Bromma WWTP is the second largest wastewater treatment plant in Stockholm, Sweden. To meet new regulations regarding nitrogen removal the plant needs to be operated with nitrogen removal all year round. In previous years, severe bulking problems during the winter have made it impossible to maintain nitrification during colder temperatures. Microscopic examination of the activated sludge has increased the understanding of bulking. The bulking is mainly due to excessive growth of filamentous organisms, mainly Microthrix parvicella. By operating the plant with a high F/M ratio and decreasing the F/M ratio when filamentous growth occurs, excessive growth of these organisms can be avoided. The nitrification rate is optimized by adjusting the concentration of mixed liquid suspended solids in the tanks so that the nitrification is complete and by operating the tanks at a DO concentration of 4 mg/l. By this process strategy it is now possible to operate the plant with nitrification all year round.

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