Ch. 5. Optimization of petroleum refinery wastewater treatment by vertical flow constructed wetlands under tropical conditions: plant species selection and polishing by a horizontal flow constructed wetlands

114 Typha latifolia-planted vertical subsurface flow constructed wetlands (VSSF CWs) can be used to treat petroleum refinery wastewater. This study evaluated if the removal efficiency of VSSF CWs can be improved by changing the plant species or coupling horizontal subsurface flow constructed wetlands (HSSF CWs) to the VSSF CW systems. The VSSF CWs had a removal efficiency of 76% for biological oxygen demand (BOD₅), 73% for chemical oxygen demand (COD), 70% for ammonium-N (NH₄ ⁺-N), 68% for nitrate-N (NO₃ ⁻-N), 49% for phosphate (PO₄ ^3--P), 68% for total suspended solids (TSS) and 89% for turbidity. The HSSF CWs planted with T. latifolia further reduced the contaminant load of the VSSF CW-treated effluent, giving an additional removal efficiency of 74, 65, 43, 65, 58, 50 and 75% for, respectively, BOD₅, COD, NH₄ ⁺-N, NO₃ ⁻-N, PO₄ ^3--P, TSS and turbidity. The combined hybrid CW showed, therefore, an improved effluent quality with overall removal efficiencies of, respectively, 94% for BOD₅, 88% for COD, 84% for NH₄ ⁺-N, 89% for NO₃ ⁻-N, 78% for PO₄ ^3--P, 85% for TSS and 97% for turbidity. T. latifolia thrived well in the VSSF and HSSF CWs, which may have contributed to the high NH₄ ⁺-N, NO₃ ⁻-N and PO₄ ^3--P removal efficiencies. T. latifolia-planted VSSF CWs showed a higher contaminant removal efficiency compared to the unplanted VSSF CW. T. latifolia is thus a suitable plant species for treatment of secondary refinery wastewater. Also a T. latifolia-planted hybrid CW is a viable alternative for the treatment of secondary refinery wastewater under the prevailing climatic conditions in Nigeria.