Graduate Student Exit Seminar: Caitlin Asato, Friday, July 25, 9 am, 2045 Bainer

BAE Graduate Student Exit Seminar 

Friday, July 25, 2014 
9-10 am, 2045 BAINER HALL

"Anaerobic Digestion of Bioethanol Stillage for Biogas Energy Production and Nutrient and Water Recovery" 

Caitlin Asato 
Department of Biological & Agricultural Engineering 
University of California, Davis 

Global bioethanol production has expanded significantly in recent decades. For instance, significant progress has been made in a project led by Mendota Bioenergy, LLC to establish a sugar beet ethanol production plant in central California. Yet, commercial growth is still limited by narrow profit margins. This may be partly alleviated by making beneficial use of stillage, a byproduct of ethanol production. In this study, anaerobic digestion of sugar beet stillage was investigated for producing biogas energy to supply the heat energy needed by the ethanol production processes. 

Sugar beet stillage was first analyzed for its suitability as a feedstock. Wet basis total solids (TS), volatile solids (VS), and total nitrogen contents were 4.88%, 3.96%, and 1750 ppm, respectively. Nutrient contents were in an acceptable range to avoid toxicity. Continuous anaerobic digestion was tested in a laboratory-scale digester in order to determine the maximum organic loading rate (OLR). Two three-month trials were conducted to compare different reactor configurations: an Anaerobic Sequencing Batch Reactor (ASBR) and an Anaerobic Mixed Biofilm Reactor (AMBR). Both reactors failed before a quasi-steady-state was reached at 5 g VS/L/day; thus, an OLR of 4 g VS/L/day is recommended in both cases. Most process performance parameters at OLRs of 4 g VS/L/day indicated that the ASBR was more stable; the biogas yield was 604 mL/g VS for the ASBR and 776 mL/g VS for the AMBR. The AMBR also achieved a greater VS reduction of 83%, compared to 72.5% for the ASBR. Continuous aeration was also tested in the laboratory to evaluate it as a means of digester effluent treatment. An additional VS reduction of 35% was achieved after aerobic treatment with an HRT of 3 days. 

Based on experimental data, a process model was developed using SuperPro Designer® to aid the engineering design and economic analysis. It was found that for a bioethanol plant processing 100 tons of sugar beets per day, anaerobic digestion of the stillage could supply enough methane to supply at least half of the heat energy required by ethanol processes The digester system with aeration was found to be still net energy positive, with 3% increase in capital cost and 7% increase in operating cost compared to the system without aeration. 

Coffee and cookies will be served.