Primary treatment vermifilter construction
Low cost wastewater treatment for the world
Constructing twin vermidigesters (for primary treatment)
Domestic vermidigester with twin vaults. The cladding material is fibre cement board
The vermidigester retains waste solids (faeces, toilet paper etc) on top of the substrate while the liquid percolates through the substrate and exits the digester vault.
Twin primary vermidigesters should be constructed so the inlet can be rotated between the active digester and the resting digester. The contents of one side are rested while the other digester is being used.
Constructing the digester vault
The digesters sit in an earth or concrete platform. 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. The platform walls should be no higher than the floor of the digester vault, so that the water level cannot ever raise above the floor of the vault. This would drown the earthworms. Remember, the digester is a composting vault, not a water tank!
The digester vaults themselves can be constructed from two large fruit crates (the type designed for handling with forklifts, see below). Ventilation into the digester is essential, but should also exclude entry of cockroaches, flies and vermin.
Shadecloth, windbreak or similar porous textile cloth is fixed onto the inner walls and floor of each crate and 10-20cm of coarse media substrate is then added to the bottom of the crates. Pine bark is recommended, because this decomposes much more slowly than wood chips, coarse sawdust or wood shavings.
Large fruit crates with a capacity of 1 cubic metre are ideal for constructing vermidigesters. Note the ventilated walls and "false floor", which allows ventilation underneath the vault.
These fruit crates also provide a structure to which walls and roof are attached to and are well ventilated.
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 between the walls and crate so that fresh air flows around the walls and underneath the crate. The structure sits on a concrete platform with walls that are no higher than the false floor, and a drain to remove filtered water into the sump.
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, decomposition will be slower and the digester will fill much faster.
Plastic pallets also provide a durable "false floor" between the media substrate and the drainage platform. Plastic pallets can also be used for the walls and roof structure, fixed together with cable ties. Vermin-proof cladding is then fixed to these, providing ventilation right around the vault.
Wastewater with a high solids component (grease, feces, toilet paper) will form a thick layer on the surface of the primary reactor. A large surface area is important for primary vermifilters to ensure drainage is not impeded. The vault should be wide but doesn't need to be deep. Worms work the heap from underneath and decomposition occurs in the zone that is in contact with the substrate.
Vermidigester walls and roof
The vault must be enclosed with walls and roof that are vermin-proof. Rats, dogs, flies and cockroaches should not be able to access the contents, but the vault must also be well ventilated. Vermin-proof vents are usually installed at the top of the vault, and must provide adequate air circulation to the wall cavities and under the floor.
A pervious textile cloth such as windbreak or shadecloth is fixed to the inside walls of the basket and must fully cover the walls and floor. The substrate is then added.
Wastewater percolates through the vermidigester and drains into the sump. This is a 200 litre plastic drum with an inlet and outlet. The outlet should not be too close to the top of the sump, so it can provide surge capacity. Surge capacity is the volume between the floor of the digester and level of that outlet. This volume must be sufficient so that surges of influent don't raise the water level sufficiently to overflow the platform walls.
The recirculation pump operates once a day for 20 seconds, and pumps suspended solids back into the digester. The recirculation pump is not essential, but without it fine solid particles that are suspended in the wastewater slowly settle and build up in the bottom of the sump as sludge, which would require periodic removal (e.g. annually).
By operating for a short period once a day, sediment does not settle in the bottom of the sump. The primary recirculation pump is highly recommended.
Recommended: Whale gulper 220
The primary digester requires a pump suitable for recirculating water with high levels of suspended solids.
The Whale gulper is a 12 volt DC, no clog waste pump with a large single diaphragm, pumping 12.6 litres per minute with current draw of 4 amps at 1m head. 19mm hose connections. Max discharge head of 3m.
Recommended: IO23C01 Cycle Delay Automation Timer Control
The IO23C01 is a reliable 30 amp relay timer that is easy to set up. It operates on 12 volts DC and is easy to set to operate the pump for only 25 seconds every 48 hours.
The primary recirculation pump uses its own IO23C01 relay timer, and uses the same battery and solar panel as the secondary recirculation pumps. This pump only operates for 25 seconds every 48 hours. It is important that the pump only operates during the day when the battery has not fully discharged.
Setting up the IO23C01 cycle delay timer:
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.