Tim. Seriously. You state that the water from the sewage plant will be much cleaner than the water in the river. That's true. Then you propose a continuous loop, sans portage, going with the flow from the downstream sewage plant upriver through the lake, then downstream via the river, going with the flow from the upstream sewage plants back to the downstream sewage plant. I believe that course will put you back out in the doo-doo. In the river. Which is unsafe for human contact.
Hey, man. Think about it. No joke. --Jim Schutze