This value has been widely used in many publications and textbooks. It seems to be deducted from the passway from acetate to methane (CH3COOH=CH4+CO2). If considering another passway from hydrogen/carbon dioxide to methane (4H2+CO2=CH4+2H2O) and some complex organics (like proteins, lipids), is it possible to achieve this value? If not, what's the achievable maximum COD conversion to methane for the common readily biodegradable wastewater like domestic or food wastewater?
How to achieve as much as methane conversion?
The theoretical (and experimental) COD of methane is 64 gram of oxygen per mole of methane. This (1 mole of methane per 64 gram of COD) is thus the maximum amount of methane that you can get if all COD is converted into methane (e.g. Heidrich et al. (2011) Environ. Sci. Technol. 45, 827–832). In practice the amount of methane recovered per gram of COD removed will probably be slightly less as some of the COD may be used as source of reducing equivalents for microbial growth; also not all COD may be biodegradable so in practice it will be difficult to convert all wastewater-COD into methane.
In principle/theory it does not matter whether the (source of) COD is for example H2 or acetate. In either case it is: 64 gram of COD equals 1 mole of methane.
In practice you will have a hard time reaching this value with for example lignin.
And finally a bit cheeky: if you measure your methane production at a higher temperature the volume per mole is higher than at standard temperature; so, yes, you can get more than the 0.35L/gram COD, but clearly that is cheating.
Thanks for Dr. Dolfing's clarification! For the common readily biodegradable wastewater like domestic or food wastewater, how about the achievable maximum COD conversion to methane? From publication, 83% (Hu and Stuckey (2006) Journal of Environmental Engineering-Asce 132(2): 190-198.) seemes highest record.
You have two parameters: one is the "amount of dissolved carbon" in your reactor.. yes, bacteria can convert all dissolved substrate to methane and CO2 if you give them a good reactor (some large organic molecules can also be used by the bacteria by the enzymatic reactions) .. the second is "the time you can give to the bacteria".. If the bacteria have time enough, they can convert all of the substrate to the methane and CO2.. if you raise the temperature you can get the methane quicker (retention time decreases), the reactor volume decreases (good for initial cost).. if you keep the temperature at mesophilic levels (around 35°C) , bacteria slowly convert the substrate to the biogas (retention time gets longer, initial cost increases).. On the other hand, you need also to decide if you like to increase operational costs by heating your reactor!
Therefore, not all of the substrate but an economically feasible portion of it is allowed to be converted to methane in digesters..
Wei, not every foodwaste is readily biodegrable (i.e., cheese whey) therefore you may not get full conversion of organics to the end products (i.e., methane, CO2). The reporting gas yield is also very important. In literature, some reserachers use gas yield as amount of methane formed per COD fed. However, when you report your gas yield as amount of methane formed per COD degraded, the gas yields are expected to be very similar. The differences are primarily due to design and operating conditions of the anaerobic operations.
I'd like to bring up the paper of other microbes that consume COD and compete with methane producing archaea. Depending on your substrate and biomass composition in most cases you will have Sulfate Reducing Bacteria that will utilize Hydrogen as an electron donor therefore reducing the amount left available for methane production.
yes, when all the physio-chemical parameters are kept optimum including stirring
The theoretical methane production does not includes the C and electrons needs for growing of the methanogens. Even if you have a pure archaea culture, a little part of the acetate (in the case of acetoclastic methanogens) or H2/CO2 (in the case of hydrogenotrophs) must be used by the archaeas to grow, and therefore it is impossible to achieve the theoretical methane potential of 350 mL/g COD. Only if the biomass is not growing you could achieve such value, but this situation only occurs in metabolic decoupling processes (as in inhibitory episodes) and it is not stable over time.
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