Freezing/thawing effects on anaerobic digestion of mixed sewage sludge

This study examined the effects of freezing/thawing disintegration on the characteristics of mixed (primary and waste) sewage sludge from municipal wastewater treatment plants. It also assessed the effects of freezing/thawing on anaerobic digestion, and its consequences for biogas production and digester supernatant quality. Freezing/thawing caused a decrease of more than 10% in the total chemical oxygen demand (COD), total nitrogen (TN), volatile solids (VS) and total solids (TS). A simultaneous doubling of the soluble COD and volatile fatty acids (VFA) occurred. Release of nitrogen and phosphorus compounds was also found.The biogas yield obtained from frozen/thawed sludge was 1.31 m3 kg−1 of removed VS; this exceeded approximately 1.5 times the value for the raw sludge.On the basis of the global mass balance it was indicated that freezing/thawing of the mixed sewage sludge followed by anaerobic digestion could be considered as a “double-phase digestion” rather than a pre-treatment method.