Optimization of coagulation–flocculation and flotation parameters for the treatment of a petroleum refinery effluent from a Portuguese plant

Petroleum refineries generate significant amounts of wastewater which have to be treated and processed before their discharge into water streams. The refinery wastewater treatment plants (WWTPs) employ physico-chemical processes to achieve effluents of satisfactory oil content to be further treated by biological processes. In the present study, coagulation–flocculation and flotation processes are optimised to reduce the concentrations of organic matter, oil and grease and other contaminants in order to obtain an effluent with suitable characteristics ready to be treated by an aerobic biological process. PAX-18 (17% Al2O3), aluminium sulphate (Al2(SO4)3) and ferric sulphate (Fe2(SO4)3) are chosen for affecting coagulation–flocculation as a primary treatment. NALCO 71408 is employed as flocculant. Treatment efficiency is evaluated in terms of chemical oxygen demand (COD), total organic carbon (TOC) and turbidity measurements. The experiments are conducted both in discontinuous and continuous mode to assess the feasibility of the process. Flotation is investigated as a post-treatment process for the removal of emulsified hydrocarbons and satisfactory results are obtained.

► Treatment of oil refinery wastewaters (ORW).
► A coagulation–flocculation/flotation process was applied to the treatment of ORW.
► PAX-18 and NALCO 71408 were chosen as the best coagulant–flocculant pair.
► Flotation showed good results in the removal of emulsified hydrocarbons.