Changes of flow patterns in a horizontal subsurface flow constructed wetland treating domestic wastewater in tropical regions

This work focuses on the study of the hydrodynamic behaviour of horizontal subsurface flow constructed wetlands (HSSF-CWs) for secondary treatment of domestic wastewater in Ginebra, Valle del Cauca, southwest Colombia. This study evaluates the effect of plant development (i.e., vegetation growth) on the hydrodynamic regime of an experimental wetland unit. Dispersion studies using Rhodamine WT were carried out in a pilot-scale unit with constant flow rate. The HSSF-CW was planted with Phragmites australis and operated with a single hydraulic loading rate for a 5-month period. This allowed free growth of plants and a controlled hydraulic loading rate. The experimental residence time distribution (RTD) curves showed that the real hydrodynamic behaviour of these systems varies widely from the practical assumption of ideal plug flow (PF). The data revealed a major trend to a completely stirred tank reactor (CSTR), with arbitrary flow patterns and significant degrees of dispersion. This fact indicates the inadequacy of ideal PF to describe the hydraulics of these units. Additionally, the hydrodynamic variables showed that as the system reached its maturity, the degree of mixing increased progressively within the reactor, and this reveals a direct influence of the biological (i.e., rhizospheric and plant) growth on the hydrodynamic regime of these reactors. Meanwhile, removal efficiencies of organic matter (COD and BOD) reached acceptable levels.