Effect of thermal hydrolysis and ultrasounds pretreatments on foaming in anaerobic digesters

• Foaming is a common problem in anaerobic digesters at WWTP.
• Thermal hydrolysis at 170 °C mitigated foaming in continuous pilot scale reactors.
• Thermal hydrolysis and ultrasounds are efficient tools to prevent foaming.
• Filamentous bacteria abundance is drastically reduced after pretreatments.
• Foam potential and stability parameters do not predict anaerobic foaming.

Foam appears regularly in anaerobic digesters producing operational and safety problems. In this research, based on the operational observation at semi-industrial pilot scale where sludge pretreatment mitigated foaming in anaerobic digesters, this study aimed at evaluating any potential relationship between foaming tools applied to activated sludge at lab-scale (foam potential, foam stability and Microthrix parvicella abundance) and the experimental behavior observed in pilot scale and full-scale anaerobic digesters. The potential of thermal hydrolysis and ultrasounds for reducing foaming capacity was also evaluated. Filamentous bacteria abundance was directly linked to foaming capacity in anaerobic processes. A maximum reduction of M.parvicella abundance (from 5 to 2) was reached using thermal hydrolysis with steam explosion at 170 °C and ultrasounds at 66.7 kWh/m3, showing both good anti-foaming properties. On the other hand, foam potential and stability determinations showed a lack of consistency with the bacteria abundance results and experimental evidences.