Cells of Nitrobacter sp. were enumerated in the influent to Bromma wastewater treatment plant using monoclonal antibodies in a competitive ELISA. By comparing this method with the most probable number technique we conclude that the immunological method is much more relevant for determining Nitrobacter sp. The amount of Nitrobacter sp. cells in the influent was measured once every second month during one year. Examinations concerning variations during one week and one day respectively were conducted. Finally a mass balance was constructed describing the amount of nitrifiers at different sites in the plant. The analysis showed that the amount of Nitrobacter sp. cells varies during the year but no obvious pattern could be observed. At the middle of an average week higher amounts were recorded. The major part of the nitrifiers was found in the biological unit and most of them have entered the wastewater treatment plant by the influent. Only small quantities were found in water from secondary clarifiers, filters and different sludge streams. 90% of the incoming cells of Nitrobacter sp. in the influent were captured in the plant. 5% of the bacteria were lost with the effluent and another 5% with the digested dry solids.

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