Advanced treatment of low C/N ratio wastewater treatment plant effluent using a denitrification biological filter: Insight into the effect of medium particle size and hydraulic retention time

Three denitrification biological filters (DNBFs) with different ceramsite medium particle sizes (2–4 mm, 4–12 mm and 12–20 mm) were used to treat wastewater treatment plant effluent. The influence of ceramsite particle size and hydraulic retention time (HRT) on the denitrification efficiency was compared. The results show that with increasing particle size (2–4 mm, 4–12 mm, 12–20 mm), the average total nitrogen (TN) degradation efficiency reduced from 76.9% to 46.4% and 41.6%. The filter medium with smaller particle size can enrich more denitrifying bacteria, improve the activity of denitrifying enzymes and electron transport system, and is more conducive to the removal of nitrate (NO3−-N) in low C/N wastewater. When the HRT was 3 h and the medium particle size was 2–4 mm, the denitrification effect was the best, with degradation efficiencies of TN and NO3−-N of 78.6% and 97.2%, respectively. Extending the HRT was beneficial to enhancing the denitrification efficiency of the DNBF, which could compensate for the effect of water temperature reduction on the denitrification efficiency. The total quantity of denitrifying bacteria reached 58.68% at the bottom of the DNBF with particle size of 2–4 mm, 48.06% in the DNBF with particle size of 4–12 mm and 40.17% in the DNBF with particle size of 12–20 mm. NO2−-N and chemical oxygen demand significantly influenced the microbial community structure. This study provides a basis to select a filter media particle size for denitrification treatment of wastewater treatment plant effluent with a low C/N ratio.