Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser Feasibility of using kitchen waste as future substrate for bioethanol production: A review Halimatun Saadiah Hafid a , Nor’ Aini Abdul Rahman a, ⁎ , Umi Kalsom Md Shah a , Azhari Samsu Baharuddin b , Arbakariya B. Ariff c a Department of Bioprocess Technology, Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia b Department of Process and Food Engineering, Faculty of Engineering, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia c Bioprocessing and Biomanufacturing Research Centre, Department of Bioprocess Technology, Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia ARTICLE INFO Keywords: Kitchen waste Pretreatment Saccharification Fermentable sugar Fermentation Bioethanol ABSTRACT This review highlights the utilization of kitchen wastes as substrates for bioethanol production. Kitchen wastes are commonly renewable, cheap and produced in large quantities daily. Kitchen wastes also contain a significant amount of organic matters particularly carbohydrates that can be converted into fermentable sugars for subsequent use in bioethanol fermentation. However, the advantages of kitchen wastes in biofuel production are indeed an untapped resource and poorly documented due to the challenges in the handling and disposal of kitchen wastes. Hence, a proper pretreatment and hydrolysis of the kitchen wastes by physical, chemical and biological methods is explored to increase the concentration of fermentable sugar released during the hydrolysis by enzymatic saccharification, thereby, improve the efficiency of the whole process. Furthermore, the advantages and drawbacks of each technology, challenges associated with feedstock handling and storage, government policies, and applications at commercial scale are critically discussed. 1. Introduction In Malaysia, there are approximately 2500 t of municipal solid wastes (MSW) generated per day in major cities with an average per person of 1.2 kg/day [1] consisting mainly of fermentable organic materials and kitchen wastes (71.6%), plastics (13.3%) and papers (5.8%), which comprise 80% of the overall weight [2]. It has been recorded that Malaysians produced 33,000 t of solid waste daily in 2012, exceeding the government's projected waste production of 30,000 t daily by 2020 [3]. At present, landfill system is the only waste management option for MSW in Malaysia. In order to divert as much as possible waste from landfill, MSW are recycled. However, they are not fully recovered and recycled due to the limited source separation and lack of proper recycling activity [4]. Since plastics, papers, and glasses are widely used, they become the most common recyclable items. The daily generation of kitchen waste is accelerated with substantial increase in volume due to rapid urbanisation, rapid growth of popula- tion and increase in food consumption rate. These organic wastes are discharged from various sources including households, restaurants and leftovers from food industries that consists of uneaten food as well as food preparation residues comprising of rice, meats, vegetables, fruits, bakery and dairy products [5]. The amount of kitchen waste is projected to increase due to the rapid economic expansion and population growth, especially in the Asian countries (Fig. 1) [6]. Asian economic giant, which is China has produced approximately 19,500×10 4 t/year of food waste. Meanwhile, other countries such as the United States, India, Japan, and Korea have also followed a similar trend, discarding between 624 – 3500×10 4 t/year of food waste. For instant, the developing South-eastern Asia countries including Thailand, Vietnam, and Malaysia, have generated about 440 – 712×10 4 t/year of food waste. Kitchen waste generation is a topic concern by most countries including Malaysia. This might be due to the unpleasant odour, vermin attraction and toxic gas emission generated by the decomposed kitchen waste containing high biodegradable organic compounds [7]. The heterogeneous composition of kitchen waste causes the specific content to be extremely unpredictable. Besides, in the landfill, kitchen waste with high percentage of moisture will generate leachate and require secondary wastewater treatment system [8]. http://dx.doi.org/10.1016/j.rser.2017.02.071 Received 28 September 2015; Received in revised form 20 January 2017; Accepted 21 February 2017 ⁎ Corresponding author. E-mail addresses: hs_hafid@yahoo.com (H.S. Hafid), nor_aini@upm.edu.my (N.A.A. Rahman), umikalsom@upm.edu.my (U.K.M. Shah), azharis@upm.edu.my (A.S. Baharuddin), arbarif@upm.edu.my (A.B. Ariff). Renewable and Sustainable Energy Reviews 74 (2017) 671–686 1364-0321/ © 2017 Elsevier Ltd. All rights reserved. MARK