FLINT, MI -- Heavy rains pushed more than 187 million gallons of partially-treated sewage into the Flint and Saginaw rivers last weekend, according to updated reports filed by wastewater officials in Flint, Saginaw and Bay City.

The discharges, treated with disinfection and some settling, were also heavily diluted by rain and melted snow, according to reports filed by the three cities with the state Department of Environment, Great Lakes and Energy.

Flint, Saginaw and Bay City each have combined sanitary and storm sewers, which means runoff from rain ends up at sewage treatment plants that have retention basins but limited capacity. Once that capacity is reached, the overflow is released to prevent sewage backups.

The U.S. Environmental Protection Agency calls combined sewer overflows “remnants of the country’s early infrastructure” in which communities built sewer systems to collect both stormwater runoff and sanitary sewage in the same pipe. During dry weather, the combined sewers typically carry a manageable load of raw sewage, but during heavy rainfall or snow melting, systems can be overwhelmed and overflow into streams, rivers or lakes.

The Flint River feeds into the Saginaw River, which discharges into Lake Huron.

“The wastewater treatment plant was working at upwards of five times its normal flow -- processing 77 (million gallons) through the plant,” city of Flint spokeswoman Marjory Raymer said in an email to MLive-The Flint Journal. “Normal dry weather flow varies between 15 and 20 (million gallons) per day. The wet weather created the spike in water flows, exceeding the plant’s capacity.”

Flint discharged more than 62 million gallons of stormwater and partially-treated sewage into the Flint River, according to state reports. Bay City released more than 47 million gallons in the Saginaw River after skimming and partial disinfection, and Saginaw released more than 78 million gallons of treated discharge into the Saginaw River from a series of retention basins that are used to hold overflows until capacity is reached.

Earlier this week, the city of Lapeer also reported a 2.5-million gallon discharge into the Flint River.

Saginaw also reported having treated the overflows, which “had floatable and settleable solids removed ... with chlorine prior to discharge," according to its report to the state.

“It’s the way the system was set up to work,” Brian Baldwin, superintendent of the Saginaw wastewater treatment plant, said of the use of partial treatment and releases to the river.

The EPA says approximately 860 communities with a total population of about 40 million people still operate with combined sewers, most of them in the Northeast and Great Lakes regions, particularly in Illinois, Indiana, Maine, Michigan, New York, Ohio, Pennsylvania, and West Virginia.

The agency calls combined sewers a major water pollution concern and among the major sources responsible for “beach closings, shellfishing restrictions and other water body impairments" because of concerns about bacteria.

Flint officials are among those who have questioned some requirements of the state concerning overflows, including bacteria testing. It sought an exemption from state requirements for the testing, contending its partially treated sewage discharges “are in general cleaner than the receiving stream under high flow storm conditions.”

Just last year, however, the Genesee County Health Department turned down the city’s request.

Genesee County Drain Commissioner Jeff Wright told the county Board of Commissioners Wednesday, Jan. 15, that health risks from sewage overflows are much higher when sewage isn’t as diluted as weekend spills were.

Wright said in one instance, human waste flowed into a storm drain in Shiawassee County and a child with an open cut was infected and eventually required a leg amputation.

The Genesee County Health Department recommends against any contact with the Flint River under high flow conditions that typically include overflows, particularly downstream of releases.