A Brief Review on Palm Oil Liquid Waste Conversion Into

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A Brief Review on Palm Oil Liquid Waste Conversion Into Biofuel

Journal: Environmental Reviews

Manuscript ID er-2018-0124.R2

Manuscript Type: Review

Date Submitted by the Author: 18-Jun-2019

Complete List of Authors: Zuber, Muhammad Ahmar; Universiti Teknologi Malaysia Malaysia-Japan International Institute of Technology, yahya, wira jazair; Universiti Teknologi Malaysia Malaysia-Japan International Institute of Technologyithnin, ahmad muhsin; Universiti Teknologi Malaysia Malaysia-Japan International Institute of TechnologySugeng, Dhani Avianto; Universiti Teknologi Malaysia Malaysia-Japan International Institute of Technologyabd kadir, hasannuddin; Universiti Teknologi Malaysia Malaysia-Japan International Institute of Technologyahmad, mohamad azrin; Universiti Teknologi Malaysia Malaysia-Japan International Institute of Technology

Is this manuscript invited for consideration in a Special

Issue? :Not applicable (regular submission)

Keyword: waste oil, palm oil liquid waste, renewable energy, biofuel conversion

https://mc06.manuscriptcentral.com/er-pubs

Environmental Reviews

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1 A Brief Review of Palm Oil Liquid Waste Conversion into Biofuel

2 Muhammad Ahmar Zuber, Wira Jazair Yahya*, Ahmad Muhsin Ithnin, 3 Dhani Avianto Sugeng, Hasannuddin Abd Kadir, Mohamad Azrin Ahmad

4 Advances Vehicle System, Malaysia-Japan International Institute of Technology, 5 Universiti Teknologi Malaysia, Kuala Lumpur, Malaysia

6 *corresponding author, Advances Vehicle System, Malaysia-Japan International Institute 7 of Technology, Universiti Teknologi Malaysia, Kuala Lumpur, Malaysia8

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9 Palm oil is an important edible oil due to its high content of beta-carotene and

10 vitamin E, high oil output, and solid fat content. However, its extensive

11 commercialization has resulted in a vast amount of waste, leading to challenges

12 for the development of an economically feasible conversion of palm oil waste

13 into useful products. This review focuses on exploring the various conversion

14 processes of the liquid waste produced from the palm oil processing industry.

15 The main treatment of Palm Oil Mill Effluent (POME), which can be separated

16 into fiber, wastewater, residual oils, and other impurities, involves a digestion

17 process which produces biogas, while the fiber and other impurities are often

18 converted into animal feed, soil fertilizer, fermentation media, and yeast

19 production. Residual oil found in POME, known as Sludge Palm Oil (SPO),

20 contains high levels of free fatty acid (FFA). Other residual oils resulting from

21 palm oil refining include Palm Fatty Acid Distilled (PFAD) and Palm Acid Oil

22 (PAO) that also have a high FFA content. The transesterification and

23 esterification processes are utilized to convert SPO, PFAD and PAO into fuel.

24 Keywords: waste oil; palm oil liquid waste; renewable energy;

25 Introduction

26 Most oil palm trees in Malaysia are from the species Elaeis guineensis originated from

27 West Africa and typically grow in tropical peat soil. They mature and start fruiting

28 around 30 months after planting. These trees can reach up to 20 meters in height and

29 produce up to 25kg of fruit bunches a day (Malaysian Palm Oil Council 2017). The two

30 types of oil that can be extracted from oil palm trees are crude palm oil, obtained from

31 the mesocarp, and palm kernel oil, derived from its kernel.

32 The growth of the palm oil industry is proliferating due to the ever-increasing

33 demand for palm oil compared to other types of vegetable oil (Sumathi et al. 2008). The

34 superiority of palm oil over other vegetable oils includes its lower production cost,

35 higher production yield per hectare (Silalertruksa et al. 2012), high solid fat content, and

36 other desirable properties. When compared to soybean, sunflower, and rapeseed oils,

37 palm oil exhibits the lowest production cost and market price (Berger 1986; Amiruddin

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38 et al. 2005). Palm oil also contains beta-carotene and vitamin E (tocotrienols and

39 tocophenols), which are beneficial to human health (Obahiagbon 2012).

40 The large scale of palm oil production leads to the generation of high amounts of

41 waste and by-products. Hence the primary objective of this article is to review the

42 alternatives available in liquid palm oil waste utilisation and the processes in its

43 conversion into renewable energy sources. This review will also try to highlight the

44 waste classification of palm oil mill effluent (POME), sludge palm oil (SPO), palm acid

45 oil (PAO) and palm fatty acid distilled (PFAD) and their potential uses.

46 The Palm Oil Industry in Malaysia

47 Oil palm trees were first commercially planted in Malaysia in 1917. The palm oil

48 industry expanded rapidly ever since, and several organisations were established to

49 regulate and research it, such as the Malaysia Palm oil Board (MPOB) (MPOB, 2017a).

50 According to the MPOB statistics, the plantation area in Malaysia increases each year,

51 as depicted in Figure 1. Between 1960 and 2016, the plantation area in Malaysia is

52 estimated to have increased from 0.05 to 5.73 million hectares (Awalludin et al. 2015;

53 MPOB 2017b). Figure 2 shows the increase of crude palm oil (CPO) production from

54 the year 2000 to 2016 aligned with plantation area expansion.

