AN APPROACH TOWARD RATIONAL DESIGN OF CONSTRUCTED WETLANDS FOR WASTE-WATER TREATMENT

A simulation-based design approach is used to find the optimal size a wetland constructed for wastewater treatment. The simulation scheme synthesizes submodels describing variable wastewater loadings, atmospheric moisture fluxes, contaminant fate and transport, and effluent release and recycle rules. The wetland is modeled as a lined densely vegetated prismatic open channel that behaves as a non-ideal plug flow reactor (PFR) with first-order temperature-dependent kinetics. The hydrodynamic equation for gradually varied unsteady free surface flow is linked with the one-dimensional advection diffusion decay equation for a nonconservative nonsorbing substance. The coupled equations are solved using an implicit finite difference method. The flexibility of the simulation approach is illustrated with four hypothetical examples which compare treatment performances among alternate wetland configurations and operating strategies. Results show that ambient temperature and the effluent release and recycle rule are much more important than precipitation and evaporation in determining the size of the wetland required to meet permit limits.