PORTLAND — Portland firefighters vented the last of the fumes from an accidental chemical reaction at the Portland Wastewater Treatment Plant late this afteroon.

Two chemicals, sodium bisulfite and sodium hypochlorite – about 500 gallons altogether – were mistakenly mixed in a 2,500-gallon plastic tank early this morning.

The combination created a chemical reaction that gave off heat and could have created explosive pressure, if not for the containment tank’s safety features.

The 11 workers who had been evacuated from the building at around 7:30 this morning were allowed back into the building around 4 p.m. to retrieve their belongings from the second and third floors so they could go home.

The East End Trail, which had been closed for the day, was expected to be reopened this evening.

11:10 a.m.

PORTLAND — The dangerous mixture of two chemicals used at the Portland Water District’s East End wastewater treatment plant has created a hazardous materials incident.

A delivery of sodium bisulfite was mistakenly added to a supply of sodium hypochlorite, the main chemical in bleach. Sodium bisulfite is a chemical often used to prevent corrosion within pipes.

The incident started at 7:30 a.m. Nobody was injured and meters firefighters use to test for harmful vapors have detected none outside the facility, said city spokeswoman Nicole Clegg.

“Now that we’re certain what it is, we have teams entering the building and we’re going to work our way into where the tank would be,” Clegg said.

8:30 a.m.

PORTLAND — Portland firefighters are on the scene of a chemical incident at the Portland Water District sewage treatment plant at the end of Marginal Way.

The exposure was reported at about 8 a.m. A spokeswoman for the city said the situation is contained and there is no risk to the surrounding neighborhood. She had no details on the substance involved or how it happened.

Nobody was injured.

The department’s hazardous response team has been mobilized and at 10 a.m. was preparing to enter the building and investigate. The facility has been evacuated but oontinues to operate.

The East End trail has been closed to the public.