55

56 Figure 1: Plantation area of the palm oil industry in Malaysia (Awalludin et al. 2015; 57 MPOB 2017b)58

59 Figure 2: Production of crude palm oil (CPO) (MPOB 2012; MPOB 2017c; Awalludin et 60 al. 2015)

61 Palm oil production processes

62 Harvesting

63 The production of palm oil starts with harvesting to collect Fresh Fruit Bunches (FFB)

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64 containing the palm fruit. During the collection process, Oil Palm Fronds (OPF) will be

65 cut down along with FFB. In current practice, the harvested FFB will be collected,

66 weighed and transported to a milling plant, while OPF will be left at the plantation as

67 fertiliser or processed further for animal feed and biomass (Boschma and Kwant 2013).

68 Milling process

69 At the milling plant, the FFB will be cleaned and sterilised using steam for about 75-90

70 minutes to ensure the hydrolytic enzymes that are responsible for breaking down oil into

71 free fatty acid (FFA) are disabled. Steam will also loosen the palm fruits from the

72 bunches, coagulate the mucilage for better oil cell breaking and recovery, and reduce

73 kernel cracking. The spent water, which contains traces of oil and palm fiber, is then

74 discharged to the wastewater pond (Hassan et al. 2005).

75 After sterilization, the FFB goes through the bunch stripping process where the

76 palm fruits will be separated from the bunch using a rotating drum, leaving the Empty

77 Fruit Bunches (EFB). The collected fruits will be transferred to the digester for further

78 processing, and EFB is discarded as waste and can be used as animal feed or biomass

79 (Hassan et al. 2005).

80 The fruits are then sent to the digester machine, where they are pressed and

81 crushed to break up the mesocarp into a mash-like substance. Later, water is added to

82 the mash to increase the flowability of the mash. The mash is led into a screw pressing

83 machine to extract the oil, which is denoted as Crude Palm Oil (CPO). Some oil remains

84 in the mash and will be recovered as Sludge Palm Oil (SPO) (Vijaya et al. 2013). The

85 pressed mash cake contains palm kernel and mesocarp which can be separated later.

86 The palm kernel is separated from the press cake by using a splitter machine.

87 Palm kernels can be processed further to extract the oil denoted as Crude Palm Kernel

88 Oil (CPKO). CPKO and the crushed Palm Kernel Shell (PKS) are then separated using

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89 a hydrocyclone by both the dry and wet method (Hassan et al. 2005). The separated

90 press cake and PKS are treated as a by-product and can be used as biomass. At this

91 point, the CPO and CPKO will be refined to reduce the fatty acid and remove impurities

92 by an either physical or chemical process. The physical refining process costs less than

93 the chemical refining process; hence, it is the favoured and widely used method in the

94 Malaysian palm oil industry.

95 Refining process

96 Refining is a process of converting CPO into refined, bleached and deodorised (RBD)

97 olein and stearin, while CPKO is processed into refined, bleached and deodorised palm

98 kernel (RBDPK) olein and stearin. The refining process can be either physical or

99 chemical.

100 Physical processes involve using hot water to refine the feedstock, where the oil

101 is suspended at the top of the water and is then skimmed off. Any impurities and

102 contamination will stay in the water. Chemical processes use chemicals such as

103 phosphoric acid and caustic soda to remove non-hydratable phosphatides, soap, free

104 caustic and other soluble impurities from the crude oil. The drying, degumming,

105 bleaching and deodorising process follow afterwards to produce the final product

106 (Gibon et al. 2007).

107 Several solid and liquid by-products and residue are produced during the milling

108 and refining stages, which can be processed further to generate energy or other useful

109 products. Figure 3 summarizes the palm oil production processes and the waste products

110 generated at each stage.

111 Figure 3: Summary of the palm oil production process

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112 Management and conversion of waste products from the palm oil industry

113 Globally, Indonesia is the largest palm oil producer with a market share of

114 approximately 54%, followed by Malaysia with 30% and all others amounting to 16%

115 (Global Palm Oil Production 2016). The enormous quantities of waste and by-products

116 generated by the palm oil industry have raised public concern, resulting in research

117 focusing on the proper management and recovery of palm oil waste to reduce its impact

118 on the environment (Kurnia et al. 2016).

119 It is estimated that for one-hectare of plantation area, approximately 1.7 to 6 tons

120 of CPO are produced (Rupani et al. 2010) and for each ton of CPO produced, 3 tons of

121 wastewater are generated (Borja and Banks 1994; Wicke et al. 2008; Wu et al. 2010). In

122 2016, palm oil plantations occupied almost 5.7 million hectares in Malaysia, with CPO

123 production of approximately 17 million tonnes, resulting in an estimated 51 million tons

124 of wastes water produced. Without proper management, this will make a significant

125 impact on the environment.

126 Proper methods and technologies to convert high quantities of waste into a

127 renewable energy source or useful by-products are desirable and the potential of each

128 palm oil waste product for generating energy, producing biomass, animal feedstock,

129 fertiliser, carbon capture, fermentation medium, biogas and biofuel has been previously

130 reviewed (Yusoff 2006; Sumathi et al. 2008; Singh et al. 2011; Sulaiman et al. 2011;

131 Abdullah and Sulaim 2013; Loh et al. 2014; Kurnia et al. 2016; Bello and Abdul Raman

132 2017).

133 A study conducted by Hansen et al. (2015) concluded that 60% of palm oil

134 research papers published in the years between 2004 to 2013 focused on the handling of

135 palm oil waste. The palm oil industry emits solid wastes such as OPF, EFB, PKS and

136 fiber, as well as liquid wastes such as POME. Solid wastes can be treated through

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137 physical and thermochemical processes (Awalludin et al. 2015; Kurnia et al. 2016). In

138 the physical process, shredding, densification, and drying are applied to convert the

139 waste into a usable biomass product such as soil fertiliser, animal feedstock, bio-

140 briquette, carbon capture and pressed board (Boschma and Kwant 2013; Hassan et al.

