SECONDARY SETTLING-TANK MODELING AND DESIGN .1. REVIEW OF THEORETICAL AND PRACTICAL DEVELOPMENTS

An overview of theoretical and practical developments in modelling and design of secondary tanks (SST) is presented. Historically, to overcome the lack of simple, reliable measures of sludge settleability and to incorporate sludge settleability into the design of the SST, two approaches were taken. in English-speaking countries the flux theory was adopted and the required flux theory constants (V-o and n) were determined from empirical relationships linking these constants to the simpler SSVI or DSVI sludge settleability measures; in Europe (Germany and Holland empirical design equations were developed from observed full-scale SST performance. The former approach provides only a surface area whereas the latter recognises sludge-transfer to the SST and also provides depth maximum underflow concentration SST features (e.g. inlet, outlet, sludge collection and baffling arrangement) which are known to influence SST performance via secondary effects such as hydraulic turbulence and density currents, are implicitly incorporated in local design procedures. In English-speaking countries, this has occurred through the verification/calibration process of the flux against observed full-scale SST performance, whereas, in Europe, this has occurred through the establishment of empirical design equations based on observed full-scale SST performance. In order to bring theoretical and practical developments closer together; it was identified as necessary to establish to what extent the information embodied in the European empirical design relationships flows naturally form a dynamic SST model based on the flux theory. The outcome of this evaluation is presented in three sequel papers.