Kappala association operates Kappala WWTP, one out of three main wastewater treatment plants in Stockholm, the capital of Sweden. The plant is designed for 220 000 m3/day, or 700 000 inhabitants. The treatment comprises of mechanical, biological, chemical treatment and
a final filtration step. The biological step is an activated sludge process with UCT-configuration, with the possibility to run a combined biological phosphorus and nitrogen removal. The predenitrification process is run without any addition of external carbon source. In the year of 2005 the
COD/N-ratio in the inlet was 12, and in to the biological step approximately 7. The limiting step in the biological part is the denitrification process where nitrate is converted to nitrogen gas, N2. During 2002–2003 the discharge limit of total nitrogen of 10 mg/l
in the effluent was nearly exceeded, and measures were taken to improve the nitrogen removal. It was found that the new part of the plant, receiving 2/3 of the total flow to the plant, had higher concentrations of nitrate in the outlet than the older part and the weekly variations were
greater. The reason of this was that the supernatant from the sludge dewatering, with high concentrations of nitrogen, mainly went into the new part. This resulted in a lower COD/N-ratio in to the new part of the plant due to the higher nitrogen loading, ending up in a higher concentration
of nitrate in the outlet. On-line redox measurements in the end of the anoxic zone have been used to follow the denitrification capacity of the anoxic zones at Käppala WWTP during the last couple of years. An automatic control strategy using redox measurements to control the nitrate recirculation
has been evaluated, looking at the nitrogen removal efficiency and the energy consumption. The result of the project presented in this article led to an significant improvement of the nitrogen removal, the degree of N-removal was increased from 75 to 81%, corresponding to 8,7 mg/l
as a yearly average in the outlet from the plant, well below the limit of 10 mg/l. The total energy consumption at Käppala WWTP the year of 2005 was 27,8 GWh (0,57 kWh/m3), corresponding to 24% of the operating cost of the plant. The results of an energy survey
of the plant are presented, showing that the biological step stands for 30% of the total energy consumption.
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