While membrane bioreactors (MBR) have been broadly applied to wastewater treatment, membrane ageing studies have focused mainly on: (i) understanding the chemical action of sodium hypochlorite or (ii) monitoring ageing in direct filtration facilities (drinking water, groundwater, etc.). Thus, a comprehensive investigation under full-scale MBR operating conditions is still required. In the present study, polyvinylidene difluoride (PVDF) hollow fibers were sampled from a full-scale MBR in northern Paris during 7 years of operation. In addition, pristine membranes were aged at bench scale by single soaking in hypochlorite solution at a concentration and pH similar to MBR cleaning protocols. Both bench-scale- and full-scale-aged hollow fibers were characterized using similar analytical methods to assess the contributions of chemical action and operating conditions to the ageing process. At bench scale, membranes experienced no changes in mechanical resistance, but intrinsic permeability (i) first increased as a result of the hydrophilic agent oxidation, i.e., polyvinyl pyrrolidone (PVP), increasing porosity with the appearance of small pores (diameter < 20 nm), and (ii) then the structure seemed to collapse and permeability decreased as a result.
By contrast, membranes aged at full scale experienced a 46% decline in maximum tensile strength probably due to mechanical stress induced by aeration conditions in MBR. Besides, permeability increased for the whole period as a result of a more pronounced oxidation/dislodgement of PVP molecules leading to higher porosity and the appearance of bigger pores (diameter > 40 nm). These changes favored irreversible fouling in contrast to bench-scale ageing. Therefore, bench-scale ageing is not completely representative of full-scale ageing and membrane autopsies must be preferred to this end. However, permeability and PVP content measurements may be applied after cleaning as in situ nondestructive tools in full-scale MBR to monitor membrane ageing. Plant operators should be aware that a continuous increase in intrinsic permeability may favor irreversible fouling accumulation demanding more frequent chemical cleanings.
