Do wastewater treatment plants do anything useful with the methane produced in processing the wastewater?
The quick answer to your question is "yes"methane given off during the anaerobic digestion stage of wastewater treatment is put to useful purposes. People are generally aware now that methane is a greenhouse gas and therefore should not just be expelled. The traditional way to solve that problem was to simply “flare” off the gas to avoid releasing it into the atmosphere. More frequently now, however, wastewater treatment plants are using the methane in place of natural gas for their own heating needs, to generate their own electricity, or, in a few cases, to feed back into the public natural gas system.
I called an engineer at our local wastewater treatment plant to see what they do, and to get some basic information about the process. What I found out was very encouraging. The gas that comes off the digester is about 70% methane, contaminated with carbon dioxide and hydrogen sulfide, among other gases. They put it through a cleaning process to make it more pure, then burn it directly for their boiler, which provides all their process heat, as well as space heat in the winter. Occasionally they have excess, which they use to run a generator for some of their electricity. They are now in the process of hooking up the generator so that when they have excess generation, it will be fed back out into the grid. It remains to be seen whether they will be able to supply all their energy needs, but it sounds like it may be close.
The good news, then, is that much of the methane is being used. The bad news is that wastewater treatment, as we currently practice it, is a very energy-intensive process, so it typically consumes all of the energy generated in this way. The U.S. Environmental Protection Agency Energy Efficiency for Water and Wastewater Utilities webpage provides general information on this issue.
Even Better Ways of Treating Sewage
There may be even better ways of treating sewage than the above approaches, such as constructing wetlands. Some examples:
Methane Use Resources