Worm composting is normally carried out in some kind of bin be it plastic or wood. There are many different makes and types on the market, some are better than others, some do not work at all. Most of the older designs use an enclosed bin, that is, solid bottom and solid sides, usually with a tap on the bottom to drain off excess liquid.
For a worm composting system to work effectively, the worms must have a good supply of air, from bottom to top, they need surface area not depth. Plastic bins generate a lot more liquid, mostly through condensation, than do wooden ones, this liquid collects in the base and unless it is drained off on a regular basis, it will become “anaerobic” i.e. very smelly and give off gases that are not good for the worm population. This liquid is often promoted as being a beneficial product and should be used as a liquid plant feed, it is true that as the liquid passes through the worm bin it will absorb and carry some of the nutrients with it, equally it may well absorb substances and organisms that have not been “processed” by the worms and these may well be harmful to your plants, for this reason Worms Direct U.K. would not recommend the use of this liquid as a plant feed. Proper and safe feeds can be made from the finished vermicompost, these are known as “compost teas” and will be dealt with later. The biggest drawback is once the bin is full, you have to remove the top layers, hopefully with the worm population, upend the bin to remove the compost and start all over again, a messy job at the very least.
More modern worm bin designs are based on stackable trays or flow through systems. The stackable trays consist of a base support onto this is placed the first tray, each tray has a perforated base, the worms are started off in the first tray and fed as you would in any other system, when the tray is full, an empty tray is placed on top and feeding continues, the worms will migrate from the lower tray into the next where there is fresh feed, this continues until the last tray is used, the one now at the bottom is removed, the finished compost emptied and then becomes the next one to use on the top. In this system it must be remembered that worms will not migrate up through a space so it is essential that the base of the top tray remains in contact with the surface of the food layer in the tray below.
Flow through designs consist of one bin but this has a mesh bottom, usually with some sort of mechanism to agitate the casts from below which fall into a collecting tray. The worms are started off in a bedding material, feeding to the surface continues until the bin is almost full, at this point the mechanism in the base is operated, casts are removed, the whole lot settles down and feeding continues. Usually made of wood, therefore less condensation, being ventilated at the bottom allows air to move upwards through the system, no lifting of heavy trays and handling of worms is kept to a minimum.
We have tried many systems and in our opinion, the flow through is generally the best.