Modelling and simulation of subsurface horizontal flow constructed wetlands

Abstract

As a consequence of the poor implantation of sanitation systems, more than 80% of the wastewater from human activity is dumped, which poses a threat to public health and generates water stress. In contrast, its depuration and reuse for irrigation improves the efficiency of the water management cycle and reduces the use of fresh water. In the context of grey water purification in rural areas of Europe, constructed wetlands are the preferred option. Furthermore, in developing countries, they are particularly attractive as an alternative to conventional purification systems. The design of such facilities tends to avoid the complexity of the interactions between solids, pollutants, microorganisms, and plants. In this work, a more rigorous mathematical model is proposed, which considers the mass balances of the substrate and the microorganisms, in addition to the degradation kinetics typical of biotechnological processes, the life cycles of microorganisms, and their horizontal and vertical transport processes. This model was tuned based on the responses of already installed systems located in Zapote (Costa Rica) and San Salvador (El Salvador). The model foresees start-up times close to three months, in addition to depuration efficiencies reaching above 90%, which corresponds with previously reported values. The developed simulation tool is also employed to study the response of the model to various disturbances, such as punctual stops or seasonal variations in the incoming flow.