“Weak” ultrasonic pre-treatment on anaerobic digestion of flocculated activated biosolids

This study examined how “weak” ultrasonic pre-treatment affects anaerobic digestion of waste biosolids, treated with a cationic polyelectrolyte flocculant. In relation to pre-treatment, the term “weak” used refers to the fact that the total ultrasonic energy input to biosolids is insufficient to fully disrupt its floc structure or the cell walls, as described in the literature. Methane production potential, floc characteristics (size, morphology and ζ-potential) and process parameters (chemical oxygen demands and oxidative-reductive potentials) were monitored, as the digestion was tested. The presence of polyelectrolyte flocculants enhanced methane production within 6 days of digestion (phase I), but inhibited the digestion thereafter (phase II). Following pre-treatment by sonication (0.33 W/mL, 20 min), the methane productions of both original and flocculated biosolids significantly increased. The underlying mechanisms of weak ultrasonic pre-treatment are discussed.