Start-up and HRT Influence in Thermophilic and Mesophilic Anaerobic Digesters Seeded with Waste Activated Sludge

Since thermophilic anaerobic digestion represents an efficient alternative to mesophilic anaerobic digestion, multiple studies have been developed to compare their performance and viability. One of the problems related to thermophilic anaerobic digestion is the availability of an adequate seed to start-up the process. The goal of this study is to evaluate the possibility of using waste activated sludge (WAS) as a seed for both mesophilic (35 °C) and thermophilic (55 °C) anaerobic digesters fed with a real sludge waste (primary and secondary sludge mixture) based on the gradual substitution of synthetic substrate by real feed. The obtained results show that mesophilic and thermophilic anaerobic digesters were rapidly stabilized within 60 and 85 days at a hydraulic retention time (HRT) of t = 35 and 30 d with 450 and 520 mL biogas per g CODt added and 65 % and 72 % as methane content, respectively. Moreover, HRT was progressively reduced in order to assess the maximum organic load that can be treated in the thermophilic reactor. The minimum HRT reached was t = 8 d with a VS removal efficiency of 50.32 % and a biogas yield index of 440 mL biogas per g VS added (54 % as methane content).