Effects of K⁺ salinity on the sludge activity and the microbial community structure of an A²O process

Salt ions are ubiquitous in wastewater and have significant impacts on the microbial activity and nitrogen and phosphorus removal in biological wastewater treatment processes. The effects of KCl salinity on the removal of COD, TN and PO₄^3--P were investigated in a lab-scale A²O process. Meanwhile, the effects of K⁺ concentration on the composition of extracellular polymeric substances (EPS) and the microbial community structure were demonstrated. The results showed that the pollutant removal efficiencies and the bioactivity of the activated sludge decreased and the EPS content enhanced under high concentration of K⁺, which resulted in the deterioration of sludge compactness and settleability. The microbial diversity reduced after K⁺ addition and the microbial community structure was distinct between the system with (10 g L^-1 and 40 g L^-1) and without K⁺ addition. The relative abundance of Candidatus-Competibacter, Acinetobacter and Azoarcus decreased in the anoxic zone with the increase of K⁺ concentration, which might led to the decrease in denitrifying phosphorus removal capacity. However, the relative abundance of some genera of Firmicutes (such as Fusibacter, Acetoanaerobium, Planococcus and Exiguobacterium) increased, which was coincident with the enhanced microbial salt-tolerance capacity. Proteobacteria, Bacteroides, Chloroflexi and Firmicutes were the dominant phyla irrespective of the salinity changed, which guaranteed the removal of organic compounds, nitrogen and phosphorus in salty environment.