Microfiltration process for surface water treatment: irreversible fouling identification and chemical cleaning

This paper identifies the performance of irreversible fouling during microfiltration (MF) for surface water treatment. A pilot-scale MF process was conducted and blocking models were employed to fit the fouling behaviors. Results identified intermediate blocking as the major fouling mechanism. Fouled membranes were then chemically cleaned by sodium hypochlorite (oxidant), sodium hydroxide (base) and hydrochloric acid (acid), sequentially. The order of these chemical solutions based on their ability to restore flux was as follows: oxidant > base > acid. Oxidant cleaning greatly decreased the membrane contact angle compared with base/acid cleanings. Hydrophobicity and molecular weight (MW) distribution of the organics in the cleaning solutions were determined by adsorbent resins and high performance size exclusion chromatography (HPSEC) with peak-fitting. Fouled membranes with and without chemical cleaning were analyzed via Fourier transform infrared spectroscopy (FTIR). It was found that low molecular weight acids and humics (LMWAH), humic substances (HS) and biopolymers (BP) in the fractions of both neutral hydrophilic compounds (NEU) and strongly hydrophobic acids (SHA) were responsible for hydraulically irreversible fouling (HIF); this was also associated with humic-like, protein-like and polysaccharide-like substances. LMWAH in the NEU fraction contributed to chemically irreversible fouling (CIF), which was mainly related to polysaccharide-like substances.