Integrated Analysis and Optimization of Hydraulic Performance in Baffled Horizontal Subsurface Flow Constructed Wetlands

This thesis addresses the challenge of low Hydraulic Efficiency (HE) in traditional Horizontal Subsurface Flow Constructed Wetlands (HSSF-CWs), which often results in actual residence times falling short of design expectations, thereby affecting treatment performance and maintenance. To improve HE and optimize system performance, a combination of laboratory experiments and computational modelling was conducted. First, Computational Fluid Dynamics (CFD) and tracer experiments were used to examine how different baffle configurations influence the hydraulic performance of HSSF-CWs. Results showed that increasing baffle number had a stronger impact on Residence Time Distribution (RTD) than baffle length. For example, with baffle length fixed at 0.58 m, HE improved by 58% when the number of baffles increased from 0 to 5, whereas increasing the baffle length from 0.4 to 0.58 m with 11 baffles only improved HE by 5.86%. A predictive equation was developed using dimensional analysis to estimate HE for systems with varying configurations. Building on these findings, pollutant removal experiments were performed using five baffled configurations with different HEs. A positive correlation between HE and pollutant removal was observed. ANOVA results confirmed that higher HE values (0.92 and 0.95) significantly improved the removal of total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP). Notably, TOC removal improved from 55.50% in the 3-baffle system to 74.54% in the 7-baffle system, with similar trends for TN and TP.
To integrate technical and economic considerations, a cost-performance analysis was conducted. A practical equation was proposed to guide the selection of optimal baffle configurations based on HE targets. By evaluating all combinations of baffle number and length, designers can identify cost-effective options that meet performance criteria. The generalized parameters ensure broader applicability across different wetland systems. Lastly, the effect of aeration on hydraulic behaviour in baffled HSSF-CWs was investigated using a gas-liquid two-phase CFD model based on the Euler-Euler approach in OpenFOAM. The study found that aeration positioned at the end of baffles (Position C) led to a 3.2% improvement in HE and reduced the Morrill Dispersion Index (MDI) from 1.6087 to 1.4000, indicating more uniform flow and fewer dead zones. Compared to mid-channel aeration, end-of-baffle aeration more effectively reduced short-circuiting and promoted better flow distribution. In conclusion, this thesis provides a comprehensive understanding of how baffle configuration and aeration strategies influence hydraulic and treatment performance in HSSF-CWs. The proposed predictive tools and design framework offer practical guidance for engineers seeking to enhance wetland efficiency and sustainability under real-world constraints.