A BIOGEOCHEMICAL APPROACH TO RENOVATION OF ENERGY-DERIVED PRODUCED WATER USING A PILOT-SCALE CONSTRUCTED WETLAND TREATMENT SYSTEM

Constructed wetland treatment systems (CWTSs) can effectively remove many constituents of concern that limit use of untreated produced water. The pilot-scale CWTS used for this study to treat simulated oilfield produced water consisted of three series: a free water surface flow (FWS) series, a subsurface flow (SSF) series, and a hybrid SSF series preceded by an oil-water separator (OWS). The objectives of this investigation were: 1) to assess effect of mass loadings of oil and grease (O & G) on treatment performance in pilot-scale SSF and FWS CWTSs and 2) to evaluate effect on treatment performance of adding a pilot-scale OWS to a SSF CWTS.

Increased mass loading of O & G resulted in more reducing hydrosoil conditions and lower dissolved oxygen concentrations in the water column. Removal of nickel and zinc by biogeochemical pathways promoted by reducing conditions increased, while removal of O & G, iron, and manganese by pathways promoted by oxidizing conditions decreased. Removal rate coefficients for O & G ranged from 0.893 to 0.905 d^-1 at inflow concentrations of 49.8 to 52.3 mg/L, compared to 0.479 to 0.722 d^-1 at 98.7 to 99.3 mg/L O & G inflow concentrations. Rate coefficients were 0.455 d^-1 for nickel and 1.01 d^-1 for zinc at 99.3 mg/L O & G inflow concentration, compared to 0.074 d^-1 and 0.196 d^-1, respectively, at an inflow concentration of 49.8 mg/L O & G. Rate coefficients for iron and manganese were 0.523 d^-1 and 0.542 d^-1 respectively at 99.3 mg/L O & G inflow concentration and 0.544 d^-1 and 0.970 d^-1 at 49.8 mg/L inflow concentration.

The hybrid SSF series produced lower outflow concentration of O & G (<1.4 mg/L) compared to a SSF series not containing an OWS (12.3 mg/L), with both series operating at an inflow concentration of 100 mg/L O & G. This study demonstrated that treatment performance depends on initial concentrations of O & G. When increased mass loads of O & G are encountered, inclusion of an OWS in a hybrid CWTS can contribute to effective treatment.