Water biological denitrification in batch and anoxic/aerobic-membrane bioreactor operations: carbon-to-nitrogen ratio, nitrate concentration and hydraulic retention time

The current research aimed to investigate the biological denitrification (BD) of high-nitrate drinking water using acetic acid as a carbon source in both batch and continuous modes. In batch mode, optimal denitrification was observed at carbon-to-nitrogen ratios (C/N) of 2 or higher without nitrite accumulation, and at C/N = 2, for nitrate concentrations of 150 mg/L or greater, nitrate and chemical oxygen demand (COD) removal efficiency was 97.5 ± 1.3% and 73.5 ± 1.9%, respectively. In continuous mode, the water BD process was assessed in a sequencing anoxic/aerobic-membrane bioreactor (MBR) over two stages with hydraulic retention time ratios (HRT_MBR/HRT_Anoxic) of 34/17 in stage one and 8/8 in stage two. Results indicated that the limited carbon in the MBR led to a significant reduction in biomass, resulting in increased ammonia production and higher nitrate concentrations during the final days of both stages. The recycling of produced nitrate and dissolved oxygen into the anoxic reactor reduced the effective C/N ratio, which decreased the specific denitrification rate (SDNR). Although nitrate concentration rose more quickly in stage two, this stage achieved a higher SDNR (312 ± 48 mg NO₃/g MLVSS.d) and COD removal efficiency (100%). Overall, the hybrid system effectively removed contaminants and rejected E. coli, producing an effluent that met World Health Organization drinking water standards.