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Engineered wetlands solution for tailings problem

An environmental engineering firm is planning to design and construct an engineered wetland as a pilot project for Newmont Mining Corporation’s closed Golden Giant Mine near Marathon, Ontario.

As part of the mine’s closure plan, Newmont was looking for solutions to clean up seepage and excess water from its tailings pond. Ammonia and molybdenum were particular concerns.

Jacques Whitford Stantec, a leader in engineered wetlands, has developed technology using anaerobic and aerobic bioreactors within the context of a constructed wetland. These semi-passive systems act as biological machines creating environments in which anaerobic or aerobic bacteria thrive while removing leaching pollutants out of wastewater that has been contaminated from past industrial activity.

The engineered wetland cells are designed to act as the secondary stage in a wastewater treatment process, similar to the activated sludge process that occurs in a mechanical wastewater treatment facility. However, engineered wetlands are easier and cheaper to operate, according to Dr. James Higgins, director and senior consultant with Jacques Whitford Stantec’s ecological engineering division.

Higgins said the company came up with the technology as a result of a Bioreactor Engineered Wetland  (BREW) research project funded by the National Research Council’s Industrial Research Assistance Program.

Several outdoor and indoor units were constructed at the University of Guelph’s Alfred College campus near Ottawa. The objective was to upgrade the performance of constructed wetlands, remove 100 per cent of the pollutants, operate during the winter with ambient air as low as minus 40 degrees Celsius, increase water flow capacity, and reduce the physical size of the constructed wetland.

Higgins explained that there are three types of constructed wetlands:
• shallow aerobic ponds with emergent vegetation such as cattails and reeds around the outside
• free-water surface wetlands, which are artificial marshes; and
• subsurface flow wetlands (SSFW), a specialty of Jacques Whitford Stantec.

The SSFWs are designed to have water flow under constructed beds of gravel and other material in which wetland plants grow.

“You can walk around on the surface (of SSFWs) and keep your feet dry. It’s great for treating things that you don’t want exposed to the public or environment,” Higgins said.

During the BREW research program, it became obvious that all sorts of aerobic and anaerobic reactions occur together in one constructed wetland cell, Higgins said.

“We recognized how much pollutant a SSFW could treat depended upon how much air could come out of the plants, because wetland plants transfer air into the water through their root systems.”

Aeration

In order to speed up the aerobic-bacterial process, an aeration device was added. The company has since developed and patented aeration technology that allows the cells to operate during severe winter temperatures and to handle increased wastewater flow rates of up to tens of thousands of cubic metres per day.

“With an engineered wetland, you can make one cell all aerobic and get very high performance, then make the next cell anaerobic, and get high performance of those things that require aerobic conditions to be removed. Then we realized we can add all sorts of components to a wetland, like heat, chemicals or dilution…that is where the concept of the engineered wetland evolved.”

Described as advanced subsurface flow wetlands, engineered wetlands are actively monitored, and managed, and may constitute a multi-cell system in which physical, chemical, aerobic and anaerobic processes are carried out in a series of steps.

Since the BREW project, the company has developed the anaerobic bioreactor (ABR) cell, an ecotechnology for removing contaminants such as dissolved metals, sulphates and other contaminants resulting from mine drainage. The ABRs use a carbon source like manure, wood chips or pulp and paper biosolids, a byproduct of the wastewater treatment plant from a pulp and paper mill. Mixed with gravel, it creates a fertile environment for the bacteria that break down pollutants like sulphates into hydrogen sulphide They, in turn, react with any dissolved metals precipitating out into the substrate, thereby removing the metals from the water. The operational life of the ABR depends on the amount of metal content in the wastewater. Where there are high concentrations, the metals can be recovered for profit, and the beds replaced.

“After a while, the bed becomes saturated with them (metals),” Higgins said. “You dig them out and you have the richest ore you can ever imagine.”

To date, Jacques Whitford Stantec has constructed and commissioned about 75 engineered wetlands from small home units to sites treating huge amounts of industrial wastewater. One site at Buffalo International Airport in Upstate New York is a four-cell unit designed to treat 1.2 million gallons of storm water contaminated by glycol, which was used for de-icing aircraft in the middle of winter. It takes about one year to build and establish plant growth, depending on the size of the engineered wetland.

Golden Giant

The proposed engineered wetland for Newmont’s Golden Giant Mine is a 500 cubic metre per day unit consisting of anaerobic and aerobic cells.

“First we took water from their pond, took it down to our test units at Alfred and demonstrated that we could remove molybdenum,” Higgins said. “We tested having an anaerobic cell and an aerobic cell. The anaerobic cell took out the molybdenum and any other metals in the water down to below provincial water quality objectives. Downstream of the anaerobic cell we had the aerobic cell remove biological oxygen demand, ammonia or anything else that was oxidizable.”

Higgins said Newmont is interested in seeing this technology in operation because of its potential for widespread use at the company’s other mines throughout North America.

One advantage that may be appealing to mine sites is that engineered wetlands are relatively low maintenance. It only requires periodic checking to make sure the aerator is operating and the water flow continues, according to Higgins. Since engineered wetlands are designed to treat wastewater at higher concentrations initially, after 10, 15 or 20 years, the concentrations in the effluent decline to the point at which the wetland can evolve into a more passive, constructed wetland system. Over more time, it could be left to transform into a natural wetland with a “full walk-away.”

Higgins sees engineered wetlands as a cost-efficient way to deal with wastewater, particularly for mine closure plans which are becoming more stringent and financially demanding.

www.jacqueswhitford.com

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