Influence of cationic polyacrilamide flocculant on high-solids' anaerobic digestion of sewage sludge under thermophilic conditions

Treatment of sewage sludge (SS) by biodegradable polyacrylamide-based flocculants (PAM) is considered to be an effective way to increase total solids' (TS) content prior to anaerobic digestion (AD). However, data on how PAM addition influences the efficiency of AD process are quite contradictory; moreover, no data are available for thermophilic AD (TAD). This study showed that at an optimal inoculum-to-substrate ratio (ISR, 55/45), PAM addition resulted in some decrease in initial methane production during the TAD of SS due to the formation of large flocs (up to 2-3 mm in diameter), which deteriorated the mass transfer. However, at non-optimal ISR (40/60), which led to the destabilization of TAD, PAM addition (40 mg/g TS) could restore the methanogenesis despite the inhibiting accumulation of volatile fatty acids (14-15 g/l) and pH drop (5.5). The observed positive effect of PAM-forced flocculation proposes a new interesting alternative for recovery of 'soured' reactors.