141 2013; Awalludin et al. 2015; Kurnia et al. 2016; Rivera-Mendez et al. 2017). In the

142 thermochemical process such as gasification, syngas can be produced to generate

143 electricity and heat (Li et al. 2009a; Li et al. 2009b; Nipattummakul et al. 2012; Atnaw

144 et al. 2013; Ariffin et al. 2015; Sivasangar et al. 2015; Ariffin et al. 2016a; Ariffin et al.

145 2016b; Samiran et al. 2016). Another method of managing the wastes from the palm oil

146 industry is to produce soil fertilisers through composting (Vakili et al. 2015), where the

147 use of worms can promote the composting process (Rupani et al. 2010; Singh et al.

148 2011).

149 Liquid waste from the palm oil industry

150 The palm oil industry’s liquid waste can be further classified into four different types:

151 palm oil mill effluent (POME), sludge palm oil (SPO), palm acid oil (PAO) and palm

152 fatty acid distillate (PFAD). POME is the wastewater which is discharged into the waste

153 pond, while SPO is the oil residue found in the POME. PAO and PFAD are classified as

154 by-products rather than wastes and can be found in refining plants. SPO, PAO, and

155 PFAD are classified as residual oils and contain different percentages of free fatty acid

156 (FFA). The wastewater and residual oils have the potential to be processed into

157 renewable fuels at a reasonable cost.

158 Wastewater is generated during the processing of palm oil in the milling plant

159 and refinery plant, but the wastewater generated by the refinery plant is less polluting

160 due to the absence of oil, grease and low organic loads (Hassan et al. 2005). The

161 wastewater mix from milling plant contains water, residue oil, microorganisms, palm

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162 fiber, solid fat, and other impurities. According to Onyia et al. (2001) and Hassan et al.

163 (2005), the brown slurry wastewater resulting from palm oil milling plants is non-toxic

164 but has an unpleasant odour. The wastewater consists of approximately 4 to 5% organic

165 solids, 0.5 to 1% residual oils, and a high concentration of nitrogen. Madaki and Seng

166 (2013) described the wastewater as a water-soluble component of palm oil fruit that

167 contains suspended materials such as palm fiber and oil residue. Even though the water

168 is non-toxic, it must be treated appropriately before being discharged into the

169 environment due to its acidity and high biological oxygen demand number. The two

170 methods that are widely employed by the palm oil industry for wastewater treatment is

171 the ponding system and the open digester with ponding system (Hassan et al. 2005).

172 In the making of biodiesel, raw materials are the main contributor to production

173 costs. Therefore, by using waste oil, residual oil, low-grade oil and fat instead of

174 vegetable oils, the cost of raw materials for biodiesel production can be reduced

175 significantly (Hayyan et al. 2013). Some examples of low-grade oils that are suitable for

176 biodiesel production include waste cooking palm oil, low industrial grade palm oil,

177 acidic crude palm oil and PFAD (Hayyan et al. 2013). SPO, which is extracted from

178 POME, can be used as biodiesel’s raw material, while the remainder of POME has a

179 high content of biological components that can be used to produce biogas.

180 Palm Oil Mill Effluent (POME)

181 POME is a denotation for wastewater from the processing of palm oil in the milling

182 plant. It consists of 95-96% water, 0.6-0.7% suspended oils and 4-5% total solids.

183 Around 2-4% of the suspended solids found in POME comes from the sterilisation,

184 sludge separation and wet hydrocyclone processes (Ma 2000). Table 1 shows the

185 characteristics of a typical POME in Malaysia, highlighting that POME typically has a

186 high number of COD, BOD, total solids and other impurities that make it harmful to

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187 discharge into the environment.

188 Table 1: POME characteristic189190 Direct discharge of POME into the environment is harmful to soil, water, and

191 aquatic life due to the high biological oxygen demand (BOD) and chemical oxygen

192 demand (COD) (Aluwi et al. 2013). Hence POME needs to be properly treated before it

193 can be released into the environment. Various POME treatment methods will be

194 discussed in the next section.

195 Treatment Methods

196 There are four major classifications of POME treatment including 1) the ponding

197 system which is the most widely used method, 2) anaerobic and aerobic digestion, 3)

198 physicochemical and, 4) membrane filtration (Wu et al. 2010). Each one of these

199 treatment methods has its advantages and trade-offs. The ponding system is the most

200 common method used by the palm oil industry due to its low capital and operating

201 costs, but has a high hydraulic retention time, requires a large area, and it releases

202 methane gas into the environment (Hassan et al. 2005; Wu et al. 2010; Madaki and Lau

203 2013; Liew et al. 2014; Bello and Abdul Raman 2017).

