Biotreatment of petroleum refinery wastewater in vertical surface-flow constructed wetland vegetated with Eichhornia crassipes: lab-scale experimental and kinetic modelling

The objectives of this study were to evaluate the potential of vertical surface flow constructed wetland (VSF-CW) vegetated with Eichhornia crassipes to treat petroleum refinery secondary wastewater under tropical conditions. Also, to provide a comparative evaluation of biotreatment kinetic models (traditional first order and other alternative kinetic models) proposed to describe the removal kinetics of organics (biochemical oxygen demand (BOD) and chemical oxygen demand (COD)), nitrate-nitrogen and total petroleum hydrocarbons (TPH) in wetland systems. The refinery secondary wastewater was characterized and treated in five VSF-CWs. Eichhornia crassipes were planted in three VSF-CWs and the remaining two VSF-CWs served as the unvegetated control. The wastewater relatively had high levels of turbidity (18.30 +/- 3.88 NTU), BOD (20.40 +/- 2.20 mg/L), COD (86 +/- 6.0 mg/L), TPH (16.6 +/- 1.76 mg/L), oil and grease (18.4 +/- 2.00 mg/L), heavy metals (Cadmium (0.034 +/- 0.01 mg/L), Lead (0.12 +/- 0.05 mg/L), Chromium (0.47 +/- 0.01 mg/L), Iron (1.54 +/- 0.25 mg/L) and Nickel (0.09 +/- 0.01 mg/L)) and Chloride (1412 +/- 9.6 mg/L). The vegetated VSF-CWs significantly performed better than the unvegetated control and resulted in the removal efficiencies of 91.5% turbidity, 94.6% BOD5, 80.2% COD, 92.6% TPH, 90.4% oil and grease, 94% cadmium, 92.5% lead, 93% chromium, 94.8% iron, 92.2% nickel, and 57.7% chloride. The results suggested that the proposed kinetic models were applicable for understanding the removal mechanisms of organics, nitrate-nitrogen and hydrocarbons in VSF-CW systems. Thus, Eichhornia crassipes planted VSF-CW has the potential of treating refinery secondary wastewater to discharge permissible limits.