|WANG, HUI - University Of Wisconsin|
|LARSON, REBECCA - University Of Wisconsin|
Submitted to: Environmental Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/9/2019
Publication Date: 6/25/2019
Citation: Wang, H., Larson, R., Spencer, S.K., Borchardt, M.A. 2019. Effect of mixing duration on biogas production and methanogen distribution in an anaerobic digester. Environmental Technology. DOI: 10.1080/09593330.2019.1621951.
Interpretive Summary: Anaerobic digesters are used by some livestock farms to process manure and produce methane for energy. Finding the operating conditions for optimal methane production is an active area of research. Using bench-scale digesters we showed mixing of the manure during digestion increased methane production compared to no mixing. The optimal mixing time was 50%, in which the mixer was turned on for 15 minutes out of every 30 minutes. Mixing at a lower rate, 15 minutes for every 60 minutes, significantly altered the composition of methane-producing bacteria in the digester. Methane was still produced, but less efficiently than at a higher mixing rate. Overall, the increase in methane production with mixing was small and the decision to mix or not in full-scale digesters would need to be weighed against the energy costs for running the mixer.
Technical Abstract: Mixing has been shown to have effect on biogas production in anaerobic digestion systems. To further examine this impact, a study was designed to evaluate nearly continuous mixing (mixing for 15 min every 30 min, CON), intermediate mixing (mixing for 15 min every hour, INT) and no mixing (mixing for 0 min, NO) on biogas production in three 208 L pilot-scale tank reactors. The experiments were conducted in triplicates at a controlled temperature of 37 ±1o C, with a total solids percentage of 5% and a hydraulic retention time (HRT) of 21 days. Digesters with NO mixing had greater solids build up in the bottom quarter of the digester after four weeks’ retention time. The methane percentage in biogas produced from digesters with INT and CON mixing were 63% and 62%, respectively, which were 7% to 8% percent higher than that from digesters with NO mixing (58%). The specific methane yield for digesters with NO, INT and CON mixing was 206, 169, and 186 mL-methane per g-VS destroyed, however, those differences were not statistically significant (p > 0.05). Methanosarcinales had the least amount of copies in the digester under all mixing conditions. However, the Methanosarcinales, Methanosarcinaceae, Methanomicrobiales, and the total amount of methanogens were less in digesters with INT mixing compared with the ones with NO and CON mixing.