There’s a certain amount of nitrogen in sewage, and a bit of it is released as nitrous oxide during treatment. One problem is that nitrous oxide is a powerful greenhouse gas (about 300 times stronger than carbon dioxide).
It is reported that researchers at Stanford believe they might one day increase that amount, separate it, and then burn the nitrous oxide in a rocket thruster. On the downside, the increase in nitrous oxide results from increasing anaerobic activity, which could cause bad odors.
On the upside, there’s the idea that this process could be used to get a lot of energy out of wastewater. (But I doubt it.) On the more realistic upside, nitrous oxide is in fact also known as laughing gas, so the faculty and students can at least have some fun with tanks of it at parties.