Songklanakarin J. Sci. Technol. 42 (2), 406-414, Mar. - Apr. 2020 Original Article Evaluation of enhanced bioremediation for soils contaminated with used lubricating oil in soil slurry system Naruemon Meeboon 1 and Suppasil Maneerat 2* 1 Department of Biological Innovation, Faculty of Science and Technology, Nakhon Si Thammarat Rajabhat University, Tha Ngio, Mueang, Nakhon Si Thammarat, 80280, Thailand 2 Biotechnology for Bioresource Utilization Laboratory, Department of Industrial Biotechnology, Faculty of Agro-Industry, Prince of Songkla University, Hat Yai, Songkhla, 90110 Thailand Received: 22 July 2017; Revised: 14 January 2019; Accepted: 18 January 2019 Abstract This study was conducted to evaluate the biodegradability of used lubricating oil (ULO) in soil slurry system using bioaugmentation and biostimultion approaches. Bioaugmentation with SC9 consortium isolated from ULO-contaminated soil and biostimulation with biosurfactant obtained from Acinetobacter calcoaceticus subsp. anitratus SM7 as well as nutrients were investigated. Soil concentration, initial pH, type and concentration of nutrients, and inoculum size of SC9 consortium affected by the rate of microbial degradation of ULO in soil slurry system. Maximal degradation rate of ULO (63.3%) was obtained when SC9 consortia was incubated under the optimized conditions for 7 days. The maximum biodegradation and bacteria growth were 88.6% and 10.9 log CFUml -1 in unsterilized soil slurry supplemented with SC9 consortium and crude biosurfactant after 30 days of incubation. Biosurfactant derived from A. calcoaceticus subsp. anitratus SM7 and/or SC9 consortium are significant factors in supporting indigenous microbial growth and added to soils to enhance degradation of ULO in soil slurry bioremediation. Therefore, both SC9 consortium and biosurfactant have potential for apply in soil slurry bioremediation of ULO and other hydrocarbons- contaminated soil. Keywords: biodegradation, biosurfactant, consortium, soil slurry, used lubricating oil 1. Introduction Billions of gallons of used lubricating oil (ULO) are generated every year in the world. ULO is considered to be hazardous waste materials. ULO contains heavy metals, alipha- tic hydrocarbons, aromatic hydrocarbons and polycyclic aro- matic hydrocarbons (PAHs) that would contribute to chronic hazards including carcinogenicity (Dindar, Şağban, & Başkaya, 2016). Currently, ULO generated from mechanical workshops is becoming a visible problem in Thailand (Meeboon, Leewis, Kaewsuwan, Maneerat, & Leigh, 2017; Pimda & Bunnag, 2012). Bioremediation methods have been developed to support and increase the degradative activities of native microbial populations, thus allowing a reduction in time re- quired for the expected bioremediation. In order to bioreme- diation of a contaminated soil to succeed, it is necessary to have favorable environmental conditions that foster microbial acti- vity and contaminant biodegradation, such as soil matrix, sufficient nutrients, indigenous soil microbes and no inhibitors of microbial metabolism (Chikere, Okpokwasili, & Chikere, 2011). Accordingly, this study was conducted to accelerate the biodegradation of ULO in soil slurry system by using con- sortium bacterial culture and biosurfactant. Bacterial diversity of SC9 consortium was characterized with PCR-DGGE. Fac- tors affecting ULO degradation by SC9 consortium in soil slurry including soil concentration, inoculum size, nutrient source and initial pH of soil slurry were studied. In addition, *Corresponding author Email address: suppasil.m@psu.ac.th