A PILOT-SCALE STUDY TO APPLY BIOGEOCHEMICAL PROCESSES OF NATURAL WETLANDS TO TREATING IMPAIRED WATERS USING CONSTRUCTED WETLAND TREATMENT SYSTEMS

Wetlands offer natural environments harboring unique biogeochemical reactions. Constructed wetland treatment systems (CWTSs) can be designed to allow required reactions to take place for treatment (i.e. transfers and transformations) of targeted constituents in waters needing renovation. We investigated specific wetland conditions that decrease aqueous concentrations of metals, metalloids, and oil & grease in pilot-scale experiments for application to design of full-scale constructed wetland systems for treating waters for surface discharge or use. Pilot-scale CWTSs were constructed with a unique combination of sediments, plants, size, and other design parameters to produce conditions that promote preferred biogeochemical treatment pathways (e.g. biotransformation, oxidation, reduction, sorption). Explanatory parameters (e.g. pH, redox potential, dissolved oxygen concentration, temperature) were monitored during the experiments. Organic compounds in the waters can be retained and chemically altered by abiotic and biodegradation processes in the CWTSs. Some metals (e.g. Ni, Zn) are removed from the water by binding to organic detritus and microbes, followed by decomposition resulting in negative redox conditions and metal-sulfide precipitation in the presence of reduced sulfur. Other metals (e.g. Fe, Mn) can be removed by oxidation. The targeted removal pathway for selenium in our experiments was microbial reduction in anaerobic conditions using microbial assemblages capable of dissimilatory selenium reduction. Monitoring of treatment performance considered goals for beneficial use or discharge of the water as well as goals for function of the system. After treatment in the pilot-scale CWTSs, concentrations of iron, manganese, nickel, zinc, and oil & grease decreased to below guideline concentrations for irrigation and livestock watering. The goal of 5 µg Se/L in outflow water was achieved and maintained in the pilot-scale CWTS using a nutritional supplement for microbial growth. The design strategy for CWTSs, based in fundamental biogeochemistry of natural wetlands, represents a novel approach for renovating impaired waters by converting constituents of concern to less bioavailable and less toxic forms often sequestering them in sediments.