K_t of Facultative Ponds in Series and the Modified Marais—Shaw Equation Zhou Fan Yang Weiquan Zhang WeidongThe faculative ponds in series is designed with Marais-Shaw equation in which it isone of the basic supposes that k is a constant.But it is shown in the test that k is not a con-stant.It is affected by temperature and is a function of hydraulic retention time(HRT).Ac-cording to the result of the test,the modified Marais-Shaw equation is proposed in thispaper.This equation may be used to design as reference.Lagoon;;Facultative;;Series;;Equation.