Methane Production from Animal Manure is a Two Stage Microbially Mediated Process

Greenhouse gas emission from our manure storages in Canada is relatively small due to our colder climates and manure management systems. It is now 30 years since I first began my research with greenhouse gas production from agriculture, which began with a conversation with Dr. Naveen Patni in Ottawa in 1985.

Dr Patni’s findings in the early 1980s (Patni and Jui 1985) is more profound now, with the increased interest in greenhouse gas emissions. At the time, we discussed the effect of temperature on volatile fatty acid accumulation, the disappearance of the volatile fatty acids later in the summer, and the pH of the manure throughout the year. The ideas coming from this one research paper affect our understanding of both methane and nitrous oxide emissions from animal manure, and to anaerobic digestion including:

1. Biogas from manure is a two stage process, the first being the production of organic acids (VFA), primarily acetic acid, and the second being methanogenesis by methane producing bacteria – who are much more sensitive to temperature, as well as to ammonia and propionate accumulation in the manure.
2. VFA production also produces carbon dioxide – the manure carbon is the electron acceptor to form VFAs and hydrogen – this is why we see high carbon dioxide, low methane in manure studies at low temperature
3. Methane production is a much slower process and is affected by storage time and temperature
4. Although one would expect VFA production to lower manure pH (like a silage), it doesn’t because protein breakdown releases ammonia which raises pH
5. When methane producing bacteria begin to produce methane, we observe  a pH increase due to loss of the VFA
6. pH of manure is high after biodigestion because of the loss of VFA and the accumulation of ammonia/ammonium
7. We can expect lower N2O after biodigestion because we have removed the VFA or the available carbon which increases N2O emission during both nitrification and dentrification

This conversation greatly influenced my research. The following comes directly from my MSc thesis, completed in 1988.

“Anaerobic digestion to methane involves a two stage process, the acidogenic stage and the methanogenic stage. Cellulose, hemicellulose, lipids and proteins are degraded to volatile acids, hydrogen, and carbon dioxide during the first, or “acid forming” phase. The volatile fatty acids are converted to methane and carbon dioxide by the methanogenic bacteria in the second phase of anaerobic digestion.

Mosey (1983) summarized the acid forming phase of anaerobic digestion during carbohydrate fermentation. He stated that the preferred product of fermentation was acetic acid. The production of propionic and butyric acids were the bacterial response to an increasing acid load and a decreasing redox potential.

Farm slurries differ from anaerobic digesters in that the conditions are rarely optimum for the second phase of anaerobic digestion, methanogenesis. The result is that high concentrations of volatile fatty acids accumulate in the manure. The methane formers grow at slower rates and are more sensitive to adverse conditions than the acid formers.

Methane is not produced in most anaerobic manure storage tanks in Canada because the temperature is below optimum, the loading rate of the manure is too high, inhibiting methanogenesis by allowing toxic concentrations of volatile acids and ammonium to accumulate.

Van Velsen (1977) concluded that methanogens adapt to high ammonium concentrations, but are inhibited when the rate of ammonium formation is more rapid than the ability of the methane formers to acclimatize to the change.

Ianotti and Fischer (1983) measured the effects of volatile fatty acids on the growth of methanogens isolated from a swine manure digester. They found propionic acid to be more toxic to methanogens than either acetic or butyric acids. Van Velsen (1977) also observed digester failure when the propionic acid began to accumulate.

Methanogens are also sensitive to pH. In some anaerobic digesters, failure of the digester results in decreased pH as a result of organic acid production. This is not a problem with most manure digesters because of the high concentrations of ammonium in the manure. Schmid and Lipper (1969) observed only a slight decrease in pH when the organic acid concentration increased to 15,000 mg per L in a swine manure digester. They concluded that the high concentration of ammonium buffered the manure pH near neutral.” (Paul 1988)

We measured a direct correlation between manure slurry pH and volatile fatty acid concentration in various liquid dairy cattle, swine, beef and poultry manure slurries sampled during May in Ontario. (Paul and Beauchamp 1989). The data suggests that very little methane production had occurred.

There are suggestions that the methane production process is a more complicated one than simply the two stage process described: the acid forming stage and the methane forming stage. The reality is that there is a very diverse and fascinating group of microbes operating to allow methane production to occur. Lyberatos and Skiadas (1999) have a great summary of various models of anaerobic digestion. There are some slightly different versions of a four step methane production process.

The acidogenic microbial community is not as sensitive to temperature as the conversion of volatile fatty acids to methane, hence we see an accumulation of volatile fatty acids in many liquid manure storage tanks, without the corresponding methane production.

We will review research on the temperature affects on methane production during manure storage, with the goal of estimating methane production from our manure storages in Abbotsford.

References
Ianotti, F.L and J.R. Fischer. 1983. Effects of ammonia, volatile acids, pH and sodium on growth of bacteria isolated from a swine manure digester. Dev. Ind. Microbiol. 25:741-747.

Lyberatos, G., and L.V. Skiadas. 1999. Modelling of anaerobic digestion – a review. Global Nest: the Int J. 1: 63-76

Mosey, F.E. 1983. Mathematical modelling of the anaerobic digestion process: regulatory mechanisms for the formation of short chain volatile acids from glucose. Water Sci. Tech. 15: 209-232.

Patni, N.K. and P.Y. Jiu. 1985. Volatile fatty acids in stored dairy cattle slurry. Agricultural Wastes 13: 159–178.

Paul, J.W. 1988. Fermentation products of manure and plant residues as carbon sources for denitrifying bacteria in soil. M.Sc thesis. University of Guelph.

Paul, J.W. and E.G. Beauchamp. 1989. Relationship between volatile fatty acids, total ammonia and pH in manure slurries. Biol. Wastes. 29: 313-318.

Schmid, L.A. and R.I Lipper 1969. Swine wastes, characterization and anaerobic digestion. In Animal Waste Management: Cornell Univ. Conf. Agric. Waste Management. R.C. Loehr, chair pp. 50-57.

Van Velsen, A.F. M.  1977. Anaerobic digestion of piggery waste. 1. Influence of detention time and manure concentration. Neth. J. Agric.Sci 25: 151-169