PERFORMANCE OF CONSTRUCTED WETLAND TREATMENT SYSTEMS DESIGNED TO RENOVATE FRESH OILFIELD PRODUCED WATERS FOR BENEFICIAL USES

Produced waters (PWs) are generated in relatively large volumes (~3.3 million m³ annually), can range from fresh (<5,000 mg Cl/L) to hypersaline (>40,000 mg Cl/L), and contain constituents that may hinder beneficial use. Fresh oilfield produced waters (FOFPWs) can contain metals, metalloids, and oil. Some FOFPWs may be renovated for surface water discharge and irrigation using efficient and effective approaches such as constructed wetland treatment systems (CWTSs), which can be designed to promote removal pathways resulting in sequestered or non-bioavailable forms of constituents. In CWTSs, pathways and processes for removing metals (Cd, Cu, Ni, Zn) and oil are dependent on specific sediment redox conditions. Sediment redox potential can be controlled by changes in water depth and may affect performance (i.e. rate and extent of removal) of a CWTS. For this experiment, replicate pairs of pilot-scale CWTSs with four sequential cells were designed to provide oxidizing or reducing sediment conditions conducive for removing constituents in simulated FOFPW. The systems were operated with five discrete water depths (15, 23, 33, 46, 56 cm) and a hydraulic retention time (HRT) of 24 h for each wetland cell. Oxidizing conditions (+15.3 to +250.1 mV) were produced in 15, 23, 33 cm water depths along with greater removal rates for oil (0.01-0.056 day^-1) than the 46 and 56 cm water depths (~0.001 day^-1) likely due to differences in rates of aerobic and anaerobic oil degradation. Reducing environments (-65 to -212 mV) were generated in 46 and 56 cm water depths and supported greater removal rates for metals (0.01-0.185 day^-1) than 15, 23, 33, cm water depths (no change-0.077 day^-1), which may be attributed to precipitation by sulfides from dissimilatory sulfate reduction. If sediment redox potential is not within specific ranges conducive for targeted pathways to operate, removal of constituents may not occur regardless of HRT. Water depth can influence sediment redox conditions, which may enhance or suppress pathways necessary to transfer or transform constituents to non-bioavailable forms and achieve the desired performance in a CWTS.