Insights into nitrite accumulation and microbial structure in partial denitrification (PD) process by the combining regulation of C/N ratio and nitrate concentration

Abstract

The influence of nitrate (NO3--N) concentrations (40, 80 mg/L marked as R40 and R80) and carbon/nitrogen (C/N) ratios (0.8,1.5, 2.5, 3.5, 4.5) on nitrite (NO2--N) accumulation in partial denitrification (PD) process was investigated. The results proved that R40-3.5 and R80-2.5 systems showed the optimal NO2--N contents and nitrate-to-nitrite transformation ratio (NTR) values of 14.88, 29.77 mg/L and 52.87 %, 59.12 % with strong resistance to low temperature (12.5–3.8 °C). Batch tests displayed the NO2--N accumulation mechanism via substrate transformations, and the metabolic pathways of nitrogen and carbon for PD and complete denitrification were also compared. Based on the Monod equation, the half-saturation constants of 58.33 and 61.20 mg/L were estimated with zero-order reaction for NO3--N and NO2--N reduction, showing a tighter affinity to NO3--N concentration. Proteobacteria, Bacteroidota, Acidobacteriota and Chloroflexi were the four dominant phyla with total percentages of 85.16–94.55 % and 90.81–96.85 % in R40 and R80 groups, while Thauera, Flavobacterium contributing to NO2--N accumulation reached up to 42.79 % and 52.13 % in the R40-3.5 and R80-2.5 systems. The above discussion demonstrated that lower C/N ratio and higher NO3--N concentration were more favorable for the PD formation.