Effects of coexisting Na+, Mg2+ and Fe3+on nitrogen and phosphorus removal and sludge properties using A2O process

Abstract

Na+, Mg2+ and Fe3+ are generated in industrial production and human life such as petroleum refining, textiles process and seawater flushing. They can affect sludge activity and operation performance after entering sewage treatment plant. The effects of coexisting Na+, Mg2+ and Fe3+ on nitrogen and phosphorus removal and sludge flocculation were investigated and the distribution of alkaline phosphatase in the sludge was analyzed using A2O process. Fourier transform infrared spectroscopy (FTIR) and three-dimension excitation emission matrix (3D-EEM) were used to explore the changes of extracellular polymeric substances (EPS), and Brunauer-Emmett-Teller (BET) and scanning electron microscopy (SEM) were used to analyze the sludge microstructure, respectively. The results showed that when Na+ concentration was 0.5 g/L and mole ratio of Mg2+/Fe3+ was 3, removal efficiencies of COD, TN and TP were the highest (95%, 89% and 90%), and flocculation ability (FA) was the optimum (43.28%, 44.64% and 43.18%) in anaerobic zone, anoxic zone and oxic zone, respectively. Distribution rule of alkaline phosphatase was Pellet > tightly bound extracellular polymeric substances (TB-EPS) > loosely bound extracellular polymeric substances (LB-EPS) > supernatant. Coexisting Na+, Fe3+ and low concentration of Mg2+ (0.54 mmol/L) increased alkaline phosphatase activity and sludge surface roughness and reduced specific surface area of sludge. Furthermore, these coexisting ions increased EPS concentration, but did not change main compositions and functional groups of EPS.