Efficient municipal wastewater treatment by oxidation ditch process at low temperature: Bacterial community structure in activated sludge

Temperature is a key element affecting the activity of microorganisms in activated sludge. Low water temperature generally leads to decreasing wastewater treatment efficiency and destroying sludge settleability. In this study, activated sludge samples from a municipal wastewater treatment plant (WWTP) implementing oxidation ditch process was used to investigate the bacterial community characteristics of a system that operates well in a cold region (Xinjiang, China) by high-throughput 16S rRNA gene sequencing. The results showed that the influent temperature was 7–12 °C in winter and 13–17 °C in summer, while the sludge volume index (SVI) of samples was between 51 and 74 mL/g. The average removal efficiencies for chemical oxygen demand (COD), biochemical oxygen demand (BOD5), suspended solid (SS), ammonia nitrogen (NH4+-N), total nitrogen (TN), and total phosphorus (TP) were 94%, 95%, 95%, 91%, 73% and 89%, respectively. The bacteria were distributed in 32 phyla and 559 genera. The dominant phyla were Proteobacteria (28.85–48.45%), Bacteroidetes (20.00–31.22%), Chloroflexi (3.59–12.23%), Actinobacteria (1.58–15.54%) and Firmicutes (1.38–10.49%). The dominant genera were Saprospiraceae_norank (4.41–12.23%), Comamonadaceae_unclassified (3.82–8.83%), Anaerolineaceae_norank (1.39–9.35%), Dokdonella (1.13–11.26%), Candidatus_Microthrix (0.26–7.50%), Flavobacterium (0.32–8.14%), Ferribacterium (0.36–5.19%) and Nitrospira (0.084–5.37%), which were different from those found in warm-region WWTPs. Contrary to previous studies, the relative abundance of ammonia-oxidizing bacteria (AOB; Nitrosomonas and Nitrosomonadaceae) and nitrite-oxidizing bacteria (NOB; Nitrospira) increased when the temperature decreased. The successful operation of this WWTP suggests that cold-region WWTPs can achieve good pollutants removal efficiency by simultaneously maintaining an ultra-low sludge load and high dissolved oxygen concentration in the oxidation ditch. The findings of this study provide fundamental knowledge required for an efficient and stable operation of WWTPs in cold regions.