Based on the batch settling tests,the discipline of zone and compression settling activated sludge are studied.The parameters of Vesilind form used for estimation of stirred zone settling are determined by the experimental measurement and experiential formula,and the significant differences between them are found.According to the regression relation between height of the sludge blanket(SBH) and time,a mathematical model describing compression settling is(advanced) and the methods for its parameters estimation are structured.Furthermore,the validation of the model is(approved) by the experiments. From the model,the authors deduce a theoretical equation for determining the change-over point where the compression settling commences,and a new experiential velocity function of compression settling related to the settling time,initial sludge concentration and SBH.