Modern sewer systems have two kinds of pipes: sanitary sewers for conveying sewage from homes and businesses to wastewater treatment plants and storm sewers for conveying untreated rainwater to nearby streams to prevent flooding.

But many older urban areas have combined sewers — sanitary sewers and storm sewers sharing one pipe. These systems are a major pollution source because, during heavy rains, a mixture of rainwater and untreated sewage overflows into the environment. 

To fix these combined sewer overflows (CSOs) sewer districts in Cleveland and Akron are spending hundreds of millions of dollars on facilities (such as huge storage tunnels deep underground) to store and treat polluted stormwater. CSO programs will be the region's big water pollution control investment in the next 30 years.

Resources
Green strategies for controlling stormwater and CSOs
Map of CSO locations in Cleveland area
Northeast Ohio Regional Sewer District CSO program
Ohio EPA CSO program

Northeast Ohio developments
NEORSD has reduced combined sewer overflows from 9 billion gallons a year to 4.74 billion gallons a year.  Pace of future reductions depends on EPA agreement and cost.  Big bills in balance as sewer talks step up, Plain Dealer, September 26, 2009.