Effects of sodium hypochlorite on structural/surface characteristics, filtration performance and fouling behaviors of PVDF membranes

Sodium hypochlorite (NaClO) is a common chemical cleaning agent in water treatment due to its effective removal of irreversible pollutants that cannot be removed by physical cleaning. In this research, PVDF ultrafiltration (UF) membranes were exposed to NaClO for 120–960 h, and then the structural/surface characteristics of the control and exposed membranes were compared. Results showed that different exposure doses lead to variations in PVDF membrane structural/surface characteristics according to different mechanisms and further influence the permeability and fouling behaviors of PVDF membranes during ultrafiltration (UF) processes. Reversible fouling was the most common fouling behavior of membranes and exposure to NaClO resulted in higher degree of irreversible fouling. The irreversible flux decline of the membranes could not only caused by physically irremovable fouling but also irreversible compaction of the membranes due to the formation of macro-voids in the membrane skin layer. The changes in membrane mechanical properties caused a sharp increase in the inherent membrane resistance (Rm) which was generally considered as a constant. ‘Dynamic resistance mechanism’ could be proposed based on the impact of different NaClO exposure doses on the total membrane resistance (Rt ), which is the sum of the changes in Rm and the variation of fouling resistance (Rf) resulted from the changes in surface characteristics of the membranes. According to this concept, the filtration operating conditions and cleaning processes could be optimized in water treatment.