Physical pretreatment of petroleum refinery wastewater instead of chemicals addition for collaborative removal of oil and suspended solids

Abstract

The conventional petroleum refinery wastewater pretreatment process, usually consisting of an American Petroleum Institute (API) separator and air flotation tank, mainly achieves suspended solids (SS) and oil removal but also presents many shortcomings, such as chemical consumption, flotation scum production and land occupation. A physical method coupled by hydrocyclone-intensified filtration (HIF) and mixed fibrous coalescer was first proposed, followed by a lab-scale experimental investigation of the feasibility of collaborative suspended solids (SS) and oil removal. A 1 m3/h pilot-scale physical pretreatment unit was also set up to comprehensively evaluate the collaborative removal efficiency of oil and SS and the operation stability in raw petroleum refinery wastewater. The effect of the initial oil concentration, mainly emulsified oil, on the removal efficiency was thoroughly explored by the physical pretreatment process. The mean oil and SS concentrations of the petroleum refinery wastewater was decreased from 1078.9 mg/L and 214.2 mg/L to 9.7 mg/L and 31.2 mg/L, respectively, due to the physical pretreatment process. Additionally, the mean effluent oil concentration of the pilot-scale physical pretreatment unit decreased to 11.1 mg/L, and that of the flotation tank effluent was 18.8 mg/L, which also indicated that the physical pretreatment process had better performance in coping with influent fluctuations (with mean values from 14.4 mg/L to 5214.1 mg/L). SS and most of the oil in the petroleum refinery wastewater were efficiently separated by the hydrocyclone-intensified filter based on interception and adsorption by the filter media and thorough surface renewal of the filter media in the hydrocyclone, followed by further emulsified oil removal due to the X-shaped heterogeneous fibers. No flocculants were added to the physical pretreatment unit, thus achieving remarkable savings of 80.64 t/year poly-aluminum chloride (PAC) and 53.12 t/year polyacrylamide (PAM). Compared with a conventional pretreatment process with a capacity of 800 m3/h, the physical pretreatment process saves approximately 196 thousand USD/year due to chemicals consumption elimination and the oil sludge reduction arising therefrom. The physical pretreatment process instead of chemicals addition can help achieve the environmentally friendly goals of near-zero consumption of chemical flocculant, a near-zero increase in oil sludge and near-zero emission of VOCs.