Primary vermidigester

Primary treatment of domestic wastewater - the vermidigester

Domestic wastewater flow enters the vermidigester in the top. Solid waste (poo, toilet paper, kitchen waste, hair etc) is filtered out of the wastewater flow and retained in the vermidigester, where worms reduce this to humus. The liquid filters through the substrate and exits the vermidigester at the bottom.

A "wet" compost heap forms as solids accumulate in the digester. Worms work this from underneath, reducing the bulk by many magnitudes as they consume it.


Twin digesters

Twin digesters offer a resting period each side before humus is removed. One digester is active while the other rests and the inlet is rotated after several years to allow the humus in each side to age and become pathogen free. Even helminth eggs are completely destroyed in that time.

When the decomposed contents are dug out from one side, these will be fully digested with no fresh fecal matter present.

Domestic vermidigester with twin vaults

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 (parallel) digesters is easily achieved by making a concrete sump 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 not allow cockroaches and flies entry 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, 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 at least 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. There is insufficient air inside the heap for it to undergo aerobic decomposition, but the worms introduce air into the heap from underneath. Because earthworms consume the solids from underneath, the wider and shallower the heap is, the greater the decomposition zone is that the worms and microorganisms 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. 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 a small to average households, 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 walls to which walls and roof are attached to.

Plastic pallet

Plastic pallets provide a durable "false floor" between the media substrate and the sump, where liquid drains away.

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 higher 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, or installed in the shade in hot climates.
  • Air must circulate around and underneath the digester.

The sump

The vermidigester retains solids on top of the substrate while the liquid drains through and exits the reactor. This liquid must drain away and cannot be allowed to build up to the level of the false floor. The sump provides an outlet below the false floor for water to exit the reactor.

A sump has one exit point for water to drain away, so needs a platform that directs all water to the exit point. The sump can be as simple as a slightly sloping earth platform with a drain that always allows water to exit from the sump.

An earth platform is only suitable where there is no risk of wastewater entering the water table. Usually a steel-reinforced concrete platform is constructed that directs the water to a drain or a pipe where water exits the sump.

It is important that water exits the sump freely at all times. If water builds up above the false floor the worms will be killed and the system will become anaerobic, fail and stink.

If there is fall away from the digester, then gravity can be used for removing water at the outlet.

If your vermidigester 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. A reliable pump with a float switch must be installed in a sub-soil sump, to remove the water from the sump and ensure the water level never reaches the false floor.

Vermidigester walls

The reactor must be enclosed with walls and roof that are vermin-proof. Vermin that shouldn't have access to solid waste includes rats, dogs, flies and cockroaches.

The reactor must also be well ventilated. Vermin-proof vents are usually installed at the top and bottom of the reactor, to ensure adequate air circulation.

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.

A pervious textile cloth such as windbreak or shadecloth is fixed to the walls and floor of the vermidigester. This must fully cover the walls and floor. The substrate is then installed.

Substrate

The substrate is usually coarse organic material that is slow to decompose, like coarse pine bark, contained in a coarse textile cloth lining such as shade cloth or windbreak cloth. It must be thick enough (e.g. 10-20cm) to retain the solids influent, filter larger suspended solids out of the wastewater flow and provide habitat for the worms.

Woodchips work well because of good porosity, but decompose over time and may require replenishing. Composted sawdust and wood shavings may be suitable for use as substrate in a primary digester because of low porosity - compost made from woody material is better than leafy material as it tends to retain adequate porosity. Coir fibre works well and peat may work, but a coarse media with good porosity is recommended.

If a suitable organic material is not available for a substrate, inorganic materials such as coarse gravel can be used. Soil doesn't tend to have sufficient porosity - the water needs to filter through relatively unimpeded.