Effect of coexistence of Na+ and K+ on sludge properties and microbial community structure in A2/O process

A large amount of salty wastewater is produced from industrial processes such as food processing, tanning, textiles, aquaculture and oil production. After the salty wastewater entered the activated sludge system and contacted with the activated sludge, the sedimentation, microbial activity and community structure of activated sludge were influenced, which could result in the change of pollutant removal efficiency. A2/O process could achieve simultaneously nitrogen and phosphorus removal, but it was unclear how Na+ and K+ influenced the microbial communities and the removal efficiency of pollutants in the anaerobic, anoxic and oxic zones. In order to reveal the removal mechanism of the pollutants and the characteristics of activated sludge in the biological treatment system under the coexisting multiple metal ions, the effect of the coexisting Na+ and K+ on the removal efficiency of pollutants was investigated and the properties of sludge and microbial community in the anaerobic, anoxic and oxic zones were analyzed by high-throughput sequencing. Combined with the changes of removal efficiency of nitrogen and phosphorus and sludge properties, the succession regulation of dominant populations were explored based on the analysis of microbial community structure under different Na+/K+ molar ratios to distinguish the effect of the coexistence of Na+ and K+ on the removal efficiency of the pollutants in the salty wastewater from the point of microorganisms. The results showed that when the influent Na+/K+ molar ratio was 2, 1 and 0.5, the removal efficiencies of COD were 80%, 84% and 86%, respectively. The removal efficiencies of TN were 73%, 77% and 80%, respectively. The increase of K+ concentration alleviated the inhibition of Na+ on the removal efficiency of COD and TN. The release rates of TP in anaerobic areas were 70%, 73% and 74%, respectively. The phosphorus uptake rates in the anoxic zone were 53%, 55% and 58%, respectively. The phosphorus uptake rates in the oxic zone were 70%, 72% and 75%, respectively. The Zeta potential in anaerobic zone is -25.4, -23.2 and -14.7 mV, respectively. The Zeta potential in the anoxic zone is -33.0, -26.6 and -13.7 mV and the Zeta potential in oxic zone is -30.4, -18.6 and -11.0 mV, respectively. The positive charge adsorbed by sludge floc increased. Moreover, the biological flocculation of sludge increased gradually and the deflocculation ability of Na+ was inhibited. The richness and diversity of microbial communities in anaerobic, anoxic and oxic zones decreased and the differences of microbial communities were significant. The relative abundance of Proteus increased by about 30% and the relative abundance of Bacteroides phylum and Chloroflexi decreased gradually. As the dominant genera, the relative abundance of nitrogen-fixing Thauera and Azoarcus gradually increased and the proportion of unknown bacteria decreased, which were beneficial to the removal of pollutants. Therefore, the increase of K+ concentration contributed to increase the removal efficiency of nitrogen and phosphorus under the coexistence of Na+ and K+. It was also beneficial to the improvement of dehydrogenase activity of sludge in anoxic zone and oxic zone and the enhancement of particle size and flocculation ability (FA) of sludge.