Rapid Biofilm Start-Up and Treatment of Oilfield Produced Water Using a Modified Polyurethane Biosponge Carrier

Oilfield-produced water contains petroleum hydrocarbons, suspended solids, salts and other refractory constituents that challenge conventional biological treatment. Suspended activated sludge systems can suffer from microbial washout, poor retention of slow-growing functional microorganisms and unstable settling, whereas conventional ceramic biofilm packings often provide limited surface area and slow biofilm start-up. Here, a modified polyurethane biosponge was evaluated as a three-dimensional carrier for immobilized microbial growth during the treatment of real oilfield produced water. Three parallel systems were compared under identical start-up and operating conditions: suspended activated sludge without a carrier, Raschig ring-packed biofilm reactors, and the biosponge-based composite system. The biosponge system developed a visible and stable biofilm within 7 days, whereas the Raschig ring system showed only sparse and uneven biofilm coverage. During an 8-day treatment test with real produced water, the biosponge system decreased the COD concentration from 800 to 13 mg L−1 in the aerobic zone and from 1500 to 16 mg L−1 in the anoxic zone, corresponding to removal efficiencies of 98.4% and 98.9%, respectively. In contrast, the activated sludge and Raschig ring systems showed much lower COD removal under the same test period. 16S rRNA gene sequencing indicated that the biosponge selectively enriched petroleum hydrocarbon-associated genera, including Sphingopyxis, Achromobacter and Gordonia, in the aerobic zone, together with fermentative and anaerobic genera such as Dysgonomonas and Clostridium in the anoxic zone. PICRUSt2-based functional prediction further suggested the coexistence of aerobic hydrocarbon degradation, chemoheterotrophy, fermentation and nitrate respiration potentials. These results indicate that the modified polyurethane biosponge provides a favorable carrier for rapid biofilm establishment and short-term removal of organic pollutants from oilfield produced water. Further long-term operation, adsorption controls and direct functional gene validation are recommended to confirm the engineering robustness and degradation mechanisms of the system. The system also avoided sludge bulking and maintained short-term stability, providing a promising low-cost strategy for treating high-salinity refractory oilfield produced water.