How it works

The revolution in domestic wastewater treatment...

Vermifilters purify wastewater and they do this at low cost. The "filter" is a media substrate such as bark or woodchips or sawdust, maintained by earthworms to remain aerobic and porous. Vermifilters are not just a filter, they are also an incredibly diverse living ecosystem made up of worms and microorganisms living in balance, consuming pathogens and organic substances from the water as it passes through the substrate. The worms, as caretakers of the system, aerate the media with their burrowing, maintaining the porosity and producing castings (humus), the perfect media additive. 

The process requires virtually no maintenance, and being aerobic, generates no smell and is fast. The purified, odour-free, nutrient rich water is suitable for surface irrigation to crops or pasture. Nutrients are recycled and household water is reused for growing crops or trees. Learn more about the nutrient cycle.

Vermifiltration is therefore well suited to both domestic wastewater treatment and also community-based treatment using simplified sewers.

Primary treatment of domestic wastewater - the vermidigester

The first stage removes solids from the wastewater and these are digested by earthworms. 

Wastewater from toilets, kitchen, bathroom etc enters the vermidigester vault at the top. Solid waste (poo, toilet paper, kitchen waste, hair etc) is retained in the vermidigester and accumulates on the media surface, where worms reduce this to humus. The liquid filters through the substrate and exits the vermidigester at the bottom.

Accumulating solids form a spreading "wet" compost heap in the digester vault. Worms consume this heap from underneath in a continual process as more waste is added. 

Twin digesters

Twin digesters provide a resting period before humus is removed, i.e. one digester is active while the other rests. This means when the contents are dug out from the digester, these will be fully decomposed.

The inlet is rotated from one digester to the other after several years to allow the humus in each side to age and become pathogen free. 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. Helminth (parasitic worm) eggs are completely destroyed in two to three years of resting. Capacity of twin vaults must therefore be sufficient so that the other side does not fill with solids before that time.

By digesting the solids, the worms 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 for the worms to digest. 

Domestic vermidigester with twin vaults. The cladding material is fibre cement board

Then, the filtered wastewater exits the digester vault and either enters the secondary vermifilter stage to be further treated, or enters a properly constructed subsurface soakage field to infiltrate into the soil.

Secondary treatment of domestic wastewater - the vermifilter

Secondary vermifilters remove oxygen demand and pathogens, to produce clean, nutrient rich water suitable for surface irrigation of crops. 

Wastewater exiting the primary vermidigester has most of the solids removed. This "primary treated" effluent then enters the secondary treatment reactor, or vermifilter. 

The wastewater trickles through the media, filtering out fine suspended solids and organic substances. These are rapidly digested by microorganisms and worms in the reactor's aerobic environment, purifying the wastewater to be suitable for surface irrigating pasture, trees or crops. Rich in plant nutrients (nitrogen, phosphorous and potassium - NPK), the water is best described as liquid plant food, completing the nutrient cycle.

Vermifilters can be set up in series depending on treatment level required (secondary or tertiary). Usually one or two in series are sufficient for treatment of domestic wastewater for surface irrigation. 

The illustration below shows two gravity flow vermifilters in series:

Note the ventilation cavity between the external wall of the reactor and the media basket.

The water trickles downwards through the media, aerating it and filtering out suspended solids, which are rapidly digested by worms and microorganisms in the aerobic environment. Earthworms and microorganisms attached to the media also remove pathogens, nutrients and dissolved organic substances from the wastewater, reducing biological oxygen demand (BOD). 

Importantly, the worms also maintain the porosity of the media by borrowing through it and depositing their castings. The worm castings mix with the media, eventually forming a homogenous, porous, humus substrate that requires no maintenance. 

The worms are best described as "caretakers" of the system. They burrow in the substrate and ensure this remains aerobic and well drained. Worms consume and digest surplus biofilm and generate humus, which becomes part of the substrate, an aerobic home for the biofilm to attach to and a comfortable residence for the worms.

Domestic vermifilter reactors are easily made from 250 litre plastic drums:

40 gallon plastic drum secondary treatment reactor. The ventilated basket is made with windbreak cloth, plastic netting, polythene pipe and cable ties.

Oxygen demand and vermifilters

Because the vermifilter substrate is rich in oxygen, microorganisms attach to this substrate and proliferate to form a "biofilm", which in turn provides a food source for the worms. The worms graze the biofilm which maintains the health of the fauna, similar to cattle grazing pasture.  The fauna consumes pathogens and degrades organic compounds while at the same time oxygen dissolves into the water. Respiration and oxidation takes place without depleting dissolved oxygen, purifying the wastewater by removing biological oxygen demand (BOD).

Learn more about Oxygen and wastewater »

Capacity and sizing

Reactor capacity must take into account the volume of wastewater being treated, including peak flows. Sizing of reactors tends to be directly proportional to the number of users. As wastewater volume increases, design of reactors can either be modular (more reactors) or scaled (larger reactors).

Learn about constructing a domestic vermifilter wastewater treatment system »