Wastewater treatment and flow patterns in an onsite subsurface flow constructed wetland

Abstract

Subsurface flow constructed wetlands (SFCWs) are becoming increasingly common as a secondary treatment of onsite domestic wastewater. Even though SFCWs are being used widely, sufficient data has not been collected to determine how parameters such as wastewater depth and plants affect treatment. This study was conducted to determine how the sizing of a SFCW and the addition of plants influences the quality of effluent it releases. Data was also collected to determine how wastewater flows in a SFCW at different depths, with and without plants, and in relation to dosing volume. Quality of effluent wastewater was measured for different biological oxygen demand (BOD₅) loading rates, wastewater depths, and with and without Cyperus alternifolius (umbrella palms). Blue dye and bromide tracers were used to determine flow of wastewater through a SFCW at different depths and with plants. Results showed that BOD₅ load reductions were 10% better at a 25 cm vs. a 40 cm wastewater depth when the surface area remained the same. Reductions of influent BOD₅, TSS, and NH₄⁺ concentrations were also 13, 13, and 43% better, respectively, when umbrella palms were growing in the SFCW. It was determined that wastewater mixed with depth as it moved through the SFCW, both when the wastewater inlet pipe was near the wastewater surface, and when it was placed on the bottom of the SFCW having a 17 or 25 cm wastewater depth. Increasing the wastewater depth to 40 cm resulted in less uniform flow of wastewater with depth. More water seemed to move through the middle depths at a 40 cm wastewater depth. Bromide performed better as a tracer to follow water movement than blue dye, since blue dye movement was retarded in the SFCW. Wastewater depth had a significant effect on wastewater treatment, and plants increased wastewater treatment efficiency. This study suggests that the position of the inlet pipe (top or bottom of the gravel bed) is not critical for adequate wastewater treatment. In addition to this, if plants are included, the size of the SFCW could probably be reduced by 10 to 15%. From this research it seems that a small (one family) SFCW for on-site domestic wastewater treatment would have a wastewater depth between 20 and 25 cm.