204 Aerobic and anaerobic digestion are two different methods of POME treatment

205 which produce biogas. The decomposition of organic material by microorganisms to

206 produce methane involves a series of reactions, including hydrolysis, acidogenisis

207 (including acetogenesis) and methanogenesis (Poh and Chong, 2009). The advantages

208 of aerobic digestion is a shorter retention time, whereas the disadvantage is the need of

209 an aeration system which increases the energy requirement and capital cost (Poh and

210 Chong 2009; Wu et al. 2010; Gobi and Vadivelu 2013). In contrast, some of the

211 advantages of the anaerobic digestion system includes lower capital cost and a lower

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212 energy requirement, resulting in a faster return on the investment, but does require a

213 longer retention time (Poh and Chong 2009; Wu et al. 2010; Madaki and Lau 2013;

214 Hasanudin et al. 2015).

215 The three most commonly used physicochemical POME treatment methods are

216 sedimentation and centrifugation, coagulation and flocculation, and flotation and

217 adsorption (Wu et al., 2010). Coagulation and flocculation can also be used as a pre-

218 treatment process to eliminate up to 58% of pollutants, BOD, lignin-tannin, and

219 ammonia nitrogen in POME (Zahrim et al. 2014). Photocatalysis is a process of using

220 light energy (activation agent) that is absorbed by a photo-catalyst to degrade an organic

221 compound. The treatment of POME by photocatalysis has been examined using titania

222 doped with platinum (Cheng et al. 2015) and silver modified with titania (Cheng et al.

223 2016; Ng et al. 2016b) as catalysts. A review on the photocatalytic treatment of POME

224 has been discussed in detail by Alhaji et al. (2016), but due to its low BOD reducing

225 capability, this approach was deemed only to be practical when used as a post-treatment

226 process. Some of the disadvantages of this method are that it requires large amounts of

227 chemicals such as coagulants and adsorbents to cater the high organic loading, which

228 can increase the cost, and the non-biodegradability of the chemicals used (Wu et al.

229 2010; Bello and Abdul Raman 2017). To reduce the cost, a list of lower cost materials

230 that are biodegradable are summarized in Table 2.

231232 Table 2: Low-cost coagulants and adsorbents used in POME treatment 233234 Another POME treatment method that shows promise is the membrane filtration

235 method, but POME contains high solid impurities such as fiber that can cause clogging

236 of the membrane pores, rendering the membrane ineffective (Wu et al. 2010; Bello and

237 Abdul Raman 2017). Membrane fouling increases system maintenance, leading to a

238 high total cost of ownership of such a system (Bello and Abdul Raman 2017). The

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239 membrane filtering system can be improved by introducing an adsorption process as a

240 pre-treatment step (Azmi and Yunos 2014). It has been proven that membrane systems

241 can remove 99.4% of the suspended solids in POME provided a pre-treatment step is

242 installed to reduce the sludge, suspended solids, BOD and COD, effectively lowering

243 the maintenance of the membrane (Shah and Singh 2003).

244 Several researchers have suggested that the physicochemical and membrane

245 filtration approaches are more effective as a tertiary treatment or polishing method than

246 as a primary treatment of POME (Gobi and Vadivelu 2013; Liew et al. 2014; Bello and

247 Abdul Raman 2017). Excessive fouling can be reduced by decreasing the organic

248 content in POME before employing these treatments. The POME polishing process

249 involves a combination of activated carbon and ultrasonic cavitation that can remove up

250 to 100% of COD and total suspended solids within a short time limit (Parthasarathy et

251 al. 2016). A combination of pre-treatment processes of coagulation, flocculation and

252 active carbon adsorption (Othman et al. 2014), followed by a membrane separation

253 process, ultrafiltration and reverse osmosis has been found to treat POME into clear

254 water effectively (Ahmad et al. 2003; Ahmad and Chan 2009). The membrane filter

255 plays an important role in reducing the colour of treated POME until it becomes clear

256 and eliminate unwanted chemicals and impurities (Ali Amat et al. 2015).

257 Advanced oxidation process (AOP) is a recent breakthrough in POME polishing

258 that degrades the organic substance in POME through the application of an OH radical

259 (Liew et al. 2014; Taha and Ibrahim 2014; Ahmed et al. 2015; Alhaji et al. 2016; Bello

260 and Abdul Raman 2017). The OH radical can be generated by a combination of ozone,

261 hydrogen peroxide, radiation and ultrasound (Bello and Abdul Raman 2017). The

262 review by Bello and Abdul Raman (2017) stated that AOP as a POME polishing

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263 method is promising despite the challenges, as this method can improve the POME

264 treatment significantly.

265 The most desirable POME treatment choice is the anaerobic digestion system

266 because it is easy to set up, has a low investment and maintenance cost, has a high

267 organic loading and high methane production. There are disadvantages, however,

268 including high retention time and large space requirements. A combination of POME

269 anaerobic treatment and polishing treatment can increase the effectiveness of waste

270 removal that will be better able to meet discharge regulations. Some examples of

271 methods that can improve anaerobic digestion and overall performance of the process

272 include the use of sorbents (Mohammed and Chong 2014), electrolysis (Aluwi et al.

273 2013), membrane filtering (Shah and Singh 2003), adsorption and magnetic field

274 (Mohammed et al. 2014), and ultrasonic and hydrogen peroxide (Manickam et al. 2014).

275 An anaerobic reactor with the addition of bacteria to enhance digestion was

276 introduced to reduce retention time and space requirements. The bacteria can

277 breakdown POME effectively, but they require proper monitoring as they are sensitive

278 to changes in their environment (Madaki and Seng 2013). Alternatively, the reactor can

279 employ both physicochemical treatments and anaerobic digestion at a high reaction rate

280 (Zinatizadeh et al. 2006). For more controlled and faster digestion that improves the

281 efficiency of the treatment and increases biogas production, Khemkhao et al. (2015)

282 developed a reactor which can accept multiple variables such as changing the digestion

283 process, modifying organic loading rate and continuous stirring to achieve the optimum

284 treatment process.

