For counting bacteria of selected metabolic groups in activated sludge several methods such as the MPN-method, the pour plate method, the surface plate method, and the membrane filter technique are available. Population densities of heterotrophic saprophytes were assessed with the MPN-method, the pour plate and the surface plate method. The surface plate method yielded higher bacterial counts compared to the pour plate method. The MPN-method is not suitable because of the ambiguity of results leading to large statistical errors. The membrane filter technique yielded significantly higher counts of ammonifying bacteria compared to the MPN-method. The population density of nitrate reducing bacteria was estimated by the MPN-method and the membrane filter technique. Higher counts were found with the MPN-method. However, both methods are not satisfactory for determining nitrate reducers. Methodological problems are discussed. For counting coliform bacteria the MPN-method has to be preferred over plate count methods for MacConkey agar-medium selects aeromonads instead of enterobacteria.

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