TREATMENT OF SELENIUM IN ENERGY-DERIVED WATERS USING A NUTRIENT-AMENDED PILOT-SCALE CONSTRUCTED WETLAND TREATMENT SYSTEM

Waters containing elevated levels of selenium (Se) can adversely affect biota in receiving aquatic systems and require treatment prior to discharge. In this study, pilot-scale constructed wetland treatment systems (CWTSs) targeted removal of Se from a simulated energy-derived produced water (SEDW). The objectives of this research were: 1) to determine the ability of nutrient-amended pilot-scale CWTSs to consistently achieve removal of dissolved Se (50 µg/L) from a generic SEDW to levels permitting surface water discharge (≤ 5 µg/L); 2) to determine the relationship between concentrations of nutrient amendments to the CWTSs and Se removal; and 3) to determine the effect of seasonal temperature change on CWTS performance. The experiment was a regression design with 18 CWTSs divided into 6 groups of 3 systems each. The design included a control group and 5 treatment groups amended with a series of concentrations (inflows 60 to 265 mg/L) of a nutritive fermented yeast product (AquaSmart^TM, Diamond V^®, Cedar Rapids, IA), which served as a carbon and nutrient source for Se-reducing bacteria. The CWTSs were designed to maintain specific conditions favorable for dissimilatory Se reduction. Se removal in the CWTSs was influenced by both nutrient amendment concentration and seasonal temperature. The targeted removal goal of 5µg/L was achieved consistently in the 265 mg/L AquaSmart^TM treatment during August, September, and October. During these months, Se removal efficiency correlated positively with amount of AquaSmart^TM added to the CWTSs. Removal efficiencies remained consistently high (> 90%) in the 265 mg/L AquaSmart^TM group, while removal efficiencies decreased in the control group from 73.2% in September to 21.9% in October. Se removal efficiencies in the 265 mg/L AquaSmart^TM treatment decreased during the November and December sampling periods. The decrease in removal efficiency correlated with lower air temperature. Results to date indicate that temperature is a key design parameter when using CWTSs for remediation. The data collected in August to December illustrate that removal in CWTSs is proportional to the concentration of nutrient amendment added to the system.