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

•Low concentration of Mg2+ (< 3 mmol/L) promotes nitrification activity of sludge.•The distribution of Mg2+ in the activated sludge floc was as follows: pellet > supernatant > LB-EPS > TB-EPS.•The number of groups in EPS increased with the Mg2+ concentration.•Aromatic PN-like substances and humic acid-like substances were identified in LB-EPS and TB-EPS.