I wish to add that, the temperature will be related to metabolic activity of methanogens, but at consistently lower average temperature (
How is designed and where is located your digestor? Why you cant achieve at least 35C?
Most of the bacteria used for methanogens are very efficient above 30 deg . May be worth to look at the example at Germany where average temperature is below 20 deg.
It's a complex question.
1. According to general rule from the Arrhenius equation (growth rates or metabolic activity shown by biogaz production doubles every increase of 10 °C) . Thus, at lower temperature biogaz production will be slower.
2. At the same temperature growth rate or biogaz production may vary between species.
3. There are species abele to grow and produce methan in low temperatures as low as 0 °C (Methanogenium frigidum) and about half of all known methanoges is able to grow within the range 20-27 °C.
4. Most of anaerobic digesters are inoculated with cow manure so microorganisms adapted to cow temperature which is about 39.
5. Digesters microorganisms consortia are quite stable (it is not easy to modify their structure to different conditions ex. temperature)
6. Whole consortium contributes to anaerobig degradation ending in biogaz production and often it is the first step related to degradation of polymers which is limiting. Thus, not only methanogens should be adapted to lower temperature.
It's possible to achieve good efficiencies at mesophilic temperatures also. Explain your technology a little more.
Based on my experience, higher temperature is more homogenous because changing of the rheology properties, the microbe is more easier to mobilize. Normally in digester the critical factors is mixing. Mixing and Temperature has a correlation
At 30°C the predominant microorganisms are acetoclastics (Methanosarcina and Methanosaeta), whose activity is lower than activity of hydrogenotrophic (dominants at thermophilic temperatures). Revise: http://doi.wiley.com/10.1002/elsc.201000064 (thermophilic) and http://dx.doi.org/10.1016/j.biortech.2011.03.004 (psychrophilic)
Methanogens are not very diverse but they are extremophiles. It is not necessary to have temperatures >30C for efficient methanogenesis - just look at landfill operations! There are extensive methanogenic communities in permafrost in the Northern Hemisphere - as pointed out by another commentator. The Q10 (mentioned above) will determine the rapidity of reaching methanogenesis (warmer systems get there faster) but they are no more efficient in terms of conversion to methane than temperate /cold systems and once they are all at peak efficiency there is no real difference in output.
The wastewater content will affect which stage, hydrolysis or methanogenesis will be the rate-limiting stage. For complex organics higher temperatures, even termophilic range will be required. You have to check on the VFA concentration and species. If you do not get sufficiently high acidification you also won't get high methanation. Also, if propionic acid accumulates than the slower stage is the acetogenic conversion of p.a. to acetic acid.
Dear all, please see updated question’s description for more discussion. Thank you.
In my opinion, this problem is related with the microbial consortia in the digester (variety of methanogens). The hydrogenotrophic methanogens are faster than acetoclastic methanogens (their minimum doubling time has been estimated to be 6 hours, compared with 2,6 days for the slow-growing acetoclastic methanogens). Thermodynamically, the hydrogenotrophic pathway (4H2 + CO2 -> CH4+ 2H2O; Gibb's energy -135 KJ·mol⁻¹) is more favourable than the acetoclastic (CH2COOH -> CH4 + CO2 ; Gibb's energy -31 KJ·mol⁻¹) at standard temperature. Furthermore, the acetate oxidation pathway become more favourable at higher temperatures (>30°C).
There are other important factors, such as acetate and VFA levels, and the fact that the temperature's effect on the methane yield is higher when the substrate has high protein content.
Also hydrogenotrophic methanogens are less sensitive to environmental changes than acetoclastic methanogens.
Are you calculating yield based on COD or VSS degraded? Are you getting similar organic removal rates in summer and winter? Which VFA is higher in your reactor? What is your wastewater/waste? If you're treating complex organics than your VFA will not be produced at elevated levels at low temperature, because sufficient hydrolysis will not take place and low level VFA will be converted to methane.
The generation of methane by methanogenic bacteria is dependent upon he availability of the various volatile fatty acids produced by other bacteria, fungi, and yeast within the anaerobic digestive process. Essentially, the lowering of the temperature means there is less energy readily available from the environment to facilitate the decomposition of organic waste into volatile fatty acids and then into methane. Anaerobic digestion is not done as a monoculture. It requires a multi-species ecosystem to work.
I don't believe that temperature greater than 30degrees is a necessary prerequisite for efficient methanogenesis in all cases. The specie of methanogen, its adaptative ability can make some methanogens do very well even at lower temperature in some instances. If not, how do we account for large natural gas deposits even in the northpole,
Microbes in nature adapt to their environment to survive. The question raised basically asks which microbial ecosystem produces more methane... those a higher or lower temperature, what is the influence of temperature over wide range on the decomposition process. The other question raised is just what is efficient methanogenesis? The methanogenic microbial action is driven by the availability of volatile fatty acids which come from other microbes. Perhaps study needs to be done on these supporting microbes to determine their efficiency in volatile fatty acid production. I am not aware of any study on this issues.
Methanogens databas:
http://metanogen.biotech.uni.wroc.pl/
should be helpful in data mining concerning growth requierments, efficienyc etc. I especially find useful charts.
http://metanogen.biotech.uni.wroc.pl/
Anna Schnurer, Biogas Production: Microbiology and Technology (2016)
DOI: 10.1007/10_2016_5
Biogas production can proceed at different temperatures, typically mesophilic (35–42C) or thermophilic (46–60C). Biogas production can also proceed at psychrophilic temperature (15–25C). Temperature, together with substrate, is the most strongly determining parameter for stability and process performance. As mentioned above, the temperature impacts strongly on community structure, but also on microbial diversity, degradation pathways and degradation rate [108, 127]. In general, anaerobic digestion at thermophilic temperatures gives higher methane production rates and higher methane yield, but this is not always the case [108, 128]. Moreover, thermophilic digestion results in comparatively higher reduction of pathogens [108, 129] and gives lower viscosity, resulting in less energy consumption for stirring [130]. Disadvantages with higher temperatures include lower microbial diversity, with an accompanying risk of a less stable process and less efficient degradation of certain chemical compounds, such as phenols [108, 128]. Moreover, a higher process temperature needs a higher energy input in the form of heating. Processes operating at mesophilic temperature are generally considered to be more stable and less sensitive to inhibitory components such as ammonia. The microbial community, specifically the methanogens, are sensitive to temperature variations and experience from large-scale operation shows that temperature fluctuations should not exceed 2–3C for best results and to avoid instability [131]. However, biogas production is possible at a wide range of temperatures, even in the range between mesophilic and thermophilic temperatures, and it is also possible to shift from mesophilic to thermophilic temperature and vice versa [90, 132, 133].
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