ABSTRACT
Petroleum refining industries extract large volumes of freshwater for the process of refining crude oil. Thus, large volumes of wastewater are generated. These wastewaters contain contaminants that are neurotoxic, carcinogenic and mutagenic. Petroleum related industries have to comply with strict regulations by regulatory authorities, these have led them to explore many treatment technologies for effluent management. However, conventional wastewater treatment technologies have higher energy and capital requirement than constructed wetland treatment systems. In addition, their processes can lead to incomplete decomposition of contaminants which can generate by-products that are often toxic to both humans and the environment. Thus, these by-products require further treatment. This paper reviews literature specifically on the use of constructed wetlands (CW) systems for treatment of petroleum contaminated wastewater from conventional oil and gas industry. CWs can effectively treat multiple contaminants in secondary and tertiary wastewater with less production of sludge. They also have the advantages of providing an ecologically friendly approach with low energy demand and operational cost. Therefore, they are a viable alternative to conventional wastewater treatment systems. Plants, microorganisms, substrate media and different flow types play important roles in the removal of pollutants from the refinery wastewater in CWs, however, microorganisms play the most crucial role in the transformation and mineralization of nutrients and organic pollutants. Many studies reported significantly higher removal efficiencies of petroleum pollutants in planted compared to unplanted gravel beds, and that subsurface flow CWs can achieve more biological treatment than surface flow CWs due to the higher specific surface area of the gravel bed. CWs can be intensified with forced aeration to enhance aerobic biodegradation rates of petroleum hydrocarbons in contaminated wastewaters to non-detectable levels.
