Rainfall and Temporal Influences on the Hydrodynamics of a Vertical Subsurface Flow Constructed Wetland Treating Domestic Sewage

This study aimed to evaluate the hydrodynamic behavior of a vertical subsurface flow constructed wetland (VSSF-CW) treating domestic sewage by applying a saline tracer, comparing system performance in operational Years 3 (NR-3) and 5 (NR-5), and assessing the influence of a rainfall event (R-5). Electrical conductivity monitoring was used to construct residence time distribution (RTD) curves for all tests, enabling detailed characterization of hydraulic behavior. As a result, the system exhibited highly dispersed flow (d > 1.21; N < 2.07) with a tendency toward continuous stirred tank reactor (CSTR) behavior. A comparison between NR-3 and NR-5 tests revealed significant differences (p < 0.05, t test) in the hydrodynamic parameters. The rainfall event (R-5) had a statistically significant effect (p < 0.05, t test), decreasing hydraulic retention time, increasing dilution, and enhancing dispersive flow within the treatment unit. These findings highlight the importance of long-term hydrodynamic monitoring in VSSF-CW systems and demonstrate how operational conditions and external factors such as rainfall can influence treatment performance.