A single pass vermifilter is designed for the wastewater to only flow through it once. Careful design is required, especially media porosity. Hydraulic loading rate must not exceed hydraulic retention time, but hydraulic retention time should be as long as possible. This means that posity of the media should be "just right" for the organic loading rate so that adequate treatment is achieved. The depth of the media can be adjusted along with the porosity to achieve the required retention time for a required level of treatment.
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.
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.