A System Dynamics Approach to Modelling the Degradation of Biochemical Oxygen Demand in a Constructed Wetland Receiving Stormwater Runoff

The objective of this research was to develop a tool to aid the Air Force Environmental Manager in the identification of the design parameters of a constructed wetland system that may be optimized to provide a desired biochemical oxygen demand (BOD) removal efficiency during the treatment of Air Force stormwater runoff. The objective is achieved through the development and use of a system dynamics model which simulates the hydrological functions of a constructed wetland as well as the processes within the wetland responsible for degradation of BOD. Based on literature review, the primary mechanism responsible for the degradation of BOD within a constructed wetland system is degradation due to microbial populations in the form of both suspended biomass and biofilm found on the surface of vegetation and the wetland floor. The model was run for constructed wetlands of various surface areas, each subjected to a range of stormwater influent rates and influent concentrations. The hydraulic retention times, organic loading rates and BOD removal efficiencies were determined for each case. Scatter plots of both hydraulic retention times and organic loading rates vs. removal efficiency indicated a clear relationship between both hydraulic retention time and removal efficiency as well as organic loading rate and removal efficiency. Several runs of the model also indicated that larger surface areas, greater length to width ratios and depths contributed to lower BOD concentrations in the water column