Evaluation of forward osmosis membrane performance and fouling during long-term osmotic membrane bioreactor study

• Activated sludge was treated with CTA and TFC FO membranes over 100 days.
• CTA had higher water flux, lower RSF, and lower SRSF than TFC.
• Biological and organic foulants dominated and were similar for both membrane types.
• TFC had a higher fouling propensity despite smoother and more hydrophilic surfaces.
• Fouling and performance was independent from MLSS, calcium, and TDS concentrations.

Forward osmosis membrane performance and fouling was studied during 100 days of continuous activated sludge treatment. The purpose of the study was to compare the performance and fouling of commercial cellulose triacetate and newly developed polyamide thin film composite membranes that treated high salinity and low salinity activated sludge from two membrane bioreactors. Water flux, reverse salt flux, and specific reverse salt flux were measured to evaluate the performance of virgin and fouled membranes. Membrane autopsy was used to investigate foulant composition and compare physicochemical membrane properties before and after fouling. The results indicated that both membrane types attained steady-state water flux over 100 days, characterized by an initial decline and subsequent steady-state period. Biofouling and organic fouling caused overall water flux decline, in which foulants were identical between membrane and activated sludge types. Water flux results were similar for the two activated sludge types and demonstrated that FO membrane performance and fouling was independent of total dissolved solids, calcium, and mixed liquor suspended solid concentrations. Lastly, virgin membrane properties (i.e., hydrophilicity and surface roughness) did not contribute substantially to membrane fouling. Cellulose triacetate membranes outperformed thin film composite membranes, with lower fouling propensity, higher water flux, lower reverse salt flux, and lower specific reverse salt flux.