Organic Matter, Turbidity, and Apparent Color Removal in Planted (Typha sp. and Eleocharis sp.) and Unplanted Constructed Wetlands

Abstract

Constructed wetlands (CWs) are wastewater treatment ecotechnologies which use plants as a source of pollutants removal in the system. In these systems, the root zone acts in pollutant removal, interacting with the support medium, water, microorganisms, and contaminants. This study investigates the hypothesis of the absence of significant differences in the removal efficiencies of organic matter, turbidity, and apparent color between planted (Typha sp. and Eleocharis sp., individually and combined) and unplanted constructed wetlands with horizontal subsurface flow on the secondary wastewater treatment. The study was conducted on a pilot-scale system at a wastewater treatment plant (WWTP) located in São Paulo, Brazil. The experiment consisted of four treatment tanks: T, the feeding tank, filled with raw sewage coming from the WWTP’s preliminary treatment; T1, containing gravel and Eleocharis sp.; T2, containing gravel and Typha sp.; T3, containing gravel, Eleocharis sp., and Typha sp.; and T4, containing only gravel (control tank). The system was monitored for three months and samples were collected from three locations in each tank: the influent, the middle of the process, and the effluent. Analyses of biochemical oxygen demand (BOD5,2), turbidity, and apparent color are carried out on these samples. According to the studied parameters, there is an effective removal by the four constructed wetlands systems. Because these systems are designed for secondary sewage treatment, i.e., they mainly aim for organic matter removal, the four tanks perform equivalently, having over 9% BOD5,2 removal efficiency. Color and turbidity removal efficiencies vary from 55% to 71% and from 43% to 77%, respectively, and tanks T1 and T4 have higher levels of removal efficiency. The study validates partially the hypothesis because there is no significant difference in organic matter removal between the tanks cultivated with different plant species (isolated or combined) and the control tank (with gravel only). An ANOVA test shows no statistical difference (p>.5) in organic matter removal among the tanks. The similar organic matter removal efficiencies for the four tanks demonstrate that the physical (filtration and sedimentation) and biological (biofilm) mechanisms play an important role in constructed wetlands wastewater treatment.