Most municipal sewer systems in the United States have primary or secondary treatment, which filter the human waste and break down its biological content. However, this level of treatment does not remove most of the reactive nitrogen. Only about 5% of homes in the US are attached to municipal sewer systems with tertiary treatment, which further break down waste to a non-reactive form (N2) that does not harm the environment.
Knowing the level of sewage treatment at your local sewage treatment plant is very important because it can impact your N footprint by about 10%. To find out if you have tertiary sewage treatment, contact your local municipal sewage treatment plant.