Factors affecting the hydraulic performance and purification efficiency of free-water-surface constructed wetlands based on EFDC model

To investigate the effects of aspect ratio, water depth, and hydraulic loading rate (HLR) on the performance of free-water-surface constructed wetlands (FWS CWs) and their quantitative relationships, this study established a hydrodynamic-water quality coupling model for free-water-surface constructed wetlands by Environmental Fluid Dynamics Code (EFDC), and the model was calibrated and validated. Based on the constructed model, a full factor simulation experiment with 3 factors and 6 levels was set up. By analyzing 216 sets of experimental results, the impacts of wetland aspect ratio, water depth, HLR, and their interactions on wetland hydraulic performance and purification effect were studied. Then, a multivariate nonlinear regression model was used to establish a quantitative relationship between the multi-factor impact and 7 indicators. In August and September 2021, wetland tests were conducted at the Hubei Provincial Irrigation Experiment Center Station in Zhanghe Irrigation District, Jingmen City, Hubei Province (30° 50'N, 112° 11'E). A total of 9 free-water-surface constructed wetlands were used for the experiment. Plant species (lotus, water bamboo, cattail) and aspect ratio (2, 4, 8) were designed as 2 factors with 3 levels each. Tracer tests were used to study the hydraulic characteristics of water flow. The hydraulic performance evaluation indicators included effective volume ratio, hydraulic efficiency, Morril dispersion index (MDI), moment index (MI) and short-circuit index φ₁₀. The purification effect test mainly reflected the purification effect of wetlands on pollutants by measuring the water quality concentration at the inlet and outlet. Removal rate and mass removal rates per cubic meter were considered as purification efficiency indicators. An EFDC model suitable for free-water-surface constructed wetlands was constructed based on the test data. Based on the EFDC model, a full factor simulation test was conducted with aspect ratio, water depth, and HLR as design factors, with 6 levels for each factor. Regression equations were constructed between the evaluation indicators and design parameters based on the results of the full factor test. The results showed that the interactions between aspect ratio, water depth, and HLR were significant on the hydraulic performance and purification efficiency. The regression model of the constructed parameters and evaluation indicators showed that hydraulic efficiency, effective volume ratio, and MDI were exponentially correlated with aspect ratio, while the removal rate of total nitrogen(TN) was exponentially correlated with water depth. The modeling determination coefficients of the regression equation were all greater than 0.9, and the relative error verified by another tests in 2022 was below 20%, indicating that the model was reliable. The results could provide valuable information for the design and operation of FWS CWs.