Methane emissions from manure
Part 3: Manure digestion to produce methane
Anaerobic digestion of manure has been researched and implemented on farms successfully for hundreds of years. The process of anaerobic digestion involves anaerobic microbes breaking down organic matter into intermediate substrates that can then be utilized by a second population of microbes (methanogens) and turned into biogas (Parsons, 1984). Biogas is a blend of methane (CH4) and carbon dioxide (CO2) with some trace amounts of ammonia (NH3), hydrogen sulfide (H2S), and water. The methane that is produced can be converted into electricity, thus the goal is to maximize methane production.
Several important considerations when designing an anaerobic digester are pH, temperature, and retention time (Franco et al., 2007). Maintaining a neutral pH, close to 7.0, is critical to support methanogens. Temperature is also important, but there are several classes of methanogens that thrive at different temperatures. In general, higher temperatures promote faster microbial growth and produce more biogas from a given amount of substrate; however, there is an added cost of maintaining this higher temperature and economics typically favor operating at 77-100°F. Retention time is the amount of time that substrate is in the digester before being removed as effluent. Longer retention times (> 15 days) result in more complete breakdown of the organic material and thus improved methane yields, even with lower temperatures (Varel et al., 1980). Shorter retention times (< 10 days) result in more substrate passing through the digester, and thus more total methane produced.
Historically, anaerobic digestion has utilized "clean" dairy or swine manure that is collected from cement pits, has no soil contamination, and is 80 to 85% organic matter (OM). Within Nebraska, the feedlot industry produces significant amounts of manure. This manure can vary between 15 and 65% OM, with the remainder being ash that will not degrade within a digester. Two studies done at UNL suggest that with adequate daily cleanout, open lot beef cattle manure can be used for anaerobic digestion, although increasing the ash content of the manure may decrease methane production by up to 25% (Watson et al., 2013; Watson et al., 2015). Methane production decreased from 22.4 gallons per lb of OM fed to the digester with 65% OM manure to 16.7 with 15% OM manure.
Quality of manure, measured as OM, is a good measure of anaerobic digestion potential of manure. Utilizing open lot beef cattle manure for anaerobic digestion could be very advantageous in Nebraska as this manure is readily available and effluent removed from digesters following digestion is a valuable fertilizer that can be applied to crop fields (Field et al., 1984).
University of Nebraska - Lincoln
- Field, J.A., J.S. Caldwell, S. Jeyanayagam, R.B. Reneau, W. Kroontje, E.R. Collins. 1984. Fertilizer recovery from anaerobic digesters. Transactions of the ASAE 27: 1871-1876.
- Franco, A., A. Mosquera-Corral, J.L. Campos, E. Roca. 2007. Learning to operate anaerobic bioreactors. Communicating Current Research and Educational Topics and Trends in Applied Microbiology 618-627.
- Parsons, R.A. 1984. On-farm biogas production. NRAES-20. Northeast Regional Agricultural Engineering Service, Cornell University, Ithaca, NY.
- Varel, V.H., A.G. Hashimoto, Y.R. Chen. 1980. Effect of temperature and retention time on methane production from beef cattle waste. Applied and Environmental Microbiology 40: 217-222.
- Watson, A.K., G.E. Erickson, T.J. Klopfenstein, S.C. Fernando, J.L. Harding. 2013. Anaerobic digestion of finishing cattle manure. Nebraska Beef Cattle Rep. MP 98:98-99.
- Watson, A.K., A.L. Shreck, T.J. Klopfenstein, G.E. Erickson. 2015. Anaerobic digestion of feedlot manure. Nebraska Beef Cattle Rep. MP 100:TBD.