In my experiment, VFAs concentration is higher in thermophilic than in mesophilic anaerobic digestion, but the methane yield is not much different.
Does the microbe use VFAs for other purpose like cell maintenance especially in high temperature?
Do you have VFA speciation for meso- and thermo-philic systems? that would help to answer your query.
What is your substrate? Thermophilic anaerobic digestion is known to improve solids hydrolysis. It means that more VFA should be produced.
I measured acetic, propionic and butyric acid for both mesophilic and thermophilic.
The substrate is seaweed.
(VFA) concentration in thermophilic is almost doubled in mesophilic digesters. Is that mean that the methane production in thermophilic should be twice the methane production in mesophilic? Thanks
In thermophilic system, one should expect high degradation rate and different VFA speciation (in terms of quantity). How different the acetic acid ratio to total VFA in two systems?, although you get more VFA the methane production may be affected if the conditions are not favourable (e.g. if you get more acetic acid, then more methane is possible). Also other factors govern the methane production, e.g. Ammonia and sodium chloride. Ammonia toxicity under thermophilic conditions is common than mesophilic systems, that affect the overall methane production (refer my review article "bioenergy recovery from high-solid organic substrates by dry anaerobic bioconversion processes"). In summary, thermophilic systems are more sensitive than mesophilic systems and therefore operation of thermophilic system required critical understanding. Sodium chloride also affect the community structure, that will be reflected in VFA species (qualitatively), alter the methane production.
In theory and practical, thermophilic system should yield more methane than mesophilic system (but definitely not twice as you expect). So, you need to watch your system parameters to find out reasons for not getting more methane.
Thank you Dr.Obuli Karthik,
The ratio of acetic acid between thermophilic and mesophilic is 2:1. So I am expecting to get two folds methane on thermophilic AD. The pH profile seems pretty normal, dropping during hydrolysis and then rose gradually until stationary stage. Unfortunately, I do not have chance to measure the ammonia concentration.
Dear Aqil,
In my experience with anaerobic digestion of OFMSW at thermo and mesophilic temperatures I may point-out the following regarding VFA's
- The medium, subtrate and innocculum will have influence on methane productivity and VFA's. It is hard to generalize that a behavior is expected only depending of whether it is thermophilic or mesophilic regime you are working on, in order to do so you should have you microbial community analyzed.
- You should already probably known that methane may be evolved by two metabolic pathways: acetotrophic or hydrogenotrophic. There are some reports that suggest the acetotrophic to be the dominant, but this depends again on substrate, medium and community structure. Either the dominant pathway in your process may influence in your VFA's profile.
-In fact, a high VFA concentration is indicatory of an accelerated yet incomplete metabolism. For some reason (e.g. microbial community structure, ammonia concentrantion) your digestion in thermophilic regime is somewhat 'inefficient'.
-The only VFA that s used for cell maintenance is acetic as it may be used by acetil-CoA. The other acids are just intermediaries. Another indication of incomplete metabolism.
If you want to improve your results in thermo regime, try to:
- Low the OLR, or increment you mass retention time if you have a continuous or semi-continuous process. If batch, try to build-up a better thermophilic innocculum.
Cheers!
It is normal to see higher VFA concentration at thermophilic temperature than mesophilic, and thermo do have a higher reaction rate. However, the yield may not be higher because theoretically, the yield is determined by the nature of feedstock, and the temperature only affect the kinetics. You may also want to check for possible inhibitions at thermo, such as ammonia, which may also reduced your yield. Finally, the VFAs you measured are the remaining VFAs, not those have been converted to methane. So if you are doing batch reactors, you may want to try to extend your digestion time to have the process completed and compare the results; if you are doing continuous, you may want to reduce the organic loading.
VFA's are the product of the cellulosic bacteria, fungi, and yeast breaking down the organic matter. The production of methane is a function of the concentration of methanogenic bacteria. The productivity of the methanogens is a function of the pH. The higher the pH, the more the VFA's are converted to methane. The VFA's are composed of acetic, proprionic, valeric, butyric, and hexanic acids. The piece that is missing is the concentration of protozoa on which the methanogenic bacteria reside. The amount of fluid in the reactor will influence this number. The other factor is the temperature. We have found a temperature of about 40 C is the most productive.
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