Thermophilic sidestream anaerobic membrane bioreactors: The shear rate dilemma

Anaerobic biomass retention under thermophilic conditions has proven difficult. Membrane filtration can be used as alternative way to achieve high sludge concentrations. This research studied the feasibility of anaerobic membrane bioreactors (AnMBRs) under thermophilic conditions. A sidestream MBR was operated at crossflow velocities up to 1.5 m/s. For comparison, a thermophilic upflow sludge blanket reactor also was operated. Results confirmed that biomass retention may limit the performance of sludge bed reactors during long-term operation. During MBR operation, cake formation was identified as the key factor limiting the applicable flux. Low levels of irreversible fouling were observed. Even though high shear can provide an increase in particle backtransport, exposure of the sludge to a high shear stress produced a reduction of particle size, affecting the attainable flux. The concept of "shear rate dilemma" is introduced to describe this dual effect of shear during the operation of MBRs.