Seasonal Performance of a Hybrid Pilot-Scale Constructed Wetland Treatment System for Simulated Fresh Oil Field-Produced Water

Abstract

Seasonal changes (e.g., temperature, photoperiod, microbial activity) can affect treatment performance of constituents (oil, Cd, Cu, Ni, and Zn) in a constructed wetland treatment system (CWTS). The purpose of this research was to measure seasonal differences in performance of a hybrid free water surface CWTS for renovating simulated fresh oil field-produced water (FOPW) for surface water discharge and irrigation. The hybrid CWTS was constructed outdoors and consisted of an oil/water separator and two pilot-scale wetland series with four cells each. Oil removal activity (oil-specific agar plates) as well as acid-volatile sulfide (AVS) and simultaneously extractable metal concentrations were measured in the laboratory. For this experiment, water temperatures ranged from 0 to 30.2 °C in the wetland cells. There were no measurable seasonal differences in performance during this study. From inflow to outflow, the hybrid CWTS decreased oil marker compound concentrations by ≥99 % and Cd, Cu, Ni, and Zn concentrations by ≥98 %. There were no significant (p ≥ 0.5634) differences in biodegradation activity of oil nor in production of AVS during the year-long study, which supported our findings of consistent removal of the targeted constituents. With a hydraulic retention time of 4 days, oil marker compound and Cd concentrations in the treated simulated FOPW were greater than surface water discharge criteria, but did not cause toxicity to Ceriodaphnia dubia and Pimephales promelas (e.g., decreased survival or reproduction). Data from this pilot-scale hybrid CWTS indicated that performance goals for the removal of targeted constituents from simulated FOPW could be achieved throughout a seasonal study.