Long-term fouling and performance of forward osmosis membranes treating activated sludge and oil and gas produced water

Abstract

The thesis is comprised of two studies that compared the performance and fouling of cellulose triacetate (CTA) and polyamide thin film composite (TFC) forward osmosis (FO) membranes. The first study used CTA and TFC membranes to treat high-salinity and low-salinity activated sludge for 100 days, and the second study used CTA and improved TFC membranes to treat oil and gas produced water for three weeks. Both studies evaluated virgin and fouled membrane performance by measuring water flux, reverse salt flux (RSF), and specific reverse salt flux (SRSF). Membrane autopsies were performed to understand the importance of physiochemical membrane properties on fouling potential and to determine the organic and inorganic composition of fouling layers. The second study expanded on the first study and used chemically enhanced osmotic backwashing for membrane cleaning, streaming potential analysis of membrane surfaces, and gas chromatography-mass spectroscopy for evaluating the rejection of volatile and semi-volatiles hydrocarbons by the FO membranes. The studies found that CTA membranes outperformed TFC membranes with higher water flux, lower RSF, and lower SRSF. TFC membranes had more ideal virgin membrane properties (e.g., smooth, hydrophilic, and neutrally charged surfaces), but were more prone to fouling and demonstrated that conventional characterization techniques do not accurately predict fouling potential. In both studies, the organic composition of the fouling was similar for both membrane types, but the inorganic fouling varied according to membrane type, wastewater composition, and cleaning. The second study found over 90% organic compound rejection for both membranes, in which TFC membranes had superior rejection of neutral hydrophobic compounds.