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In 2008, the City of Superior was facing year-round, seasonally-varying effluent ammonia limits imposed by their most recent NPDES permit renewal. To comply with these impending limits, Donohue & Associates, Inc. and LHB Corporation (Donohue/LHB) evaluated alternatives and developed a facility plan for the 20-year planning horizon. This project won the Best of State award in ACEC's annual Engineering Excellence Competition.
The conventional approach would have been to build a nitrification facility, which would have produced excellent effluent quality year round, but at a cost of $30 million. However, using an interactive evaluation process and process modeling, the Donohue/LHB team developed an innovative alternative to nitrify the ammonia-rich side stream coming from the anaerobic digesters. This solution, which has only been used at a handful of facilities in the US, works by exploiting the warm temperature of the side stream to achieve year-round nitrification in only the flow with the highest content of ammonia. This solution would have allowed the City to reliably nitrify enough flow year-round to meet permit limits while reducing capital and operating costs, a savings which translated to more than $26 million compared to the baseline alternative.
But we did not stop there. When modeling the expected performance of this low cost alternative, it became apparent that the City would likely benefit from monthly rather than seasonal effluent ammonia limits. The City worked with WDNR to develop new limits that are representative of the City’s specific receiving waters. By working together with WDNR, the City received a significant relaxation in the limits while still protecting the valuable waters of Lake Superior. As a result, a new, lower cost alternative could be implemented that saved the City approximately $28 million in capital cost and $300,000 in annual operating costs.
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| On hand to receive the award at the ACEC banquet in Kohler were (from left) Ed Nevers, Donohue; Jon Shamla, City of Superior; and Bill Marten, Donohue. |
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