System design involves understanding the following parameters:
- Wastewater volume per day
- Required treatment level
- Treated effluent disposal
Modular systems are the easiest way for implementing quantity-based design goals.
Capacity of the reactors must take into account the volume of wastewater influent and organic loading rate. The quantity of solids and grease must also be taken into account.
Sizing of reactors tends to be directly proportional to the volume of wastewater or number of households.
As wastewater volume increases, design of reactors can either be modular (more reactors) or scaled (larger reactors).
A key design parameter is media porosity. If the hydraulic loading rate exceeds the hydraulic retention time then the reactor will overflow. Porosity is achieved by:
- using a media that drains well. Compost produced from woody vegetation (e.g. wood chips, bark) is generally more porous than compost produced from green vegetation.
- Surface area. A high solids component in the wastewater flow can form a layer on the surface of the reactor that impedes media porosity and limits hydraulic loading rate. Therefore surface area is a key design parameter for primary vermifilters (but not secondary vermifilters, because most of the solids have already been removed from the wastewater flow). Media depth is the most important parameter for secondary reactors - greater media depth give better treatment levels.
System design should aim to consistently achieve a desired treatment level. Modular systems offer flexibility - the system might start with a small number of modules in series and be scaled up to achieve the desired treatment level.
Domestic system with twin primary vermidigesters, followed by a sediment trap, then two secondary vermifilters in series. Two small pumps (5 watt) recirculate the wastewater through each vermifilter and treatment level improves by employing vermifilter modules in series.
The final stage is a pump-out tank with a pressure pump and float switch, that discharges to surface irrigation lines.
This system uses a pump to raise the primary-treated wastewater and remaining suspended solids/sediment up into elevated vermifilters (in series or stacked on top of each other), with the outlet high enough to gravity discharge the treated wastewater to surface irrigation. The outlet should have some head for pressure on the irrigation nozzles, for even distribution through the active lines.
This system can gravity feed to land or crops, so no pump is required. The secondary vermifilters are either employed in series (above) or stacked directly on top of each other. This design suits land with fall between the wastewater entry and treated water exit.
The primary treated wastewater is pumped from the sump to the top of a secondary vermifilter and via gravity back to the sump. Multiple secondary vermifilters can be in series or stacked on top of each other, depending on the head of the pump. Water from the sump can be recirculated at set intervals through the vermifilter using a timer, or can be constantly recirculated through the reactor.
The sump overflows into a pump-out tank with a pressure pump and float switch (or alternatively gravity feeds to surface irrigation). Sumps can also be connected in series, each one with a recirculating vermifilter, to achieve greater levels of treatment.
Treated effluent disposal
Treated wastewater must still be discharged to the environment. Discharge should never be to waterways, but always to land. Because vermifiltration systems are low cost, scaling down offers greater opportunity for valued disposal of treated wastewater. The rule is "dispersal is always better than concentration".
Once pathogens and BOD have been removed using vermifiltration, the water remains rich in nutrients such as nitrogen, phoshorous and potassium (NPK). This water has value for irrigating crop plants, to provide abundant growth.
- Discharge should only be to soil that does not have a high water table.
- Secondary treated effluent can be discharged to the soil surface using drippers, whereas primary treated effluent must be discharged to subsurface effluent fields.