The production of volatile fatty acids from Napier grass via an anaerobic leach bed process: The influence of leachate dilution, inoculum, recirculation, and buffering agent addition

Abstract

The production of volatile fatty acid (VFA)-rich leachate as feedstock for methane production is an essential and rate-limiting step in two-stage grass biogasification. Batch experiments were conducted to investigate the influence of leachate dilution, inoculum, recirculation, and buffering agent addition on the acidification of Napier grass in anaerobic leach bed reactors (AnLBRs). Four AnLBRs with 14.17 L effective volume were operated at ambient temperature for 28 days in each experiment. The results clearly demonstrate the significance of the control parameters, particularly leachate dilution and inoculum, in the regulation of grass solubilization. Different dilutions of leachate produced different grass degradation rates. These different dilutions generated 95.62–112.63 g of soluble chemical oxygen demand (SCOD) or 0.52–0.62 g SCOD/g VSadded for those diluted at 1.0–7.0-day intervals compared to 51.50 g SCOD or 0.28 g SCOD/g VSadded for those without dilution. Adding inoculums of 10–30% increased the SCOD to 67.01–115.65 g SCOD or 0.40–0.59 g SCOD/g VSadded. While leachate recirculation and the addition of a buffering agent had less influence on solubilization, which increased by only 6.80–16.41% of SCOD produced. These produced leachates contained orderly acetic, propionic, and butyric acids. The proposed conditions (20% of added inoculum and leachate dilution at a 3.0-day interval), yielded practical degrees of solubilization and acidification of 46.61% and 62.21%, respectively, or 0.56 g SCOD/g VSadded and 0.26 g VFA/g VSadded. These results demonstrate that beneficial control conditions can promote the rapid and practical production of VFAs from grass biomass via an AnLBR.