Complex sentence: Hydraulic retention time determines the actual time the wastewater is in contact with the filter and is proportional to the depth of the vermifilter witch may increase over time due to the accumulation of earthworm vermicastings.
Split into simpler sentences: Hydraulic retention time determines the actual time the wastewater is in contact with the filter. It is proportional to the depth of the vermifilter witch may increase over time due to the accumulation of earthworm vermicastings.