Probably the best homemade composting design is the multiple bin system where separate compartments facilitate continuous decomposition. Each bin is about four feet on a side and three to four feet tall. Usually, the dividing walls between bins are shared. Always, each bin opens completely at the front. I think the best design has removable slatted separators between a series of four (not three) wooden bins in three declining sizes: two large, one medium-large and one smaller. Alternatively, bins may be constructed of unmortared concrete blocks with removable wooden fronts. Permanently constructed bins of mortared concrete block or wood may have moisture-retentive, rain-protective hinged lids.
!br>
There are two workable composting systems that fit these structures. Most composters obtain materials too gradually to make a large heap all at once. In this case my suggestion is the four-bin system, using one large bin as a storage area for dry vegetation. Begin composting in bin two by mixing the dry contents temporarily stored in bin one with kitchen garbage, grass clippings and etc. Once bin two is filled and heating, remove its front slats and the side slats separating it from bin three and turn the pile into bin three, gradually reinserting side slats as bin three is filled. Bin three, being about two-thirds the size of bin two, will be filled to the brim. A new pile can be forming in bin two while bin three is cooking.
When bin three has settled significantly, repeat the process, turning bin three into bin four, etc. By the time the material has reheated in bin four and cooled you will have finished or close-to-finished compost At any point during this turning that resistant, unrotted material is discovered, instead of passing it on, it may be thrown back to an earlier bin to go through yet another decomposition stage. Perhaps the cleverest design of this type takes advantage of any significant slope or hill available to a lazy gardener and places a series of separate bins one above the next, eliminating any need for removable side-slats while making tossing compost down to the next container relatively easy.
A simply constructed alternative avoids making removable slats between bins or of lifting the material over the walls to toss it from bin to bin. Here, each bin is treated as a separate and discrete compost process. When it is time to turn the heap, the front is removed and the heap is turned right back into its original container. To accomplish this it may be necessary to first shovel about half of the material out of the bin onto a work area, then turn what is remaining in the bin and then cover it with what was shoveled out. Gradually the material in the bin shrinks and decomposes. When finished, the compost will fill only a small fraction of the bin抯 volume.
My clever students at the Urban Farm Class, University of Oregon have made a very inexpensive compost bin structure of this type using recycled industrial wood pallets. They are held erect by nailing them to pressure-treated fence posts sunk into the earth. The removable doors are also pallets, hooked on with bailing wire. The flimsy pallets rot in a couple of years but obtaining more free pallets is easy. If I were building a more finished three or four bin series, I would use rot-resistant wood like cedar and/or thoroughly paint the wood with a non-phytotoxic wood preservative like Cuprinol (copper napthanate). Cuprinol is not as permanent as other types of wood preservatives and may have to be reapplied every two or three years.
