Effect of sawdust and polonite addition to multi-module biochar filters for the enhanced removal of nitrate and phosphorus

ABSTRACT A high level of nutrient removal in onsite wastewater treatment systems is critical for avoiding eutrophication and groundwater contamination, especially in phosphorus- and nitrogen-sensitive areas. This study evaluates modifications to a pilot-scale Multi-module Biochar Filter (MmBF) system aimed at enhancing nitrogen and phosphorus removal through material amendments. A preliminary column experiment identified sawdust as an efficient carbon source for supporting denitrification, motivating its use in anoxic modules containing biochar and 10 wt% and 30 wt% sawdust. Polonite, a calcium-silicate material, was also tested at the same dosages in the final aerobic module for phosphate removal. The results showed that biochar with 30 wt% sawdust significantly enhanced nitrate removal (up to 83%) during the initial three weeks of operation, while biochar with 30% polonite addition achieved an average phosphate removal of 38%, with a peak of 95%. However, these improvements were not sustained over the full sampling period (199 days), suggesting the need for periodic media replacement. The MmBF system maintained high COD and nitrogen removal efficiencies even under doubled hydraulic loading (100 L m−2 day−1), though phosphate removal declined under these conditions. These findings support the potential of MmBF systems with targeted media amendments as a flexible and modular onsite wastewater treatment.