Causes and effects of foaming in anaerobic sludge digesters

Full scale anaerobic digesters treating mixed sludge containing primary sludge and thickened waste activated sludge were investigated for causes and effects of foaming. Sludge samples were collected from different depths of two full scale digesters, one gas-mixed, and the other mechanically-mixed, to determine the extent of foaming and its effects on anaerobic digestion; both digesters were fed the same feed sludge (3.4% TS) and at the same feed rate (about 2.2 kg TS/m3.day ). The average depth of the surface foam layer in the gas-mixed and mechanically-mixed digester were 2.4 and 1.3 m respectively. Higher total solids concentrations were found at the surface (about 5% TS) than those found at the bottom (about 2% TS) in both gas-mixed and mechanically-mixed digesters, indicating an inverse total solids profile. Presence of excessive levels of Nocardia filaments (> 106 numberlg VSS) in the activated sludge caused thicker foam layer at the surface, and this effect was more pronounced in the gas-mixed digester than in the mechanically-mixed digester. Gas-mixed digester (0.74 m3/kg VS destroyed) produced less sludge gas than the mechanically-mixed digester (0.93 m3/kg VS destroyed), however, gas-mixed digester (62% VS reduction) destroyed more volatile solids than the mechanically-mixed digester (54% VS reduction). These results indicate that gas-mixed digesters are more prone to foaming than mechanically-mixed digesters, and that the foaming can increase when excessive levels of Nocardia filaments are present in the feed sludge causing decreased digester performance.