Microbial population shifts during disturbance induced foaming in anaerobic digestion of primary and activated sludge

Foaming during anaerobic digestion (AD) of sewage sludge is poorly understood and remains an uncontrollable operational obstacle for sewage treatment systems globally, causing mechanical damage, increased hazards and reduced biogas recovery. Foams during AD commonly occur after process disturbances, such as organic loading shocks. However, it is still unclear whether these foam events are biologically driven and linked to the abundance of organisms like filamentous or hydrophobic bacteria. A time-series study was conducted, comparing digestion performance, microbial community succession, metagenomes, and metabolomes in six anaerobic continuous stirred-tank reactors (CSTRs): a control group fed normally (n = 3), and one treated group inhibited through organic shock loading of more than twice the steady state loading rate with glycerol (treatment, n = 3). As soon as microbial activity and methanogenesis recovered after inhibition, significant volumes of foam accumulated simultaneously in the reactor headspace of the three treated CSTRs. Microbial abundance profiles (16S rRNA, V3-V4) from 165 days of operation showed that filamentous or mycolic acid-producing organisms were not associated with this foam event. Shock loading led to acidification, biomass decline and microbial imbalance, contributing indirectly to the foam event. During that period, metabolomes and functional pathway abundances indicated that the stressed microbial biomass was enriched in long-chain fatty acids prior to foaming. This biomass, combined with pH changes, may have modified the physicochemical properties of sludge, leading to the fractionation of organic mass once gas production resumed. More research is needed to understand how abiotic and biotic interactions contribute to foam formation.