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.