Treatment of petroleum refinery wastewater with constructed wetlands

The use of constructed wetlands (CWs) for polishing of petroleum refinery wastewater in Nigeria was evaluated. Secondary treated petroleum refinery wastewater from a refinery (Kaduna, Nigeria) was characterized with different types of organic and inorganic pollutants (Chapter 3). Vertical subsurface flow (VSSF) CWs planted with locally available macrophytes (Cyperus alternifolius and Cynodon dactylon) were designed and built for polishing of secondary treated refinery wastewater in terms of organic matter, nutrients and suspended solids removal (Chapter 4). The tertiary treated refinery wastewater did, however, not meet effluent discharged compliance limits in terms of total suspended solids (TSS), biochemical oxygen demand (BOD5), chemical oxygen demand (COD) and ammonium-N (NH4+-N) removal.

Typha latifolia planted-VSSF CWs could, however, treat TSS, BOD5, COD and NH4+-N in the petroleum refinery wastewater to below World Health Organization and Federal Environmental Protection Agency (Nigeria) effluent discharge limits of 30 mg/L for TSS, 10 mg/L for BOD5, 40 mg/L for COD and 0.2 mg/L for NH4+-N (Chapter 5). T. latifolia-planted VSSF CW achieved higher removal efficiencies for all parameters measured in comparison to C. alternifolius and C. dactylon planted-VSSF CWs. In addition, the T. latifolia-planted VSSF CW had the best heavy metal removal performance, followed by the C. alternifolius-planted VSSF CW and then the C. dactylon-planted VSSF CW (Chapter 6). The accumulation of the heavy metals in the plants accounted for only a rather small fraction (0.09 - 16 %) of the overall heavy metal removal by the wetlands. Coupling a horizontal subsurface flow (HSSF) CW to the VSSF CW (hybrid CW) further improved effluent quality with an overall BOD5 and PO43--P removal efficiency of, respectively, 94% and 78% (Chapter 5).

Diesel contaminated wastewater was treated in the hybrid CWs spiked with three different nutrient concentrations. Numerical experiments were performed to investigate the biodegradation of the diesel compounds in the synthetic contaminated wastewater by the duplex-CWs using constructed wetland 2D. The VF CWs had a higher removal efficiency than the HFF CWs and the hybrid CW showed higher removal efficiencies in the days with nutrient application than the days without nutrient application (Chapter 8).

This study showed that VSSF CWs planted with T. latifolia, C. alternifolius and C. dactylon can be used for the removal of suspended solids, organic contaminants and heavy metals from secondary refinery wastewater under tropical climate conditions. Especially T. latifolia-planted hybrid CWs are viable alternatives for the treatment of secondary refinery wastewater to below standards of the World Health Organization and Federal Environmental Protection Agency (Nigeria) under the prevailing climatic conditions in Nigeria.