Influence of substrates proportion and concentration on biogas composition and yield in the co-digestion of solid and liquid waste from coffee and swine farms

The present study aimed to investigate the anaerobic co-digestion of liquid swine manure (LSM), coffee wastewater (CFW) and coffee husk and pulp. The fixed operating conditions of batch reactors were pH 7, 3 g L−1 of coffee pulp and husks, 37 °C and 130 RPM. A single central composite rotational design with two factors was used to evaluate the influence of the CFW percentage (6–54 %, in relation to LSM) and the concentration of dissolved organic matter (OMD) (3–17 gCOD−1). It was observed that lower percentages of CFW (7–13 %) and/or lower concentrations of OMD (3–10 gCOD L−1) results in the production of biogas rich in CH4 without H2. Higher proportions of CFW (30–54 %) with higher OMD concentrations (10–17 gCOD L−1) favored hydrogen production. The highest CH4 yield was verified with 6 % CFW and 3 gCOD L−1 (423.04 mLCH4 g−1VS) in which the COD/N ratio was equal to 32. The main genera identified in the Archaea domain were Methanosaeta sp. (34 %) and Metanolinea sp. (33 %). Regarding hydrogen production, a higher yield was observed with 53 % CFW, 17 gCOD L−1, COD/N ratio of 48 and use of pre-treatment for Archaea inhibition (93.02 mLH2 g−1VS), the main genera identified were Lactobacillus sp. (75 %). The CH4 and H2 production were also evaluated in terms of energetic balance for a coffee and swine farm. Only CH4 production resulted in a positive energy balance and would promote the farm's energy self-sufficiency.