Biodiesel Production from Palm Oil, Its By-Products, and Mill Effluent: A Review

: The sustainability of petroleum-based fuel supply has gained broad attention from the global community due to the increase of usage in various sectors, depletion of petroleum resources, and uncertain around crude oil market prices. Additionally, environmental problems have also arisen from the increasing emissions of harmful pollutants and greenhouse gases. Therefore, the use of clean energy sources including biodiesel is crucial. Biodiesel is mainly produced from unlimited natural resources through a transesteriﬁcation process. It presents various advantages over petro-diesel; for instance, it is non-toxic, biodegradable, and contains less air pollutant per net energy produced with low sulphur and aromatic content, apart from being safe. Considering the importance of this topic, this paper focuses on the use of palm oil, its by-products, and mill efﬂuent for biodiesel production. Palm oil is known as an excellent raw material because biodiesel has similar properties to the regular petro-diesel. Due to the debate on the usage of palm oil as food versus fuel, extensive studies have been conducted to utilise its by-products and mill efﬂuent as raw materials. This paper also discusses the properties of biodiesel, the difference between palm-biodiesel and other biodiesel sources, and the feasibility of using palm oil as a primary source for future alternative and sustainable energy sources


Introduction
Biodiesel produced from different triglyceride sources is an alternative fuel to petro-diesel.The American Society for Testing and Materials (ASTM) defines biodiesel as mono-alkyl esters produced from various lipid feedstocks including vegetable oils, animal fats, etc.Furthermore, it has been accepted as a fuel and fuel additive worldwide and registered with the U.S. Environmental Protection Agency (EPA).Owing to the worries about petroleum availability and the current increase in petroleum price, the usage of biodiesel in conventional diesel engines has attracted much attention.The history began in the 1900s when Sir Rudolf Diesel successfully run conventional diesel engines using vegetable oil without any modification.In the 1930s and 1940s, vegetable oil was utilised as diesel fuel, particularly in the emergencies [1].However, further investigation has verified that the direct usage of vegetable and animal oils as diesel fuel is impractical due to their large molecular mass, low volatility, and high kinematic viscosity, which reduce the performance of the engine and raise other problems including thickening, gelling, and sticking of the oil [2].To overcome these problems and allow its application as a fuel, several methods have been implemented such as blending with petro-diesel, microemulsification, pyrolysis, and transesterification, as summarised in Table 1 [3][4][5][6][7][8].
Then the resulted oil-diesel mixture was applied into the diesel engine.
Does not required any chemical process (non-polluting), absence of technical modifications, and easy implementation.
High viscosity, unstable, low volatility, and increase in vegetable/animal oil portion resulted in improper spraying pattern, poor atomization, incomplete fuel combustion, and difficulty in handling by conventional engines.

Microemulsification
The vegetable/animal oils were solubilized in a solvent (alcohol) and surfactant until the required viscosity was obtained.
Simple process and pollution free.
High viscosity, low stability (the addition of ethanol can enhance the quantity of surfactant required to maintain the state of microemulsion), and could lead to sticking, incomplete combustion, and carbon deposition.
Pyrolysis (thermal cracking) The vegetable/animal oils were preheated and decomposed at elevated temperature (more than 350 • C) whether or not the catalyst is present.Different products (gas and liquid) were analysed based on their boiling temperature range to determine the exact product.
The process is effective, simple (not required washing, drying or filtering), wasteless, and pollution free.
Required high temperature and expensive equipment and produce low purity of biodiesel (contain heterogeneous molecules including ash and carbon residues).

Transesterification
The vegetable/animal oils and fats were reacted with alcohol (ethanol or methanol) and catalyst (alkali or acid).Then the mixture of methyl/ethyl esters (biodiesel) and glycerol (byproduct) will undergo separation and purification steps before further usage.
High conversion with relatively low cost, mild reaction conditions, product properties are closer to the petro-diesel, and applicable for industrial-scale production.
Required low free fatty acids (FFAs) and water content in the raw material, extensive separation and purification steps, possibilities of side reaction to occur, and generation of a large amount of wastewater.

