Influence of carbon-to-nitrogen ratio on microbial activity and sludge characteristics in a microaerobic sequencing batch reactor for nitrogen removal

Graphical abstract shows a microaerobic SND-SBR operated at C/N ratios of 12, 10, and 5. Optimal TN removal occurred at C/N 10. Lower ratios improved nitrification but reduced denitrification, affecting sludge composition and overall performance. Microaerobic sequencing batch reactors (SBRs) enable the efficient removal of organic carbon and nitrogen under reduced aeration demand. This study investigated the effect of influent carbon-to-nitrogen (C/N) ratio on reactor performance, sludge characteristics, and formation of soluble microbial products (SMPs) and extracellular polymeric substances (EPSs) in a microaerobic SBR. Total organic carbon (TOC) removal remained high (>97%) across all C/N ratios. Total nitrogen (TN) removal was strongly dependent on the C/N ratio, with the highest TN removal achieved at an optimal C/N ratio of 10. Lower C/N ratios enhanced nitrifying activity and sludge stability while reducing heterotrophic denitrifying activity. SMP increased at low C/N ratios, whereas maximum EPS production was observed at a C/N ratio of 10. The influent C/N ratio is a key operational parameter that regulates microbial metabolism, EPS and SMP secretion, and sludge structural properties, thereby governing nitrogen removal in microaerobic SBRs. The novelty of this study lies in systematically evaluating the effect of the C/N ratio on nitrogen removal performance, together with microbial metabolic activity indicators and sludge characteristics in a microaerobic SBR system. These findings offer practical insights for designing compact, energy-efficient wastewater treatment systems capable of achieving high nitrogen removal under limited aeration conditions.