Secondary treatment of sewage - the vermifilter
Wastewater that flows from the vermidigester (or a septic tank), having had most of the solids removed, then enters the secondary reactor or vermifilter. Vermifilters can be set up in series depending on treatment level required. More vermifilters means greater treatment level, but usually one or two are sufficient.
The illustration below shows two gravity fed vermifilters in series:
The vermifilters above feature "splashers". The water drops onto these and splashes, distributing the water over the surface of the media. Unlike nozzles, these cannot block from buildup of biological slime.
The water then trickles downwards through the media. The media filters out suspended solids which are rapidly decomposed in the aerobic environment. Microorganisms attach to the media surface and these remove nutrients and organic substances, reducing biological oxygen demand and removing pathogens.
Vermifilter reactors are easily made from plastic drums for domestic applications, or using plastic or concrete tanks for larger-scale community treatment reactors. Construction is simple but to ensure the media remains aerobic diameter should be as small as practicable. Media volume must be matched to influent volume and it is best to increase media volume with reactor height rather than diameter.
The reactor should have a false wall that separates the media from the wall of the tank. This is to allow ventilation so the media remains fully aerobic and effective at treating water with a high oxygen demand. The false wall must allow air flow through it. Vents should also be installed so that sufficient air flows through the reactor.
The media should be organic, such as wood chips, sawdust, peat or bark. The worms add humus (vermi-cast) to this so having an organic media means that eventually the media will be all humus as organic material breaks down. More media will have to be added at regular intervals as the substrate composts and reduces. Inorganic substrates also work, such as stone chips, pumice and scoria.
Porosity of the media is important to get right. If the vermifilter is single-pass, then the media should not be too porous, otherwise the retention will not be sufficient to provide a high level of treatment. Recirculating vermifilters can have a porous media because the wastewater passes through multiple times - each time the treatment level improving.
This video shows a secondary treatment vermifilter built in India for an office building. The plant treats 20,000 litres per day of wastewater from a septic tank, to a secondary level for surface disposal. Note the individual baskets that provide excellent levels of aeration to the media.