Fluorescent compounds retained by ultrafiltration membranes for water reuse

Abstract

Ultrafiltration (UF) membranes are increasingly being applied in water treatment and reuse processes because they provide effective removal of microorganisms, suspended solids, and high molecular weight compounds. However, membrane fouling necessitates backwashing and periodic chemical cleaning and, as a result, causes an increase in the amount of energy and overall cost for operation. Dissolved organic matter (DOM) has been identified as one of the major causes of biofilm formation and fouling during membrane ultrafiltration. Yet there is still much to be understood regarding how DOM interacts with biofilms, with some studies suggesting that labile DOM fractions may be preferentially removed by biogeochemical interactions with the biofilm. In this study, we evaluated the effects of biofilm formation on the retention of different organic compounds by measuring dissolved organic carbon, total dissolved nitrogen, DOM fluorescence peak intensities, and also performed parallel factor analysis (PARAFAC) modeling of fluorescent components to characterize the feed, permeate, and backwash during UF membrane fouling. We used a pilot scale, inside-out flow, UF membrane with a 0.03 μm pore size and different synthetic wastewater solutions as feed water. To evaluate the role of biofilms in preferential removal of DOM, unsterilized and sterilized synthetic wastewater solutions were run for several cycles. Results indicate that ultrafiltration for both solutions preferentially removed the biologically labile tryptophan-like components compared to the humic-like components. The similar percentage of removal of tryptophan-like components in both experiments suggest that the membrane biofilm on polyethersulfone membranes has little influence on the removal of biodegradable organic constituents. These findings also demonstrate that fluorescence spectroscopy can be a valuable tool to discriminate between different DOM fractions retained and passing through UF membrane filters.