METHODS FOR DETERMINING THE SETTLING VELOCITY PROFILES OF SOLIDS IN STORM SEWAGE

Recently, research has shown that a settling process is an efficient t reatment for the removal of suspended solids from storm sewage. In ord er to design settling tanks, there is a need to determine the settling velocity characteristics of these solids. Devices and test methods fo r measuring Settling velocities of solids in storm sewage have been de veloped by researchers. A literature review has revealed that the sett ling velocity profiles obtained with some methods (Chebbo, 1992), (Mic helbach and Wohrle, 1993) and (Tyack et al., 1993) are very different. In order to explain why the results are different and to quantify the influence of the experimental procedure on the settling velocity grad ing curves, we have compared the selected methods when tested with the same sample and we have studied the effects of the conservation of th e sample before the test, of the concentration of solids in the device and of the nature of the water on the settling velocity profiles. The results of the comparative tests indicate that the settling velocitie s are significantly lower with both the Cergrene and Aston methods tha n with the UFT test procedure. Moreover, the study has shown that the settling characteristics of solids change with time, that the use of s ewage liquor rather than demineralised water or drinking water can cha nge the results of the measurement and that the higher the concentrati on of solids, the greater the velocity of settled particles. Copyright (C) 1996 IAWQ. Published by Elsevier Science Ltd.