Assessment of wetting phenomena and treatment performance in anaerobic membrane distillation bioreactor (an-MDBR) for pulp and paper industry

Graphical abstract summarizing the An-MDBR treatment concept for pulp and paper wastewater. The image shows wastewater entering an anaerobic CSTR, followed by membrane distillation using a PTFE membrane, producing treated water and biogas. A temperature comparison section presents 35, 45, and 55°C operating conditions with flux, COD removal, fouling, contact angle, and wetting behavior. The graphic highlights 45°C as the optimal condition and shows membrane characterization methods including FTIR, Raman, XRD, and contact angle analysis. This study investigates the performance of a lab-scale anaerobic membrane distillation bioreactor (An-MDBR) for treating synthetic pulp and paper (P&P) wastewater under mesophilic (35–45 °C) and thermophilic (55 °C) conditions. The An-MDBR integrates an anaerobic continuous stirred tank reactor (CSTR) with a submerged membrane distillation (MD) module in a side-stream configuration. Wastewater with chemical oxygen demand (COD) of 1,300 ± 130 mg/L was treated to evaluate COD removal, biogas production, membrane fouling, and operational stability. Daily COD removal of 23.1 g/day (76–84% from CSTR) was observed at a flow rate of 21.6 mL/day, yielding specific methane (CH4) production of 0.89–1.42 mL CH4/g COD removed. Optimal performance occurred at 45 °C, balancing flux stability (17 LMH), high organic removal, and methane-rich biogas (50.5% CH4; 1.2 ± 0.1 mL CH4/g COD). Biomass parameters (VFA/alkalinity <0.5; mixed liquor volatile suspended solid/mixed liquor suspended solid >0.7) indicated stable microbial activity, with near-complete removal of PO43−, >90% SO42−, and approximately 40% total Kjeldahl nitrogen (TKN). Membrane fouling, dominated by organic deposition and high temperature-induced inorganic scaling confirmed by Fourier Transform Infrared Spectroscopy, Raman, and X-ray diffraction analyses, caused progressive hydrophobicity loss, with contact angle decreasing from 111.6° (pristine) to 10.3° at 55 °C, leading to irreversible pore wetting and operational instability at thermophilic conditions.