The Cascade County zoning board of adjustment approved a special use permit for the Belt Water Treatment Plant during their May 18 meeting.

The plant is designed to remove metals, primarily iron and aluminum, from mine-impacted water through chemical precipitation processes, commonly known as the high-density sludge process, according to Drew Herrera, of HGL, the engineering team on the project.

Right now, those metals are being discharged into Belt Creek and the dissolved metals in the mine-impacted water oxidize and cause the orange, red and white staining downstream of the Anaconda Adit mine impacted discharge location, Herrera told the board.

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Coal mining began around Belt in 1877 and operated as late as 1963 with underground block and pilar coal mines, leaving about 250 acre-feet of mine-impacted water discharged into Belt Creek annually, according to HGL.

In 2012, the Montana Department of Environmental Quality abandoned mine lands program completed a water treatment assessment to look at coal mine discharges in the Great Falls area and prioritize treatment. The assessment identified the coal mines around Belt as the high priority for treatment due to their proximity to Belt Creek, according to HGL.

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Herrera told the zoning board that the first step of the high-density sludge process, hydrated lime is added to the water to create a slurry that is combined with recycled sludge from the treatment process. The combination is then added to the mine-impacted water to remove suspended solids, biochemical oxygen demand and dissolved metals by precipitating high-density sludge, Herrera told the board.

High-density sludge is metal hydroxides, gypsum, carbonate, unused lime and water, all of which are non-toxic and insoluble under neutral-to-alkali pH conditions, according to Herrera.

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The high-density sludge from the plant will be disposed of in unlined trenches on-site, according to the design plans.

According to HGL, the metal won’t leach from the sludge since the excess alkalinity in the sludge is enough to sustain natural pH and the high residual lime and precipitated carbonate in the sludge ensures long-term buffering capacity and mineral stability.

The trenches will be on-site where surface water can be managed to reduce the potential for stormwater contact, but the metal hydroxides are insoluble and won’t leach, according to Herrera.