When you are confronted with the job of cleaning difficult mine wastewater, your primary goal is an environmental optimization strategy — keeping costs down while meeting environmental requirements. This optimization requires reaching a balance that facilitates permit compliance while avoiding nuisance reactions which only increase complexity and costs. Experienced laboratory professionals can help you make confident decisions about your treatment goal.
Mine Wastewater Treatment: No One Size Fits All
Mining reclamation projects particularly benefit from on-site laboratory testing since the solutions are unique and can be highly variable. A single treatment approach does not work for every site. On-site testing is required to match solutions to a customized treatment strategy. The strategy helps you choose the most appropriate processes, design parameters, chemical requirements, chemical dosages, and equipment sizing.
A perfect example of this occurred at the Cresson Mine in Pennsylvania, a state where coal mining helped fuel the Industrial Revolution but created acid mine drainage that continues to impair over 4,000 miles of streams. Pennsylvania is the worst state in the nation for acid mine drainage. Dangerous runoff from these mines can include arsenic, selenium, other heavy metals, and oxygen completing compounds that severely impact the environment. Untreated, these contaminants make water uninhabitable for fish and wildlife. Consequently, these wastewaters are subject to significant regulations.
Pennsylvania’s Bureau of Abandoned Mine Land Reclamation (BAMR) constructed a mine drainage treatment system to help restore several miles of impaired streams and estuaries. While the majority of Pennsylvania mine runoff can be characterized as low pH with elevated metal concentrations, the Cresson treatment plant received pH 5 mine drainage with a combined iron and aluminum concentration of 10-15 mg/L.
A traditional treatment design approach would have been to use the high-density sludge (HDS) process, widely used in the industry and originally developed at Bethlehem Steel nearby. The HDS process is relatively simple, but requires recirculating a portion of the previously settled solids from the clarifier and dosing the solids with lime before mixing the solids/lime slurry with the untreated mine drainage. The process takes advantage of surface chemistry reactions that cause dissolved pollutants to adsorb and precipitate on the surface of the recirculated sludge to increase density and settling rates.
However, the Cresson Mine did not select the HDS process because it would add unnecessary costs. Instead, the plant was designed to use a solids CONTACT CLARIFIER™ to treat the acid mine drainage wastewater.
Here’s how the treatment process works at the Cresson Mine:
Initially, the mine drainage is decarbonated to remove dissolved carbon dioxide that, otherwise, would have consumed lime and increased cost.
Next, hydrogen peroxide is added to oxidize ferrous iron.
Then, the water enters the solids CONTACT CLARIFIER where the water is dosed with high-density lime slurry to increase pH and cause ferric iron precipitation.
Concurrently, the solids CONTACT CLARIFIER is concentrating the suspended solids through internal sludge recirculation which increases flocculation, settling, and clarity. The concentration of solids by the solids CONTACT CLARIFIER was instrumental in achieving solid/liquid separation for mine drainage containing low metal concentrations.
During start up, the clarifier experienced poor solid-liquid separation and BAMR requested WesTech’s laboratory team to conduct on-site testing and develop a solution.
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To improve settling and underflow concentrations, WesTech made a site visit to see if testing could improve settling and underflow concentrations.
A flocculant screening and HDS testing were both done to see if either would improve performance of the clarifier. The testing revealed a problem in the flocculant feed system. Fortunately, they concluded that when properly flocculated, the system was effective in treating the mine wastewater to acceptable limits for discharge.
Customer Confidence Leads to Testing of Other Mine Runoff Projects
Anthracite Region Plant: Later that year, WesTech was invited back by the same customer — the Pennsylvania Department of Environmental Protection (PADEP) — to perform testing at two other sites. The first was the Quakake drainage tunnel in the Anthracite Region of Pennsylvania. This mine drainage was unusual because the primary contaminant was aluminum.
On-site testing was performed to design a system to remove aluminum. It showed that after neutralizing the tunnel drainage, the precipitated solids could be effectively flocculated and settled via either internal or external recirculation of solids. As a result, PADEP is currently developing a treatment solution.
South of Pittsburgh Plant: The second site was a crumbling, aged acid mine drainage treatment plant south of Pittsburgh. The purpose of the testing was to confirm the correct sizing of replacement equipment and determine if there was enough space to combine two separate treatment plants onto this one site. The testing showed that solids recirculation either internal or external was again the key for optimal performance.
In each of these cases, detailed on-site testing of the mine drainage water helped confirm the most effective treatment method for the varying levels of acidity and metals.