Numerical simulation of emerging pollutants removal in constructed wetlands

Abstract

The progressive water demand and the shortage of water resource in recent years have attracted increasing interest in finding alternative techniques for wastewater treatment rather than traditional cleaning systems. Constructed wetlands are natural systems of wastewater treatment, which consist of an impermeable basin filled with gravel and planted with macrophytes. The physical, chemical and biological processes taking place inside the system lead to the purification of wastewater, which can later be used in various applications such as agriculture or discharged in watercourses without causing pollution. The objective of this work is the implementation of a mathematical model that can simulate the flow and concentration of several emerging pollutants (i.e. ibuprofen, carbamazepine) within subsurface flow constructed wetlands through the software COMSOL Multiphysics 4.0. The pilot plant used for this study is located in Les Franqueses de Vallès (province of Barcelona, Catalonia), which treats urban wastewater. The model was calibrating using bromide as tracer test. After, were simulated response curves of ibuprofen and carbamazepine and these results were compared with the experimental curves. Furthermore the model was validated by comparing it with the respective curves obtained experimentally in another treatment wetland. The model is able to reproduce with sufficient accuracy the experimental tracer curve.

First-order removal rates for ibuprofen and carbamazepine were found for the wetland in Les Franqueses Les Vallès. The values are k = 0.045 [1/d] for ibuprofen and k = 0,035 [1/d] for carbamazepine. The results show a good agreement between simulated and experimental data, which confirms the validity of the developed model. The results obtained by validation showed how the response curve of ibuprofen is similar to the sperimental curve, but the response curve of carbamazepine is different. The carbon source, as well as the design features of the wetland were found to have a significant effect on the removal capacity of ibuprofen and carbamazepine. Subsequent studies should be devoted to investigating separately the various factors that contribute to the removal of these pollutants from the wastewater.