Assessment of reactor hydrodynamics and treatment effectiveness of secondary treated refinery wastewater in an airlift photobioreactor†

Abstract

Oil refineries are among the topmost polluting industries due to the large amount of wastewater generated during crude oil refining. The wastewater contains toxic substances that can bioaccumulate in aquatic organisms. Microalgae are versatile and have the capability to metabolize the organic matter and nutrients present in refinery wastewater. A diesel-acclimatized algal culture, comprising of green algae and cyanobacteria, was used for start-up of an airlift batch photobioreactor using a 16 : 8 h light : dark cycle. The optimal gas flow rate was determined through hydrodynamic assessments. When treating secondary treated refinery wastewater, the microalgae achieved significant growth, reaching a maximum chlorophyll a concentration of 5.35 mg L⁻¹ after 18 days. The initial chemical oxygen demand (COD) of 412.8 mg L⁻¹ and ammoniacal nitrogen of 70.9 mg L⁻¹ were reduced with removal efficiencies exceeding 79% and 78.5%, respectively, while nitrate and nitrite levels remained below 1 mg L⁻¹. Two-dimensional gas chromatography-time of flight mass spectrometry (GC×GC-TOF MS) analysis revealed that at the end of the cycle, removal efficiencies of various hazardous organic compounds followed the order biphenyls > naphthalenes > phenols > n-alkanes > iso-alkanes > S-heterocyclics > N-heterocyclics > benzenes. This is the first study reporting the use of an airlift photobioreactor for treatment of refinery wastewater.