Combined removal of nutrients and suspended solids in a dual-media post-denitrification filter as additional process step of wastewater treatment plants

Operation of a pilot-scale dual-media biological filter as post-denitrification step in a municipal wastewater treatment plant was investigated for 28 months. In order to identify key design parameters, filtration rate, external carbon dosing concentration and strategy as well as backwash frequency were varied. The results show that dual-media biological filtration is able to achieve effluent concentrations of total suspended solids (TSS) ≤2 mg/L and NO₃-N ≤ 5 mg/L. TSS removal also leads to a reduction of particulate bound phosphorus and chemical oxygen demand without dosing any precipitant. Soluble reactive phosphorus is required for growth of the denitrifying bacteria and reduced from 0.4 to 0.3 mg/L in the filter effluent, corresponding to approximately 0.02 g P/g NO_x-N removed. Depending on NO_x-N loading and carbon dosage, average denitrification rates of 0.5–1.0 kg NO_x-N/m³*d were achieved in different operational phases. Seasonally varying nitrite formation and breakthrough in the filter effluent were observed and could not be controlled by adjusting carbon dosage and backwash frequency. Effective operational strategies to prevent nitrite breakthrough at NO_x-N loads in the range of 1–2 kg NO_x-N/m³*d and high influent O₂ levels are therefore needed.