Long-term performance of a constructed wetland/filter basin system treating wastewater, Central Florida

Abstract

This study investigated the efficiency of a constructed wetland/filter basin (CW/FB) treatment system to improve the chemical composition of waste and surface waters. The system was constructed in closed phosphate mines used for clay settling and sand tailings. Monitoring was carried out for 18 months to evaluate the CW/FB performance under a variety of climatic conditions. Water samples were taken bi-monthly. To evaluate possible groundwater input into and water leaking out of the wetland 6 monitor wells were installed along the flow path and sampled monthly. In order to estimate the change of water chemistry along the wetland flow path, water samples along a transect were taken during the dry and rainy seasons. The samples were analyzed for pH, T, oxidation–reduction potential (ORP), conductivity, total dissolved solids (TDS), dissolved oxygen (DO), Fe(II), H2S, major anions, major cations, arsenic, fecal and total coliform.

The study showed the following changes in water quality between the input and output: (1) Substantial decrease of water temperature (up to 10 °C); (2) Significant change in pH from about 9 to 6.5–7; (3) Negative ORP confirming the reducing conditions of the treatment system; (4) Substantial increase of H2S (up to 1060 µg/L); (5) Reduction of As from 5 to <2 µg/L (mostly <0.5); (6) Substantial reduction of SO4, F, Cl, NO3, NO2, Br, Na, K, Ca, and Mg; (7) Reduction of fecal and total coliform from 30–730 and 1000–7000 to <2 and <100 count/100 mL, respectively. In general, the performance of the CW/FB treatment system showed great potential to improve the water quality of industrial and municipal wastewater. Despite significant seasonal variations with respect to temperature, rainfall and humidity, the chemical/microbiological composition of the wetland output remained relatively constant.