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Overflows of combined sanitary and storm sewers Wednesday spilled an estimated 336.2 million gallons of untreated wastewater, mostly water with sewage, into local rivers and Lake Michigan, the Milwaukee Metropolitan Sewerage District said Friday.

Combined sewers in central Milwaukee and eastern Shorewood overflowed to waterways for 19.5 hours Wednesday after thunderstorms poured 2 inches or more of rain on those neighborhoods in one hour between 1 a.m. and 2 a.m., the district says in a report to the state Department of Natural Resources. This was MMSD's second combined sewer overflow of the year.

Overflows began shortly after 2:30 a.m. Wednesday when the district's deep tunnel system started filling quickly in the heavy downpour.

More than 100 million gallons of water flowed into the main tunnels in just 12 minutes, between 1:52 and 2:04 a.m. Wednesday, according to flow monitors. The torrent took up 23% of the main tunnels' capacity in those few minutes.

MMSD Executive Director Kevin Shafer ordered Jones Island sewage treatment plant operators to start an emergency measure to reduce volumes in the tunnels. Starting at 2:15 a.m., wastewater was pumped directly out of the tunnels and diverted around some treatment to the final step of disinfection where chlorine was added to kill bacteria and pathogens. There it was mixed with fully treated wastewater and discharged to the lake.

But the emergency action could not keep pace with incoming flows. At 2:30 a.m., Shafer ordered the closing of gates that enable combined sewer overflows to drop into the tunnel. This was done to reserve remaining space in the tunnels for overflows from separate sanitary sewers and to prevent backups of sewage into basements. Those combined sewer overflows were directed to waterways until 10 p.m. Wednesday when gates to the tunnel were reopened.

The Jones Island plant diversion ended a few hours later, at 1 a.m. Thursday. An estimated 60 million gallons was pumped out of tunnel during the emergency, reducing combined sewer overflows to waterways by that volume, according to Peter Topczewski, MMSD director of water quality protection.

The district also reported two separated sanitary sewer overflows in Wednesday's storms.

A pump station at N. 35th St. and Roosevelt streets poured an estimated 660,000 gallons of untreated wastewater to Lincoln Creek, Topczewski says in the report to the DNR. Pumps operated intermittently before 3:49 a.m. Wednesday to draw down excess flows in the sanitary sewer there and prevent basement backups in nearby residences and businesses.

A second sanitary sewer overflow occurred between 10:30 a.m. and 2:30 p.m. Wednesday at N. Greentree Road at the Milwaukee River in River Hills. An estimated 140,000 gallons of untreated wastewater poured into the river during that time, the report says.

Six communities in southeastern Wisconsin — Milwaukee, Brown Deer, Fox Point, Bayside, Mequon and Port Washington — also reported overflows of sanitary sewers on Wednesday.

Milwaukee public works officials said a total of 179,105 gallons of untreated wastewater overflowed from six sanitary sewers to local rivers.

No estimates of volumes were available Friday from the other five municipalities.

In MMSD's first combined sewer overflow this year, on May 12, an estimated 200,000 gallons of untreated wastewater spilled into Lincoln Creek from one regional sewer on Milwaukee's north side.

A state permit allows the district up to six combined sewer overflows a year.

State permits prohibit sanitary sewer overflows, but there are exceptions for protecting private property and public safety.

Michael Luba, DNR wastewater program supervisor for the southeast region, said Friday that he and other department officials would be reviewing all reported sanitary sewer overflows for possible permit violations.

Rainfall totals, potential for basement backups, history of overflows at each location, as well as duration and volume of each overflow will be considered in deciding whether enforcement action is taken, Luba said.

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