Journal of Hazardous Materials 203–204 (2012) 195–203 Contents lists available at SciVerse ScienceDirect Journal of Hazardous Materials jou rn al h om epage: www.elsevier.com/loc ate/jhazmat Oil recovery from refinery oily sludge via ultrasound and freeze/thaw Ju Zhang a , Jianbing Li a,∗ , Ronald W. Thring a , Xuan Hu b , Xinyuan Song a a Environmental Engineering Program, University of Northern British Columbia, Prince George, British Columbia, Canada V2N 4Z9 b College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China a r t i c l e i n f o Article history: Received 23 February 2011 Received in revised form 30 August 2011 Accepted 2 December 2011 Available online 13 December 2011 Keywords: Freeze/thaw Oil recovery Oily sludge Petroleum hydrocarbons (PHCs) Ultrasound a b s t r a c t The effective disposal of oily sludge generated from the petroleum industry has received increasing con- cerns, and oil recovery from such waste was considered as one feasible option. In this study, three different approaches for oil recovery were investigated, including ultrasonic treatment alone, freeze/thaw alone and combined ultrasonic and freeze/thaw treatment. The results revealed that the combined process could achieve satisfactory performance by considering the oil recovery rate and the total petroleum hydrocarbon (TPH) concentrations in the recovered oil and wastewater. The individual impacts of five different factors on the combined process were further examined, including ultrasonic power, ultrasonic treatment duration, sludge/water ratio in the slurry, as well as bio-surfactant (rhamnolipids) and salt (NaCl) concentrations. An oil recovery rate of up to 80.0% was observed with an ultrasonic power of 66 W and an ultrasonic treatment duration of 10 min when the sludge/water ratio was 1:2 without the addition of bio-surfactant and salt. The examination of individual factors revealed that the addition of low concentration of rhamnolipids (<100 mg/L) and salt (<1%) to the sludge could help improve the oil recovery from the combined treatment process. The experimental results also indicated that ultrasound and freeze/thaw could promote the efficiency of each other, and the main mechanism of oil recovery enhancement using ultrasound was through enhanced desorption of petroleum hydrocarbons (PHCs) from solid particles. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The effective disposal of oily sludge wastes generated from petroleum industry during crude oil transportation, storage and refinery process is a worldwide problem. Generally the oily sludge is a complex water-in-oil (W/O) emulsion, typically including 30–50% of oil, 30–50% of water and 10–12% of solids by mass [1,2]. Due to the existence of high concentration of petroleum hydrocarbons (PHCs), oily sludge is considered to be hazardous to environments and human health, thus requiring effective remedia- tion [3]. However, the emulsion and high PHC concentration could make the conventional sludge treatment process (i.e. landfarm- ing, landfilling, incineration) to be time-consuming, ineffective and expensive [4,5]. Given the high oil concentration in oily sludge, oil recovery before disposal would be considered as one feasible method to improve the performance of those conventional treat- ment processes [6,7]. In fact, the treatment of sludge containing over 10% of oil could result in economic benefit from oil recovery [1]. The oil recovery would significantly reduce the PHC concen- tration and the volume of sludge for further treatment, thus the efficiency of conventional process such as landfarming could be ∗ Corresponding author. Tel.: +1 250 9606397; fax: +1 250 9605845. E-mail address: li@unbc.ca (J. Li). improved through significantly reducing soil/sludge mixing ratio in landfarm and providing a PHC concentration non-toxic to micro- organisms [8]. Solvent extraction has been applied to recover oil from waste oily sludge. For example, Zubaidy et al. [7] applied methyl ethyl ketone (MEK) and LPG condensate (LPGC) for oil recovery from sludge generated from the storage of crude petroleum, and found that these two solvents could achieve an oil recovery of 39% and 32%, respectively, when using the optimal 4:1 solvent-to-sludge ratio; Avila-Chavez et al. [9] used the supercritical fluid extraction apparatus to investigate the extraction of hydrocarbons from a crude oil tank bottom sludge with supercritical ethane at varying pressure and temperature conditions, and an extraction yield of up to 58.5% was obtained; Taiwo and Otolorin [10] reported an oil recovery of about 67.5% from the accumulated sludge in oil storage facilities by using hexane and xylene extraction. Although being applied to a number of oil recovery studies, the solvent extrac- tion method is still associated with relatively lower oil recovery efficiency and requires the use of massive volume of solvents which may then restrict its application [7]. In addition to solvent extraction, a number of other studies have been reported to focus on physical approaches for oil recovery, including air flotation, thermal desorption, sonication, electrical and microwave heating [1,11–13]. Among these methods, ultrasonic irradiation has been proved as an effective treatment of removing adsorbed materials 0304-3894/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2011.12.016