ANALYSIS OF THE METHODS FOR DETERMINING THE SETTLING CHARACTERISTICS OF SEWAGE AND STORMWATER SOLIDS

In order to optimise the design and the management of sewer networks a nd settling tanks, there is a need to determine the settling velocity characteristics of sewage and stormwater solids. Devices and test meth ods for measuring settling velocities of these solids have been develo ped (Chebbo, 1992), (Michelbach and Wohrle, 1993) and (Tyack et al., 1 993). In order to quantify the influence of the experimental procedure on the settling velocity grading curves, we have compared three selec ted methods (Cergrene, Aston, Camp) when tested with the same sample w ith the method developed by UFT (Michelbach and Wohrle, 1993). The res ults of the comparative tests indicate that the settling velocities ar e significantly higher with the UFT test procedure than the other meth ods. This is mainly explained by the fact that the UFT method takes on ly settleable solids into account. When the calculations of the four m ethods are made with the total mass of SS, the study has shown that th e settling velocity curves of the same samples are different. The diff erences vary between the nature of the samples and the test procedures which were compared. The influence of the experimental procedure for separating a sample by settling velocities fractions;is demonstrated. In conclusion, we explain the need to develop an international test pr ocedure in order to be able to compare the results. (C) 1998 IAWQ. Pub lished by Elsevier Science Ltd.