Are there dangers of swimming in a river with sewage in?

Bad idea. Swimmers in sewage-polluted water could contract any illness that is spread by ingestion. Be careful.
Of Course. I would not take the plunge. It must be recently checked and deemed fit for swimming by the appropriate health department . We have had triathlon swimming legs cancelled due to unsafe sanitary conditions. Be smart -- don't take the chance.
Yes. There are bacteria, viruses, fungi that can make you very sick. I can't see any reason you would want to swim in a river if you know or suspect there's sewage floating in it.