A combined sewer processes both household waste water and rain water. In the past few decades, American cities have by and large upgraded from combined sewers to sanitary sewers that only treat household waste water, but there are still quite a few larger American cities that operate combined sewer systems. Under normal circumstances, the water treatment facility can process the water that flows into it, but during heavy rainstorms, a combined sewer cannot handle the increase, and the water treatment plant must divert the excess sewage somewhere else. Whan a combined sewer overflows, the plant operators have two options: let the excess sewage back up into people's toilets and drains, or divert it into a river or other waterway. Fortunately, they usually choose the second option, which also used to be the only option in the bad old days before water treatment plants existed at all.
Although everyone agrees that combined sewer overflows are a bad thing (after all, who wants their local river to become a Superfund site) they're becoming increasingly difficult to prevent. Not only does heavy rain threaten to cause CSOs, but the mere act of developing land typically neutralizes the land's ability to naturally filter rainwater into the water table underground. As the amount of land covered with sidewalks, asphalt, and roofs increases, so does the amount of rainfall that flows into the combined sewer.
In areas that still face the risk of CSOs, it's common for the city to fund campaigns encouraging people to block up the storm drains beneath their roof-mounted rain gutters. Unfortunately, there have been instances where well-intentioned, overzealous emissaries persuade oblivious homeowners to plug up their storm drain with cement, without really knowing if the grade of the land around the house can handle the increased runoff. When it can't, the house faces several very real risks, including erosion, basement flooding, and possibly even foundation damage.
Other approaches to reduce CSOs are less controversial, including rain barrels and pavers. Unlike sidewalks and asphalt, pavers contain tiny seams between each cobblestone that will allow water to penetrate to the ground beneath. However, sustained or heavy rain will most likely overwhelm the pavers and wind up flowing into a nearby gutter or drain.
Someday, combined sewers (and, consequently, CSOs) will be eradicated, but the substantial cost and disruption of upgrading entire cities to sanitary sewers means that the problem will persist for at least the next ten to twenty years.