Membrane filtration-based recovery of extracellular polymer substances from excess sludge and analysis of their heavy metal ion adsorption properties

Excess sludge production by wastewater treatment plants (WWTPs) represents a serious environmental problem that necessitates the development of effective sludge treatment/disposal methods. Excess sludge contains valuable materials such as extracellular polymer substances (EPSs), and their recovery is of great practical importance. However, conventional recovery methods afford EPS solutions with moisture contents of ∼100% and the dehydration of these solutions via direct drying methods is considerably energy consuming. Here, we investigate the separation of EPSs extracted from excess sludge by microfiltration (0.5 μm) and ultrafiltration (10 kDa), revealing that Ca2+ addition reduces the extent of membrane fouling during EPS isolation since the interactions between Ca2+ and EPS carboxylate groups increase the size and reduce the concentration of colloids/polymers in EPS suspensions. Additionally, we establish an ultrafiltration-based EPS separation protocol, demonstrating that it allows highly efficient EPS recovery (>90%) irrespective of the Ca2+ concentration, and show that EPSs extracted from excess sludge samples derived from two different WWTPs exhibit similar filtration resistances at Ca2+ concentrations of 0-4 mM. Finally, we demonstrate that the EPSs recovered from excess sludge can be used as adsorbents for the removal of diverse heavy metal ions (Pb2+, Cd2+, Cu2+) from wastewater, exhibiting performances that are on par with those of commercial adsorbents and are unaffected by the employed extraction procedure. Based on the obtained results, Ca2+ addition is confirmed to be a viable method for reducing filtration resistance during the membrane recovery of EPSs, since this ion does not affect the adsorption capacity for heavy metal ions.