Stillwater Mining uses a nitrification/denitrification process to treat its mine water, requiring several water retention cells fitted with underwater aeration grids to facilitate the process. A recent modification to the system required placement of a new aeration grid; to meet the site challenges of the installation, which include extreme temperature variations and immersion in corrosive liquids, the company selected Aquatherm polypropylene-random (PP-R) piping.
Stillwater Mining uses a nitrification/denitrification process to treat its mine water, requiring several water retention cells fitted with underwater aeration grids to facilitate the process. A recent modification to the system required placement of a new aeration grid; to meet the site challenges of the installation, which include extreme temperature variations and immersion in corrosive liquids, the company selected Aquatherm polypropylene-random (PP-R) piping.

Stillwater Mining Co. mines, smelts, refines, and markets palladium, platinum and byproduct metals contained in ore from a geological formation in southwestern Montana, USA, known as the J-M Reef. Stillwater, which employs 1,600 people, is the only U.S. producer of palladium and platinum.

The Stillwater mine has been operational since 1986. Mine operations extend laterally approximately six miles east to the west and vertically more than a mile at elevations of 2,000 to 7,500 ft above sea level. The J-M Reef is accessed by a 1,950-ft vertical shaft, ramps and declines.

Josh Harris, Stillwater's environmental compliance specialist, said he and his team are constantly seeking innovative technologies that could help keep the mine on the leading edge of environmental advancements. For example, Stillwater was one of the first mining companies in the U.S. to use the nitrification process (known at SMC as the "bug plant") to treat mine water and remove ammonia from its water stream.

Stillwater has used a moving bedbio-reactor (MBBR) for the denitrification of mine water since 1996. This process employs "cells" or basins to provide the required retention time to effectively treat mine water. Untreated mine water is pumped from a lined storage pond to the uppermost treatment cell where it passes through plastic media housing bacteria.

Mine water is gravity fed through a series of six nitrification/denitrification cells. This process enables total nitrogen removal in a compact space through high-density nitrifying and denitrifying bacteria populations. The bacteria creates a biofilm that clings to the surface area of the media, which effectively nitrifies mine water to treat ammonia (when supplied with dissolved oxygen) or denitrifies mine water when supplied with a carbon source. Dissolved oxygen for nitrification is achieved through the use of high-volume blowers and an underwater aeration grid at the bottom of the cell.

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The grid installation sequence, from start to finish.

In late 2013, Harris and members of the design team decided to split the uppermost 137,368-gal cell (cell-5) into two nitrification cells (they would be renamed: No.5A and No.5B). The existing cell was 68 ft by 30 ft by 10 ft deep. A 10-ft-tall concrete wall would be erected in the center, dividing the basin into two 68,500-gal-capacity cells.

As the team was designing the aeration grids for this project, which would measure 30 ft by 22 ft and employ 6-in. pipe mains and 1.5-in. branches, Harris recalled a conversation with MDM Supply Co.'s Bob Gaughen. MDM is a division of Dakota Supply Group and a full-line wholesale plumbing, heating and industrial PVF supply house with locations in Billings, Helena, Kalispell, Bozemanand Missoula. Gaughen had mentioned Aquatherm polypropylene-random (PP-R) piping systems to Harris as something to consider for future projects.

The grids would be located at roughly 5,200 ft above sea level, in a remote area prone to extreme temperature swings, and also would be exposed to a corrosive environment. Since the mining operation makes extensive use of high-density polyethylene (HDPE), the team was familiar and comfortable with the heat fusion process for permanently joining a pipe and fitting using pressure at high heat. This is the same connection method used to connect Aquatherm piping systems.

"We have used a lot of HDPE and it holds up pretty well. Since this installation was going to be outside and underwater in a somewhat corrosive environment, we were open to considering some alternatives," said Harris. Gaughen explained that Aquatherm pipe doesn't expand and contract as much as many other pipe systems, and because it is hydrophobic and chemically inert, it resists scaling and "getting fouled up."

