Secondary treatment of sewage - the vermifilter
Wastewater that discharges from the vermidigester (or alternatively from 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. Note that a ventilation cavity is provided between the external wall of the reactor and the media basket
The vermifilters above feature "splasher" inlets. The water drops onto the splashers from a single low pressure entry point and splashes to distribute the water over the surface of the media. Unlike nozzles, these cannot block over time 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 attached to the media also remove nutrients and organic substances from the wastewater, reducing biological oxygen demand (BOD) and removing pathogens.
The media should be organic, such as wood chips, sawdust, peat or bark. As organic material breaks down more media should be added at regular intervals. The worms add humus (vermi-cast) to this, so eventually the media will become 100% humus as organic material breaks down. Although inorganic substrates, such as stone chips, pumice and scoria, are suitable and don't break down, organic substrates are more effective.
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 time will not be long enough to provide a high level of treatment. Recirculating vermifilters can have a porous (free draining) media because the wastewater passes through multiple times for a high level of treatment. Recirculating vermifilters are recommended because retention time can be adjusted based on recirculation frequency.
Domestic scale reactor
Vermifilter reactors are easily made from 250 litre plastic drums for domestic applications.
40 gallon plastic drum reactor. The ventilated basket is made with windbreak cloth, plastic netting, polythene pipe and cable ties.
The basket filled with composted pine bark and with a splasher in the centre. Note the well ventilated cavity between the wall of the reactor and the media-filled basket. Vents are installed at the bottom and top of the reactor to provide fresh air around the basket.
Shadecloth or windbreak cloth used for vermifilters must be open enough to freely drain water but also not too open as it needs to hold the media in place.
Plastic drainage netting - this is strong and rigid enough for making baskets for secondary domestic vermifilters.
Reactor columns made from plastic drainage cells make the most cost-effective community-scale secondary treatment vermifilters. Drainage cells offer a modular system for matching media volume to influent volume, which is scaleable for increased demand.
Reactor columns can be installed in parallel or in series, or a comination of both. Individual colums should not have a surface area of more than 0.7m2 but individual columns can be more than 2 m high. Each reactor column should have ventilation right around the column, so the media remains fully aerobic - to effectively treat influent with a high oxygen demand to the full depth of the media. Vents should also be installed so that sufficient air flows through the reactor.
Plastic drainage cells are the best option for constructing ventilated columns. They are tied together with cable ties into a structure:
The structure can be as wide and as high as required.
A textile "sock" is made from windbreak or shadecloth and inserted into each cavity. The sock is then filled with media for each column, with ventilation between each column. Walls may be attached to the outside of the structure and a roof constructed if necessary.
A false floor is required under the structure. This can be made from plastic pallets.
Usually the pallets would sit on a steel-reinforced concrete pad that directs the treated water to an outlet.
This video shows a secondary treatment vermifilter built in India for an office building. The plant treats 20,000 litres per day of wastewater generated from a septic tank, to a secondary level for surface disposal. Note the individual baskets - these provide excellent levels of aeration to the media, but are not as cost-effective as drainage cells.