TREATING SELENIUM IN ENERGY-DERIVED PRODUCED WATERS FOR SURFACE WATER DISCHARGE USING CONSTRUCTED WETLAND TREATMENT SYSTEMS

Large volumes of energy-derived produced waters (EDPWs) are generated from a variety of sources and may contain constituents that originated from geological formations as well as from process chemicals that were added. Selenium (Se) is a common constituent requiring treatment prior to discharge or beneficial reuse of these waters. To meet stringent water discharge limits established under the National Pollutant Discharge Elimination System (NPDES) and the Clean Water Act, an effective and reliable approach is needed to treat Se in EDPWs. A pilot-scale constructed wetland treatment system (CWTS) was designed and built at Clemson University to evaluate removal of Se from simulated refinery effluent (SRE). Specific objectives were to: 1) chemically and physically characterize a specific petroleum refinery effluent for confirming Se as a constituent of concern and simulating EDPW, 2) design and conduct bench-scale experiments to measure Se removal in response to organic carbon additions, 3) design and build a pilot-scale CWTS using information from the bench-scale experiments, and 4) measure performance of the pilot-scale CWTS in terms of rate and extent of Se removal in response to organic carbon additions. Samples were collected from the inflow and outflow of each wetland cell in the pilot-scale CWTS. The following parameters were measured to determine rates and extents of Se removal and factors that may influence performance: elemental analysis of Se, dissolved oxygen concentration, conductivity, pH, alkalinity, hardness, and temperature. The Se concentration in the pretreatment SRE ranged from 42–44 μg Se/L. With organic carbon additions to the SRE inflow and after a 96-hour hydraulic retention time, outflow Se concentrations ranged from 3.4 to 9.8 μg/L depending on the experimental treatment. Se removal efficiencies (i.e. percent decrease in aqueous Se concentration) ranged from 78 to 92%. This pilot-scale study illustrates that CWTSs can enhance Se removal from a SRE and that performance (i.e. removal of Se to <5 μg/L) required to meet stringent discharge limits can be achieved.