Effect of organic loading rates on the membrane performance of a thermophilic submerged anaerobic membrane bioreactor for primary sludge treatment from a pulp and paper mill

The effect of organic loading rates (OLRs) on membrane performance and mixed-liquor suspended solids (MLSS) properties were examined using a laboratory-scale thermophilic submerged anaerobic membrane bioreactor (ThSAnMBR) that treats primary sludge from a thermomechanical pulp mill. Membrane performance was evaluated by monitoring its flux and corresponding transmembrane pressure (TMP). Changes in membrane chemical and physical properties resulting from operating conditions were investigated using Fourier transform infrared (FTIR), scanning electron microscopy (SEM), contact angle, energy-dispersive X-ray analyzer (EDX), and pore size measurement. While MLSS properties are analyzed by X-ray photoelectron spectroscopy (XPS), zeta potential, and particle size distribution (PSD) analysis. Membrane performance and MLSS properties were studied at different OLRs ranging from 1.5 to 3.9 kg-TSS/m(3)d, and hydraulic retention times (HRTs) of 3-8 d. This study showed that the increased OLR reduced the membrane performance and degradation of the primary sludge, as well as the biogas yield. This result indicates that OLR of less than 1.5 kg TSS/m(3)d, HRT of 8d, and solids retention time (SRT) of 32d should be maintained to achieve stable membrane performance and better biogas production. Also, results showed that important changes occurred on the membrane morphology and MLSS characteristics under the tested operating conditions. High-resolution SEM images reveal distinct differences in the pore morphology between the virgin and used membranes indicating the effect induced by the operating conditions. Thermophilic temperature led to an expansion of membrane pores. Overall, the primary sludge from pulp and paper mills can be treated successfully by ThSAnMBR for methane production with stable membrane performance and high treatment efficiency.