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 and settling tanks, there is a need to determine the settling velocity characteristics of sewage and stormwater solids. Devices and test methods for measuring settling velocities of these solids have been developed (Chebbo, 1992), (Michelbach and Wohrle, 1993) and (Tyack et al., 1993). In order to quantify the influence of the experimental procedure on the settling velocity grading curves, we have compared three selected methods (Cergrene, Aston, Camp) when tested with the same sample with the method developed by UFT (Michelbach and Wohrle, 1993). The results of the comparative tests indicate that the settling velocities are significantly higher with the UFT test procedure than the other methods. This is mainly explained by the fact that the UFT method takes only settleable solids into account. When the calculations of the four methods are made with the total mass of SS, the study has shown that the settling velocity curves of the same samples are different. The differences 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 procedure in order to be able to compare the results. (C) 1998 IAWQ. Published by Elsevier Science Ltd.