Question

How can we calculate purityof methane gas by theratically?

Answer 1

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.