They've been around for hundreds of years. Actually thousands. It used to be very common. These were built before sewage treatment plants even existed. The whole thing would just dump into a river or lake. Turds in the lakes and river, turds in flooded basements. We had that happen several times in the 50s.
Then, when treatment became common, the rebuilding of the whole thing was very expensive, so they ran everything into the treatment plant. It worked fine until a heavy rainfall arrived. When the plant was taking in more water than it could handle, the overflow went into the lake or river.
There are still a lot of them around. Some cities have spend huge amounts of money to separate the combined systems. An alternative is to build massive tunnels to store storm water.
Search "combined storm sanitary system" or "CSO" for more info.
This article says there are still almost 800 communities in the US with such systems. https://en.wikipedia.org/wiki/Combined_sewer
For many years Cleveland (and other cities) has been digging many enormous sewer lines that can store storm/sewage water and feed it to the treatment plants at a rate they can handle. Here's an article about another one that's just beginning. http://www.cleveland.com/metro/index.ssf/2012/06/massive_sewer_drilling_machine.html
Another $200 million chunk in the $3 billion storage tunnel project.
Stuck on Stupid...double dumb!
What a bunch of liberal EPA dysfunctional gubmint egghead claptrap.