above example is the wedge method. A row is created in the inside corner where a concrete floor and concrete wall come together. As an example, visualize a row 3' high against the wall and 3' wide on the floor. It has a width of zero at the top and 3' at the bottom. After feeding along the side of it for a while, keeping it 3' high, it might have a width of 2' at the top and 5' at the bottom. With an empty space at the end of the row, material on the outside can be shuffled down, and then the original material (which has turned to vermicompost) removed. There is no shoveling involved. But measures should still be used to prevent cross contamination.
One variation of the wedge method uses a migrating windrow. The row can be started as a windrow. Or it can be started as a layer as in the first example, and then grown into a windrow. As in the last example, a loader is used to feed one side of the row, keeping it the same height and length, but making it wider. After a while, a loader is used to remove vermicompost from the side opposite the one being fed. And then after another while, vermicompost is removed from that same side again. The row migrates laterally as it is being fed along one side and harvested along the other. At any time, the feeding and harvesting sides can be switched to change the migration direction.
Windrows work great in places where temperatures are just right most of the time. If the windrows are outside, they should ideally be under a shade structure on a slightly elevated concrete slab. If there is no shade structure, or if the shade structure does not shed rain, then compost covers need to be ready for use during heavy or frequent rain. Compost covers are heavy and awkward, usually requiring two or more people to handle. If there is no concrete, there still needs to be some type of impervious surface such as asphalt or certain types of clay. And in all cases, a plan has to be in place to deal with runoff, whether there is runoff or not.
The Dirt Maker Owner's Manuals go into more detail about the operation of flow-through worm bins. They also discuss considerations which are important to any vermicomposting operation; preparation of bedding, feeding, watering, and temperature control.