Review Development of anaerobic digestion methods for palm oil mill effluent (POME) treatment P.E. Poh, M.F. Chong * School of Chemical and Environmental Engineering, Faculty of Engineering, The University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor, Malaysia a r t i c l e i n f o Article history: Received 10 April 2008 Received in revised form 11 June 2008 Accepted 12 June 2008 Available online 25 July 2008 Keywords: POME Anaerobic treatment UASB UASFF a b s t r a c t Palm oil mill effluent (POME) is a highly polluting wastewater that pollutes the environment if dis- charged directly due to its high chemical oxygen demand (COD) and biochemical oxygen demand (BOD) concentration. Anaerobic digestion has been widely used for POME treatment with large emphasis placed on capturing the methane gas released as a product of this biodegradation treatment method. The anaerobic digestion method is recognized as a clean development mechanism (CDM) under the Kyoto protocol. Certified emission reduction (CER) can be obtained by using methane gas as a renewable energy. This review aims to discuss the various anaerobic treatments of POME and factors that influence the operation of anaerobic treatment. The POME treatment at both mesophilic and thermophilic temperature ranges are also analyzed. �� 2008 Elsevier Ltd. All rights reserved. 1. Introduction In the process of palm oil milling, POME is generated through sterilization of fresh oil palm fruit bunches, clarification of palm oil and effluent from hydrocyclone operations (Borja et al., 1996a). POME is a viscous brown liquid with fine suspended solids at pH ranging between 4 and 5 (Najafpour et al., 2006). The char- acteristics of POME could be referred to the Data for Engineers: POME (2004). Direct discharge of POME into the environment is not encouraged due to the high values of COD and BOD. Further- more, with the introduction of effluent discharge standards im- posed by the Department of Environment in Malaysia, POME has to be treated before being released into the environment (Federal Subsidiary Legislation, 1974). Anaerobic digestion has been employed by most palm oil mills as their primary treatment of POME (Tay, 1991). More than 85% of palm oil mills in Malaysia have adopted the ponding system for POME treatment (Ma et al., 1993) while the rest opted for open digesting tank (Yacob et al., 2005). These methods are regarded as conventional POME treatment method whereby long retention times and large treatment areas are required. High-rate anaerobic bioreactors have also been applied in laboratory-scaled POME treatment such as up-flow anaerobic sludge blanket (UASB) reactor (Borja and Banks, 1994a) up-flow anaerobic filtration (Borja and Banks, 1994b) fluidized bed reactor (Borja and Banks, 1995a,b) and up-flow anaerobic sludge fixed-film (UASFF) reactor (Najafpour et al., 2006). Anaerobic contact digester (Ibrahim et al., 1984) and continuous stirred tank reactor (CSTR) have also been studied for treatment of POME (Chin, 1981). Other than anaerobic digestion, POME has also been treated using membrane technology (Ahmad et al., 2006, 2007), aerobic activated sludge reactor (Vijayaraghavan et al., 2007), and evapora- tion method (Ma et al., 1997). 1.1. Clean development mechanism (CDM) The utilization of methane gas as a renewable energy from the anaerobic digestion can be used to obtain certified emission reduc- tion (CER) credit by clean development mechanism (CDM) under the Kyoto protocol (Tong and Jaafar, 2006). Besides helping to re- duce carbon emission to the environment, CDM has the advantage to offer developing countries such as Malaysia to attract foreign investments to sustain renewable energy projects (Menon, 2002). Thus, palm oil mills could earn carbon credits as revenue by the utilization of methane gas as renewable energy from anaerobic digestion of palm oil mill effluent. More emphasis has been given to develop anaerobic treatment for POME since the implementa- tion of CDM. Currently, there are two CDM projects that have been registered to recover methane from palm oil mill effluent which are hosted by Kim Loong Power Sdn. Bhd. (Project, 0867) and Uni- ted Plantations Bhd. (Project, 1153). Subsequent to this, investigation by Yacob et al. (2006a) on the methane emission from anaerobic pond shows that 1043.1 kg/day/ pond of methane gas is emitted. Based on the ponding system 0960-8524/$ - see front matter �� 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.06.022 * Corresponding author. Tel.: +603 8924 8347 fax: +603 8924 8017. E-mail addresses: MeiFong.Chong@nottingham.edu.my, chong_mei_fong@ yahoo.com (M.F. Chong). Bioresource Technology 100 (2009) 1���9 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

investigated, there were 4 anaerobic ponds, which in total pro- duced 4172.4 kg/day of methane gas. It is estimated that approxi- mately RM 1,027,975 per year (��� 228,438.9) can be generated as revenue if methane gas emitted from the anaerobic ponds are cap- tured as renewable energy. Calculations are based on 300 working days and carbon credit price of ��� 10 per ton of carbon quoted by Menon (2007). Payback period for investment on anaerobic bioreactors can be short if carbon credit prices remain high (Menon, 2007). Consider- ing the revenue and advantages achieved through capturing meth- ane gas, palm oil mills could switch to anaerobic bioreactor for POME treatment. 