Sludge char-to-fuel approaches based on the hydrothermal fueling IV: fermentation

Sewage sludge was subjected to hydrothermal fueling (HTF) (330 °C for 40 min), obtaining hydrochar at 13.5 MJ kg−1. The higher heating value (HHV) of the raw sludge was related to its fatty acid content. The results showed that although the higher heating value (HHV) of the raw sludge was related to its fatty acid content, with the intensification of HTF, the increase in aliphatic/cyclic amino acids determined the production of HHV in the hydrochar. In order to increase the content of fatty acids and amino acids, the sludge was fermented. However, the Bacteroidetes consumed the organic matter too early, which was detrimental to the production of HHV. Therefore, appropriate sludge fermentation is recommended to restrict excessive Bacteroidetes proliferation, decompose lipids to saturated fatty acids, and convert proteins to aliphatic/cyclic amino acids to increase the efficiency of converting sludge to fuel. HIGHLIGHTS Hydrochar of 13.5 MJ/kg produced at 330 °C for 40 min can be used as fuel.; HHV of sludge relates to fatty acids, while HHV of hydrochar relates to amino acids.; Saturated fatty acids, aliphatic & cyclic amino acids play a role in HHV production.; Over-proliferation of Bacteroidetes harmful to HHV production is to be controlled.