Biodiesel Production Routes
Among the various methods, the most common method used in producing biodiesel is a transesterification process.In this process, lipid feedstocks are converted into biodiesel.One mole of triglyceride reacts with three moles of alcohol to produce three moles of mono-alkyl ester and one mole of glycerol.To improve the rate of reaction and biodiesel yield, a catalyst is usually added with excess alcohol, which shifts the equilibrium to the product side since the reaction is reversible [1,9,10].Traditionally, the transesterification process utilises solvents including ethanol or methanol and homogeneous catalysts such as KOH, NaOH, and H 2 SO 4 [11][12][13].However, this method has several shortcomings such as extensive separation process, wastewater generation, and equipment corrosion [13][14][15][16].To overcome these problems, various studies have been conducted such as the implementation of heterogeneous catalysts.To date, many types of heterogeneous catalysts such as solid acid and base, zeolite, and polymer have been widely utilised so that the recovery process becomes straightforward and the purification steps are minimised [17][18][19][20].Nonetheless, as compared to homogeneous catalysts, heterogeneous catalysts are less effective due to long reaction time (up to 24 h), low reaction and conversion rate, easy deactivation, and high viscosity that increases the mass transfer resistance.In addition, heterogeneous catalysts also require high reaction temperature, pressure, catalyst concentration, and alcohol-to-oil molar ratio, which basically lead to the increment in the overall production cost [13,14,16,21,22].
The use of biocatalysts, i.e., enzymes and microbial cells, in the transesterification reaction is also possible [23,24].For instance, the use of Pichia pastoris yeast whole cell catalyst was implemented for biodiesel production from waste cooking oils [23].The study found that up to 82% biodiesel production could be obtained within 84 h.Alternatively, lipase can also be used as a biocatalyst for production of biodiesel [24].This process takes place in two steps involving the hydrolysis of an ester bond and esterification with the second substrate.The implementation of the biocatalyst for biodiesel productions remains challenging since it has several issues such as high preparation cost, reusability issues, low stability, and durability of the native enzymes [25][26][27].
Recently, a new type of chemical, the so-called ionic liquids (ILs), and non-catalysed routes under supercritical conditions have gained attraction among researchers [28][29][30][31].Even though both strategies have been successfully performed on a laboratory scale, their industrial acceptance and application are still limited due to their high capital and operational cost, non-conformity of the recovery methods, low reusability of ILs, and several technical and operational challenges, including the requirement of heating instrument and high-pressure vessels resulting in high energy demand [32][33][34][35][36]. Thus, the transesterification using homogeneous catalyst is still the most favourable and applicable process for producing biodiesel on an industrial scale.

Properties of Biodiesel
In recent years, the research, development, and commercialisation of biodiesel have been boosted due to the urgency of finding the best solution to the world energy crisis.We cannot continue to rely heavily on crude petroleum as the primary source of transportation fuels and electricity.Although many alternative energies such as solar, wind, biomass, and geothermal, only biofuel or biodiesel can be used on a large scale, especially for transportation, due to its reliability and economic feasibility [37].Moreover, biodiesel presents several advantages such as non-toxicity and eco-friendly properties, compatibility with existing diesel engines without extensive engine modifications, and the huge amount of renewable sources currently available worldwide [38,39].The quality of biodiesel can be evaluated on the basis of two major guidelines, i.e., the U.S. Specification (ASTM Standard) and the European Specification (EN Standard).Table 2 summarises the major properties of biodiesel based on both standards.
The use of biodiesel significantly diminishes the emissions of harmful GHG, particulate matter, and hydrocarbons but slightly rises fuel consumption and reduces the engine power.Although NOx emissions are increased in some cases, this can be minimised using exhaust gas recirculation (EGR) or other additives [40].A comparison of biodiesel properties is tabulated in Tables 3 and 4. It was confirmed that biodiesel produced from natural and renewable resources is an excellent alternative for existing petro-diesel, especially for transportation.As shown in Table 3, biodiesel has superior properties to petro-diesel in many areas: for example, higher cetane number (a significant advantage regarding engine performance and emissions), low ash content, and low carbon residue, whereas the other properties can be improved by using blending processes [41].Moreover, Table 4 shows an interesting feature of biodiesel (rapeseed methyl ester) due to low exhaust emissions of CO and NOx, which is one of the major concerns for any engine or fuel [42].Datta and Mandal [43] reviewed the effects of various types of biodiesel on major performance parameters of compression ignition engine.When biodiesels were used instead of petro-diesel, they discovered that brake fuel consumption and thermal efficiency were generally decreased.On the other hand, exhaust gas temperature and emissions of CO, hydrocarbon, and smoke reduced with the use of biodiesel.[4,9,15,17,32,38,40,41].

