The decision to upgrade the treatment plants of the City of Copenhagen to include nutrient removal was made in 1989. At the same time, it was decided that intensive pilot plant studies should be initiated in combination with wastewater characterisation. The pilot plant studies were designed to suit kinetic interpretation of plant performances including wide variations of operation to identify the process limitations. It soon became apparent that the limiting factors with respect to nitrogen removal were temperature, rate of nitrification and the availability of sufficient carbon and energy sources for denitrification. The study included a literature survey to identify a reference growth rate of nitrifiers, study of temperature conditions of future plant operation during winter periods, nitrification rates in the pilot plant under temperature control and identification of nitrification inhibition, plus analysis of carbon and energy sources for denitrification. The result was that the inhibition was significant, and several performance break-downs were experienced from 1989 to 1991. Intensive programmes have identified inhibition from industrial regions in the city and internal recycles in the treatment plant. The inhibition has been reduced considerably over a six-year-period.

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