The aim of the present work was to evaluate two different strategies to convert a continuous mesophilic anaerobic reactor into thermophilic, treating sewage sludge. The compared strategies were a single direct temperature change, from 35° to 55°C, and a multi-step temperature change (35°–43°–50°–55°C). The single direct step strategy was shown to be as effective as the more conservative multi-step strategy, recovering the system efficiency after 20 days in terms of methane yield, although there was a higher volatile fatty acids accumulation and higher propionate/acetate ratio values during the temperature transition. From the results of the multi-step strategy it could be stated that the critical process performance during temperature transition occurred between 43 and 50°C, which should be taken into account to define more accurate future transition strategies. Sewage sludge thermophilic vs mesophilic biodegradability was assessed by means of anaerobic biodegradability tests, and no significant statistical differences in biodegradability or methane yields were found.

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