the elimination of biological phosphorus is the residence time t bp of the waste water in the anaerobic phase which is normally about 11 2 hours the actual required residence time t bp depends of course on the temperature on the quantity of acidifying bacteria on the quantity of acidifiable substances and their composition on the o 2 potential and also on the possible o 2 supply and other factors as yet unknown but which are no doubt essentially functions of time consequently the optimal exposure time may be different from case to case leading to dilemmas with respect to dimensioning of the basin regions even at the planning stage therefore there has long been a need for a waste water purification system capable of adaptation in terms of optimal anaerobic stage residence time even after the system has been built if the anaerobic stage residence time is too long the nuisance of odors in the surrounding area may often become unacceptable in addition active sludge in the waste water is damaged by anaerobic exposure and this must be also be avoided these problems are solved quite simply in accordance with the invention by providing a variable volume for the anaerobic region in which biological phosphorus elimination is carried out i e the so called bp stage this solution to the problem will explained further with reference to fig1.