285 To find the optimum reactor parameter, Zinatizadeh et al. (2006) used the

286 response surface methodology (RSM). By varying the feed flow rate and up-flow

287 velocity in up-flow anaerobic sludge fixed film, COD removal time was improved

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288 (Zinatizadeh et al. 2006). Other parameters affecting POME treatment include the

289 oxygen flow rate, catalyst loading time and the initial concentration of POME (Ng et al.

290 2016a). Also, RSM was used to determine the optimum parameter for incubation time,

291 enzyme concentration and impeller speed for the recovery of sludge palm oil (SPO) in

292 POME (Noorshamsiana et al. 2013). Fuzzy optimisation is another method used to find

293 a suitable parameter for the reactor process (Chan et al. 2015).

294 Potential POME Product: Biogas

295 Typically, palm oil mill discharges POME into the pond for anaerobic processes only to

296 comply with and satisfy environmental quality regulations, because this process will

297 lower POME’s COD (Chin et al. 2013). To avoid biogas from escaping into the

298 environment, a proper collection method should be employed such as a closed tank

299 reactor or a closed ponding system, where the biogas can be collected directly into a

300 storage tank (Nasrin 2016). Anaerobic treatment in a bioreactor is more favourable for

301 producing biogas than ponding, as it is both the most cost-efficient and has the lowest

302 time trade-off (Poh et al. 2010). The resulting biogas contains approximately 60%

303 methane, which is suitable for heat and power generation (Borja and Banks 1994; Foo

304 and Hameed 2010; Chin et al. 2013).

305 The biogas that is produced from POME treatment can be further enhanced into

306 biological compressed natural gas (bio-CNG) by a series of processes. A recent study

307 by Nasrin et al. (2017) explains the process of upgrading biogas into bio-CNG through

308 three steps: pre-treatment, upgrading and storing. In the pre-treatment process, a

309 combination of biological and chemical methods was used to reduce the hydrogen

310 sulphide content to less than 10 ppm. The biogas was then compressed, and a membrane

311 technology was used to remove the carbon dioxide in the upgrading process to ensure

312 that the methane content in the biogas is similar to the composition of natural gas (>

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313 94%). After this condition had been met, the biogas is then stored and distributed. Table

314 3 compares the specifications of bio-CNG to natural gas and biogas. It is shown that the

315 bio-CNG properties are almost the same as natural gas. Hence bio-CNG can be used to

316 replace natural gas (Mohtar et al., 2017).

317 Table 3: Comparison of Bio-CNG to biogas and natural gas (Nasrin 2016)318319 Another method used to enhance biogas is the addition of hydrogen gas and

320 studying the effect it has on the flame behaviour. Hydrogen enrichment showed an

321 increase in flame temperature, flame stability, and flame length, but NOx emissions did

322 increase slightly due to the rise in temperature (Hosseini et al. 2015). The biogas

323 produced can be used to power self-preheated reactors, and the power generated from

324 the reactor was increased when hydrogen gas was added (Hosseini and Wahid 2015).

325

326 Sludge Palm Oil (SPO)

327 When POME is discharged into the pond, residual oils that leached out during the

328 milling process floats on the water. This oil is known as the sludge palm oil (SPO),

329 which is a foul smelling-dark brown substance which solidifies at room temperature.

330 SPO contains a significant amount of FFA, usually around 20-80%, depending on the

331 time of its exposure to the environment. It has a low deterioration of the bleachability

332 index (DOBI), which renders it unusable as a food source. SPO has a total fatty matter

333 content of at least 95%, moisture and impurities at a maximum of 3% and FFA content

334 of at least 50%, with palmitic acid as the main component (Mohamad and Yahya 2012).

335 Chow and Ho (2002) reported composition of natural lipids in SPO to be approximately

336 84% which contain 70.14% triglycerides, 6.72% diglycerides, 0.42% monoglycerides

337 and 6.72% FFA suitable for microbial growth during the production of biosurfactants.

338 Table 4 shows the details of SPO characteristics summarize from Ainie et al. (1995).

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339 Table 4: SPO characteristics (Ainie et al. 1995)340341 A refining process can transform SPO into an oil product that can be used as fuel

342 or detergents. For example, SPO oil products can be used directly in a burner as fuel,

343 biodiesel production feedstock or as FFA feedstock in soap manufacturing (Wafti et al.

344 2010; Abd. Wafti et al. 2012). SPO can be refined further through degumming (using

345 phosphoric acid), bleaching (using hydrogen peroxide and sodium hypochlorite) and

346 deodorising (to remove short-chain acid) (Wafti et al. 2010). However, palm acid oil

347 (PAO) and palm fatty acid distilled (PFAD) has better quality as feedstock for the

348 refining process to yield the same products (Wan Nawawi et al. 2010).

349 Potential SPO Products: Biodiesel

350 In lipid technology, transesterification is the chemical process involved in converting

351 FFA into FAME. However, it only works well if the FFA content is below 2% (Liu

352 1994). Above an FFA content of 2%, soap is generated, preventing the separation

353 between glycerine and ester (Canakci and Gerpen 2001). The high content of FFA in

354 SPO, therefore, needs to be reduced through an esterification process before the

355 transesterification process can be undertaken. The esterification process reduces the

356 FFA content to around 2%.

357 The esterification process for FFA reduction usually involves short chain

358 alcohols such as methanol or ethanol in the presence of an acid catalyst. There are two

359 groups of catalysts, namely homogeneous and heterogeneous acids. Homogeneous acids

360 such as sulphuric acid are the most commonly used in SPO esterification compared to

361 heterogeneous acids because they are less costly (Jain and Sharma 2010; Hayyan et al.

