The roof of the sludge concentration building at Chicago's Calumet Water Reclamation Plant collapsed after an explosion Aug. 30 around 11 a.m. Ten people were hurt and successfully evacuated to area hospitals for treatment by the Chicago Fire Dept. The plant is located in south suburban Riverside and is the oldest of the seven Metropolitan Water Reclamation District of Greater Chicago wastewater treatment facilities. It opened in 1922 and provides wastewater treatment to approximately 1 million homes and businesses in southern Cook County.

According to a statement from the city/county agency, two people were trapped and were extricated from the building by Chicago Fire Department emergency crews and transported to local hospitals along with the other eight injured personnel. Firefighters to had to tunnel more than 40 ft through the rubble to extract one of the trapped, injured workers; that rescue took nearly two hours.

"It was a very dangerous situation," said William Vogt, a deputy district chief for the CFD, at an afternoon press conference.

On Aug. 31, the Chicago Fire Dept Office of Fire Investigations said in a statement that the cause of the explosion was use of a worker's torch in an area with a significant amount of methane gas present, which caused ignition and a shock wave that lifted the roof.

MWRD's website said that, currently, biogas is used to produce steam in boilers for plant heating at Calumet, but that the changes at Calumet that the agency is pursuing could increase biogas production by 160% over current volume.

"The MWRD applauds the City of Chicago's emergency responders for their commitment and expertise in rescuing and treating our workers. We will thoroughly investigate this incident to determine how this happened and identify measures to prevent something like this from happening again," MWRD President Mariyana Spyropoulos and all of the agency's commissioners said in a statement. The Calumet plant remains in operation, and the MWRD said in its statement that there is no threat to the public or the drinking-water supply of the communities it serves.

Biogas is being used to produce plant heating at other MWRD facilties, such as the Stickney Water Reclamation Plant.