Evaluating energy efficiency in anaerobic membrane bioreactors: Effect of filtration/backwash cycle duration on net energy balance

In this study, a dynamic model based on the AM2b anaerobic digestion framework was modified to simulate an anaerobic membrane bioreactor, including the return of detached biomass to the bioreactor bulk during physical backwashing. The identified model accurately captured the evolution of soluble microbial products, achieving a determination coefficient of 0.88, and its relevance was further confirmed through a post-operation characterization of the fouling layer, with relative errors between predicted and measured transmembrane pressures ranging from only 3.2% to 16.6% following refinement of fouling-related parameters. Then, the model was used to optimize the net energy balance (NEB) by simulating scenarios operated at filtration/backwash cycle durations (T_cyc) ranging from 5 min to 8 h. Results indicate that while total energy consumption remains relatively stable, methane production and NEB are affected by cycle duration. Longer cycles result in decreased biogas recovery due to membrane fouling and reduced microbial activity. Although the permeate pump represents only 6% of total energy use, its contribution increases slightly with extended cycles. The NEB reaches its maximum at short cycles (5-10 min) and declines progressively, with losses of up to 9% at 8-hour cycles. These results underscore the critical role of T_cyc optimization and the underlying attachment-detachment dynamics of the fouling layer.