Primary vermidigester

« How it works

Primary treatment of domestic wastewater - the vermidigester

Solids (poo, toilet paper, kitchen waste) are filtered out of the wastewater flow and retained in the vermidigester, where worms reduce this to humus. The toilet and other wastewater influent must be generated from higher than the entry into the digester.

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 offer a resting period before removal of humus. One digester is active while the other rests and they are rotated after three to four years to allow the humus to age and become pathogen free. Even helminth eggs are completely destroyed in that time.

Substrate is usually coarse bark or a similar highly porous organic substrate contained in a coarse textile cloth such as shade cloth or windbreak cloth. The substrate only needs to be thick enough to provide habitat for worms and to filter larger suspended solids out of the wastewater flow.

Sufficient capacity is required to allow the heap to spread unconstrained. The worms consume the solids from underneath, so the wider and shallower the heap is, the greater the surface area is for the worms to digest. Although there is insufficient air inside the heap for aerobic decomposition, the worms introduce air into the heap from underneath where digestion occurs.

For household-sized digesters a minimum surface area of 1 square metre is recommended.

Construction of twin (parallel) digesters is easily achieved by making a concrete sump that fits two 1 cubic metre 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. Air flow into the digester is required but gaps should not be sufficient for cockroaches and flies to gain entry. 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.

The digester contents should not freeze or become too hot for survival of the earthworms. Shade or insulation may be necessary.

Earthworms are added to the digester after the contents have been allowed to build up for a month or so. Species of earthworms should be locally sourced compost worms such as red wrigglers.

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