The short answer is of course yes, they utilise the anaerobic digestion process for the treatment (albeit only partial and not to watercourse discharge standards) of sewage from isolated locations where inhabited dwellings are located remote from public sewers into which a connection would otherwise be made.
However, they are in my expereince not normally discussed as being anaerobic digestors, and I have never heard them mentioned as methane emittors. If methanogenisis does occur it will be at low temperatures for the methanogens to be working.
However, in hot countries we do know that methane gas can be produced very effectively from domestic effluent type anaerobic digestors and is used for cooking etc. Such installations are used throughout China and India in very large numbers.
So, should we in the west be collecting methane from our septic tanks?
We always love feedback and comment, but feedback on this one appreciated more than ever.