Study on the long-term operation results of horizontal subsurface flow constructed wetlands for COD and nitrogen...

In recent years, process-based models have gained prominence in investigating and quantifying the internal purification mechanisms of constructed wetlands (CWs). However, most existing process-based models focus primarily on short-term simulations of pollutant removal performance. To enable long-term simulations and predictions of the purification efficiency of CWs, this study developed a comprehensive process-based model that incorporates sub-models of hydraulics, reactive-transport, bacterial kinetics, plant dynamics, and media clogging. The model effectively simulates effluent concentrations of COD and nitrogen in long-term operation (about two and a half years) of horizontal subsurface flow (HSSF) CWs. Furthermore, it simulates the accumulation of influent particulate components, bacterial growth and detachment, seasonal plant growth and nutrient uptake, root oxygen release, and wetland clogging within the wetland bed. Importantly, the model's consideration of wetland clogging encompasses not only clogging caused by inert particulate solids, but also bio-clogging due to plant root development and bacterial growth, along with the subsequent effects of clogging on water flow and bacterial distribution. Simulation results show that the influent particulate components rapidly settle and concentrate within a 2-m range at the wetland's inlet, with their distribution along the wetland bed gradually decreasing in the direction of water flow. Biofilm stabilizes approximately 20 days after startup, with its distribution along the wetland bed gradually decreasing in the direction of water flow. The peak concentration of the biofilm gradually shifts towards the wetland outlet over time. Wetland plants exhibit distinct seasonal fluctuations and can influence the treatment performance of HSSF CWs. The porosity of the wetland bed decreases gradually over time, with a more significant reduction near the inlet. The primary contributors to wetland clogging, in descending order of impact, are the accumulation of inert particulate solids, plant root development, and biofilm growth.