Effect of Substrate-to-Inoculum Ratio on Methane Production with a Batch Anaerobic Digestion of Bean Curd Residue

Abstract

This study focused on the batch methane fermentation of Bean Curd Residue (okara) using anaerobic digesters under mesophilic (36°C) conditions. Inoculum (anaerobic digestion sludge) was mixed with different amounts of Substrate (okara), and their final moisture content was adjusted to 90%. Methane production at 6 different substrate-to-inoculum ratios (S/Is), ranging from 0.3 to 2.0, were determined. Methane yield reached its maximum at S/I values of 0.6. 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. Microbial behavior in anaerobic digester was studied by using the developed molecular technique-PCR-DGGE. Appearances of acid-forming bacteria coincide with the acetic acid and propionic acid production at S/I values of 1.6 and 2.0, therefore accumulated acetic acid and propionic acid directly inhibited methane production. After 19 days of mesophilic digestion, maximum methane yields of 495 ml CH4 g VS-1 was achieved at S/I values of 0.6.