Northern Systems Water Reuse & Recycling
digs deep for sewage treatment
By Ken Johnson, AECOM
There are strange things done in the midnight sun by the engineers who design for the cold, The arctic trails have their secret tails that will make your bid dollars explode, The arctic nights have seen queer sights, but the queerest they ever did see, Was the night on a nook of a Klondike brook there was a deep hole dug to treat Dawson “pee.” (With apologies to Robert Service)
(Cartoon by Wyatt Tremblay was originally seen in the Yukon News. Used with permission.)
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Sewage treatment in Dawson City, Yukon Territory has had an interesting history, and for the past 30 years a rather controversial one. Prior to 1979, sewage from Dawson discharged directly into the Yukon River, through a series of over a dozen independent wood stave pipe outfalls, without any treatment. Sewage treatment entered the picture in 1979 with the completion of a “screening plant” that provided something better than preliminary treatment (the removal of two-by-fours and bicycles), but not quite primary treatment. This was a logical improvement to the sewage infrastructure serving Dawson City, and was followed by the replacement of much of the wood stave piping with insulated plastic (HDPE) piping. From an environmental impact perspective, this improvement was not considered to be a significant improvement, but from an aesthetic perspective, the removal of the “floatable” component of the sewage was very significant. As well, the sewage discharge configuration was changed from the many shore discharges to a single submerged discharge near the centre of the Yukon River. The relocation and opportunity for increased dispersion of the sewage, through the current mixing regime, provided a significant public health improvement to the shore discharges. Dawson was apparently left alone until 1983, when they were first directed, as part of their water licence compliance, that they would have to clean up their act if toxicity could be established – and 26 years later this controversy still rages on. Limited arguments were made that preliminary treatment did not go far enough in Click here to return to Table of Contents
1979 to improve the effluent discharge into the Yukon. However, given the nature of the overall improvements at the time in Dawson’s water and sewer system (which included the complete replacement of all of the piped water and sewer and the elimination of the raw sewage discharges into the Yukon River) the improvements at the time were considered to be an appropriate increment. Improvements to the Dawson City wastewater treatment system have been at various stages of planning over the past two decades, and the work advanced to the detailed design of a SBR system in 2003. However, the construction of this system was never tendered because the estimated operation and maintenance costs exceeded $600,000 per year – this was unsustainable for a community with a permanent population of less than 2,000. On the regulatory front, the pressure was maintained, and it reached a climax with a police raid on the Dawson City municipal offices in 2003. Dawson was subsequently charged under the Fisheries Act for the discharge of a deleterious substance, and court order was placed on the community. A judge has been monitoring the progress of the work ever since. From an engineering perspective, the project switched gears, and aerated lagoon technology entered the picture. Aerated lagoon technology has been operating successfully in Alaska and in the northern reaches of the provinces, so the process was a logical alternative to mechanical treatment. The main obstacle was the level land on which to build the lagoon. At first glance, Dawson appears to have a relative abundance of land, in spite of the mountainous terrain. However, with placer mining claims and aboriginal land claims, the abundant land is reduced to mere morsels. Ultimately, the lagoon option failed to advance when a land-use referendum vetoed the chosen site in 2008. The referendum put an end to the government’s proposed site for an aerated surface lagoon at the junction of the Dome Road and the Klondike Highway. Instead of commissioning a consultant to complete yet another design, and then tendering it for construction, the Yukon Government charted a new course by issuing a design/build
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request for proposals, where the solution and risk associated with the solution were put to the private sector. The outcome of the design/ build process was the selection of Corix Water Systems as a proponent for an innovative solution for Dawson City. The key to the system proposed by Corix is the patented Vertreat process used by Noram Engineering. Vertreat is a technology that employs a “deep shaft” as the primary treatment vessel instead of the more common aerated basin at the ground surface. A similar system has been operating in Homer, Alaska for almost 20 years (see photo of Homer’s UV system above).
The advantages to the deep shaft version of an aeration basin were presented by Corix-Noram at a public information session in September in Dawson City. The system was described by Corix-Noram as being a good fit for an area where there are concerns about space limitations, extreme low temperatures, fluctuating sewage loads, seismic activity, and proximity to residential areas. Corix-Noram also claims that the Vertreat system uses about half the power of a conventional mechanical sewage treatment system. In the operation of the Vertreat process, the influent is directed to a vertical 85 metre aeration shaft, where air is injected under high pressure, and supersaturates the influent with oxygen. From the aeration shaft the influent flows into a flotation clarifier, where the sewage sludge is separated before UV disinfection and discharge. With the construction of the deep shaft, and the record setting capital cost of $25 million, Dawson City and the Yukon Government are “digging deep” literally and financially for the new wastewater treatment facility. In spite of the continuing controversy about wastewater treatment in Dawson City, Yukoners maintain their sense of humour. This was very clearly demonstrated by Wyatt Tremblay’s April cartoon in the Yukon News.
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