Mg2+ distribution in activated sludge and its effects on the nitrifying activity and the characteristics of extracellular polymeric substances and sludge flocs

Divalent cations act as bridges among extracellular polymeric substances (EPS) and form cross-linkage for the self-immobilization of microbial biomass. However, their effects on the nitrification performance during the biological nitrogen removal are still unclear. In the present study, the effects of Mg2+ on the nitrifying activity, EPS and floc characteristics were investigated using a lab-scale sequencing batch reactor. The distribution of Mg2+ was quantified at different level of sludge floc. The results indicated that the nitrification activity was significantly improved when influent Mg2+ was below 1.1 mmol/L, but suppressed at 3 mmol/L. The overall performance characterized by COD, NH4+-N and TN, the particle size and sludge flocculation ability rapidly increased with the increase of Mg2+ concentration. Mg2+ was mainly distributed in the pellet and changed slightly in supernatant, LB-EPS and TB-EPS. The four fluorescence peaks detected by three-dimensional excitation-emission matrix spectra were attributed to PN-like substances and humic acid-like substances in the LB-EPS and TB-EPS. The results of XPS analysis demonstrated that LB-EPS and TB-EPS comprised similar elements. Therefore, the types of EPS functional groups was unchanged under varied Mg2+ concentrations, while their proportions changed and LB-EPS/EPS was key factor for the changes of bioflocculation.