A small explosion sparked a fire late Tuesday morning at a sprawling waste treatment plant in St. Paul, leaving one person slightly injured, authorities said.

The fire started about 9:45 a.m. and broke through the roof of the Metropolitan Council facility at 2400 Childs Road, located along the Mississippi River and across the water from the St. Paul Downtown Airport.

The fire’s intensity at one time forced firefighters to battle the blaze strictly from the exterior, a department official said. Employees were gathered outside as plumes of smoke rose skyward over the 80-acre complex and grounds.

The blaze was considered contained within an hour, and crews were tending to hot spots as the morning drew to a close, said Fire Marshal Steve Zaccard.

Met Council spokeswoman Bonnie Kollodge said the plant’s operations were uninterrupted.

A Met Council official said the blast was caused when a contract employee cut into odor-fighting equipment as it was being demolished.

The Total Mechanical Services worker was taken by a fellow contract employee to Regions Hospital for treatment of minor injuries, Zaccard said.

The worker “shouldn’t be cutting in an atmosphere of explosive vapors,” the fire marshal said.

The fire occurred in “an older part of the complex that is being decommissioned, so it wasn’t populated,” Kollodge said.

Zaccard described the situation as “pretty dangerous,” particularly in the first 30 minutes until officials located an employee that had been unaccounted for.

The 77-year-old facility, one of eight operated by the Met Council, is the largest wastewater treatment site in Minnesota, with a capacity of 251 million gallons a day.