Methane production from anaerobic co-digestion of vinasse and molasses: effects of substrate proportion, COD and alkalizing agent

Methane production using sugarcane industry effluents remains challenging due to the numerous operational variables that can affect the process. Nevertheless, it is essential to understand how different factors influence the performance of reactors used for this purpose. This study investigated the effect of interactions among different parameters in the anaerobic co-digestion of vinasse and molasses: (X1) proportion of vinasse and molasses (V/M = 27/75, 50/50, 75/50), (X2) COD concentration (3, 6, 9 g/L), and (X3) addition of alkalizer (0.5, 1, 1.5 gNaHCO₃/gCOD). A Box-Behnken experimental design was performed, employing fifteen 310 mL batch reactors, with 150 mL consisting of a 90% substrate mixture and 10% inoculum (sewage sludge), stirred at 120 rpm, and maintained at 35 ºC. Results showed that reactors exhibited COD removals from 2.92 to 58.41% and total volatile solids removal between 15.74 and 69.12%. Maximum methane production potential of 1055.77 mLCH₄ occurred for V/M = 25/75, 6 gCOD/L and 0.5 gNaHCO₃/gCOD. The alkalizer presented a negative linear effect on methane yield and production rate, which achieved maximums of 297.85 mLCH₄/gTVS_appl and 12.81 mLCH₄/gTVS_appl/d under V/M = 50/50, COD = 9 g/L, and 0.50 gNaHCO₃/gCOD. V/M and COD factors were not statistically significant at the 95% confidence level. Nevertheless, higher proportions of vinasse favored the responses at lower COD concentrations. Therefore, the current investigation verified that the co-digestion of vinasse and molasses is a technically feasible option for renewable energy production. However, the efficiency of this process is notably influenced by the addition of NaHCO₃.