February 2013

Pollutant-hungry wetlands

Patience key to future of passive water treatment

By Herb Mathisen

A passive treatment system at the Empire Mine State Historic Park in California has been in operation since November 2011 and has already been able to reduce arsenic, iron and manganese concentration levels below their respective 10, 300 and 50 micrograms per litre effluent limits | Courtesy of California State Parks

When Jim Gusek first began studying passive water treatment systems in 1988, research materials were hard to come by. The only resource he could find was a book about wetlands in Siberia, translated from Russian to English. “It was a thin book, but it still cost us $104,” he recalls, laughing.

Now, 25 years later, the senior consultant with Golder Associates has turned the tables. Gusek has become one of the most prolific authors on how to use bacterial systems to treat mine-contaminated water and acid mine drainage without continuous human involvement. He has also brought roughly 10 such systems into full-scale operation. Gusek says the passive technology mimics a process Mother Nature has used for billions of years – water that passes through a natural environment is slowly altered by resident bacteria. “She has figured it out,” he notes, “but we just need to figure her out.”

The specific designs of passive treatment systems take time to develop. But once ironed out, they are particularly attractive for remediation of either remote or abandoned mines in perpetuity. Unlike lime-treatment plants, they require no power and need only limited personnel for monitoring, while eliminating the cost of burying or storing high-pH sludge that results from plant treatment. “Most mining companies have these legacy sites, and this is the best practical technology to handle them,” says Gusek.

No added chemicals, no problem

The way a passive system treats contaminated water depends on the metals to be removed and on the available space on site. The multi-step process typically in­volves nurturing a quasi-­symbiotic bacterial environment in one or multiple biochemical reactor ponds. For example, to remove byproducts from water influenced by hard rock mining operations, autotrophic bacteria first metabolize carbon dioxide to produce complex fixed carbons (acids or sugars). These fixed carbons subsequently serve as carbon sources for heterotrophic bacteria that reduce sulphate to produce hydrogen sulphide. Finally, the hydrogen sulphide combines with metals to produce insoluble metal ­sulphides. These sulphides are sequestered in the neutral-pH, anaerobic ponds. Metals are also adsorbed to organic materials and the bioreactors themselves.

Gusek recently partnered with California State Parks on a semi-passive system (it uses a pump) to treat 4,740 litres per minute of water contaminated with arsenic, iron and manganese at the abandoned Empire gold mine in the foothills of the Sierra Nevada Mountains. Parks manager Dan Millsap says a treatment plant was initially considered, but its construction costs were roughly twice that of a passive system. Add to that the plant’s estimated US$600,000 annual operating budget, which did not include chemical, hauling and waste disposal costs, and the passive system – requiring just US$20,000 annually to power the pump – started to look pretty good. The system has been running since November 2011 and while it was not expected to reach its peak efficiency until wetland plants were established, it has already met the park’s final discharge requirements. Not only is the system cheaper, Millsap says, but “it looks better too” as it blends in with the natural surroundings.

“Wetlands are nicer to look at than a big steel structure,” affirms Al Mattes, research director with Nature Works Remediation Corporation. In the 1990s, Teck Cominco contracted Mattes to use his passive treatment expertise to deal with a mildly acidic leachate – containing zinc, arsenic and cadmium – that seeped from a historic lead-zinc smelter landfill near Trail, B.C. Rainwater percolated through the landfill, picking up dissolved metals before hitting bedrock and flowing towards a creek. This water was captured before reaching the creek and a portion was pumped to Mattes’ system for treatment.

Leachate would initially flow through two oxygen-­deficient bioreactor ponds lined with limestone and quartz sand and containing waste pulp and paper biosolids used to energize the sulphate-reducing bacteria. From there, it would continue through three plant-cell wetlands and finally, 15 days later, on to a holding pond, where the treated water was used to irrigate a tree farm. The wetland area had been a desert of blasted rock, Mattes says, adding, “It’s now full of trees and grass, deer and elk, bears and rabbits.”

From 2003 to 2007, the system, which cost $700,000 to build, treated 2,990 kilograms of arsenic, 7,698 kilograms of zinc and 86 kilograms of cadmium, with reductions in concentration of 99.4, 98.5 and 99.6 per cent, respectively.

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