A nitrogen generator is like 30k, which in building projects is a drop in the bucket. They probably prefer tanks cause they are more mobile and simpler, anybody can run a line and twist a valve.

Lots of armchair speculation incoming: In the article, it mentions the tanks some times hit their pressure release causing “snow”, so they must not be pushing much gas as I’m guessing the heat of the sun is causing the tank to over pressure. That leads me to believe they aren’t trying to pressurizing the whole sewer system, but create a positive pressure in the conduit or sleeve the lines run through in the sewer system.

They must have to do a ton of venting and gas checks before climbing down into any of the manholes when this system is running anywhere in the vicinity.

Nitrogen makes up most of the atmosphere and is a relatively cheap gas to capture and compress.