Although developments in wastewater treatment over recent decades have focused on the biological reactor, the secondary settling tank (SST) also plays a major role in achieving increasingly stringent effluent quality standards. This report collates many of the significant developments that have taken place in SST theory, design, modelling and operation. Earlier SSTs were designed only by empirical hydraulic criteria such as overflow rate, which do not recognize sludge concentration and settleability. Today, there are available not only much improved design procedures but also hydrodynamic...