Characteristics of NO2--N accumulation in partial denitrification during granular sludge formation

In this study, a partial denitrification (PD) system was built in an expanded granular sludge bed (EGSB) to treat low-concentration (30 mg/L) nitrate wastewater. By controlling for low COD/NO3--N (C/N, 2.0–3.5) and progressively lower HRT, the characteristics of NO2--N accumulation during granular sludge formation was examined using particle characterization, extracellular polymeric substances (EPS) and microbial analysis. Results showed that when the C/N was 3.0 and HRT was 2.0 h, the NTR reached 91.99% and the particle size reached 1.44 mm. When the C/N was 3.5, the NTR dropped to 72.75% but the particle size further rose to 1.72 mm. With the growth of sludge particle size, the inorganic substances contained in granular sludge were gradually enriched and the ratio of MLVSS to MLSS decreased to 62.45%. Analysis of EPS showed that the rise in particle size exhibited a good correlation with the rise in the PN content of EPS (r = 0.99, P < 0.001). Microbial high-throughput sequencing showed that Chloroflexi, Bacteroidetes and Proteobacteria were dominant in the PD granular sludge systems, with the main NO2--N accumulating genera in the reactor being norank_f__A4b (33.20%), Candidatus Competibacter (3.67%) and Defluviicoccus (2.49%). This study confirmed the feasibility of the rapid formation of PD granular sludge from activated sludge at low C/N and nitrogen loads.