Reactive transport modeling of an innovative nature-based solution for domestic sewage treatment

The discharge of inadequately treated sewage is still a worldwide problem that contributes to the deterioration of receiving water bodies. Especially in urban environments of less developed countries this threatens drinking water availability and, therefore, puts human health at risk and impedes sustainable urban development. Aerated treatment wetlands are innovative nature-based solutions that have been successfully applied in treating domestic, municipal and industrial effluents. The advantage of these technologies is their simplicity which translates into low operation and maintenance requirements and robust treatment. Aerated wetlands can be easily integrated into  decentralized water infrastructure to serve the demand of changing and fast-growing urban environments.

Aerated wetlands mimic natural processes to treat wastewater. Air is injected into these systems to provide an aerobic environment for increased aerobic biodegradation of pollutants. However, quantitative knowledge on how aeration governs oxygen transfer, organic matter and nitrogen removal within aerated wetlands is still insufficient.

In this study, we developed a reactive transport model for horizontal sub-surface flow aerated wetlands using the open-source multi-physics simulator OpenGeoSys. The model was calibrated and validated by pilot-scale experiments with real domestic sewage including steady-state operation and induced aeration failures. In both cases, the model achieved an acceptable degree of simulation accuracy. Furthermore, the experiments including short—term aeration failure showed that horizontal flow aerated wetlands can fully recover from such operational disruptions.

We then analyzed several simulation scenarios and found out that increasing aeration alters and shifts water quality gradients for organic carbon and nitrogen downstream. This can, for instance, be exploited to provide specific effluent qualities for different demands in an urban environment such as irrigation or groundwater recharge. We identified that the aeration rate required to provide an efficient and robust treatment efficacy for organic carbon and nitrogen of domestic wastewater is 150–200 L m² h¹. The developed model can be used by researchers and engineers to support the design of horizontal flow aerated wetlands in the context of applications in urban environments. Furthermore, our research highlights the suitability of horizontal flow aerated wetlands as a resilient treatment technology with potential application for water pollution control in urban environments.