The effect of UV pre-treatment on biofouling of BWRO membranes: A field study

Abstract

The use of biocides, particularly chlorine, in reverse osmosis (RO) desalination is widely practiced despite documented evidence that although biocides may be advantageous in controlling microbial counts in the water, in some cases they can actually exacerbate biofouling of the membranes. The adverse effects, associated with widespread biocide use, have spurred the need for finding alternative RO pre-treatment disinfection methods. Ultraviolet (UV) disinfection, and specifically medium pressure ultraviolet (MP-UV) disinfection has been considered a possible alternative and is now gaining recognition as a viable disinfection method applicable to RO desalination; however documentation on the effects of UV pre-treatment on RO membrane biofouling is scarce. This paper reports the findings from a four month field study conducted at a brackish water reverse osmosis (BWRO) desalination plant, treating groundwater, in the north of Israel, in which MP-UV was applied as a pre-treatment disinfection step prior to RO desalination. The plant contains two double stage desalination trains that operate in parallel — one train served as a reference, while in the other an Atlantium HOD™MP-UV system was installed. Both trains were run in parallel, and for the duration of the study, all normalized performance parameters were collected and microbial counts monitored. At the end of the run previously replaced sacrificial membranes, situated in the front of the first stages, were autopsied and various biofilm analyses were conducted to elucidate cell/extracellular polymeric substances (EPS) content and microbial speciation. The overall results suggest that MP-UV pre-treatment prolonged the train performance, which manifested itself in a lower relative normalized permeate flux decline vs. the train which received water without MP-UV pre-treatment (11% vs. 17%, respectively). Significantly less EPS was found on the RO membrane which received MP-UV treated water. The differences in biofilm thickness and cell density counts (cells/cm2) between the two membranes were notable, in favor of UV pre-treatment, yet less significant. The MP-UV pre-treatment also had a substantial effect on biofilm community composition; the RO membrane that received MP-UV disinfected water exhibited a biofilm in which the diversity was reduced by 30% and more, and did not contain certain phylogenetic groups that were detected on the RO membrane that received water without MP-UV pre-treatment. It can be concluded that pre-treatment disinfection using MP-UV may be a promising option for combating biofouling of RO membranes and prolonging operation of the trains between cleaning regimes.