362 2013). However, homogeneous acid catalysts require a high alcohol molar ratio and a

363 longer reaction time to convert FFA to FAME (Jain et al. 2011), while heterogeneous

364 acids such as PTSA are more desirable due to their higher catalytic activity (Guan et al.

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365 2009a), no soap is produced and FAME is more easily separated from the product

366 mixture (Guan et al. 2009b; Thinnakorn and Tscheikuna 2014). The major drawback of

367 the heterogeneous catalyst is the resistance in mass transfer among three phases (oil,

368 catalyst and methanol) during the conversion process, but this shortcoming can be

369 overcome by increasing the stirrer speed (Thinnakorn and Tscheikuna 2014b).

370 Another commonly used heterogeneous catalyst is an enzyme called Candida

371 cylindracea lipase (Ricca et al. 2013), which can convert SPO into FAME. The

372 advantages of using an enzyme catalyst are that it can be a catalyst for both

373 esterification and transesterification processes (Yan et al. 2012), easy separation of

374 FAME and glycerol, and it requires less alcohol (Chen et al. 2011; Yan et al. 2012;

375 Zheng et al. 2012). The disadvantage of using an enzyme catalyst is the high cost of the

376 enzyme (Ricca et al. 2013). Also, a higher enzyme loading, which is favourable for

377 biodiesel synthesis, results in unfavourably high water content in the product

378 (Nasaruddin et al. 2014).

379 The effectiveness of the esterification process is greatly influenced by the

380 feedstock’s acid content, the alcohol to oil molar ratio, the reaction time, the reaction

381 temperature, and the stirrer speed (Hayyan et al. 2010b). The optimum parameters are

382 desirable to reduce alcohol use and reaction time. Table 5 lists publications describing

383 the conversion of SPO into biodiesel using various parameters and catalysts. The

384 shortest reaction time for esterification process was achieved through the use of a

385 heterogeneous acid called trifluoromethanesulfonic acid (TFMSA) at 0.75% (w/w), 10:1

386 molar ratio of methanol to SPO, and 60°C reaction temperature for 40 minutes. Almost

387 similar efficiency was achieved by using toluene-4-sulfonic monohydrate acid (PTSA)

388 for 60 minutes, instead of the 40 minutes of TFMSA. A less expensive option is through

389 the use of homogeneous acids is achieved by using 0.75% (w/w) sulphuric acid and 8:1

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390 molar ratio methanol at 60°C for 60 minutes. The usage of an enzyme in SPO

391 conversion into biodiesel required longer reaction time but at lower ethanol

392 concentration and need further investigation.

393394 Table 5: The esterification process parameter to convert SPO into biodiesel395396 Each method has its merit. Increasing the acid content does not improve the

397 esterification process further as the optimum parameter is adequate to produce the same

398 yield amount. This can be explained by its chemical kinetics, which is already at

399 equilibrium. This is the same as increasing reaction time, reaction temperature, and

400 stirrer speed. While having a higher molar ratio of alcohol can prevent the reversible

401 reaction, this will cause waste as more alcohol is needed, and it will require a long

402 separation time to remove the alcohol.

403 Following esterification, the SPO will contain less than 2% FFA, which allows

404 the transesterification to proceed. There is not much variability in the choice of methods

405 and catalysts that can be selected for the transesterification process. Typically, 1%

406 (w/w) potassium hydroxide is used as a catalyst, with a methanol molar ratio of 10:1

407 and a reaction temperature of 60°C for 60 minutes to produce FAME that meets the EN

408 14214 and ASTM D6751 requirements (Hayyan et al. 2010a; Hayyan et al. 2011;

409 Hayyan et al. 2013; Hayyan et al. 2010b).

410 Potential SPO Product: Burner fuel411

412 SPO can also be used directly in a burner as a fuel alternative. In a recent study (Zuber

413 et al. 2018), measured the average calorific value of SPO and compared it to diesel,

414 concluding that SPO has on average, a lower calorific value (38MJ/kg) than diesel

415 (45MJ/kg). SPO’s viscosity is much higher than that of diesel and is often semi-solid

416 and unable to flow or spray properly. Hence, heating the SPO will increase its

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417 flowability by reducing the viscosity significantly. During the combustion process, it

418 has been reported that the flame temperatures of SPO were lower than that of diesel,

419 resulting in lower emissions of nitrogen oxide (NOx) and carbon monoxide (CO)

420 compared to diesel. This study indicated that SPO could be used as fuel without the

421 need to convert into biodiesel as it only required heating to reduce the viscosity.

422 Palm acid oil (PAO)

423 Palm acid oil is a by-product of the chemical refining process of CPO and is widely

424 used in laundry soap and calcium soaps for animal feed (Kuntom et al. 1994). During

425 the chemical refining process of CPO using alkaline solution, soapstock will be

426 produced as a by-product. The soapstock contains emulsified neutral oil which can

427 easily be separated from the soapstock due to different densities. The acidification

428 process will then turn the emulsified neutral oil into PAO.

429 The Malaysia Palm Oil Board (MPOB) classifies PAO as a waste from the

430 chemical CPO refining process, whereas palm fatty acid distilled (PFAD) comes from a

431 physical method. Since the chemical refining process uses alkaline, PAO is less acidic

432 compared to PFAD (Kuntom et al. 1994). Since most palm oil refining in Malaysia uses

433 the physical refining process, the production of PAO is smaller compared to that of

434 PFAD.

435 PAO is a hydrophobic compound with an energy content of 38MJ/kg (Khan et

436 al. 2015), which consists of free fatty acid, neutral oil and moisture (Kuntom et al.

