Membrane fouling analysis and chemical cleaning optimization via membrane autopsy for membrane bioreactor from lab to engineering application

Chemical cleaning is critical to maintain the long-term stable membrane filtration for membrane bioreactor (MBR) process. Membrane fouling analysis is the prerequisite for optimizing chemical cleaning. Membrane autopsy including scanning electron microscopy with energy disperse spectrum (SEM-EDS), attenuated total reflection Fourier transform infrared spectrum (ATR-FTIR), front-face fluorescence excitation-emission matrix (FF-EEM) and specific clean water flux (SCWF) was employed in this study to reveal the combined organic–inorganic-biological fouling of hollow fiber ultrafiltration membrane from a large MBR plant, where average 160k m³/d mixed municipal (85%) and industrial (15%) wastewater was treated. The lab-scale fouled membrane cleaning tests showed that the optimal concentration of sodium hypochlorite (NaClO) was active chlorine of 2400 mg/L with pH of 11 and the optimal cleaning time was 12 h, the optimal concentration of oxalic acid (H₂C₂O₄) was 10 g/L (equivalent pH of 1.42) and the optimal cleaning time was 8 h, and the optimal cleaning sequence was H₂C₂O₄ followed by NaClO. H₂C₂O₄ cleaning mainly removed inorganic foulants and destroyed the connection between inorganic substances and organic substances, which was beneficial to the effective removal of organic-biological foulants by the following NaClO cleaning. The engineering-scale cleaning test in a 300 m³ membrane tank in the MBR plant demonstrated that the above-mentioned optimal cleaning was superior to the routine cleaning of NaClO followed by citric acid (C₆H₈O₇), the reagent cost was reduced by 43.1%, and the cleaning time was reduced by 37.5%. Thus, NaClO should be the dominant cleaning reagent and acid cleaning should be implemented before NaClO cleaning if necessary. The SCWF measurement revealed that the upper membrane unit suffered less fouling due to its lower flux caused by more sludging in membrane fibers than the lower membrane unit in double-deck cartridge. Therefore, air scouring along membrane fibers should be uniformly distributed especially for the upper membrane unit to release its filtration capacity.