Enhanced biogas from sewage sludge digestion using iron nanocatalyst from Vitex negundo leaf extract: response surface modeling

Abstract

In this study, Vitex negundo leaf extract (VN) were utilized as a potential reducing agent to enable greener synthesis of iron nanocatalyst (VN-FeNCs). The synthesis time and ratio of extract to precursor and pH of extract for green synthesis of VN-FeNCs were optimized. Furthermore, the synthesized VN-FeNCs were characterized for morphological, functional and spectral properties. VN-FeNCs were supplemented for the anaerobic digestion of the sludge. Augmentation of VN-FeNCs improvised the methane yield with reduction of chemical oxygen demand up to 86% and volatile solids up to 93.6% with a retention time of 12 days. The highest biogas yield of 0.45 (L/g VS reduced) is obtained with VN-FeNCs aided digestion whereas digestion with bulk salt and control yielded 0.13 (L/g VS reduced) and 0.07 (L/g VS reduced) respectively. The central composite matrix under the response surface optimization was employed to estimate the optimal process conditions, and the outcome reveals that reactions when maintained at pH 7.0 with 0.2 g/L of VN-FeNCs escalated good biogas production on 6th day. Conclusively, the remarkable reduction in the digestion time would contribute to increase the life span of biogas digester and will reduce the overall maintenance cost.