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.

[Effect of Ozone Dosage on Sludge Settleability and Biological Nutrient Removal in SBR System].

The effect of ozone dosage on sludge settleability and biological nutrient removal performance in a sequencing batch reactor was investigated by inoculating the bulking sludge with the SVI of 280 mL·g-1 from a wastewater treatment plant in winter. The filamentous mycelium was interrupted, and the SVI was decreased to 125 mL·g-1 after ozone dosage with a low concentration of 0.085 g·g-1(O3/MLSS) for 20 days, which indicated the disappearance of the sludge bulking. The performance of nitrification and phosphorus removal efficiency was not affected obviously. However, the sludge settleability deteriorated with a high dosage of ozone, and the phosphorus removal efficiency was decreased to around 60%. Further study showed that PS/PN had a positive correlation with SVI with the correlation coefficient of 0.9381, which can be used to characterize sludge settleability. A low ozone dosage not only interrupted the filamentous mycelium, but it also affected the content and composition of the EPS, which led to improved settleability.