Recirculating vermifilters are secondary treatment vermifilters that use very small pumps to recirculate the wastewater through the vermifilter continuously. They are very energy efficient, low maintenance and low cost.
Hydraulic retention time is not important, so media tends to be coarse and porosity high. The wastewater might pass through the vermifilter multiple times before being discharged, so treatment levels can be very high, controlled by the volume of water recirculating and the number of stages.
Two recirculating vermifilters in series. Each has a recirculation pump
Multiple recirculating vermifilters can be positioned in series with each vermifilter providing additional treatment. The outlet is near the top of the tank and the inlet goes to the bottom of the next tank to settle out any suspended sediment.
Full secondary treatment system with recirculating vermifilters:
Note that a series of settling tanks precede the recirculating vermifilters. These settle any suspended solids out before the wastewater gets to the recirculating pumps, so filters are not required. Pump filters are a bad idea for wastewater because growth of biological slime will block them in short time.
Note also that the inlet to the Removal tank sets the water level for the full series of tanks. This is important because the water level needs to be higher than the outlet in each settling tank! Understand displacement - the water level always stays the same in the tanks - as more influent arrives in the first tank, the same quantity exits into the removal tank.
Also keep in mind that the recirculating pumps don't affect the water level in the tanks, they just recirculate the water. If they fail the system still works, but without the treatment that recirculation provides.
This brushless mini water pump is 12 volts, consuming 4.2 watts of electricity. It has a static head of 3m and pumps 240 litres per hour, with a life span of 30000 hours.
The cost per unit is less than $10. One of these pumps is used per 250 litre reactor.
These pumps are ideal for a domestic recirculating vermifilter system. They can either be run directly from small solar panels or using a 12v battery and solar panel so they run 24/7.