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United States Department of Agriculture

Agricultural Research Service

Title: Anaerobic Digestion of Animal Waste: Effect of Mode of Mixing

Authors
item Karim, Khursheed - WASHINGTON UNIVERSITY
item Hoffmann, Rebecca - WASHINGTON UNIVERSITY
item Klasson, K Thomas
item Al-Dahhan, M - WASHINGTON UNIVERSITY

Submitted to: Water Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 22, 2005
Publication Date: October 1, 2005
Citation: Karim, K., Hoffmann, R., Klasson, K.T., Al-Dahhan, M.H. 2005. Anaerobic digestion of animal waste: effect of mode of mixing. Water Research. 39:3597-3606.

Interpretive Summary: Reactors containing cow manure were mixed using either an impeller, a pump, or through bubbling gas. A control reactor was not mixed. The amount gas coming out from these reactors was measured as different quatities of manure was added to the reactor on a daily basis. The results show that when thin manure was added, the mixing method mattered little; even the unmixed reactor produced gas well. When thick manure was added, reactors that were mixed performed better than the unmixed reactor. The type of mixing mattered was not found important.

Technical Abstract: Laboratory-scale digesters were operated to study the effect of mixing (via biogas recirculation, impeller mixing, and slurry recirculation) on biogas production. Three sets of experiments were performed using cow manure slurry feed with either 50, 100, or 150 g/L total solids (TS) concentrations (referred in the text as 5%, 10%, and 15% manure slurry). The experiments were conducted at a controlled temperature of 35C and a hydraulic retention time of 16.2 days, resulting in TS loadings of 3.1, 6.2, and 9.3 g/L d for 5%, 10%, and 15% manure slurry feeds, respectively. Results showed that the unmixed and mixed digesters performed quite similarly when fed with 5% manure slurry and produced biogas at a rate of 0.84-0.94 L/L d. The methane yield was found to be 0.26-0.28 LCH4/g volatile solids loaded. However, the effect of mixing and the mode of mixing became important when the digesters were fed thick manure slurry feeds (10% and 15%). Digesters fed with 10% and 15% manure slurry and equipped with external mixing produced about 10-30% more biogas than the unmixed digester. While the mixed digesters produced more biogas than unmixed digesters, digester mixing during start-up was not beneficial, as it resulted in lower pH, performance instability and prolonged start-up time. Mixing using biogas recirculation system was found not to be effective in the case of 15% manure slurry feed under the experimental conditions studied.

Last Modified: 8/27/2014
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