Effects of Vermi-Plant Bed Substrate Composition on Excess Sludge Treatment in Vermi-Ecological Filters

A vermi-ecological filter (VEF) enables in situ reduction in excess sludge, which is particularly suitable for small municipal wastewater treatment plants. This study aimed to investigate the substrate configuration of a vermi-plant bed influencing the performance of a VEF when treating excess sludge. For this, three composite substrates, namely, high organic (HO), organic–inorganic mixed (MIX), and high inorganic (HI), were evaluated by adjusting the proportions of loess, vermicompost, diatomite, perlite, and coconut fiber. The results showed that after 54 days of staged operation under low-to-high loading conditions, earthworms and plants in the MIX and HO groups demonstrated healthy reproduction and growth. In contrast, MIX showed the highest TS (Total Solids) removal efficiency during the entire operation period, reaching 87.47%. The effluent COD (Chemical Oxygen Demand) concentration and ammonia nitrogen levels in MIX were 59.39 and 1.59 mg/L, respectively. Moreover, the MIX substrate formed an optimized core functional network with the lowest network density but the highest proportion of positive associations (>75%). However, the HO and HI groups displayed operational instability and ecological collapse, respectively. Overall, the results suggest that an organic–inorganic composite substrate can effectively enhance VEF performance when treating excess sludge via constructing a virtuous micro-ecosystem.