Characteristics of membrane fouling by consecutive chemical cleaning in pressurized ultrafiltration as pre-treatment of seawater desalination

• Effect of chemical cleaning on pressurized UF as pre-treatment for desalination
• NaOCl and oxalic acid showed the optimum results for recovering flux rate
• The optimum cleaning combination was oxalic acid–NaOCl–oxalic acid.
• Inorganic salts were found to adhere more on the membrane surface.
• Hydrophobic foulants were removed easily by chemical cleaning on the surface.

Chemical cleaning of membranes is one of the most important strategies in pressurized hollow fiber ultrafiltration (UF) as a pre-treatment for seawater desalination. Various physical cleaning strategies such as backwashing, aeration and air-scrubbing or chemically enhanced backwashing (CEB) have been investigated in order to remove foulants from the UF membrane. However, the limitation in their cleaning effects being found during long-term operation leads to the need of cleaning in place (CIP) for the recovery of membrane performance. In this study, we used oxalic acid and sodium hypochlorite as chemical cleaning agents. The cleaning in series of oxalic acid–sodium hypochlorite–oxalic acid showed the optimal cleaning efficiency and was applied for the consecutive chemical cleaning. The recovery efficiency of the CIP after first, second, third and fourth cleanings was 96.8%, 95.8%, 98.3% and 99.9%, respectively. It was almost fully recovered to the previous recovered value. However, membrane surface structure was deformed by contact with chemical cleaning agents during cleaning time, because, hydrophilic inorganic foulants are still adhered on the membrane surface even after several cleanings although hydrophobic organic foulants were removed easily by chemical cleaning. An improved CIP strategy should be developed to remove hydrophilic foulants for long-term operation of desalination plants.

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