You'd probably find that it would cost far more to store the sewage, wait for it to produce gas, extract this and purify it, than you'd ever actually get out of selling it.
There are various things like this around that could, in theory, provide some kind of usable resource, but often, the limitation is the amount it would cost to actually process it into something usable, so it doesn't get done.
For another example, it is estimated that there are millions of tonnes of gold dissolved in the oceans- enough for each person on the planet to have 9 pounds of pure gold. That's a lot of gold right there, ripe for the taking. Meanwhile, the asteroids are chock-full of expensive metals that could be mined.
The catch with either of these is that all that gold and metal is so hard to extract, process and transport that nobody has found a cost-effective way of doing it. So for instance, if we really wanted to, we could start pumping seawater through the lab to extract that dissolved gold- with current technology it would not be impossible to do. However, it would cost far more in terms of pumping the water, processing it and extracting the gold, than the gold would ever be worth. Likewise, by the time you got a rocket out to the asteroids, moved tonnes of metal back to Earth, landed it and sold it, you'd have spent far more per kilogram of metal than you would ever get for it on the market.