Influence of substrate-to-inoculum ratio on the batch anaerobic digestion of bean curd refuse-okara under mesophilic conditions

Bean curd refuse-okara, a byproduct of tofu (soybean cake) production, contains more than 20% total solid (TS), making it difficult to use as a substrate for anaerobic methane fermentation without dilution. This study focused on the batch methane fermentation of okara in anaerobic digesters under mesophilic (36 °C) conditions. Inoculum was mixed with different amounts of okara, and their final moisture content was adjusted to 90%. Methane production at 13 different substrate-to-inoculum ratios (S/Is), ranging from 0.1 to 3.0, were determined. The results indicated that despite the TS loading of 10%, which is significantly higher than that of traditional methane fermentation, okara was fermented to methane at the highest methane-based degradation of 93%. When the S/I values ranged from 0.1 to 0.6, methane yields indicated a linear relationship with the S/I value. Methane yield reached its maximum at S/I values of 0.6-0.9. In contrast, methane yield decreased when the S/I exceeded 1.0, due to higher accumulation of volatile fatty acid (VFA), which significantly inhibits fermentation. After 19 days of mesophilic digestion, maximum methane yields of 478-495 ml CH₄ g VS-1 were achieved at S/I values of 0.6-0.9, which corresponds to 90-93% methane-based degradation of okara.