Improvement of the biomethanization of sewage sludge by thermal pre-treatment and co-digestion with strawberry extrudate

Abstract

Abstract The management of sewage sludge is an important issue in developed countries due to the highly polluting character of this waste. Biomethanization is a widely employed technology for this purpose, although it has several disadvantages such as low methane yield, poor biodegradability, and high sensitivity to nutrient imbalance. In this paper, a thermal pre-treatment (120 °C, 2 atm) is proposed to improve the biomethanization yield of sewage sludge and strawberry extrudate by solubilizing organic matter from the residual raw materials. Additionally, the co-digestion of sewage sludge with strawberry extrudate is evaluated as this combined treatment allows enhancing the nutrient balance and diluting inhibitors from sewage sludge. Therefore, the main aim of this study is the joint evaluation of the pre-treatment and co-digestion of this waste, which has never been described before in literature. The individual thermal pre-treatment of sewage sludge for 15 min was found to increase the soluble carbon concentration (mainly as volatile fatty acids), nitrogen, and phosphorus by 165%, 16%, and 24%, respectively. In contrast, the variation in the concentration of soluble carbon, nitrogen, and phosphorus for strawberry extrudate was as low as 10%, 32%, and 43%, respectively. Subsequent individual biomethanization tests showed a positive relationship between substrate solubilization and methane yield enhancement, with an increase of around 29% and 16% for sewage sludge and strawberry extrudate, respectively. Moreover, the co-digestion of sewage sludge and strawberry extrudate, both without pre-treatment, enhanced the stability and diluted the nitrogen concentration inside the digesters, although the methane yield was slightly lower than expected. In contrast, the most suitable combination was found to be the co-digestion of pre-treated sewage sludge and raw strawberry extrudate as it was also stable, but showed a synergy in methane production (237 ± 29 mLSTP/g VS; STP: 0 °C, 1 atm). This combined treatment might be considered an interesting alternative for the combined treatment of both polluting wastes.