Biological Treatment Helps Remove Nitrate, Sulfate from Mine Runoff
Nitrate and sulfate are common contaminants in surface water and groundwater associated with mining operations.
By Mark Reinsel
Nitrate and sulfate are common contaminants in surface water and groundwater associated with mining operations. Three biological treatment systems have successfully removed nitrate and sulfate at the Kettle River Operations near Republic, WA, since their construction in 2005-06. These facilities are operated by Kinross Gold Corp. Mine water and mining-impacted groundwater are treated to meet State of Washington antidegradation standards for groundwater discharge. Treatment is accomplished with a combination of engineered reactors and in situ treatment.
Nitrate is commonly removed from wastewater in both industrial and municipal applications with biological treatment using nitrate-reducing (denitrifying) bacteria. Added methanol or a comparable carbon source typically serves as an electron donor for the bacteria. In this reaction, denitrifying bacteria reduce nitrate to nitrogen gas, oxidize methanol to carbon dioxide and create more bacteria (biomass). Including methanol demand for dissolved oxygen, approximately 3 mg/L of methanol is required for each mg/L of nitrate-nitrogen. Nitrogen gas and carbon dioxide, which are natural constituents of the atmosphere, passively vent from the treatment system.
Cell #1 containing plastic media is shown shortly after start-up in November 2006.
For sulfate reduction, controlled experiments have shown that approximately 0.45 mg/L of methanol is required to reduce 1 mg/L of sulfate. Sulfate is reduced to hydrogen sulfide or elemental sulfur. Therefore, the total methanol demand is the sum of that required for denitrification and sulfate reduction. Addition of excess methanol is desirable from all aspects other than possibly cost, since excess nutrients may provide in situ denitrification and sulfate reduction.
A photo of the system, comprising three reactors in series, is shown in December 2005 after insulation was applied.
The selected biological treatment system uses sized rock media or engineered plastic media for bacterial attachment. This is a “packed bed” system with a very high biomass quantity per unit volume, which reduces the footprint of the treatment system. Packed bed systems are much more resilient and less prone to upsets than “suspended growth” systems, which are commonly used in municipal wastewater treatment plants.
The “Underdrain” is a groundwater capture system downgradient of the tailings impoundment at the Key Mill facility. A low flow of groundwater seeping from the tailings impoundment contains slightly elevated levels of nitrate and sulfate. The goal of this project was to remove nitrate and sulfate from captured groundwater, and infiltrate this treated water to reduce concentrations at downgradient monitoring wells.
After demonstrating in bench tests that biological treatment would meet the project goals, Apex Engineering was contracted to design a treatment system with the following design criteria:
- A flow rate of 10 gallons per minute (gpm),
- A nitrate-nitrogen concentration of 6 g/L, and
- A sulfate concentration of 120 mg/L.
This treatment system was constructed in late 2005 and “inoculated” with bacteria from another mine water treatment plant. The system rapidly developed to remove 100 percent of the nitrate and up to 95 percent of the sulfate in captured groundwater. Treatment upsets periodically occurred when methanol flow was interrupted; this was addressed by modifying the methanol pumping system.
Infiltrating treated water at the Underdrain site also reduced nitrate and sulfate concentrations at downgradient monitoring wells such as TP-2, which was the treatment system’s goal. A photo of the system, comprising three reactors in series, is shown in December 2005 after insulation was applied. Each reactor contains plastic media (3-1⁄2" diameter Flexirings manufactured by Koch-Glitsch), PVC distribution piping at the bottom and PVC collection piping at the top. After water is pumped to the first reactor, it flows by gravity through the rest of the treatment and infiltration system.
Key Mine System
The Key Mine is a closed facility where groundwater was negatively impacted by infiltration through a reclaimed development rock stockpile. Concentrations at downgradient monitoring wells exceeded the State of Washington standards for nitrate, sulfate and total dissolved solids (TDS).
Based on the success of the Underdrain treatment system, Apex Engineering was contracted to design a similar treatment system to handle a larger flow with higher concentrations. Golder Associates designed the groundwater capture and treated water infiltration systems.
Design criteria for the Key Mine system were:
- A flow rate of 50 gallons per minute (gpm),
- A nitrate-nitrogen concentration of 33 g/L, and
- A sulfate concentration of 800 mg/L.
At the Key Mine treatment system groundwater captured in a French drain was amended with methanol and phosphate (an essential nutrient in addition to the carbon found in methanol), and pumped into three treatment cells in series. The first cell, designed for denitrification, was much smaller than the others and contained plastic media similar to the Underdrain system. Cells #2 and #3, designed for sulfate reduction, were much larger and contained rock media. Rock media requires approximately twice the media volume, but is less expensive than plastic media.
|Figure 4. Process Flow Diagram-Key Mine|
Cell #1 containing plastic media is shown shortly after start-up in November 2006 (see page 22). Cells #2 and #3, containing rock media covered with snow, are shown in the background. Insulated covers were later added to all three cells, which were rectangular concrete tanks built into the ground. As at the Underdrain system, water was pumped to Cell #1 and then gravity-flowed through the rest of the system. Excess methanol was added to promote in situ treatment.
Nitrate reduction began almost immediately after start-up and sulfate reduction activity increased over the first two months of system operation. This was at a low flow rate of 2-5 gpm during the dry winter season. As flows increased in the spring to as much as 30 gpm, sulfate reduction decreased until April, when the flow stabilized and an increased amount of biomass in the reactors again became effective. Sulfate concentrations in the effluent continued to drop throughout 2007, even as the influent concentration increased to above the design concentration of 800 mg/L.
The water temperature in the Key Mine treatment system was as low as 2oC during the winter. A biofilm was apparent at the closest downgradient monitoring well, presumably from in situ biological activity.
K2 Mine System
A third treatment system was installed at the operating K2 Mine in late 2005. The purpose of this system was to remove nitrate from captured groundwater and mine water to reduce nitrate concentrations at downgradient wells. This system performed as designed but ran only intermittently.
All three treatment systems are simple and inexpensive to operate. A technician checks on each system and takes samples two or three times a week. The Underdrain and K2 systems were inoculated from a denitrification system at the Stillwater Mine in Montana, while the Key Mine system was inoculated later with biomass from the Underdrain system.
The Underdrain system installed cost was approximately $150,000, with $80,000 for materials and $70,000 for construction. The Key Mine installed cost was $1.05 million, including $350,000 for materials and $700,000 for construction. However, costs at the Key Mine were higher than typical because of the remoteness of the site, and also included the groundwater capture and infiltration systems. The estimated installed cost for the treatment system alone was approximately $500,000.
Biological treatment completely removed nitrate from both the Underdrain and Key Mine waters. Significant sulfate removal was noted in treated effluent at both sites, except after methanol flow interruptions. Groundwater concentrations at downgradient monitoring wells at both sites were reduced, which allows Kinross Gold to meet its groundwater discharge standards. With addition of excess carbon, it is believed that in situ treatment may reduce groundwater concentrations even further.
About the Author: Mark Reinsel, Ph.D., P.E., is president of Apex Engineering PLLC, a small consulting firm in Helena, MT. Dr. Reinsel has 25 years of experience in consulting, industry and academia, with recent projects focusing on treating industrial wastewater, groundwater and drinking water through biological, chemical and physical methods. He may be contacted at email@example.com.