Denitrification for acidic wastewater treatment: Long-term performance, microbial communities, and nitrous oxide emissions

Acidic nitrogenous wastewater often requires alkali pretreatment before biological treatment, which results in increased system complexity and operating costs. The demonstration of denitrification under acidic conditions would provide a theoretical basis for the direct treatment of such wastewater. In this study, the denitrification performance, microbial community, and nitrous oxide (N2O) emissions under acidic conditions were investigated using a sequencing batch reactor. When the influent pH decreased from 5.5 to 4.5, the sequencing batch reactor removed 99.8 ± 0.2% of the nitrate and 92.5 ± 1.6% (n = 171) of the chemical oxygen demand, and the production efficiency of N2O increased significantly to 11.45%. This was 2.6-fold higher than that observed at pH 5.5. The long-term denitrification treatment of acidic wastewater (pH 4.5) led to the formation of granular sludge, and Thauera, Allorhizobium-Neorhizobium-Parararhizobium-Rhizobium, and Diaphorobacter became the dominant microbes with a collective abundance of 81.3%. More importantly, only 0.25% of the nitrate was denitrified as N2O, and the batch test revealed that the emissions of N2O decreased with the increase in sludge size. These results indicate that denitrifying granular sludge formed under acidic conditions and denitrifying bacteria capable of N2O reduction proliferated, which both resulted in a significant reduction in the release of N2O. [Display omitted] •After long-term operation at acidic pH, only 0.25% of nitrate was reduced to N2O.•Granules formed in denitrification SBR for acidic wastewater treatment.•Thauera becomes the dominant bacterium in acidic denitrification systems.•N2O release significantly decreases with increasing sludge particle size.