There aren't a lot of places that use anaerobic digesters (fermenting waste to methane) to treat wastewater. Anheuser-Busch does it at their breweries because their wastewater is particularly rich. If more municipalities built those at their wastewater treatment plants, then it would be a sizeable amount of carbon-neutral fuel. There are already 1200 municipal wastewater treatment plants in the US that do this according to the EPA, but there are many major cities that do not. <a href="https://www.epa.gov/anaerobic-digestion/types-anaerobic-digesters" rel="nofollow">https://www.epa.gov/anaerobic-digestion/types-anaerobic-dige...</a>