Secondary treatment vermifilter construction

Low cost wastewater treatment for the world

These construction pages provide advice for building a domestic system that irrigates domestic wastewater to the soil surface for feeding plants. 

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Constructing vermifilters (for secondary treatment)

Constructing a recirculating vermifilter from two 250 litre (40 gallon) plastic drums

Stacked secondary vermifilter

Stacked secondary recirculating vermifilters can be built from two 40 gallon plastic drums. The top is cut off the bottom drum and the other drum is stacked on top of this. These become the vermifilter and sump drums (see image above). 

The water level in the sump (red arrow) is set by the inlet to the pump-out drum. This ensures the outlet from the sump is below the water surface (learn about equilibrium water level and surge capacity).

Pine bark media used in secondary vermifilter

40 gallon plastic drum secondary treatment vermifilter. The basket is made with windbreak cloth, plastic netting, cable ties and polythene pipe for ventilation spacers. This vermifilter either sits on top of the sump (left) or sits separately above the sump with a drainage pipe feeding into the sump.

Secondary and tertiary treatment vermifilters can be added in series depending on level of treatment required.

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 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.

Composted pine bark is also 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.

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.

Settling tank

The sump also acts as a settling tank with the inlet from the previous sump entering at the bottom of the sump.

Tank fitting

Two tank fittings are installed in the sump drum for an inlet and outlet. These should be 40-40mm polythene pipe.

and one tank fitting is installed at the bottom of the vermifilter drum

Tank fitting (outside of drum)

Tank fitting inside the drum. Polythene pipe is inserted...

...and the pipe inlet goes to the bottom of the drum.

Setting up solar-powered recirculating vermifilters

Solar vermifilters operate intermittently, for example ten seconds every minute, twenty four hours per day. Four components are required: the pump, the solar panel, the battery and the timer.

1. Pumps

Secondary and tertiary vermifilters require pumps suitable for recirculating water with low levels of suspended solids.

The pump pictured here is 12 volts DC, has a water flow of 800 litres per hour, a maximum discharge head of 5m, with 1/2" male threads. The power is 19 watts and at 12 v the current is 1.6 amps.

To operate two of these pumps (secondary and tertiary vermifilters in series) and a primary vermidigester recirculation pump, a 50 watt solar panel is required, along with a 21 amp hour LiFePO4 battery.

Recommended: 19 watt brushless centrifugal water pump

2. Battery and timer

A single LiFePO4 battery is required, along with one timer control for the secondary recirculation pumps. The timer operates the recirculation pumps intermittently.

Recommended: 12.8V 21Ah Lifepo4 battery pack (4S3P, with 12x 32700 cells)

Overvoltage and undervoltage protection are required. The Small Den battery available on Ali Express has Discharge Cutoff Voltage: 10 +/- 1V and Charge Cutoff Voltage: 14.6V, with a 40 amp balance BMS suitable for direct solar charging

Cost approx. US $100

Recommended: JZ-801 or XY-J02 Cycle Delay Automation Timer Control 

12 volt, low power consumption, provides a separate "on" time and "off" time.

Cost approx. US $4

Setting up the JZ-810 timer module:

3. Solar panel

A single 50 watt solar panel is sufficient for intermittently running a secondary recirculation pump and a tertiary recirculation pump.

Make sure to use a nominal 12 volt solar panel for a 12 volt system.

Cost approx. US $100 for a 50 watt solar panel.

4. Assemble the components

See the video below for how these components can be put together in a working recirculation system.

The video above shows a domestic vermifilter with active recirculation using a battery, timers and solar panel.