Insight into simultaneous urea hydrolysis and total nitrogen removal in textile printing wastewater: Focus on the impact of sodium sulfate salinity.

The textile printing industry discharges large volumes of effluent containing high concentrations of urea and nitrogenous compounds. Anoxic-oxic (AO) treatment is a promising method for treating printing wastewater. However, the effect of sodium sulfate (NaSO) salinity on the urea hydrolysis and nitrogen removal simultaneously in the AO process has received little attention. In this study, five batch reactors were used to treat synthetic printing wastewater with high urea and nitrogen concentrations. A strategy was applied to increase the NaSO concentration from 0 to 19 g/L in the anoxic stage of each reactor. The effect of NaSO on urea hydrolysis, total nitrogen removal and COD removal, sludge characteristics, and bacterial community structure were investigated. The findings showed that urea hydrolysis increased with increasing NaSO concentration. The main mechanism of urea removal was intracellular hydrolysis, with a urea removal efficiency (URE%) of approximately 98% in all batch reactors. In addition, under the stress of NaSO, the total nitrogen and COD removal performances were partially inhibited. The most significant removal performances after AO treatment were observed at 0 g/L NaSO with nitrogen and COD removal efficiencies of 88% and 95%, respectively. When NaSO concentration reached 19 g/L, the sludge settling performance and compactness were enhanced. The extracellular polymeric substance (EPS) components in the sludge were dependent on their ability of removing organics. Bacterial community diversity analysis revealed that the enrichment of the Proteobacteria, Firmicutes, and Gemmatimonadota phyla in the anoxic stages of batch reactors was related to intracellular urea hydrolysis. Bacteriodota and Chloroflexi were responsible for total nitrogen removal in all anoxic and oxic stages. This research will develop the understanding of NaSO salinity impact on simultaneous urea hydrolysis and nitrogen removal during AO treatment process.