Volatile fatty acid fermentation of lime-treated bagasse by rumen microorganisms

Abstract

This thesis describes the design and operation of a batch, anaerobic, in vitro fermentation of sugarcane bagasse by a mixed culture of ruminal microflora. The bagasse was supplemented with a small amount of alfalfa (0.16 g alfalfa/g bagasse) to provide necessary nutrients. The volatile fatty acid (VFA) product concentrations, yields and proportions of each acid for six different bagasse concentrations (10, 20, 35, 50, 75, and 100 g/L) are reported. Bagasse was treated with calcium hydroxide to increase the digestibility of the cell wall carbohydrates. The treatment conditions were: Ca(OH)2 loading = 10 g/100 g dry bagasse, water loading = 8.5 g/g dry bagasse, temperature 100'C, and treatment time = 1 hour. Compared to untreated bagasse, the lime-treated bagasse gave higher total VFA concentrations, faster rates of acidogenesis, and more stable molar proportions of individual VFA'S. The highest total VFA concentration obtained from lime-treated bagasse was 690 mM (45 g/L). By applying the lime pretreatment, the total VFA concentrations increased over 80% for a 10 g dry bagasse/L loading fermentation (from 4.5g VFA/L to 8.5 g VFAAL) With lime pretreatment, approximately 71 to 96% of the final total VFA yields were accomplished within the initial three days of fermentation, whereas only 52 to 67% were achieved without pretreatment during the same time period. At all solid loadings, the VFA molar compositions resulting from lime-treated bagasse were quite constant: acetate, 64-70%; propionate, 21-28%; butyrate, 6.5-7.6%; and other acids were about 1% each. In this thesis, we examined the effect of higher substrate concentration up to 100 g dry bagasse/L. For untreated bagasse, the VFA yields were fairly constant regardless of substrate concentration (ca. 0.37 g VFA/g dry substrate). However, for lime-treated bagasse, the total VFA yields decreased as the substrate concentrations increased. The best total VFA yield obtained from 10 g/L lime-treated bagasse was 0.63 g VFA/g dry raw substrate (or 0.82 g VFA/g dry ash-free substrate or 0.94 g VFA/g dry ash-free, lignin- free substrate). This is greater than yields previously reported in the literature using lignocellulosic substrates, and hence demonstrates the superiority of this very effective lime pretreatment.