The decision to dump partially treated wastes into the lake in the midst of two days of rain was a judgment call made by United Water, the private firm that operates the tunnel and two local treatment plants for the Milwaukee Metropolitan Sewerage District, said district Executive Director Kevin Shafer.
The concern at the time was that large amounts of additional rainfall might have forced raw sewage dumping, Shafer said.
The decision to skip part of the treatment at the Jones Island plant was made at 6:30 a.m. Tuesday, when up to another 1.6 inches of rain was being forecast for the Milwaukee area, Shafer said.
The district's two treatment plants were operating at full capacity at the time, he said, and fully treated about 1 billion gallons of sewage over the last three days. That's nearly double the amount treated over such a period during dry weather.
Milwaukee got 0.78 inches of rain for all of Tuesday, according to the National Weather Service, and 1.98 total inches of rain from Sunday through Tuesday, as measured at Mitchell International Airport.
MMSD said it measured rainfall over the three days at 1.75 to 4.25 inches at various locations in the Milwaukee area.
"Based on available information, it appears United Water made the correct call," said Bill Graffin, a spokesman for MMSD.
Mike Link, operations manager for United Water, couldn't be reached for comment.
The deep tunnel system, completed in late 1993, can store up to 405 million gallons of wastewater during heavy rains and was built to avoid sewage dumping. MMSD generally reserves half the tunnel capacity for sanitary wastes from the suburban communities served by the district to try to avoid overflows or basement backups in those communities.
The "in-plant diversion" at Jones Island lasted 10 hours Tuesday and means that the partly treated sewage had two of the three stages of treatment, including settling and disinfection, said Shafer.
Plant operators skipped one crucial stage of treatment, in which air is pumped into wastes so that beneficial bacteria and other biological organisms consume waste particles.
Though full treatment is preferable, partial treatment is better than dumping raw sewage, Shafer said. The DNR permits partial treatment under certain limited circumstances, he noted. A statement released by MMSD Wednesday called dumping of partial sewage "standard operating procedure for a wastewater treatment plant trying to maximize the amount of wastewater treated."
Bob Boucher, who heads the local environmental group Friends of Milwaukee Rivers, called the decision to dump partially treated sewage "suspect."
"It does call into question the political choice to maintain half the (tunnel) capacity, when for the general public trust it would be in the best interest to fully treat" sewage, he said.
"If it's perfectly safe water, then maybe all the sewage commissioners should go swimming near the (Jones Island treatment plant) outfall and do surface dives to prove it to the rest of us," Boucher said.
The Jones Island plant discharges its wastewater effluent into the Milwaukee harbor.
Shafer said he was confident that even with the discharge of partially treated sewage, MMSD still would meet monthly pollution limits set by the state Department of Natural Resources.
Boucher's organization and the Lake Michigan Federation have sued MMSD in federal court for sewage dumping. The district has dumped more than 13 billion gallons of raw sewage since the deep tunnel project was completed, including about 1 billion gallons of concentrated sanitary wastes. The rest is storm and sanitary sewage from old combined sewers in Milwaukee and Shorewood.
MMSD dumped more than 100 million gallons of partially treated sewage last year over four separate occasions, according to district records.
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