Removal of heavy metal ions by ultrafiltration with recovery of extracellular polymer substances from excess sludge

Abstract

To solve the difficult problem of separating extracellular polymeric substances (EPSs) with adsorbed heavy metal ions (HMIs) from aqueous solutions, EPS recovery and HMI adsorption were combined into dead-end ultrafiltration (EPS-enhanced ultrafiltration, EPS-UF). First, an EPS solution was concentrated via UF into a cake on the membrane, which was then used to filter heavy-metal-containing wastewater. EPS-UF could effectively remove HMIs in wastewater, with a 94.8% Pb2+ removal efficiency and 85.5% EPS recovery efficiency with concentrations of 0.1 g/L EPS and 10 μM Pb2+. During concentration, the filtration resistance increased with increasing filtration pressure because of the high compressibility of the EPS cake and as filtration proceeded. During removal, a high filtration pressure (200 kPa) could not increase the filtration rate, and the removal efficiency of Pb2+ decreased markedly (78.9%). Ca2+ was proposed as a strategy for controlling membrane fouling because it was completely released from the cake after adsorbing Pb2+ and barely affected the characteristic groups in EPS. Fourier transform infrared and X-ray photoelectron spectroscopy revealed that carboxyls or esters from carboxylate and uronic acids in EPS bonded with HMIs via ion exchange or complexation, and the main characteristic groups in EPS remained unchanged, while more complex proteins formed.