Wastewater from oil extraction industries such as palm oil and olive oil mills contain high levels of oil. Oil containing wastewater is of great concern as it creates problems such as foaming, growth of filamentous bacteria and floc formation in the treatment system. Physico-chemical methods such as flotation, adsorption and skimming are employed to remove oil. Biological processes are also commonly used because most oils are biodegradable. Biological processes such as aerobic and anaerobic treatment systems and biodegradation methods using microorganisms and enzymes have been used for reducing the oil content. Aerobic systems such as activated sludge process using both acclimatized and non-acclimatized sludge are common. Among the biodegradation methods, the use of microorganisms and enzymes, either alone or in various combinations have been reported. Bacteria such as Acinetobacter sp., Pseudomonas sp., fungus such as Aspergillus sp., Mucor sp. and photosynthetic bacteria were found to remove oil. Microorganisms that produce hydrolytic enzymes include fungi such as Penicillium restrictum and Candida rugosa. Anaerobic systems involve use of reactors such as upflow anaerobic sludge blanket reactors, upflow anaerobic sludge fixed film bioreactor and modified anaerobic bioreactors. The review shows that vegetable oils present in wastewater can be readily removed by biological treatment methods.
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