Not a particularly important item, but the odor is not from methane since it has no odor, at least so says the internet:
Methane is a colourless, odourless gas which is lighter than air. It is formed by the decomposition of organic carbons under anaerobic conditions and is commonly found in or near swamps and wetland areas, peat deposits, woodwastes such as hogfuel, or in the area of old landfills.
Of course, I can well imagine a variety of sources for odor in sewage.
