Primary treatment vermidigester construction

Low cost wastewater treatment for the world

Construction:

Vermidigester for primary treatment

The whole system produces treated wastewater suitable for surface irrigation. This is a high level of treatment and achievable only with primary + secondary treatment.

The system operates off-grid and utilises solar panels and batteries to operate the pumps required to function.

If you only want a simple system (primary treatment, subsurface discharge - e.g. soak pit) then just build the primary digester (below) and a soak pit.

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 into the sump.

Twin primary vermidigesters provide for the inlet to 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 vaults

The digester vaults sit in an earth or concrete drainage platform. An earth platform is only suitable where there is no risk of wastewater percolating into the water table, or no risk that the water table could enter the sump. Usually a steel-reinforced concrete platform is constructed. The drainage platform is actually a shallow sump with walls and an outlet (see below). The walls should be no higher than the floor of the digester vault, so that the water level cannot ever raise above the level of the floor. This would drown the earthworms. Remember, the digester is a composting vault, not a tank!

Diagram of drainage platform and vault, enclosed with walls and roof

The digester vaults can be two large (1 cubic metre) fruit crates (see below). The perforated false floor sits on feet above the drainage platform, allowing air to flow underneath the vault and providing drainage through the floor.

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 vaults. Pine bark is recommended, because this decomposes much more slowly than wood chips, coarse sawdust or wood shavings. However, provided it drains, any organic media is suitable as substrate.

Large fruit crates with a capacity of 1 cubic metre are ideal for constructing vermidigesters. Note the ventilated walls and false floor.

These fruit crates also provide a structure to which walls and roof can be attached.

This twin 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 exterior walls and crate so that fresh air flows around the walls and underneath the crate.

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 pallet

Plastic pallets can be used to 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 and under the vault. Remember, you are not constructing a tank, but a vault with a floor that drains to the sump below.

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 vault and must fully cover the walls and floor. The substrate is then added.

The heap

The solids component of the wastewater (faeces, toilet paper, grease, kitchen scraps, hair) will form a thick layer on the surface of the primary digester's substrate. A large surface area is important for the primary vermifilter, to ensure drainage is not impeded. The vault should be wide but doesn't need to be deep. Worms live in the substrate and digest the heap from underneath. Decomposition occurs in the zone that is in contact with the substrate. The substrate shouldn't be so deep that it impedes drainage.

The sump

Water percolates through the vermidigester and drains through the floor to the drainage platform where it then enters the sump. This can be a plastic drum (e.g. 200 litre) with an inlet and outlet. The outlet should not be too close to the top of the sump, to ensure it has surge capacity. Surge capacity is the volume between the level of that outlet and the floor of the digester. This volume must be sufficient so that surges of influent don't raise the water level sufficiently to overflow the drainage platform walls.

Recirculation

The recirculation pump operates once every two days for 20 seconds. This pumps suspended solids that are settling to the bottom of the sump back into the digester. By operating for a short period once a day, sediment does not build up in the bottom of the sump, but is instead pumped back into the digester where it is consumed by the worms.

Recommended: Whale gulper 220

The primary digester requires a pump suitable for recirculating water with high levels of suspended (settling) 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 20 seconds every 48 hours.

The primary recirculation pump uses its own IO23C01 relay timer, and the same battery and solar panel as the pumpout tank.

See below to set up the IO23C01 relay timer module.

Setting up the IO23C01 cycle delay timer:

Design considerations

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 vault.
The wastewater inlet must be high enough above the vault floor for sufficient depth. At least 1m of fall between the inlet and outlet is required to allow the heap to build up without blocking the inlet.

Next - Secondary vermifilters »

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