2. Anaerobic digestion Anaerobic digestion is the degradation of complex organic mat- ters under the absence of oxygen. This process is time consuming as bacterial consortia responsible for the degradation process re- quires time to adapt to the new environment before they start to consume on organic matters to grow. In the process of degrading POME into methane, carbon diox- ide and water, there is a sequence of reactions involved hydroly- sis, acidogenesis (including acetogenesis) and methanogenesis (Gerardi, 2003). Hydrolysis is where complex molecules (i.e., car- bohydrates, lipids, proteins) are converted into sugar, amino acid and etc. In the step of acidogenesis, acidogenic bacteria will break down these sugar, fatty acids and amino acids into organic acids which mainly consist of acetic acid (from acetogenesis) together with hydrogen and carbon dioxide. Hydrogen and carbon dioxide will be utilized by hydrogenotropic methanogens while acetic acid and carbon dioxide will be utilized by acetoclastic methanogens to give methane as a final product. Methanogenesis is the rate limiting step in anaerobic digestion of POME (Ibrahim et al., 1984). As such, conventional anaerobic digesters require large reactors and long retention time to ensure complete digestion of treated influent. Nonetheless, high-rate anaerobic bioreactors have been proposed (Borja and Banks, 1994a,b, 1995a,b Najafpour et al., 2006 Ibrahim et al., 1984) to re- duce reactor volume, shorten retention time as well as capture methane gas for utilization. 2.1. Anaerobic and alternative POME treatment methods Aerobic treatment, membrane treatment system and evapora- tion method are the currently available alternative methods for POME treatment. The advantages and disadvantages for anaerobic and alternative treatment methods are shown in Table 1. In terms of energy requirement for POME treatment operation, anaerobic digestion has a stronger advantage over other alternative methods as it does not require energy for aeration. Furthermore, anaerobic POME treatment produces methane gas which is a value-added product of digestion that can be utilized in the mill to gain more revenue in terms of CER. Take for instance the open digesting tank for POME treatment without land application, capital cost quoted by Gopal and Ma (1986) for a palm oil mill processing 30 tons FFB/h is RM 750,000. Based on the Chemical Engineering Plant Cost Index in 2006 (Ullrich and Vasudevan, 2004) the capital cost for this system is estimated to be RM 1,147,842 in 2006. Comparing this to the capital cost for a membrane system in POME treatment for a palm oil mill processing 36 tons FFB/h at RM 3,950,000 (Chong, 2007), it is obvious that the former anaerobic treatment has better advan- tage over other treatment methods in terms of capital cost. The only two significant drawbacks of anaerobic treatment are long retention times and long start-up period. However, the problem of long retention times can be rectified by using high-rate anaero- bic bioreactors while the long start-up period can be shortened by using granulated seed sludge (McHugh et al., 2003), utilizing seed sludge from same process (Yacob et al., 2006b) or maintaining suit- able pH and temperature in the high-rate anaerobic bioreactor for growth of bacteria consortia (Liu et al., 2002). Untreated wastewater with BOD/COD ratio of 0.5 and greater can be treated easily by biological means (Metcalf and Eddy, 2003). With reference to the published values of BOD and COD in Data for Engineers: POME (2004), aerobic and anaerobic treatment is suitable for POME treatment since the BOD/COD ratio is of 0.5. In comparison of these two treatment methods, the anaerobic treat- ment can be regarded to be more suitable for POME treatment due to its lower energy consumption while producing methane as a value-added product in the process. 2.2. Anaerobic treatment methods 2.2.1. Conventional treatment systems Ponding system is the most common treatment system that is employed in palm oil mills to treat POME with more than 85% of the mills having adopted this method. Ponding system comprises of de-oiling tank, acidification ponds, anaerobic ponds and faculta- tive or aerobic ponds (Chan and Chooi, 1984). Number of ponds varies according to the capacity of the palm oil mill. Facultative or aerobic ponds are necessary to further reduce BOD concentra- tion in order to produce effluent that complies with Federal Sub- sidiary Legislation, 1974 effluent discharge standards. A typical size of an anaerobic pond in a palm oil mill which has a processing capacity of 54 tons per hour is 60.0 29.6 5.8 m (length width depth) (Yacob et al., 2006a) which is approxi- mately equivalent to half the size of a soccer field. Size of pond de- pends on the capacity of the palm oil mill as well as the area available for ponds. Anaerobic ponds have the longest retention time in ponding system which is around 20���200 days (Chan and Table 1 Advantages and disadvantages between anaerobic and alternative treatment methods Treatment types Advantages Disadvantages Reference Anaerobic Low energy requirements (no aeration), producing methane gas as a valuable end product, generated sludge from process could be used for land applications Long retention time, slow start-up (granulating reactors), large area required for conventional digesters Metcalf and Eddy (2003), Borja et al. (1996a) Aerobic Shorter retention time, more effective in handling toxic wastes High energy requirement (aeration), rate of pathogen inactivation is lower in aerobic sludge compared to anaerobic sludge, thus unsuitable for land applications Leslie Grady et al. (1999), Doble and Kumar (2005) Membrane Produce consistent and good water quality after treatment, smaller space required for membrane treatment plants, can disinfect treated water Short membrane life, membrane fouling, expensive compared to conventional treatment Ahmad et al. (2006), Metcalf and Eddy (2003) Evaporation Solid concentrate from process can be utilized as feed material for fertilizer manufacturing High energy consumption Ma et al. (1997) 2 P.E. Poh, M.F. Chong / Bioresource Technology 100 (2009) 1���9