Treatment of Selenium in Simulated Refinery Effluent Using a Pilot-Scale Constructed Wetland Treatment System

Abstract

Effective and reliable treatment is needed to treat selenium (Se) in petroleum refinery effluents to meet water discharge limits established under the Clean Water Act through the National Pollutant Discharge Elimination System. As a byproduct of petroleum refining processes, effluent containing Se may be produced due to geochemistry of the formations from which petroleum is derived. Pilot-scale constructed wetland treatment systems (CWTSs) were designed and built at Clemson University to evaluate removal of Se from simulated refinery effluent (SRE). For this study, objectives were to: (1) chemically and physically characterize a specific petroleum refinery effluent for simulation and assess Se as a constituent of concern, (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 the performance of the pilot-scale CWTS in terms of rate and extent of Se removal in response to organic carbon additions following a period of maturation and acclimation. The treatment goal (i.e., Se removal) for this study was to decrease the Se concentration in SRE from ∼50 μg/L to less than 5 μg/L. Samples were collected from the pilot-scale CWTS inflow and the outflow from each wetland cell. In order to determine rates and extents of Se removal and factors that may influence performance, parameters measured from the sampling locations included elemental analysis of Se, dissolved oxygen concentration, conductivity, pH, alkalinity, hardness, and temperature. Outflow Se concentrations ranged from 3.4 to 9.8 μg/L, depending on the experimental treatment, with an organic carbon addition to the simulated effluent inflow. This pilot-scale study illustrates that CWTSs can enhance Se removal from a simulated petroleum refinery effluent and that performance (i.e., removal of constituents of concern) can be achieved to meet stringent discharge limits.