Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman Research article Optimization of aluminium recovery from water treatment sludge using Response Surface Methodology Theam Yiew Ooi a , Ee Ling Yong a , Mohd Fadhil Md Din a,b,* , Shahabaldin Rezania c , Eeydzah Aminudin d , Shreeshivadasan Chelliapan e , Azlan Abdul Rahman f , Junboum Park c a Department of Environmental Engineering, School of Civil Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81300, Johor Bahru, Malaysia b Centre for Environmental Sustainability and Water Security (IPASA), Research Institute for Environmental Sustainability, Universiti Teknologi Malaysia, 81310, Johor Bahru, Malaysia c Department of Civil and Environmental Engineering, Seoul National University, Seoul, Republic of Korea d Department of Structure and Materials, School of Civil Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81310, Johor Bahru, Johor, Malaysia e Engineering Department, UTM Razak School of Engineering & Advanced, Universiti Teknologi Malaysia, Malaysia f Office of Deputy Vice-Chancellor (Development), Universiti Teknologi Malaysia, 81300, Johor Bahru, Malaysia ARTICLE INFO Keywords: Water treatment sludge Acid leaching Response surface methodology Aluminium recovery ABSTRACT For decades, water treatment plants in Malaysia have widely employed aluminium-based coagulant for the removal of colloidal particles in surface water. This generates huge amount of by-product, known as sludge that is either reused for land applications or disposed to landfills. As sludge contains high concentration of alumi- nium, both can pose severe environmental issues. Therefore, this study explored the potential to recover alu- minium from water treatment sludge using acid leaching process. The evaluation of aluminium recovery effi- ciency was conducted in two phases. The first phase used the one factor at a time (OFAT) approach to study the effects of acid concentration, solid to liquid ratio, temperature and heating time. Meanwhile, second phase emphasized on the optimization of aluminium recovery using Response Surface Methodology (RSM). OFAT results indicated that aluminium recovery increased with the rising temperature and heating time. Acid con- centration and solid to liquid ratio, however, showed an initial increment followed by reduction of recovery with increasing concentration and ratio. Due to the solidification of sludge when acid concentration exceeded 4 M, this variable was fixed in the optimization study. RSM predicted that aluminium recovery can achieve 70.3% at optimal values of 4 M, 20.9%, 90 °C and 4.4 h of acid concentration, solid to liquid ratio, temperature and heating time, respectively. Experimental validation demonstrated a recovery of 68.8 ± 0.3%. The small dis- crepancy of 2.2 ± 0.4% between predicted and validated recovery suggests that RSM was a suitable tool in optimizing aluminium recovery conditions for water treatment sludge. 1. Introduction Aluminium is the third most abundant element in the Earth's crust (Exley, 2009). It is often used as coagulants in water treatment process, usually in the form of alum, poly aluminium chloride, ferric aluminium sulphate and aluminium hydroxide chloride (Evuti and Lawal, 2011). During the treatment process, sludge equivalent to 4–7% of the total net of water produced was generated as by-product (Sun et al., 2015). Annually, a typical water treatment plant can yield approximately 100,000 tons of sludge comprising of silicon dioxide (SiO 2 ), aluminium oxide (Al 2 O 3 ), iron oxide (Fe 2 O 3 ) and a small fraction of other oxides (Ahmad et al., 2016a, 2016b). The composition of water treatment sludge together with the type of coagulant used from previous studies are compiled in Table S1 (Supplementary Materials). It can be observed that the types of coagulant used during the water treatment process possess some considerable effects on the sludge composition. For ex- ample, sludge treated with aluminium-based coagulants such as alum and poly aluminium chloride (PAC) will have higher aluminium con- tent, whereas those treated with iron based-coagulants such as ferric chloride (FeCl 3 ) will contain more iron. In common practice, water treatment sludge are either discharged into waterways or disposed to landfills (Geraldo et al., 2017). Malaysia employs similar practice, where water treatment sludge are dried in sludge lagoons prior its disposal to landfills that do not have adequate https://doi.org/10.1016/j.jenvman.2018.09.008 Received 30 January 2018; Received in revised form 5 August 2018; Accepted 2 September 2018 * Corresponding author. Department of Environmental Engineering, School of Civil Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81300, Johor Bahru, Malaysia. E-mail address: mfadhil@utm.my (M.F.M. Din). Journal of Environmental Management 228 (2018) 13–19 0301-4797/ © 2018 Elsevier Ltd. All rights reserved. T