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Title: Design and validation of field-scale anaerobic digesters treating dairy manure for small farms

item KLAVON, KATHERINE - University Of Maryland
item LANSING, STEPHANIE - University Of Maryland
item Mulbry, Walter
item MOSS, ANDY - University Of Maryland

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/12/2015
Publication Date: 3/1/2015
Publication URL:
Citation: Lansing, S., Klavon, K., Mulbry III, W.W., Moss, A. 2015. Design and validation of field-scale anaerobic digesters treating dairy manure for small farms. Transactions of the ASABE. 58(2):1-10.

Interpretive Summary: Anaerobic digesters are used on about 300 U.S. dairies for treatment of manure prior to storage and land application. Such digesters reduce emissions of methane (a potent greenhouse gas), reduce manure odors during land application, and provide a means to produce bio-energy and help meet renewable energy targets. There are over 30 million small-scale anaerobic digesters located throughout the developing world, concentrated mainly in China and India. In a cooperative project between ARS and the University of Maryland, researchers designed and constructed six pilot-scale plug flow digesters, using simple, successful designs used in developing countries modified to function in the temperate climate of the U.S. The first objective of this study was to compare the performance of a traditional farm-scale vertical tank digester to that of the recently installed pilot-scale digesters. A second objective was to determine the effect of adding different amounts of initial inoculum (the source of bacteria for digestion) on the amount of biogas produced over a 3-month period. Results show that biogas production and solids removal values are comparable for both types of digesters (vertical tank and unmixed bag digesters). The amount of initial inoculum (20, 33, or 50% of total volume) did not affect biogas production. Unmixed plug-flow digesters are relatively inexpensive and are commonly used in tropical climates where supplemental heating is not required. Results from this study are of interest within the dairy and livestock industries as they work to reduce its fossil fuel use and greenhouse gas emissions.

Technical Abstract: Six field-scale (FS) digesters were designed, constructed, and tested using a plug-flow design used by millions of farmers in developing countries and reconfigured for a temperate climate. Digester efficiency was analyzed based on methane (CH4) production, volatile solids (VS) reduction, inoculum to substrate ratio (ISR), construction cost, and energy input/output. Results were compared to a full-scale system (BARC) using the same manure source over an 18-week period. The ISR loading did not affect long-term CH4 production, with < 15% variance between the six FS digesters. The specific CH4 yield of the FS digesters (0.33 m3 CH4 per kg VS) was similar to BARC (0.31 m3 CH4 per kg VS). However, the FS digesters produced 2.8-fold more CH4 on a volumetric basis (0.39 m3 CH4 per m3 of digester). Both systems operated in the lower mesophilic range (25- 30°C), yet functioned at the higher range of literature values. The energy generated by the FS system was similar to the amount of propane used to heat the system, and the cost of the system ($16,800) was much higher than developing countries ($150-$400), illustrating the impact of the heating and insulation infrastructure on sustainability at the small-scale. The FS design presents a novel alternative for smaller-scale farmers in temperate climates without digestion systems.