Rapid nitrite accumulation and sludge granulation in low-C/N nitrate industrial wastewater: Specific strategies and proposed sludge transformation mechanisms

During partial denitrification (PD), the rapid realization of significant nitrite accumulation and precise addition of organic carbon are critical for reducing costs. In this study, low C/N nitrate industrial wastewater was utilized to domesticate PD sludge. The application of specific strategies resulted in rapid nitrite accumulation and high nitrate-to-nitrite transformation rates (NTRs) of 94 %, 93 %, and 96 %. Rational adjustment of the C/N ratio and HRT according to microbial activity and effluent quality is essential for the success of PD. With the rapid transformation of sludge, the sedimentation performance improved, and the sludge aggregated into particles. Microbiological analyses showed that Thauera exhibited the highest relative abundance at 60 %, which significantly contributed to the stability of the system. The results of batch tests on the effects of C/N and NO₃−-N showed that the optimal C/N for PD varies under different NO₃−-N conditions. The pH effect tests showed that the NTR was maintained above 90 % at pH 6–9, with higher pH values promoting the accumulation of nitrite. This study highlights the potential of PD and its integration with other processes for the treatment of low-C/N wastewater.