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DEQ approves plant improvements


October 9, 2019

The City of Three Forks Arsenic Treatment Plants improvements have been approved by the Department of Environmental Quality.

In early 2019, it was discovered that arsenic was breaking through the media on well #2 and it was shut down due to MCL exceedance for arsenic.

In response, the City put together a team to replace filtration media, a flow meter, turbidity analyzer, chemical feed pumps, and controls. Part of the project was submitting for DEQ approval to upgrade the chemical feed pump equipment. Over the course of the project, the City staff were also able to optimize the plant and capture knowledge that will be used in operations and maintenance going forward. In particular, optimizing the air addition during media backwash is expected to give it longer life. The refurbishments, upgrades, and optimization were recently completed, and results of arsenic laboratory analysis now show levels well below the MCL.

The original arsenic treatment system for Well #2 was installed in Three Forks in 2006 in response to EPA regulations lowering the Maximum Contaminant Level (MCL) from 0.05 mg/L (50 ppb) to 0.01 mg/L (10 ppb). Many towns and cities implemented treatment systems to their water plants at that time and the years following. The Three Forks treatment system removes arsenic by co-filtering along with iron through a pressurized media filter system. As water from Well #2 enters the building, it is first chlorinated using sodium hypochlorite. Then ferric chloride is injected into the chlorinated water where iron and arsenic are oxidized and flocculated together in contact tanks. Finally, water passes through filter media to remove the iron and arsenic

The arsenic treatment plant uses a reliable filtration technology and is expected to serve the City for at least another decade until new filtration media or minor equipment upgrades can be revisited to ensure peak performance. Regular sampling of Well #2 water for arsenic is required quarterly and will be reported to DEQ for MCL compliance.

Editor's Note: This informational article was made available by the City of Three Forks.


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