Cleaning and ageing effect of sodium hypochlorite on polyvinylidene fluoride (PVDF) membrane

Many membrane suppliers recommend sodium hypochlorite (NaOCl) as cleaning agent. Despite the popularity of this chemical for membrane cleaning, there is still a lack of publications regarding NaOCl short- and long-term effect on fouling removal. To properly conduct this cleaning study (i.e. obtaining the required level of reproducibility and confidence), a rigorous methodology was proposed and applied in this paper. The results indicated that cleaning efficiency varied between single and cyclical (i.e. repeated fouling/cleaning cycles) cleanings: While 1% of NaOCl provided 95% efficiency in single cleaning, only 87% removal efficiency was observed during cyclical cleaning. As expected, foulant was found to be more difficult to remove during cyclical cleanings. The potential effect of NaOCl on polymeric materials ageing has drawn attention and recent studies have been dedicated to assess the impact of its exposure on numerous membrane materials. Still, the effect of NaOCl on polyvinylidene fluoride (PVDF, commonly used in membrane material for water and wastewater treatment) is discussed in limited references only. Under accelerated cleaning conditions (2% NaOCl), Fourier transform infrared (FTIR) analysis showed chemical changes in membrane functional groups of the PVDF flat sheet membrane, indicating possible ageing effect. This ageing study also revealed that PVDF membrane exhibited a two-step-degradation mechanism: first, the removal of its surface modification substance, followed by the increase of its relative hydrophilicity. These results were confirmed by X-ray photoelectron spectroscopy (XPS), FTIR spectroscopy, contact angle and hydraulic measurements.