Journal of Hazardous Materials B131 (2006) 195–199 Treatment of oil-in-water emulsions: Performance of a sawdust bed filter ´ Angel Cambiella, Enrique Ortea, Guillermo R´ ıos, Jos´ e M. Benito, Carmen Pazos, Jos´ e Coca ∗ Department of Chemical and Environmental Engineering, University of Oviedo, C/Juli´ an Claver´ ıa 8, 33006 Oviedo, Spain Received 11 March 2005; received in revised form 13 September 2005; accepted 14 September 2005 Available online 2 November 2005 Abstract The effect of operating conditions on the performance of a sawdust bed filter used for the treatment of an oil-in-water emulsion was investigated. A metalworking fluid (3 vol.% oil) was used as oil-in-water emulsion and sawdust as filter medium and sorbent. Because of the high stability of the emulsion, small amounts of inorganic salt (calcium sulphate) were mixed with the sorbent, acting as coagulant to achieve the emulsion breakdown. The influence of flow rate, bed height, temperature and the amount of coagulant salt added was studied. Experimental results show that several processes are involved in oil removal from oil-in-water emulsions, i.e. coagulation, coalescence, adsorption or straining. More than 99% of oil content in the influent stream was removed. Experimental results show that low-cost sorbents like sawdust are feasible to be used in the treatment of oil-in-water emulsions if small amounts of coagulant salts are added to the filter media. © 2005 Elsevier B.V. All rights reserved. Keywords: Deep bed filtration; Oil removal; Waste emulsion; Sawdust; Coagulant salt 1. Introduction The current treatment methods of waste emulsified oils before their disposal remain unsatisfactory. Emulsifiable or water- miscible cutting oils are used in metalworking industries as lubricants and coolants yielding better surface finishes and pro- tecting tools from abrasive wear. Cutting oils consist of a sus- pension of oil droplets in water, stabilised by surfactants, and usually contain several compounds, such as biocides, defoamers, rust inhibitors, extreme pressure additives, etc. Once the emul- sions lose their functional properties, they must be treated before their disposal because of the toxic and hazardous properties of their components. Even very low oil concentrations are toxic for microorganisms responsible for biodegradation in conven- tional sewage processes, and therefore removal of the oil phase is essential before effluent disposal. The oil removal process involves usually emulsion destabilisation, which is not an easy stage, because of the emulsifying agents used in their prepara- tion. For the removal of the oil phase, which has a high organic matter content (50,000–80,000 mg/L COD), several techniques have been proposed [1] and, among them, deep bed filtration ∗ Corresponding author. Tel.: +34 98 5103443; fax: +34 98 5103443. E-mail address: jcp@uniovi.es (J. Coca). has been reported as an efficient process [2–4]. However, the feasibility of fixed beds depends to a large extent on the costs of the filter media and, therefore, research has been focused on the use of solid waste materials or cheap and abundant natural products. Sawdust is an easily available by-product in timber and paper industry and exhibits good sorbent characteristics for the treatment of wastewater containing heavy metals [5–12], dyes [13–19], or phenolic compounds [20,21]. In many cases, a chem- ical pretreatment of sawdust is needed to activate the sorption sites [12,14,22]. The aim of this work is to assess the performance of sawdust as filter media for the treatment of oil-in-water emulsions, and to study the influence of the operating conditions. A cutting oil, used in metalworking operations, was selected as oily waste. 2. Materials and methods Oil-in-water emulsions were prepared by mixing a commer- cial cutting oil Licena (CEPSA Co., Spain) in deionised water. All experiments were carried out with emulsions containing 3 vol.% of oil. The influent oil concentration was kept constant at about 24,000 mg/L. Eucalyptus sawdust was provided as powder by a paper mill company (ENCE, Navia, Spain) and was used without previous 0304-3894/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2005.09.023