Anaerobic co-digestion of rice straw and de-oiled rice bran for biomethane production

Abstract

The main aim of anaerobic co-digestion of rice straw is to enhance volumetric methane yields and process stability to achieve synergetic effects. However, high carbon to nitrogen ratio of rice straw causes poor process stability for mono-digestion. This study investigated the co-digestion of rice straw and de-oiled rice bran, a nitrogen-rich by-product of the rice milling industry. The spent bio-slurry (having total solids concentration of 2.19%) from running de-oiled rice bran which inherently contains various types of microbes was used as an inoculum for process start-up. A series of experiments were conducted to optimise the methane yield in semi-continuous anaerobic digester operating at 30 days of hydraulic retention time at mesophilic temperature conditions. Co-digestion studies of rice straw and de-oiled rice bran with three different mixing ratios were conducted by maintaining a specific total solids concentration of 5.0, 7.5, and 10.0% total solids concentration for the designated digester. The biogas production, methane contents, volatile matter degradation efficiency were determined to evaluate the stability and performance of the system. The digester maintained at 5% and 7.5% TS showed nearly similar results in terms of methane yield and volatile matter conversion efficiency. During the entire experimentation the percentage of methane content in the produced biogas was in the range of 24.8–55.4%. The average methane yield of 0.169 and 0.173 L/g-VS was found for digester maintained at 5.0% and 7.5% TS respectively. However, the volatile solid mass removal efficiency of 40.13% and 37.94% was recorded for the digester maintained at 5% and 7.5% TS respectively. Since no chemical or thermal pre-treatment was adopted during the entire process it is highly economical and adaptable for concerned industries. Ultimately, the results of the present study were compared to the previous studies on anaerobic co-digestion of RS, which shows the promising result with no extra added input cost.