437 1994). According to the Palm Oil Refinery Association of Malaysia (PORAM), FFA

438 content in PAO is above 50%. Kuntom et al. (1994) reported that FFA content in PAO

439 is more than 50%, with palmitic acid as its major component. The characterisations of

440 PAO are shown in Table 6 summarize from Kuntom et al. (1994).

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441 Table 6: PAO characterisations (Kuntom et al. 1994)

442 Potential PAO products

443 Besides converted into biodiesel through the esterification and transesterification

444 process, PAO is also applied as a coating material to upgraded low-rank coal. The

445 upgraded coal shows higher resistance to moisture reabsorption, greater compressive

446 strength, the lower inclination of low-temperature oxidation and lower likelihood for

447 impromptu combustion (Khan et al. 2015).

448 Palm Fatty Acid Distillate (PFAD)

449 During the physical refining process of CPO, FFA and approximately 4 to 5% PFAD is

450 produced as a by-product (Wicke et al. 2008; Gapor Md Top 2010). PFAD’s market

451 price is around 15% lower than CPO (ZERO and Rainforest Foundation Norway 2016).

452 According to Gapor Md Top (2010) and Bonnie and Yusof (2009), PFAD is a light

453 brown semi-solid compound at room temperature which becomes brown liquid when

454 heated. Table 7 details the characteristics of PFAD summarize from Bonnie and Yusof

455 (2009), where it shows that PFAD has an FFA content of more than 70%, consisting

456 mainly of palmitic and oleic acids. Palmitic acid is the primary saturated acid at about

457 38.63 to 45.30%, while oleic acid constitutes the major unsaturated acid at about 33.54

458 to 44.05% (Chang et al. 2016). Other minor trace elements in PFAD are glycerides

459 (monoglyceride, diglyceride, and triglyceride) and trace metals (Cr, Fe, Ni, and Cu)

460 (Bonnie and Yusof 2009). Meanwhile, Gapor Md Top (2010) reported that moisture and

461 impurities were recorded at around 1%, while the saponifiable matter was at around

462 95%.

463 Table 7: PFAD characteristics (Bonnie and Yusof 2009)

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464 Potential PFAD Product: Biodiesel

465 PFAD also can be utilised to produce encapsulated vitamin E, squalene, and

466 phytosterols (Top et al. 1988, 2001; Gapor Md Top 2010). Hydrogenated PFAD can be

467 used as animal feed (Gapor Md Top 2010), and other products derived from PFAD

468 include food emulsifiers, flavouring and aromatic agents, product from the

469 oleochemical industry such as candles, and pharmaceutical products (Top et al. 2001;

470 Wicke et al. 2008; Bonnie and Yusof 2009; Gapor Md Top 2010; Chang et al. 2016;

471 ZERO and Rainforest Foundation Norway 2016). The utilisation of PFAD as feedstock

472 for FAME faces fierce competition from many industries, rendering the conversion of

473 PFAD to FAME less economically feasible (ZERO and Rainforest Foundation Norway

474 2016). Where PFAD serves a market of its own, it is wise not to disturb this market with

475 biofuel production to maintain its price and demand (Gapor Md Top 2010)

476 PFAD has the potential as a feedstock for biodiesel production. A study by

477 Chongkhong et al. (2007) using the esterification process with sulphuric acid (1.834%

478 w/w) and methanol (8:1) at 60 minutes and 70°C was found to be the optimum

479 parameter which resulted in FFA reduction from 93% to less than 2%. This procedure

480 was then followed by the transesterification process with sodium hydroxide and

481 methanol at 65°C for 15 minutes that resulted in the biodiesel produced meeting the

482 ASTM D6751-02 and Thai biodiesel quality standards. Since PFAD has a high FFA

483 content, esterification and transesterification processes are suitable approaches for

484 biodiesel conversion, similar to SPO and PAO.

485 The Rainforest Foundation Norway (ZERO and Rainforest Foundation Norway

486 2016) assessed the relevance of using PFAD as a biodiesel feedstock and found that the

487 triglycerides and fatty acids in PFAD are useful sources for renewable hydrotreated

488 vegetable oil (HVO) production. The low price and the nature of PFAD as a residue

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489 allow for the manufacturing and sale of HVO from PFAD to receive incentives and tax

490 exemptions in Norway.

491 Concluding remarks

492 Wastewater from the palm oil industry has the potential to be used as feedstock for

493 biofuel production. POME can be treated through various methods and processes to

494 produce biogas. Also, residue oils such as SPO, PAO, and PFAD, can be processed to

495 produce biodiesel, products such as animal feed, food flavouring, and laundry soap, as

496 well as products from the oleochemical and pharmaceutical industries. Treated SPO can

497 even be directly used as fuel for combustion. PAO and PFAD can be treated and utilised

498 as a biodiesel feedstock. However, only limited research on SPO, PAO, and PFAD has

499 been completed to date.

500 Since PFAD and PAO have been fully utilised in the industry and have specific

501 market demands, it is wise to avoid using both in any other particular products, especially

502 in biofuel and energy production. Since POME can be used to produce biogas, this leaves

503 SPO without a demand in the industry, opening up opportunities to make use of SPO for

504 biofuel and biodiesel production. The lower price and abundant availability of SPO

505 without any other major market demands make SPO a prime candidate to be used as fuel

506 and raw material for biodiesel production.

