Three-dimensional computational fluid dynamic modelling of a facultative lagoon

A series of facultative lagoons operated by Thames Water treating industria l wastewater in Thailand were found to be performing poorly, particularly w ith respect to the removal of biological oxygen demand (BOD). A review of t he design parameters for the site found that all the lagoons are of a suffi cient area for the flow and BOD load. However, observations of the lagoons suggested that there may be significant hydraulic short-circuiting. Computa tional fluid dynamics (CFD) modelling was therefore carried out on one of t he lagoons to establish the hydraulic regime. Two consecutive simulations w ere carried out, both with and without baffles; the first to establish stea dy flow conditions, and the second using a chemical species transport model to obtain the residence time distribution (RTD). The results of the modell ing indicate that the lagoons do currently suffer from significant short-ci rcuiting, and large dead-zones are present. The installation of baffles in the CFD model improved the plug-flow characteristics of the lagoons, substa ntially reducing the short-circuiting and the size of the dead-zones. it ha s therefore been concluded that the installation of baffles in the lagoons will lead to an improvement in their performance, by increasing the retenti on time of the system.