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

## TL;DR

This study analyzed how rainfall and time affect the flow of wastewater through a constructed wetland system, showing that rainfall can significantly change how the system processes sewage.

## Contribution

The study introduces a detailed hydrodynamic analysis of a vertical subsurface flow constructed wetland over time and under rainfall influence.

## Key findings

- The wetland showed high dispersion and behavior similar to a continuous stirred tank reactor.
- Rainfall decreased hydraulic retention time and increased dispersive flow.
- Root growth and biofilm reduced dispersivity over time while maintaining volumetric efficiency.

## Abstract

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.

VSSF‐CW operated with high dispersion, close to the CSTR model, but with significant deviations from ideality.The system exhibited preferential flows and hydraulic short circuits.The feeding regime induced daily oscillations, alternating advective transport (daytime) and diffusive transport (nighttime).Between Year 3 and Year 5, root growth and biofilm reduced dispersivity, whereas volumetric efficiency remained stable.Event R‐5 reduced HRT, altered dispersion parameters, and promoted dilution and flushing, increasing advective flow.

VSSF‐CW operated with high dispersion, close to the CSTR model, but with significant deviations from ideality.

The system exhibited preferential flows and hydraulic short circuits.

The feeding regime induced daily oscillations, alternating advective transport (daytime) and diffusive transport (nighttime).

Between Year 3 and Year 5, root growth and biofilm reduced dispersivity, whereas volumetric efficiency remained stable.

Event R‐5 reduced HRT, altered dispersion parameters, and promoted dilution and flushing, increasing advective flow.

The constructed wetland hydrodynamic study indicated the formation of preferential flow paths between Years 3 and 5. Rainfall reduced the retention time and increased dispersive flow in the sewage treatment unit.