Activated sludge settling properties can be characterised with two time-varying parameters. A simple method is presented for online identification of these parameters. The method is obtained as an exact solution of the filtration problem; it is obtained on the basis of the settling process description and the parameters variation process description. The method is suitable for practical application. It is a more exact solution of the identification problem than the alternative regression analysis method. An easily implementable system is proposed for automatic characterisation of the sludge settling properties