Anaerobic digestion of aerobic granular biomass: Effects of thermal pre-treatment and addition of primary sludge

BACKGROUND: Management of the sludge generated in the wastewater treatment plants accounts for more than 50% of their operational costs. To minimise these costs, technologies capable of reducing the production of sludge in the plant need to be developed, such as aerobic granular systems. The aggregation state of aerobic granular sludge (AGS) could be a limiting factor for its further anaerobic digestion. Therefore in this work the feasibility of anaerobic digestion of AGS has been studied under three conditions: (1) raw AGS, (2) thermal pre-treated AGS and (3) a mixture of thermal pre-treated AGS with primary sludge. RESULTS: The values obtained for anaerobic biodegradability and reduction of solids in the case of raw AGS were 44% and 32%, respectively. Thermal pre-treatment of AGS at 133°C enhanced the anaerobic digester performance, in terms of solids reduction, by approximately 47%. The mixture of thermal pre-treated AGS with primary sludge provided better results for solids removal than in the case with only thermal pre-treated AGS. CONCLUSION: Anaerobic digestion of AGS has a similar performance as that reported for waste activated sludge, which indicates that the aggregation of the biomass into granules does not seem to limit the anaerobic process.