The development and tasks of IAWQ specialist group on Activated Sludge Population Dynamics have been briefly described. The paper is aimed at three main topics of population dynamics, i.e., competition of nitrifying and organotrophic bacteria, population dynamics of enhanced biological phosphate removal, and activated sludge bulking and foaming. The population dynamics problems are illustrated on examples of Czech activated sludge plants. The examples were selected from a large national survey of activated sludge plants accomplished in 1995–1999. Nitrification proved to be the most difficult process to design and control in nutrient removal activated sludge systems. The survey of enhanced biological phosphate removal processes has shown that the arrangements used in the Czech Republic support well this process. The competition of PAOs with “G” bacteria in full-scale plants is less common than could be expected from previous laboratory studies. The phenomenon of anoxic phosphate uptake was also observed in full-scale plants. Traditional activated sludge separation problem of bulking was observed with much less frequency than expected. On the other hand, foaming caused by M. parvicella, N. limicola and by GALOs has become the most serious operation problem. The survey has also shown that physical rather than biological methods of foaming control are successfully applied in Czech activated sludge plants. The extent of foaming problems underlines the importance of a proper design and construction of final clarifiers.

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