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Arctic Science 2000 - Crossing Borders: Science and Community
Whitehorse, Yukon, Canada, Sept 21-24 2000
American Association for the Advancement of Science & Yukon Science Institute

Ammonia Removal in Northern Sewage Lagoons

Sewage lagoons designed for operation at latitudes north of the 60th parallel must address the challenges posed by a short ice-free season and an environmentally-sensitive receiving environment. While the lagoon is under ice, sewage treatment is essentially halted. Still, lagoons are typically the treatment of choice in the north due to inexpensive land, simplicity of operation and low operating costs.

Ammonia is one of the primary causes of acute toxicity in poorly treated sewage effluent. In mechanical treatment plants, ammonia removal can be achieved by ensuring high densities of nitrifying bacteria and good contact of this bacteria with the wastewater. In this nitrification process the nitrifying bacteria use the ammonia as an energy source and convert it to nitrate. The relatively low levels of nitrifying bacteria typically found in sewage lagoons can lead to low levels of nitrification.

However, studies carried out by Environment Canada at the new City of Whitehorse lagoons have shown that ammonia can be very effectively removed by algae and a high quality effluent produced for a once/year discharge in late summer. Ammonia was removed to low levels within the first 7 weeks of open water. This was due to a combination of direct consumption by algae or through gaseous volatilization due to elevated pH values (the highest pH measured was 11.30). The dominant mechanism was dependent upon the time of season and pH level.

The Whitehorse system was commissioned in 1996 and is comprised of three different types of treatment cell: 6 metre deep primary cells with 2 weeks retention to settle the solids; 2.5 metre deep facultative cells with 12 weeks retention, followed by; a long term storage cell which provides another 10 months of storage. The system is designed for a once/year discharge over a 90 day period commencing in August.

It was found that ammonia removal could be effectively modelled using an approach based on a model developed by E. J. Middlebrooks. The average removal rate achieved was 6.2 kg ammonia/hectare-day.

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