Solution to Secondary Sedimentation Problem by Spreadsheet and VBA

Abstract

The sedimentation process is one of the most commonly used processes in environmental engineering. Modelling and simulation of secondary sedimentation units usually requires writing mass balance equations for each layer of the settler. Then, the set of ordinary differential equations (ODEs) is solved to obtain solids concentration in each layer of the sedimentation tank. This paper presents implementation of an MS Excel add-in for solving the secondary settling problem. The add-in contains a number of Visual Basic for Applications (VBA) functions, which can be used directly on the worksheet. It is capable of calculating steady-state solution by the multivariate Newton-Raphson method. Also, the add-in is equipped with a VBA function that integrates the set of ODEs until a certain time specified by the user, providing an option for dynamic simulation of the sedimentation tank. The author uses the add-in for teaching settling models in an environmental modelling course. Both students and teachers can benefit from the tool.