ENVIRONMENTAL IMPACTS OF WATER TREATMENT CHEMICALS AT INDUSTRIAL SAND MINES

Polyacrylamide anionic flocculants are commonly used to enhance settling of solids in the clarifier associated with the wash plant at a silica sand facility. These products serve a significant benefit to the op­eration of a mine site because they reduce the overall footprint of the mine, which would otherwise require extensive settling basins. Polyacrylamide has been safely used for decades in the United States to treat public drinking water and is widely used in a variety of industries and can be found at food processing facilities, used as blast­ing agents, used for drilling mud and grout products, and used as a soil-stabilization agent. The EPA, through a treatment technology standard, limits the amount of anionic polyacrylamide used at water treat­ment plants by limiting the amount of the monomer in flocculants to less than 500 parts per million (ppm) acrylamide, used at a concentration of 1.0 ppm or less. This gives a de facto safe drinking water limit of 0.5 μg/L.

Although no adverse human health effects are associ­ated with the polyacrylamide polymer, adverse human health effects have been linked to an impurity called acrylamide, which is a monomer, or part of the poly­acrylamide molecule used in the production of the anionic polyacrylamide flocculant. A very small amount of the monomer molecule is present in the raw chemical feedstock that remains unreacted in the polyacrylamide flocculant when it is mixed in the wash process used in the mine’s wet plant. The acrylamide monomer is highly soluble in water, and has poor adsorption to minerals and organic matter. Thus, the concentration of acrylamide that will exit the clarifier in the underflow slurry will equal the concentra­tion of the process water that overflows the clarifier and is recycled in the process.

Studies dating back to 1979 have shown that the acrylamide monomer is biodegradable. Biodegradation rates vary with the availability of oxygen, temperature, concentration of acrylamide monomer, concentration of capable microorganisms, and whether the capable microbial community had previous exposure to acryl­amide monomer. Sampling at several mines has indicated that acrylamide does degrade, as it has not been detected in groundwater to date.