COOS BAY — At least 3.5 million gallons of partially treated sewage has spewed into the Coos Bay during the past four days after concentrated fluoride flowed into a sewage treatment plant, killing its bacteria-munching organisms.

Workers at the Coos Bay-North Bend Water Treatment Plant allowed a tank holding the fluoride, used in drinking water to prevent tooth decay, to overflow Sept. 28. The 400 gallons of highly acidic chemical flowed into a floor drain and into the sewer before reaching the sewage treatment plant several blocks away.

The acid killed all of the sewage treatment plant’s micro-organisms, which consume biological material in sewage. The waste was treated with chlorine before entering the Coos Bay, but it far exceeded state standards for fecal coliform, said Gary Young, project manager at the sewage treatment plant.

The treatment plant imported new microorganisms from the North Bend plant to reseed its system.

The fecal coliform count by Monday afternoon was nearly back to normal, Young said.

Neither the sewage nor the fluoride killed any fish in the bay or the Coos River, which is commonly filled with salmon fishermen this time of year, but it slowed commercial harvest of oysters in the bay.