Treatment of oil and grease in produced water by a pilot-scale constructed wetland system using biogeochemical...

Constructed wetland treatment systems (CWTSs) can effectively remove many constituents that limit beneficial use of oilfield produced water. The objectives of this investigation were: (1) to assess the effect of mass loadings of oil and grease (O & G) on treatment performance in pilot-scale subsurface flow and free water surface CWTS series having sequential reducing and oxidizing cells, and (2) to evaluate effects on treatment performance of adding a pilot-scale oil-water separator. Increase in O & G mass loading from 5 to 20 mg min(-1) caused decreases in both dissolved oxygen concentration and sediment redox potential, which affected treatment performance. Biogeochemical pathways for removal of O & G, iron, and manganese operate under oxidizing conditions, and removal rate coefficients for these constituents decreased (0.905-0.514 d(-1) for O & G, 0.773-0.452 d(-1) for iron, and 0.970-0.518 d(-1) for manganese) because greater mass loading of O & G promoted reducing conditions. With increased mass loading, removal rate coefficients for nickel and zinc increased from 0.074 to 0.565 d(-1) and from 0.196 to 1.08 d(-1), respectively. Although the sequential reducing and oxidizing cells in the CWTS were very effective in treating the targeted constituents, an oil-water separator was added prior to wetland cells to enhance O & G removal at high inflow concentration (100 mg L(-1)). The oil-water separator removed approximately 50% of the O & G, and removal extents and efficiencies approximated those observed at 50 mg L(-1) inflow concentration during treatment without an oil-water separator.