Pilot-scale conversion of manure—straw mixtures to methane

Abstract

A pilot-scale two-stage fermentation system designed to accomodate manure—straw mixtures was evaluated. The mixture was composed of 50% beef cattle manure and 50% wheat straw based on volatile solids (VS) content. The manure—straw mixture was mixed into a slurry and fermented in a hydrolysis tank for 1 day. The slurry in the hydrolysis tank was observed to autoheat from 40°C up to about 60°C, due to aerobic hydrolysis of substrate and mixing energy dissipation. Slurry from the hydrolysis tank either passed through a vibrating-screen solids separator or went directly to the anaerobic fermentor. When the solids were separated, approximately 37% of the screened solids were returned to the hydrolysis tank. The screened liquid, which accounted for 35% of the VS in the manure and straw, was pumped to the anaerobic fermentor for conversion to methane (CH 4). The fermentor, operated at 8-day hydraulic retention time (HRT), 44 to 47°C, and influent concentration of 47.3 kg VS/m 3, produced 1.81 m 3 CH 4/m 3 fermentor·day and 0.31 m 3 CH 4/kg VS fed. When solids were not removed, the fermentor was operated at 5-day HRT, 52 to 54°C and influent concentration of 68.1 kg VS/m 3. The CH 4 production rate and yield were 2.96 m 3 CH 4/m 3 fermentor·day, and 0.22 m 3 CH 4/kg VS fed, respectively. After correcting for differences in fermentation temperature and HRT, the system with solids separation produced only 58% of the daily CH 4 production of the system without solid separation.