Implications of changing solids retention time on long-term evolution of sludge filterability in anaerobic membrane bioreactors

The success and widespread application of aerobic membrane bioreactors (MBRs) stimulated the combination of membrane filtration and anaerobic processes for municipal and industrial wastewater treatment. Likewise, their anaerobic counterparts (AnMBRs) take advantage of membrane filtration for the separation of biomass from effluent and provide complete sludge retention.

The AnMBR is an emerging treatment technology for industrial wastewaters/slurries with extreme properties, which often cause biomass retention problems in conventional anaerobic granular sludge bed reactors.

However, AnMBRs inherit some constraints related to the membrane filtration process, such as membrane fouling, high investment and operation costs, and process complexity with increased automation needs due to additional pumps and pressure sensors used on membrane modules. Membrane fouling seems to be most important constraint, limiting the widespread application of AnMBRs.

In this study by researchers in the Netherlands, Turkey and France, long-term experiments were conducted to assess the impact of changing the SRT on sludge filterability in AnMBRs used to treat corn-based bioethanol thin stillage.

Well established parameters, such as capillary suction time (CST) and specific resistance to filtration (SRF), developed for sludge dewatering, were used to evaluate the SRT effect on sludge filterability.

The results clearly demonstrate that SRT is one of the most important factors influencing sludge filterability in AnMBRs. SRT affects the accumulation of fine particles and solutes, which were found to affect attainable flux and fouling, in reactor broth.

Better filterability was observed at an SRT of 20 days compared to elevated SRTs, i.e. 50 days.

However, a clear correlation between sludge filtration characteristics and membrane filtration resistance could not be established, especially at short SRTs, whereas many parameters such as total suspended solids (TSS), CST, soluble microbial products (SMP), and supernatant filterability were found to be mutually correlated.

Net membrane fluxes between 9 and 13 L/m2h were obtained at 0.5 m/s crossflow velocity, and the long-term fouling was controlled by using frequent filtration and backwash cycles.

Water Research, Volume 59, 1 August 2014, Pages 11–22.