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961

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Figures 1

2

Figure 1: Plantation area of palm oil industry in Malaysia (Awalludin et al. 2015; MPOB 3

2017b) 4

5

6

Figure 2: Production of crude palm oil (CPO) (MPOB 2012, MPOB 2017c; Awalludin et 7

al. 2015) 8

0

1

2

3

4

5

61

96

0

19

65

19

70

19

75

19

80

19

85

19

90

19

95

20

00

20

05

20

10

20

15

20

16

Pal

m o

il p

lan

tati

on

are

a in

Mal

aysi

a(M

illio

n H

ecta

res)

Year

0

5

10

15

20

25

20

00

20

01

20

02

20

03

20

04

20

05

20

06

20

07

20

08

20

09

20

10

20

11

20

12

20

13

20

14

20

15

20

16

Pal

m o

il p

rod

uct

ion

in M

alay

sia

(Mill

ion

to

nn

es)

Year

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9

10

Figure 3: Summary of palm oil process 11

Digester

Crude palm oil

Refining

Kernel

Chemical process alkali

Physical process steam

Palm kernel oil

Plantation

Sterilization

Fresh fruit bunch

Stripping

PKS + waste water

Fiber + waste water

Waste water

EFB

Process and product

Waste

LEGEND

Waste water

PFAD PAO

OPF

Digester Press cake

Separation

Nut cracking

Separation

Fiber

Potential Renewable Energy (Biogas, Biodiesel, Biofuel)

POME: Biogas SPO: Biodiesel, Fuel



PFAD: Biodiesel

PAO: Biodiesel

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1 Tables

2 Table 1: POME characteristic Concentration (mg L-1 except pH)

Parameter Ahmad and Chan (2009)

Wong et al. (2009)

Chin et al. (2013)

Norfadilah et al. (2016)

pH 4.7 4.15-4.45 4-5 3.4Oil and grease 4,000 1077-7582 4000-9341 -BOD 25,000 21500-

2850025000-65714

37750

COD 50,000 45000-65000

44300-102696

69500

Total solids 40,500 - 40500-72058

-

Suspended solids

18,000 15660-23560

18000-46011

47690

Total volatile solids

34,000 - 34000-49300

30870

Ammoniacal nitrogen

35 - 35-103 -

Total nitrogen 750 - 750-770 -

3

4 Table 2: Low cost coagulant and adsorbent used in POME treatmentCoagulant Reference Adsorbent ReferencePolyaluminium Chloride

(Othman et al. 2014; Poh et al. 2014)

Banana peel (Mohammed and Chong 2014)

Mango pit (Asadullah and Rathnasiri 2015)

Natural clay (Said et al. 2016)

Chitosan (Ahmad et al. 2006; Parthasarathy et al. 2016)

Resin (Bello et al. 2013; Bello et al. 2014)

Alum (Malakahmad and Chuan 2013)

Activated carbon (Azmi and Yunos 2014; Mohammed et al. 2014; Othman et al. 2014; Alkhatib et al. 2015)

56

7

8

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9 Table 3: Comparison of Bio-CNG to biogas and natural gas (Nasrin 2016; Mohtar et al., 10 2017; Hosseini and Wahid 2015)

Properties Methane (%)

Carbon dioxide (%)

Hydrogen sulphide (ppm)

Pressure (MPa)

Calorific value

(MJ/Nm3)Biogas (POME)

55-65 35-40 2500-4000 0.0002-0.0005

21-23

Bio-CNG >94 <4 <10 25 35.95Natural gas >92 <2 <3 25 36.14-

36.6111

12 Table 4: SPO characteristics (Ainie et al. 1995)Parameter ValueFree fatty acid (FFA) (%) 44.43Iodine value (meg/kg) 49.81Peroxide value (meg/kg) 9.98Moisture (%) 0.99Saponification value (mg) (KOH/g) 197.47Unsaponifiable matter (%) 0.35

13

14 Table 5: The esterification process parameter to convert SPO into biodiesel Reference Catalyst Alcohol Reaction

time (minute)

Reaction temperature (°C)

Stirrer speed (rpm)

(Hayyan et al. 2011)

Sulphuric acid (0.75% w/w)

Methanol (8:1)

60 60 400

(Škrbić et al. 2015)

Sulphuric acid (4.6% w/w)

Methanol (6:1)

120 65 -

(Hayyan et al. 2010b)

PTSA (0.75% w/w)

Methanol (10:1)

60 60 -

(Hayyan et al. 2010a)

PTSA (0.75% w/w)

Methanol (10:1)

60 60 400

(Abd. Wafti et al. 2012)

PTSA (0.35% w/w)

Methanol (1:1)

60 65

(Hayyan et al. 2013)

TFMSA (0.75% w/w)

Methanol (10:1)

40 60 -

(Nasaruddin et al. 2014)

Candida cylindracea lipase (10U/25g of SPO)

Ethanol (4:1)

- - -

(Ricca et al. 2013)

Candida cylindracea lipase (10U/25g of SPO)

Ethanol (4:1)

1440 40 250

1516

17

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18 Table 6: PAO characteristics (Kuntom et al. 1994)Parameter ValueFree fatty acid (FFA) (%) 50Iodine value (meg/kg) 40-50Peroxide value (meg/kg) <5Moisture (%) <2Saponification value 72-197Unsaponifiable matter 0.04-1.67

19

20 Table 7: PFAD characteristics (Bonnie and Yusof 2009)Parameter ValueIodine value (g/100 g) 46.3-57.6FFA (palmitic, %) 72.7-92.6Titre (°C) 46-48Water content (%) 0.3-0.24Saponifiable value (mg KOH g-1 of sample) 200.3-215.4Unsaponifiable matter (%) 1.0-2.5

21

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