A single pass vermifilter is designed so the wastewater only flows through it once. The depth of the media can be adjusted along with the porosity to achieve the required retention time for a required level of treatment. Careful design is required, especially media porosity, so that hydraulic loading rate does not exceed hydraulic retention time while also ensuring hydraulic retention time is as long as possible.
Media used for a single pass vermifilter tends to be fine and with a low porosity, such as sawdust and wood shavings. Coarse sand and fine gravel is also suitable, but material must be tested for porosity and the system carefully designed.
The recirculation vermifilter is perhaps easier to calibrate to the level of treatment required (e.g. BOD and suspended solids). This is beacause the wastewater can be passed through the vermifilter as many times as required to achieve the required level of treatment, adjusted by pump volume.
The simplest design is the gravity flow single pass vermifilter:
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.