The mining newspaper for Alaska and Canada's North
The mining newspaper for Alaska and Canada's North
Salt, metals, hydrocarbons and viruses can be removed by membrane systems, which are designed to meet today's permitting requirements
As the mine permitting process gets ever more rigorous, advances in technology are helping companies comply with environmental requirements. Clean water is usually at the top of the list when it comes to mining concerns, and membrane treatment systems are the wave of the future, according to G. Paul Schuitt of Anchorage-based GPS Environmental, which represents several water and wastewater equipment manufacturers.
Membrane treatment systems are fairly new to the industry, having arrived in the past 10 years, Schuitt said in a presentation to the Alaska Miners Association Sept. 13. The various systems can provide four different levels of filtration, depending on the purity of water required and the particles contained in it. Micro-filtration removes particles that are between 1.0 and 0.1 micrometers in size, ultra-filtration's range is 0.1 to 0.01 micrometers, nano-filtration's is 0.01 to 0.001 micrometers and reverse osmosis's is 0.001 to 0 micrometers.
Conventional filtration systems use bag filters or sand filters, for example, and are able to remove pollutants such as sand and algae. Material is pushed through these systems until the filter fails and has to be replaced, Schuitt said. With membrane systems a scouring system using injected air bubbles continually wipes debris off the membrane.
Membrane systems can make salt water into fresh water, and are able to remove viruses, humic acids, metals, hydrocarbons and very fine particles like silt, he said.
"Viruses are really, really small, so that's a major leap forward in technology to be able to remove viruses with a filter of this nature," Schuitt said. "Twenty years ago we couldn't very effectively remove viruses with sand filters and chemical flocculation treatment systems. They struggled to do that."
An outbreak of the bacterium Cryptosporidium in Milwaukee that killed over 100 people in 1993 was caused by a malfunctioning conventional filtration system, Schuitt added.
Temperature issue with membrane systems
Temperature is a very important issue with membrane systems, as the colder the water, the denser it is and the harder it is to push it through the membranes. Sometimes the water has to be heated, which can add to costs.
Membranes can typically handle water in the temperature range of 32-105 degrees Fahrenheit.
Greens Creek mine near Juneau uses Pall hollow fiber micro-filters, but Red Dog mine near Kotzebue doesn't use a micro-filtration system because of innovations it has developed, Schuitt said.
"They drilled down 20, 30 feet into the gravel on the beach and they're drawing brackish water into their RO (reverse osmosis) system," Schuitt said. "Well, they have a nice, continuous filter bed, so they don't need micro-filtration. They're using the beach gravel to do pre-filtration of their feed water."
Membrane systems generally occupy about 50 percent less space than conventional systems, which is good news when the cost of construction in some of Alaska's more remote areas can run $500 per square foot, he said.
Although membrane systems are usually about 30 percent more expensive than conventional systems, conventional systems are designed for the regulatory environment of the 1960s and '70s, Schuitt said.
Membrane systems meet and exceed today's permitting requirements, he said.
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