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I wanted to experiment with a flow through worm bin but had to keep it small. Portability was also a requirement. I ended up using two 20 litre plastic pails nested together. Both pails were in less than pristine condition. One had been my rain collection bucket which had froze and split down one side while the other was missing the bottom. To construct the bin the bottom of the bucket with the split side was cut off 10 cm above the bottom and below the split. The upper section was then telescoped over the bottom by cutting several additional 10 cm slits up from the bottom edge. This resulted in a 30cm tall bucket with a lip inside 10 cm above the bottom to support the second pail which nests inside. The wire handle was removed, straightened and cut in half with the two resulting pieces inserted across the base at the level of the lip for extra support. The base is bungeed to a plant caddy for portability. The inner pail contains the bedding and worms. A 2 cm strip was trimmed off this pail to even up the bottom edge. This strip was bonded to the inside of the bottom edge to reinforce where the grate would attach to the side wall. The grate was constructed of wire from clothes hangers cut to length. I used a spacing of 2 cm and a center cross brace with the wires inserted through holes drilled in the double wall of the bottom edge. The ends are bent at 90° to hold the wires in place and are hidden safely out of sight when this pail is nested into the base pail. I believe the design of the grate is probably overkill for the weights involved. An old round aluminum tray serves as a lid and is shimmed up about 5 mm from the lip of the pail to provide some ventilation.
Setup & Operation The new flow through bin was started by placing several sheets of newspaper over the grate and adding 4.967 kg of wet vermicompost just harvested from my Rubber Maid (RM) bin. This material had been not been fed for eight weeks but would have still contained worms. A couple of weeks later two week old newspaper bedding, along with any worms that had migrated into it, was added from the 'idle' side of the RM worm bin. There were only a few wandering worms in the first week which were collected from the base and returned to the top. Approximately 100 additional worms were transferred from the RM bin in small batches over the next several months. The bin was first fed after the bedding was added in early May 2010. After this it was fed weekly except if the bin was full or I was away. Shredded newspaper was usually added to cover each feeding. Initially I had some problems with seepage accumulating in the base and also leaking between it's double wall onto the floor. To capture the liquid I added shredded newspaper in the base and a plastic sheet in the plant caddy. To ventilate the base two 4.5cm holes were drilled in the double wall just below the lip. A small solar powered computer cooling fan was used to circulate air through the base. I have also kept the liquid content in the material fed to a minimum and the seepage has all but stopped. To feed the bin the inner pail is removed and set on the lid. Any loose newspaper bedding is pulled aside, the waste is spread evenly on the surface and the bedding replaced. Any compost that has fallen through the grate, along with the old shredded newspaper from the base, is put into the inner pail, spread evenly and misted with a spray bottle as required. New dry shredded newspaper is added to the base and the bin reassembled.
I first harvested compost after three months by using a garden hand fork through the grate to remove about 10 cm from the bottom of the pail. All the remaining compost settled down onto the grate after a few days and then little more dropped through the grate. After the second harvest five months later, the remaining compost did not settle onto the grate and material continued to drop through. This may be due to the reduced moisture levels I have been maintaining to limit seepage. Having very little compost drop through the grate between harvesting is fine for my purposes as I prefer to process the vermicompost in fewer larger batches rather than small volumes weekly. I ended up having to stuff shredded newspaper up through the grate to stop the compost from continually dropping through. Any material and worms that fell through were just put back into the top of the bin. In total the FT bin had been fed a total of 16.5 kg in 30 feedings over eight months. I believe harvesting just twice a year will be possible while maintaining the same level of input of waste (approx. 28 kg/year) and without requiring any changes to the bin.
Harvested another 3.7 kg (wet weight) of very dense moist vermicompost from the bottom of the bin after another six months. Didn't notice any worms in this batch. While using the garden hand fork through the grate to remove the material I managed to break one of the small sections of the wire making up the grate. It was a short section close to the side of the pail and the break shouldn't cause any problems. I'm hopeful that this doesn't mean that the clothes hanger wire I used will rust out quickly requiring replacement after only a couple of years. Afterwards I stuffed a couple of handfuls of dry shredded newspaper up through the grate to capture any material that wants to drop down on it's own. There has not been any seepage of liquids over the last six months since the previous harvest.
Harvested 4.8 kg (wet weight) of very dense vermicompost from the bottom of the bin. A screw driver was used to remove material from 12 cm above the grate. This material was not as moist as previous harvests and contained many small skinny worms. The bin had been fed 27 times in 33 weeks with four feedings missed in the last two months. The missed feedings may partially explain the small worms. Very little additional water had been added in this cycle which resulted in the material being drier. The clothes hanger wire used in the grate is rusting and will probably have to be replaced in the next year. I was able to insert a circle of cardboard above the grate after harvesting to prevent any material from falling through the grate. There was a bad infestation of fungus gnats over a period of several months. The bin was moved outside when the weather allowed and I tried several methods to combat the infestation indoors. Sticky traps helped but keeping the bin drier by removing the lid and keeping a layer of dry shredded paper over top of the working layer eventually worked. I still use these measures to prevent a reoccurrence. The bin also had a brief growth of slime mold (link). While visually dramatic they're harmless and didn't last long. I think this came in on some fallen autumn leaves I had mixed with my shredded paper bedding.The bin is suffering another infestation of fungus gnats. This time the bin has been inclosed in a netting bag made from curtain sheers and a sticky trap has been placed inside the netting. So far it seems to be doing the job. Scroll down to the bottom of this page from photos.
Wet weights of food and vermicompost. * Assumes weight of VC initially transferred from RM bin equals unrecorded weight of first harvest and cancel each other out. The weight of shredded newspaper bedding also not factored in. 
Photos
The blue elastic band secures the netting around the rim of the bin and a twist tie or clamp closes the netting at the top.
The holder stops bedding or netting from becoming stuck on the trap.
vermicomposters.com - DIY Flow Through bins: a collection of links I'm finding all this worm stuff very interesting. If you have any questions or comments please e-mail me.
Worms are more powerful than the African elephant
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