Ok, so I just got this idea. I by no means think I'm the first to come up with this idea but I shall share it none the less. Every house with indoor plumbing has a vent pipe that allows sewer gases to escape instead of stinking up the house. Sewer gases primarily consists of methane gas, which is apparently a worse greenhouse gas than carbon dioxide, which, along with water, is what you get by burning methane. Methane gas is also the primary component of natural gas, which is used in many houses already to burn in furnaces and water heaters and stoves. It's also used in some power plants to produce electricity.
So every house in the industrialized world already has a pipe that has methane coming out of it, why not harvest it? Why not build a little device that carefully controls the burning of those escaping sewer gases to produce at least a little electricity that the house uses and thus reduce every one's power bill by some. And in the process we also let less methane into the atmosphere to do its greenhouse effect.
It's just an idea. Not sure if it would work as simply as I'm making it sound there, but that's for the engineers to figure out. There is also the issue of the other gases in sewer gas and what would happen to them in this whole process. I also have no idea just how much methane comes out of those vent pipes and if there is enough to make any significant power offset to the cost of running the device that burns it to convert it to electric power. But as I said, that's an issue for the engineers.