Twin digesters offer a resting period each side before humus is removed. One digester is active while the other rests. This means when the contents are dug out from one side, these will be fully decomposed.
The inlet is rotated after several years to allow the humus in each side to age and become pathogen free. Helminth (e.g. roundworm) eggs are completely destroyed in two to three years.
Domestic vermidigester with twin vaults. The cladding material is fibre cement board
Twin primary vermidigesters should be constructed so the inlet can be rotated between them. The contents of one side can be rested while the other side is being used.
Construction of twin digesters is easily achieved by constructing a shallow concrete sump (to the height of the false floor) that fits two large fruit crates (the type designed for handling with forklifts). The digester walls are fixed directly to the crates with an air cavity to allow circulation of air around the crate walls and underneath the false floor. Ventilation into the digester is required but this should restrict entry of cockroaches and flies into the digester. Shadecloth, windbreak or similar porous textile cloth is fixed on the inside of the crate and 10-20cm of media is placed in the bottom of the crate. The media could be pine bark, because this decomposes much more slowly than wood chips, coarse sawdust or wood shavings.
Large fruit bins are ideal for constructing vermidigesters. Note the ventilated walls and "false floor", which allows ventilation underneath the vault.
This vermidigester is made from two fruit crates, with a frame and cladding attached to the outside of the crates. A ventilation cavity is provided so that fresh air flows around the walls and underneath the crate.
Sizing of primary vermidigesters
To be safe for application to food crops the contents should be rested for three years so that any parasites present in the humus die off. Capacity of twin vaults must therefore be sufficient so that the other side does not fill with solids before that time.
Worms digest the solids and significantly reduce their volume. This "wet" composting process is not like traditional composting because there is insufficient air inside the heap for it to undergo aerobic decomposition. However, the worms introduce air into the heap as they burrow from underneath. Because earthworms consume the accumulated solids from underneath, a wider and shallower heap provides a much larger decomposition zone that the worms can digest.
A primary vermidigester must be built as wide as possible to allow the heap to spread unconstrained. The digester on the left has a heap with a large surface area in contact with the substrate. Although the deep digester on the right has a similar volume, the surface area in contact with the substrate is much smaller and therefore decomposition will be slower and the digester will fill much faster. Worms work the heap from underneath and decomposition occurs in the zone that is in contact with the substrate.
Calculating floor area
For the average household, twin fruit crates or twin pallets (each with a surface area of 1m2) are sufficient. A minimum surface area of 1 square metre is required for each digester.
For a large family or small community, eight plastic pallets (two wide and four long) will provide squares of four pallets for each side of the twin digester. That is approximately 4m2 of surface area on each side, which is sufficient for 10 or more users.
Plastic fruit crate
Plastic fruit crates provide a durable "false floor" between the media substrate and the sump, where liquid drains away. These fruit crates also provide a structure to which walls and roof are attached to.
Plastic pallets provide a durable "false floor" between the media substrate and the digester sump, where liquid drains away.
Twin digesters can share one sump and one enclosure. It is good practice to provide a means for worms to migrate from one digester to the other.
Toilet and other wastewater influent must be generated from a site more elevated than the entry into the digester.
Over time humus does slowly build up. Designing for surplus capacity is a good strategy to extend the time period between rotations.
The worms will die if conditions become too hot or too cold. In cold climates vermidigesters should be insulated to prevent freezing, and in hot climates installed in the shade.
Air must circulate around and underneath the digester.
The wastewater inlet must be high enough above the false floor for sufficient vault depth. At least 1m of fall between the inlet and outlet is required.
The vermidigester retains solids on top of the substrate while the liquid drains through and exits the reactor. The sump provides an outlet below the false floor for water to exit the reactor. This liquid must drain away and exit the sump freely at all times. If water builds up above the level of the false floor the worms will be killed and the system will become anaerobic, stink and fail.
The sump can be as simple as a slightly sloping earth platform with an exit pipe that drains water away. An earth platform is only suitable where there is no risk of wastewater entering the water table or the water table entering the sump. Usually a steel-reinforced concrete platform is constructed, with shallow walls no higher than the false floor that are also above the soil surface.
If there is fall away from the digester, then gravity can be used for removing water at the outlet. If not, pumps are required to keep the water level below the false floor at all times.
If your vermidigester sump must be installed under the soil surface, NEVER depend on effluent percolation into the soil (e.g. soakage trenches) for removal of water from the sump. Poor drainage would cause the water level in the sump to rise above the false floor. A reliable pump with a float switch must be installed in the sub-soil sump, to remove the water from the sump and ensure the water level never reaches the false floor.
This concrete sump was made slightly wider than the plastic crates. The crates were positioned, the corrugated plastic walls fixed, then the cavity between the walls and the sump were filled with concrete.