WATER TREATMENT PROCESS
Water pollution is not an option
As a mining company, we know our environmental footprint extends beyond our fence line. The most obvious example is water, which — if managed improperly — has the potential to harm the environment. Using state of the art water treatment, process control, testing, and water management capabilities we can eliminate that potential.
Early in project development, water balance models predict the amount of water necessary versus available. Process requirements, reuse opportunities, plus historical climate and hydrological data are all taken into account. The models serve as the foundation of a water management plan, which is an integral component of mine design, operation, and closure.
In an ideal situation, you would have just the right amount of water when it is needed. In Back Forty’s case, we have more water than we can use; therefore we need to collect, treat, and discharge excess water. That means our water management plan must be tailored to reflect the needs of both the facility and the community.
As a result, our plan promotes continuous improvement in the form of minimizing water loss, efficient water usage, in-process recirculation systems, and most importantly — effective water treatment.
Our mine features an on-site, state-of-the-art wastewater treatment plant (WTP) to ensure the treated water is safe for the environment, and even cleaner than the strict quality standards established in our NPDES permit. For example, our standard for mercury is 1.3 parts-per-trillion, or 0.0013 ng/L which much lower than the EPA safe drinking water standards and the average statewide observed concentrations in water bodies. The WTP is designed to treat up to 63,000 gallons of water per hour. The treated water will be reused in the mining process or discharged to the environment. The plant will begin operating during construction and continue into closure.
PROCESS DESCRIPTION
CONTACT WATER BASINS
Water that comes into contact with mining activities (e.g., mill process water, inflow to the open pit, tailings and waste rock facilities, runoff from snow and rain) will flow via gravity or pumps to lined Contact Water Basins (CWB). Design features of the CWBs include a maximum capacity of roughly 161M gallons, ability to contain a 100-year/24-hour storm event, a composite liner system, and emergency spillway to the open pit. From the CWBs, water will be pumped to the WTP to begin the treatment process.
PRECIPITATION
Treatment starts with a series of reactor tanks to remove as many dissolved solids as possible from the water. A chemical reaction within the tanks causes very fine particles to bind together to form larger particles for easier removal in the following steps.
CLARIFICATION
Here the particles, or solids, settle to the bottom of the tank. The solids are pumped to the mill to be combined with thickened tailings and sent to the tailings management facility. The water above flows to the filtration process.
FILTRATION
The water will travel through a sequence of filters to remove any fine particles remaining after the clarifier.
The first is a multimedia filter composed of elements such as sand, activated carbon, and gravel to remove solids from the water. Multimedia filters help remove suspended solids similar to how a coffee filter traps coffee grounds and allows water to pass.
Next, the water passes through a 0.1 micron cartridge filter. For a sense of scale, a human red blood cell is about 7.5 microns wide. The tiny holes in this filter allow water to move through and stop unwanted particles from continuing.
From here, water passes through another filter with adsorbent pellets designed to collect dissolved mercury. The water we discharge to the Menominee River is required to meet the quality standard of 1.3 parts-per-trillion — try to imagine a single drop in 10 million gallons of water.
Finally, water will pass through a 0.5 micron cartridge filter to prevent any remaining particles from leaving the system. From here the water is sent to the treated water tank.
TREATED WATER
The treated water storage tank will hold the water until it is reused in the mining process or discharged to the environment. Our permit allows for the release of up to 1.52 million gallons of treated water per day. The actual amount discharged will vary by the amount of water the plant receives. We will only discharge water that meets quality standards to the Menominee River. If water does not meet permit conditions, we will return it to the CWBs for re-treatment.
An on-site laboratory will analyze water quality data necessary for both operations and process control. A third-party accredited laboratory will confirm compliance with permit requirements.
MONITORING
For more than a decade, we have been studying the regional groundwater and surface water conditions. Simultaneously, we have managed a regional hydrology survey to define surface water conditions in the surrounding rivers and lakes.
Existing groundwater and surface water monitoring programs extending beyond the mine site will continue throughout operations and after mine closure. Monitoring includes water quality and the aquatic ecosystem both upstream and downstream of the discharge point to the Menominee River. Michigan Department of Environment, Great Lakes, and Energy, U.S. Environmental Protection Agency, and
Wisconsin Department of Natural Resources all determined that the conditions in our water discharge permit will protect the health of the community, wildlife and the environment — including the river.
Throughout the life of the mine, we will continue to look for ways to reduce our environmental impact. If you have additional questions or concerns, you can reach us by clicking on the Contact tab at the top of the page.
TAILINGS MANAGEMENT FACILITY EXPLAINED
What are tailings and what is Back Forty’s management plan?
