Simulation of carbon, nitrogen and sulphur conversion in batch-operated experimental wetland mesocosms

A simulation model based on Constructed Wetland Model No. 1 (CWM1) using the AQUASIM mixed reactor compartment as a platform was built to study the dynamics of key processes governing COD and nutrient removal in wetland systems. Data from 16 subsurface-flow wetland mesocosms operated under controlled greenhouse conditions with three different plant species (Typha latifolia, Carex rostrata, Schoenoplectus acutus) and an unplanted control were used for calibration and validation in this mechanistic model. Mathematical equations for plant related processes (growth, physical degradation, decay, and oxygen leaching), physical re-aeration, as well as adsorption and desorption processes for COD and ammonium were included and implemented alongside CWM1 in the AQUASIM software, while some CWM1 parameters were adjusted to better fit the model predictions to experimental data during calibration. The simulation results showed that the model was able to describe the general trend of COD (R2 = 0.97-0.99), ammonium (R2 = 0.85-0.97) and sulphate (R2 = 0.71-0.93) removal in the wetland mesocosms and also in their controls (unplanted) through the experimental temperature range of 12-24 °C. Oxygen transfer by physical re-aeration was found to be 0.05 and 0.09 g m−2 d−1 at 12 °C and 24 °C, respectively. The amount of root oxygen transfer was the highest for the planted mesocosms at 12 °C at rates of 1.91, 0.94, and 0.45 g m−2 d−1 in the Carex, Schoenoplectus and Typha mesocosms, respectively, indicating that COD of the bulk wastewater was removed mainly by anaerobic processes under the specific experimental situations. Measured COD removal was better in the planted mesocosms than in the control; differences were effectively modelled by varying the bacteria concentration. The sorption process was found to be important in simulating COD and ammonia removal under these experimental conditions.