The potential for a methane explosion from faulty aluminum struts at a Mankato sewage treatment plant could mean a doubling of repair costs.

The City Council approved spending $317,000 earlier this year to replace the 15-year-old membrane at the city's wastewater treatment plant near Riverfront Park. A geodesic dome covering the equipment takes methane gas from the sewage and turns it into fuel to heat the sludge and kill germs. But as the membrane was being replaced, workers found faulty struts supporting the dome.

"You could conceivably have a methane explosion, that would be the worst-case scenario," said Pat Hentges, Mankato's city manager.

To avoid any possible blast, Hentges said the city is seeking another $375,000 from the City Council to replace the entire structure next spring. He said there are several options to "adapt, replace or otherwise repair the equipment that has been determined to be deficient." A staff report to the City Council warned that "the sudden release of methane gas mixing with air could present an explosion hazard."