Tailings
We will process roughly 4,800 tonnes (5,300 tons) of ore per day onsite. We will use traditional crush-grind-float technology and equipment to separate zinc and gold from noneconomic rock. After removal of these valuable minerals, what remains is called tailings. Our tailings consist of particles of rock, water, and residual process chemicals (e.g., lime, copper sulfate, silica), and have the consistency of drywall mud. We will pump tailings to a double-lined tailings management facility (TMF). The TMF will cover a total footprint of 123 acres. During operation, the facility will be a maximum 118 ft tall, and after closure a maximum of 138 ft tall.
Tailings Management Facility
Water management is crucial to our design. Unlike a conventional TMF, our design is neither a pond nor does it store liquid tailings. The entire base of the TMF will be compacted and double-lined, consisting of a composite primary liner and a single secondary liner separated by a leak detection system. This lining extends under the perimeter wall and the surrounding berm. The coarse aggregate above the primary liner collects the water (e.g., residual tailings water, runoff, rain, snow) and gravity directs it to an exterior sump. We will also pump water that collects on top of the TMF to the contact water basins or the mill for reuse in the milling process. As a protective measure, if we experience an extreme storm event, an emergency spillway will channel water from the TMF into the open pit. Once mining ends, the TMF will be capped and revegetated to prevent any oxygen penetration or water percolation into the facility. Also, we will dewater tailings after capping. By doing so, the TMF will be near neutral pH and will not require perpetual care or treatment.
Performance Monitoring
Our monitoring program includes more than a dozen ongoing studies, including water balance, groundwater level and quality, perimeter wall settlement, and leak detection system analysis. Also, an engineering review by a qualified independent Geotechnical Engineer will take place on an annual basis. The performance of the facility will be reviewed closely during construction, operations, and post-closure to ensure that the design intent is being satisfied, to confirm design assumptions, and to identify any modifications that may be required.
The design mitigates known risks of traditional tailings facility construction. Similar and successful models include the Malartic Mine and the Musselwhite Mine in Canada, and the Neves Corvo Mine in Portugal. Before construction commences, EGLE will review and approve the plan.
Tailings Management Features
- Thickening tailings to 70:30 ratio of solids-to-water compared to traditional 30:70 ratio of solids-to-water.
- Reducing water in tailings recycles approximately 793M gallons of water back into the milling process on an annual basis.
- An emergency spillway channels water from the TMF into the open pit in case of an extreme storm event.
- Incorporates modern technologies and mitigates known risks of traditional TMF construction.
- Dewatering promotes tailings consolidation, increases tailings density, and extends strength characteristics of TMF.
- The entire base rests above a double-liner with a leak detection system.
- A competent and free draining perimeter wall roughly 108 ft wide made out of waste rock.
WHAT IS A NATIONAL POLLUTANT DISCHARGE ELIMINATION SYSTEM (NPDES) PERMIT?
The Clean Water Act prohibits the discharge of pollutants through a point source without a National Pollutant Discharge Elimination System (NPDES) permit. A point source is any source that is ‘discernible, confined and discrete conveyance, such as a pipe, ditch, channel, tunnel, conduit, discrete fissure, or container.’ The purpose of the permit is to ensure that a facility meets a state's mandatory standards and the federal minimums for clean water.
The Back Forty Mine requires this permit to discharge treated water to the Menominee River. Permit conditions place limits on what we can release, monitoring and reporting requirements, and other provisions to ensure that the discharged water does not harm water quality or people's health. The MDEQ, EPA, and the State of Wisconsin have reviewed and determined the water we release will meet all water quality standards applicable to the river.
Other facilities that require NPDES permits include power plants, municipal treatment plants, manufacturers, and recycling facilities. The permit is valid for five years and is available for renewal to allow the discharge to continue.
OFFICIAL STATEMENT REGARDING WETLAND PERMIT APPLICATION
STEPHENSON, Mich. Jan. 23, 2017 – Aquila Resources appreciates the community’s interest and feedback on our permitting efforts, as well as the hard work undertaken by our team on the wetland application. We’re confident the application minimizes total wetland impact and protects the environment.
It’s important to note that the United States Environmental Protection Agency (“EPA”) has delegated permitting authority to the Michigan Department of Environmental Quality (“MDEQ”). Michigan has maintained this designation by administering laws and regulations that remain consistent with the Clean Water Act (CWA). The state has been conducting a thorough review of the wetland permit application for more than two years, during which time the EPA has been involved and providing input. We look forward to regulatory approval of the application during the first half of 2018. This is the fourth and final permit required to build and operate the Back Forty Mine.