Harris and his team, which also included Kestrel Engineering Group (Billings, Montana), investigated Aquatherm and learned that the German-made piping systems have been around for four decades and used in more than 70 countries. Local Aquatherm manufacturer's rep, Ridgeline Mechanical Sales, also helped out with the education and training process.

"The Aquatherm specs looked good and the drill and tap [fusion outlets] that Aquatherm offers versus having to create custom pieces with HDPE, was a big selling point for me. Just the flexibility and all of the different fittings and connections were really big points for us on this project," Harris recalled.

Aquatherm offers more than 400 valves and fittings and claims its fusion outlets offer considerable time and material savings-being particularly well-suited for main/branch applications since they allow branch lines to be installed when the pipe is already in place. The pipe is simply drilled and the fusion outlet is welded into the drilled hole. The hole can be located in the side wall of the pipe at any point along the pipe that is accessible.

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The Aquatherm piping allowed a large number of branch
lines to be easily and quickly connected after the main
lines were installed, saving time, materials and labor,
according to the project's  installation contractor.

The job required an experienced Aquatherm contractor; Harris knew that experience with heat fusing PP-R would be essential. A local firm, Harvey's Plumbing & Heating in Bozeman, has performed several large Aquatherm installations and was selected.

"The easy prefabrication aspect of Aquatherm and the numerous connections available for it were real keys. Being able to prefab all of the fusion points for all the connections and bring it on-site for a quick and easy installation is a major feature because I can't afford to have the system down for very long," Harris said.

With a short window to perform the installation, the Harvey's installation team began by prefabricating a large portion of the job in their shop. "We saved a ton of time on this job because we were able to prefab," said owner Bob Harvey. "We fabbed the mains with outlets placed on them and capped them up. It was roughly 150 hours of shop time and maybe 50 hours on the jobsite.

"Being able to create manifolds and takeoffs in straight lengths is huge on a job like this, and plus the flow rate is way better than PEX and copper," Harvey said.

According to Harris, the installation went "…super smooth. We had such a limited window to get it in. We designed it, got the plans to Harvey's, and they prefabbed it. Then it was just a matter of making those fusions. Both sides went in in one full day," he said.

According to Gaughen, using stainless steel would have been expensive and there would have been a lot of tees. Plus the stainless steel would not have withstood the sulfur dioxide, and hand drilling 1,148 holes in the metal would have been extremely difficult and time consuming. Harvey added, "Even with HDPE, you couldn't have done the fusion outlets and that would have meant more labor."

The installation was completed in January and passed a full pressure test, but the bacteria-housing media has not been introduced yet. When the media is introduced, two Kaeser ES291C blowers located in a nearby mechanical room will provide 574 cfm per basin through the aeration grid.

A total of 575, 3/16-in.-diameter holes were drilled throughout the system laterals, providing an equal distribution of 1 cfm per hole in each basin. The Kaeser blowers are connected via flanges to 6-in. Aquatherm pipe and a series of valves controlling air supply to the aeration grid 10 ft below on the basin floor. The grid is suspended on a stainless steel bracket system a foot above the cell floor.

Harris said the Aquatherm pipe has already proven its worth at the mine site and is considering using Aquatherm for other water treatment options. He also noted that an added benefit is the environmental friendliness of PP-R. Aquatherm is fully recyclable, rust- and corrosion-free and long-lasting, and the polypropylene material used to make it require less energy for initial production than other piping materials. Additionally, PP-R is nontoxic and is a byproduct of petroleum processing.

"I like that it can be prefabricated to fit within our existing influent and effluent systems while its weight versus steel offers easier maneuverability and installation into a pre-existing space," Harris said. With the cells ready to go online and more Aquatherm potentially being installed on the site, the project was clearly successful."

"I don't think they could have used any other type of pipe and created this system as designed," concluded Gaughen.


Information for this article was provided by Aquatherm. For additional information on the company's products, visit www.aquatherm.com.

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