After months of public scrutiny of its new wastewater permit that allowed for increased pollution of Lake Michigan, BP has agreed to abide by the limits set in its old permit.

But that doesn't mean the company won't exceed those limits.

Like most water treatment plants, periodic overruns result from storms and other events, both natural and manmade.

When a winter storm on March 9, 1998, caused an areawide power outage, BP discharged 61,159 pounds of suspended solids in a day --more than 10 times what the permit allowed. During heavy rain on Jan. 21, 2003, the refinery released 35,420 pounds -- more than six times what allowed amount.

BP was not assessed any penalties for the 20 violations between 1998 and 2006. Yet, BP's violation history is still modest compared to other industrial and municipal facilities.

"Given the parameters, that doesn't sound like that bad of an operating history," said Tom Anderson, executive director of Save the Dunes Council. "I've seen sometimes dozens and hundreds of exceedances a year" in other facilities' reports.

A certain amount of upsets is to be expected, especially with large facilities, Anderson said.

"It's a lot different than someone who's not operating carefully," Anderson said.

By comparison, IDEM records show International Steel Group in Indiana Harbor violated its permit at least 20 times over four years. Michigan City Sanitary District exceeded its permit levels 15 times over about nine years. Chesterton Wastewater Treatment Plant had 19 violations over about five years.

And East Chicago Sanitary District exceeded its permit limits at least 55 times over nearly four years.

When a violation happens, a facility has to report to IDEM when and how it happened and address any problems that may have contributed to extra pollution being released.

In a letter to BP, Greg Glover, a senior environmental manager of the Compliance Evaluation Section of IDEM, asked the company to explain why it exceeded suspended solids limits by so much in January 2003.

"I am concerned about the degree of the exceedance. The discharge of 35,420 lbs. vs a limit of 5,694 lbs. was more than six times the allowable limit," Glover wrote. "I wonder why the increase in oil recoveries and solids loadings wasn't foreseen and more effectively managed."

BP spokesman Tom Keilman said the violation occurred after a storm, with rain coming at the equivalent of about 2.4 inches an hour. "It caused a sewer backup," he said.

Several times, IDEM inspectors alleged that there was a visible oil sheen on Lake Michigan by one of BP's discharge pipes.

BP said at least three of the incidents -- July 10 this year, October 2006 and November 1997 -- occurred after heavy rains caused sewer backups and wastewater accidentally entered a cooling-water system.

Internal BP emergency response teams mitigated the oil spills by deploying booms and absorbents and using vacuum trucks to suck up the oil.

On several occasions, IDEM inspectors also said BP violated its permit by discharging brown or discolored wastewater. After two alleged discoloration incidents, in 2000 and 2004, the refinery upgraded its filters in response.

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