In Situ Biofiltration of Dissolved Organic Carbon in Reverse Osmosis Membrane Filtration

Biofouling, or the formation of biofilm on membrane surfaces, can decrease the performance (decreased flux and/or increased operating pressure) of a reverse osmosis (RO) membrane system in a water treatment plant. However, biofilms have been used in water treatment systems to remove organic carbon from water via biofilters and successfully reduce biofilm growth downstream. This research investigates the possibility that the heterotrophic biofilm present on membrane surfaces removes nutrients from the treatment water, thereby making it nutrient deprived as it travels along the treatment train. This may potentially be exploited as an in situ biofilter to actively remove dissolved organic carbon (DOC) from the treatment water, thereby protecting downstream membrane surfaces from biofouling. Analysis of fouled membranes from the Dunedin water treatment plant in Dunedin, FL indicates the presence of biofilm on membrane surfaces in a gradient pattern with a higher level of fouling at the front of the element. Additionally, the community structure of the biofilm at the front of the element is unique with respect to the feed-water and downstream membrane material. Additionally, a carbon (and nitrogen) mass balance study was performed at the water treatment plant in Dunedin, FL through extensive sampling of DOC at multiple locations of the RO membrane system over a 20 month period. Plant-level mass balance results indicate a significant pool of DOC was consistently unaccounted for, and presumably assimilated or otherwise removed within the membrane system. Sampling also indicated a removal of total nitrogen. Additionally, the specific UV absorbance (SUVA) of the DOC in concentrate was consistently greater than that of the feed water, suggesting the removal of labile aliphatic carbon as the feed water travels through the feed channel of the membrane system. A pilot system was designed and built to operate under plant conditions (flow rate and pressure) to test if the biofilm on the surface of the membrane can have a protective effect for downstream membrane material. A fouled membrane element was pulled from the plant at the same time and general location as an autopsied element (to determine composition on the surface) and used in the pilot system. Feed and concentrate water from the pilot was directed to flat sheet modules for performance testing and surface characterization. This allowed for characterization of the two sections without disturbing the membrane element. Differences in performance and foulant deposition were characterized for the two sections as a function of carbon addition and flow rate. The results from this testing suggest the membrane element, or the biofilm on its surface, has both a performance and a foulant deposition benefit for downstream membranes as compared to feed membrane material. This benefit also displayed an increasing trend as the concentration of organic carbon fed into the system increases