Influence of Ensiling Time and Elephant Grass Silage Alkaline Pretreatment in Anaerobic Co-digestion with Vinasse for Methane Production

This study investigated the anaerobic co-digestion of sugarcane vinasse (V) and elephant grass silage (S) to produce methane. Box-Behnken experimental design was applied to verify the statistical effects of the elephant grass ensiling time (40, 80 and 120 days), alkaline pretreatment of elephant grass silage (0.5, 2.25 and 4.00% w/v NaOH) and S:V mixture ratio (25:75, 50:50 and 75:25) on the methane yield. The results showed that the ensiling process resulted in the low degradation of lignocellulosic substances, emphasizing the need for pretreatment using more efficient techniques, such as thermo-alkaline, to improve the breakdown of elephant grass fibres. COD removals varied between 35 and 85%, and carbohydrate consumptions ranged from 63 to 72%, with the higher efficiencies for both parameters occurring in the reactors with lower percentages of silage. Cumulative methane yield ranged from 190.77 mLCH₄/gVS (in the reactor with S:V of 75:25, 0.50% w/v NaOH and 80 ensiling days) to 1729.80 mLCH₄/gVS (in the reactor with S:V of 25:75, 2.25% w/v NaOH and 120 ensiling days). According to ANOVA, S:V ratio was the only variable with a significant effect (p < 0.05) on cumulative methane yield. Therefore, the findings indicate that the relative composition of substrates within the mixture exerted the most significant influence on the process, underscoring the critical role of vinasse as a co-substrate in enhancing methane production despite silage pretreatments.