The application of CWs in Nigeria for polishing of petroleum refinery wastewater is an unprecedented research. This PhD thesis focused on some specific objectives which were characterization of treated secondary refinery wastewater, design, construction, operation and monitoring of planted ( , and ) and unplanted vertical subsurface flow, horizontal subsurface flow and hybrid CWs for the removal of suspended solids, nutrients, heavy metals, organic matter and organic pollutants. The CWs effectively treated the petroleum contaminated wastewater to effluent compliance limits. In this study, . planted CWs had consistently higher removal efficiencies for all the measured parameters than planted CW systems. Therefore, in order to improve the wastewater quality discharged by Kaduna Refining and Petrochemical Company (KRPC) Nigeria, meet stringent guidelines and protect the recipient streams, installation of CWs at the effluent discharge point of KRPC is strongly recommended.