Potawatomi’s plant converting waste to energy violated permit

The Potawatomi’s $20 million facility in the Menomonee Valley to convert food waste to biogas fuel — heralded as cutting-edge renewable energy technology when it opened in 2013 — repeatedly has flushed too much of fats, oil and grease to sewers in violation of its wastewater permit, records show.

The Milwaukee Metropolitan Sewerage District has notified the waste-toenergy plant of a series of permit violations since April 2017, MMSD water quality protection manager Sharon Mertens said.

The recurring excessive discharges landed the Forest County Potawatomi community’s renewable energy facility atop MMSD’s 2017 list of local industries in significant noncompliance with permitted pollution discharge limits.

The district limits the amount of fats, oil and grease that businesses can discharge to sewers to prevent clogging or blocking the pipes with thick mounds of the gunk, said Mertens.

“This is not something we can totally eliminate but too much is harmful to sewer pipes,” she said.

In a Feb. 15 notice of continuing violations, the district reported a Dec. 5 wastewater sample contained 6,900 parts per million of fats, oil and grease, or 23 times more than allowed under the permit, Mertens said.

The limit is 300 parts per million. A Dec. 11 sample contained 1,300 parts per million of fats, oil and grease.

Potawatomi Attorney General Jeff Crawford said facility managers and tribal officials have been taking the violations seriously and shut down waste digesters and biogas-burning generators for two months last summer to find the cause of the problem.

Workers removed fats from the digesters and other tanks during the shutdown and since that time, the tribe has invested more than $300,000 in equipment upgrades to boost efficiency of the digesters and prevent further problems, Crawford said.

Each year, a total of 16 million gallons of food waste is mixed together before it is poured into two 1.3 million gallon digester tanks.

The facility did not discharge excessive amounts of fats, oil and grease to sewers in 2013, 2014 and 2016, MMSD records show. There was one violation of the permit limit in 2015.

“This is like a big pot of soup that we are stirring and constantly adding waste to,” Crawford said in describing the digesters. “We found out that we need to be very good chefs in this kitchen as we cook our soup, 1.3 million gallons at a time.”

Inside the digesters, bacteria release methane and other gases as they digest the slurry of wastes warmed to 98 degrees. This biogas is collected and burned in two generators to produce electricity.

But too much fat was added to the pot in 2017. The load of wastes coming in from businesses could not be stirred adequately in a mixing tank at the start of the process so fats, oil and grease clogged up the works there last fall and had to be cleaned out, according to Dec. 1 correspondence from Crawford to MMSD. Better mixers also were installed earlier this year.

Facility managers in November decided to change what Crawford calls the soup recipe by reducing the amount of slaughterhouse waste and other sources of food waste loaded with fats and grease, such as restaurant grease trap contents, that it takes in for the digesters, said Crawford.

Crawford said the waste-to-energy facility has been in compliance with permit limits on fats, oil and grease since Jan. 1 of this year. A commercial laboratory’s test of a March 28 wastewater sample found only 16.4 parts per million of the pipe-clogging contents, well below the permit limit, according to Crawford.

Mertens disagrees that the Potawatomi have corrected the problem.

Testing of a Jan. 25 wastewater sample found 315 parts per million of fats, oil and grease, slightly above the permit limit, according to a Feb. 16 notice of continuing violations issued by MMSD senior industrial waste engineer Song Tran.