An Indianapolis wastewater treatment company is under investigation by federal and state authorities on claims that it flushed oil and other pollutants into city sewers, the company's lawyer confirmed Friday.

The U.S. attorney's office filed charges earlier in the day against two former managers of Ecological Systems Inc. for violations of the federal Clean Water Act.

They have signed plea agreements and are cooperating with the investigation.

The single charge against each man centers on the company's flushing of 300,000 gallons of untreated wastewater over eight hours on Feb. 11, 2009, according to charging documents filed in U.S. District Court in Indianapolis.

The discharge "caused an oily sludge-like substance to flow out of various sewer manholes and into the yards of several Indianapolis residents," according to the documents.

The discharge allegedly occurred at ESI's Northwest side treatment facility, 4910 W. 86th St., which takes in used oil, coolants and other liquid waste from industrial clients and treats wastewater before releasing it into the sewer system.

Larry Mackey, an attorney for ESI, said the Indiana Department of Environmental Management and the U.S. Environmental Protection Agency still have an ongoing investigation of the company, which operates as ESI Environmental.

A statement issued by ESI says company officials were disappointed by Friday's charges against Michael R. Milem, 60, Westfield, and Mark R. Snow, 43, Brazil.

Milem -- operations manager and later director of plant operations -- left the company last year. Mackey said ESI fired Snow, the laboratory manager.

"ESI's core business is the protection of our environment and any failing is unacceptable," the company's statement says. "We have cooperated fully with the EPA throughout their investigation and will continue to do so."

Prosecutors said that during the February 2009 rainstorm, ESI managers directed lower-level employees to pump a tank containing oil, wastewater and other materials and flush the untreated water directly into the sewer system, violating its city permit.

The managers wanted to maintain the facility's ability to accommodate new truckloads of waste as storm water collected in its tanks, the court documents say.

Prosecutors said Milem and Snow each face up to three years in prison and a maximum $250,000 fine, though their cooperation would be taken into consideration.

After the discharge, the charging documents say, Milem and Snow were among company officials who falsely
insisted to IDEM inspectors that there had been no "bypass event."

Prosecutors also allege Snow regularly submitted falsified monitoring reports to the city, at the direction of an unidentified corporate officer, and that ESI misrepresented its available waste storage capacity to regulators.

The same facility came under scrutiny by a City-County Council committee in May.

Area residents alleged that a gaslike odor was coming from the site, a charge disputed by a company lawyer. A proposed ordinance aiming to control offensive smells coming from businesses failed to garner support from the Public Works Committee.