Technically, what you are describing is possible. It's actually not difficult, and many communities have been doing it [1].

The article title is misleading, however. Now that we know the genome of the nCoV, we know its gene products, and we can design targeted tests for it, it is a straightforward. However, this is not an early warning sign for eventual other nCoVs that may occur. This will only work for this one, and other discovered ones. This is not going to help us identify a new, unknown coronavirus outbreak.

As far as discovery goes, it's a much more involved and delicate process, and not one I'd trust to the low margin, low wage, low talent industry that unfortunately is sewage testing.

https://www.bozemandailychronicle.com/coronavirus/wastewater...