Methane production by anaerobic co‐digestion of dairy manure and cassava wastes for energy recovery

Abstract

BACKGROUND: In this study, the anaerobic co-digestion of cassava wastewater (CW), cassava bagasse (CB), and dairy manure (DM) was investigated to produce biogas with energy recovery. Mixture experimental design was performed to establish the proportion of each substrate, on the basis of total weight (%wt). The digestion was carried out under mesophilic conditions (35 °C) using a batch test. RESULTS:The best methane yields were 150.33 NmLCH 4 gVS −1 (114.46 NmLCH 4 gCOD −1 ) for the condition with 100% CW (C/N ratio = 14.04) and 130.43 normal mililiters of methane per gram of volatile solid (NmLCH 4 gVS −1 ) and 93.84 normal mililiters of methane per gram of chemical oxygen demand (NmLCH 4 gCOD −1 ) obtained with 66.7% DM, 16.7% CB and 16.7% CW (C/N ratio = 34.84). Experimental conditions containing high proportions of CB (above 33%) yielded high C/N ratios (above 52), resulting in low pH and high volatile fatty acid production, mainly acetic and butyric acids. Kinetics of methane production under optimal conditions were best described by Groot's multi-stage model.CONCLUSION: An energy balance for the best conditions were performed, leading to 452.86 MJ ton substrate −1 for a high DM content mixture experiment (DM = 66.7%, CB = 16.7%, CW = 16.7%); thereby showing that anaerobic co-digestion can replace 22.5% of the energy demanded by cassava processing industry.