Statistical optimization of methane production from brewery spent grain: Interaction effects of temperature and substrate concentration

This study evaluated the effects of thermal pretreatment of brewery spent grain (BSG) (by autoclave 121 °C, 1.45 atm for 30 min) on methane production (CH₄). Operation temperature (31-59 °C) and substrate concentration (8.3-19.7 g BSG.L^-1) factors were investigated by Response Surface Methodology (RSM) and Central Composite Design (CCD). Values ranging from 81.1 ± 2.0 to 290.1 ± 3.5 mL CH₄.g^-1 TVS were obtained according to operation temperature and substrate concentration variation. The most adverse condition for methanogenesis (81.1 ± 2.0 mL CH₄.g^-1 TVS) was at 59 °C and 14 g BSG.L^-1, in which there was increase in the organic matter concentration from 173.6 ± 4.94 to 3036 ± 7.78 mg.L^-1) result of a higher final concentration of volatile fatty acids (VFA, 2662.7 mg.L^-1). On the other hand, the optimum condition predicted by the statistical model was at 35 °C and 18 g BSG.L^-1 (289.1 mL CH₄.g^-1 TVS), which showed decrease in the organic matter concentration of 78.6% and a lower final concentration of VFA (533.2 mg.L^-1). Hydrogenospora and Methanosaeta were identified in this optimum CH₄ production condition, where acetoclastic methanogenic pathway prevailed. The CH₄ production enhancement was concomitant to acetic acid concentration decrease (from 578.9 to 135.7 mg.L^-1).