Water flow patterns in subsurface flow constructed wetlands designed for on-site domestic wastewater treatment

Bypass flow in subsurface flow constructed wetlands, which may be related to several wetland characteristics, reduces detention time and may result in inadequately treated wastewater. Subsurface flow constructed wetlands, 2.3 m wide by 4.8 m long and containing a gravel matrix, were used to investigate the impact of wetland depth, inlet pipe location, loading volume, and plants on water flow. Flow patterns were determined using blue dye or bromide as tracers. The blue dye adsorbed to the gravel and was not an effective tracer for following water movement. Water dispersed as it flowed through the wetland, and approximately two pore volumes of added water were required to displace approximately 99% of the bromide tracer added as a pulse. In 17 and 25 cm deep wetlands, water flow was uniform with depth, and inlet depth had little influence on water flow patterns. Water flow in a 40 cm deep wetland was not uniform with depth for either inlet pipe placement. The presence of plants caused preferential water flow around root masses, thereby, limiting their potential to interface with wastewater. Water mixing by dispersion from the surface to deeper depths may enhance aeration. Bromide was first detected in effluent after only 0.5 pore volumes of tap water had been added. This indicates that detention time for some wastewater would be less than expected, since plug flow is usually assumed in subsurface flow constructed wetlands. This occurrence should be considered for time-dependent treatments, such as fecal coliform and biological oxygen demand reduction.