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 degrees C and ultrasounds at 66.7 kWh/m(3), 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. (C) 2014 Elsevier Ltd. All rights reserved.