Design of two-storey final settling tanks using mathematical models A. I. Stamou, M. Latsa and D. Assimacopoulos A. I. Stamou (corresponding author) Department of Civil Engineering, National Technical University of Athens, Iroon Polytechniou 5, 15780 Athens, Greece E-mail: stamou@central.ntua.gr M. Latsa D. Assimacopoulos Department of Chemical Engineering, National Technical University of Athens, Iroon Polytechniou 5, 15780 Athens, Greece ABSTRACT A mathematical model is applied to the design of two-storey final settling tanks. Computations show that the flow and suspended solids (SS) concentration fields for the upper and bottom tanks are similar.The flow field has the ‘two-layer’ structure, observed in real and laboratory settling tanks, consisting of a bottom current and a free surface return current with approximately equal heights. The SS concentration field is governed by the flow field (and vice versa). The SS concentration profiles are uniform in the major part of the tanks. The hydraulic and SS removal efficiencies improve with decreasing flow rate. In both tanks the outlet SS concentrations are lower than the maximum permissible value (20 mg l -1 ), with the upper tank showing a better performance than the bottom tank. Key words | final settling tanks, two-storey tanks, mathematical models, settling INTRODUCTION In urban areas with high land prices and limited available area, it is highly desirable to minimize the space for the construction of new or the upgrading of existing sewage treatment plants (STP). Significant space saving is offered by the use of multiple-storey deep units, with common walls. Moreover, the construction cost of multiple-storey units is found to be lower than that of the single-storey tank of comparable performance (Kleffner 1972). Japan has considerable experience with multi-storey STP facilities. Osaka City is a typical example, where initially, two-storey primary settling tanks (PST) have been constructed, followed by two-storey secondary set- tling tanks (SST); then aeration tanks vertically combined with SST and finally three-storey SST (Yuki et al. 1991). Early use of two-storey SST in the USA includes Mamar- oneck (New York), the first to be operational in the USA, and Deer Island (Massachusets), which have been operat- ing since 1993 and 1995, respectively (Getter etal. 1998). Multi-storey SST have also been constructed in Singapore and Europe (Kleffner 1972). The design of multi-storey SST can be optimized with mathematical models (MMs). MMs are used to calculate (i)theflowfield,(ii)theSSconcentrationfieldand(iii)the SS removal efficiency of a specific SST. Calculations can be performed for various combinations of (i) geometrical characteristics (including the inlet and outlet structures), (ii) flow conditions and (iii) SS characteristics to deter- mine the optimum (mainly geometrical characteristics) of the SST in the design phase, i.e. prior to its construction. MMs can also be used to investigate the effect of various modifications in existing SSTs to improve their efficiency, prior to the performance of the actual modi- fications. Generally, the MMs are considered to be too sophisticated to be directly applied in the design by sanitary engineers. The design of SSTs is still performed empirically using typical values for theoretical design parameters, such as the overflow rate, the solids loading and the detention time. In most of the cases, these values are chosen with a large safety factor to ensure that the resulting expensive constructions achieve the required SS removal efficiency, without any optimization. In 1997 a procedure was presented (Stamou 1997) for the optimization of the design of single-storey SSTs using 235 © IWA Publishing 2000 Journal of Hydroinformatics | 02.4 | 2000 Downloaded from http://iwaponline.com/jh/article-pdf/2/4/235/392115/235.pdf by guest on 29 December 2021