Vertical subsurface flow constructed wetlands for polishing secondary Kaduna refinery wastewater in Nigeria

• Kaduna refinery wastewater was characterized by a great range of organic and inorganic pollutants.
• Vertical subsurface flow constructed wetlands under tropical field conditions were used for this study.
• C. alternifolius and C. dactylon (L.) Pers. are two locally available plants found in Nigeria. They were used for the polishing of the secondary refinery effluent.
• Both plants were able to reduce the level of concentrations of pollutants in the refinery wastewater with C. alternifolius performing better.

Secondary wastewater discharged by the Kaduna refining and petrochemical company in Kaduna (Nigeria) was characterized and treated in six vertical subsurface flow constructed wetlands (VSF CW) under field conditions. The secondary refinery wastewater had high levels of BOD (106 ± 58.9 mg/L), COD (232 ± 121.2 mg/L), TSS (86.1 ± 99.7 mg/L), TDS (278.8 ± 112.7 mg/L) and a turbidity of 56.8 ± 59.2 NTU. Cyperus alternifolius and Cynodon dactylon (L.) Pers. were planted in four VSF CWs and the last two VSF CWs served as the unplanted control. Good reduction efficiencies in the concentration of the contaminants were achieved which corresponds to removal rates of 54%, 85%, 68%, 65%, 68%, 58% and 43% for TDS, turbidity, BOD, COD, ammonium-N, nitrate-N and phosphate-P respectively, for the C. alternifolius planted vertical flow constructed wetland and 50%, 82%, 70%, 63%, 49%, 54% and 42% for the C. dactylon planted wetland system. Hence, C. alternifolius and C. dactylon planted VSF CWs were shown capable of treating secondary refinery effluent to discharge permit limits. For most of the parameters considered, the performance of the C. alternifolius and C. dactylon planted vertical flow constructed wetland systems was not significantly different from each other, however, they performed significantly better than the unplanted control.