System design

Low cost wastewater treatment for the world

System design needs to consider level of treatment required and whether only primary treatment of the effluent is required (removal of solids followed by disposal to soak pit or soakage trenches) or whether secondary treatment is required (to produce an effluent quality suitable for surface irrigation).

Capacity and sizing

Capacity of the reactor 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).

A key design parameter is media porosity, or its ability to drain. If the hydraulic loading rate exceeds the hydraulic retention time then the reactor will overflow. Pine bark is the best media but this is available screened to different sizes. Coarser media is more porous but if its too porous then hydraulic retention time will be too low and treatment levels poor.

Treated effluent disposal

Discharge of treated wastewater should never be into or near water bodies. The golden rule is "dispersal is always better than concentration". Unfortunately centralised systems tend to concentrate effluent, usually into water bodies. The advantage of small scale and domestic vermifiltration is that households can irrigate their own plants or trees at low cost. Once pathogens and BOD have been removed using vermifiltration, the water remains rich in nutrients such as nitrogen, phosphorous and potassium (NPK). This water has value for irrigating and feeding crop plants.

Important considerations include:

  1. Discharge should only be to soil that does not have a high water table.

  2. Secondary treated effluent can be discharged to the soil surface using drippers, whereas primary treated effluent must be discharged to subsurface effluent fields.

The primary vermidigester

Wastewater with a high solids component (grease, feces, toilet paper) will form a thick layer on the surface of the primary reactor. A large surface area is important for primary vermifilters to ensure drainage is not impeded. The vault should be wide but doesn't need to be deep.

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 in contact with the substrate. 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.

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. 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 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 that the worms can digest.

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.

Calculating floor area

  • For the average household, 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.

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 a site more elevated 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, and in hot climates installed in the shade.

  • Air must circulate around and underneath the digester.

  • 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.

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 a structure to which walls and roof are attached to.

Plastic pallet

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

The secondary vermifilter

Secondary vermifilters should be deep rather than wide, because media depth improves level of treatment and most of the solids have already been removed from the wastewater flow.

Composted pine bark is the best starting media because it is very slow to decompose. As organic material breaks down, more media should be added. Eventually the media will become 100% humus as the added organic material breaks down and worm castings build up. Although inorganic substrates such as stone chips, pumice and scoria are suitable and don't break down, organic substrates tend to provide larger surface areas for micro-organisms to attach to, and are more effective.

Although substrate layering is often practiced, the value of this is dubious. Worms will eventually mix the layers and incorporate their castings into the media, which determines porosity. Having a single type of substrate works well, provided its composition provides the correct porosity. The media needs to drain well enough so that the volume of influent ("hydraulic loading rate") does not exceed the volume being drained ("hydraulic retention time"), but doesn't drain too fast for the suspended solids to be well filtered.

Testing drainage rate of the starting media is essential. Once the optimum media composition is determined, basket size needs to have adequate surface area to provide sufficient drainage for the peak influent load.

The basket filled with composted pine bark and with a plastic splasher in the centre. Note the well ventilated cavity between the wall of the reactor and the media-filled basket. Ventilation is required to provide a source of fresh air into the reactor.


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.

System types

Three system types are described below:

  • Gravity-flow vermifilter: This requires no energy to operate and can only irrigate crops down hill from the effluent source (passive).

  • Single-pass vermifilter: This uses a single pump (active) followed by gravity flow (passive).

  • Recirculating vermifilter: This uses recirculation pumps to pass wastewater through a vermifilter multiple times (active).

Gravity flow or recirculating vermifilters?

Gravity flow vermifilters offer a simple method for secondary treatment where there is fall between one reactor and the next reactor (or the effluent field being irrigated). By size, gravity flow vermifilters achieve a lower level of treatment than recirculating vermifilters, but are simpler to construct and maintain. Pumps are required for recirculating vermifilters but no fall is required between vermifilters.

Gravity flow vermifilter

The example below shows a gravity flow vermifilter system with primary and secondary treatment:

This is the simplest system, which can gravity feed treated effluent to land or crops, so no pump is required. Multiple secondary vermifilter reactors can be employed in series.This design suits land with fall between the wastewater entry and treated water exit. Additional fall would be required for sufficient pressure for a bell siphon and irrigation lines.

However, an elevated site with sufficient fall is not always available, for example where the land is flat or the wastewater flow is at or below ground level. Where the effluent needs to be raised into the inlet of the vermifilter (or to the soakage field), a pump is required.

Single pass vermifilter

This uses a single pump, followed by gravity flow. The example below shows a single-pass vermifilter with primary and secondary treatment:

This system uses a pump to raise the primary-treated wastewater up into an elevated vermifilter, with the outlet able to gravity discharge the treated wastewater to surface irrigation. The outlet should have sufficient head for a bell siphon to generate pressure on the irrigation lines for even distribution of water through the drippers.

However, the recirculating vermifilter is perhaps the easiest to match to the land form and level of treatment required.

Recirculating vermifilter

A recirculation pump passes wastewater through a vermifilter reactor multiple times. The example below shows a primary vermidigester followed by a secondary recirculating vermifilter:

The primary treated wastewater is pumped from the sump to the top of a secondary vermifilter and returns via gravity back to the sump. The sump overflows into a tank with a pressure pump and float switch, or alternatively gravity feeds using a bell siphon to surface irrigation.

Construction

Because Vermifilter.com are focussed on active solar systems that produce high quality effluent for irrigating crops, pasture or trees, we have put together a section focussed on constructing off-grid solar recirculating vermifilter systems.

Vermifilter system construction