The study of structure of anaerobic granules and methane producing pathways of pilot-scale UASB reactors treating municipal wastewater under sub-mesophilic conditions

This study was carried out to investigate the relationship between the methane producing pathways and the characteristics of anaerobic granules treating municipal wastewater. For this purpose, two pilot scale upflow anaerobic sludge blanket reactors with different granule size distribution (1-2 mm and 3-4 mm) were investigated at operating temperatures of 20 °C and 28 °C for 239 days. There was an increased and stable biogas production when temperature was elevated to 28 °C likely due to reduction in methane solubility. Larger granules had multi-layered internal microstructures with higher acetoclastic methanogenic activities (250-437 mL CH₄ g^-1 VS d^-1) than smaller granules (150-260 mL CH₄ g^-1 VS d^-1). The relative abundance of acetoclastic methanogens of larger granules was higher, confirming acetoclastic methane producing pathway was more prominent. However, there was no significant difference in the performance of the two reactors because they were operating below their capacities in terms of organic loading rate to volatile solids ratio.