ASTM D6751 EN 14214
Flash point, min (  In short, biodiesel has attracted much attention due to its various environmental benefits.However, the main challenges include its production cost and availability of suitable raw materials.Although biodiesel can be synthesised from different types of raw materials, their price and availability are among the most significant factors that must be considered.Aarthy et al. [44] mentioned that the raw materials account for 60-80% of the total production cost of biodiesel.High production cost affects the processing economic feasibility and selling price of biodiesel.For instance, the current selling price of biodiesel (B99-B100) in the USA is around 15-30% higher than that of diesel, as shown in Table 5.Despite the ongoing progress of the biodiesel industries, their sustainability may be restrained by the difficulty in securing cheap raw materials.Minimising the production cost is crucial so that the biodiesel selling price can be more competitive.Thus, this paper focused on the utilisation of palm oil, its by-products, and mill effluent as raw materials for producing biodiesel.It is believed that the biodiesel production cost can be reduced by using wastes apart from improving the quality of the environment.

Parametric Effects for Biodiesel Production
In the biodiesel production process, many parameters can affect the yield.Examples include reactor type, agitation speed, reaction time, temperature, catalyst type and concentration, solvent-to-oil ratio, solvent type, and types of raw materials.Table 6 summarises some of the important parameters which have been put forward by previous studies and can be used as guidelines for future studies.Table 6 also shows that the most commonly-used solvent and catalyst are methanol and NaOH, respectively.According to Ejikeme et al. [46] and Leong et al. [47], NaOH is well-accepted by biodiesel producers because of its low price, corrosivity, and temperature requirements, in addition to a high conversion rate.These properties are especially true for vegetable oils.On the other hand, although short-chain alcohols like ethanol, methanol, butanol, and amyl alcohol can be applied as solvents, methanol is the most favoured due to its greater polarity, easy recovery, and cheapness, apart from being able to facilitate higher reaction rates [48,49].
Additionally, the selection of the processing parameters, especially solvent and catalyst types, is mainly influenced by the types of raw materials because different raw materials consist of different types of components such as free fatty acids (FFAs), contaminants, etc. [50,51].This can be observed from a study reported by Halim and Kamaruddin [52], who compared the biodiesel production using palm oil and waste cooking oil.They stated that the production yield from waste cooking oil was lower than palm oil due to the high amount of water content which resulted in substrate hydrolysis and hence reduced the yield [53,54].Thus, apart from the cost and availability of raw material, many other factors related to the raw material selection need to be considered to ensure a high yield and quality of biodiesel.

Types of Raw Materials
Vegetable oil has been identified as the best raw material due to its sustainability, renewability, high-energy content, and energy security, which are almost similar to the petro-diesel [58].Biodiesel produced from vegetable oils also very much alike to regular diesel, which can be utilised as a fuel blend or substitute.Moreover, various studies have been performed to explore other potential and alternative (relatively cheap) raw materials aiming to reduce production cost, such as non-edible oils [59][60][61], microbial lipid and cellular biomass [62,63], animal waste and fat [64,65], waste cooking oil [66][67][68], municipal waste and sewage sludge [50,69], rice straw and bran [70,71], and grease waste [72,73].Although the use of various wastes could mitigate harmful environmental problems, the reaction processes could lead to the side formation of soaps, low production quality and yield, complicated downstream processes, and difficulty in the recovery of the glycerol formed, especially when homogeneous alkaline catalysts are applied [74][75][76].This is due to the presence of a high amount of FFAs, contaminants, and water content in the waste materials [77].
On top of that, Ribeiro et al. [78] stated that the initial FFAs content could have a dominant effect on the biodiesel production when alkaline catalysts are applied.Besides, it has been highlighted that when the concentration of FFAs is high, the mass production is reduced, and the formation of soap is favoured.In fact, alkali-catalysed transesterification could be only performed when the FFAs content is less than 3%, whereas acid-catalysed reaction is preferable for raw materials containing high FFAs, which are low in grade and cost-effective.Additionally, when dealing with high FFAs raw materials (>3%), a two-step transesterification process is employed.This technique involves an acid-catalysed reaction as the first step to esterify the FFAs to fatty acids methyl esters (FAMEs) followed by the second step using the alkali-catalysed transesterification process.It has been reported that the two-step process using alkali-catalysed reaction in both steps can maximise the overall yield at room temperature.Thus, the proper FFAs level in the raw materials should be between 0.5% and 3% for alkali-catalysed transesterification to achieve the desired conversion rate [79].
Table 7 summarises the list of raw materials utilised in the transesterification process together with their percentage of fatty acid composition [44,48,64,65,69,[80][81][82][83][84][85][86][87][88][89][90][91]  According to Katre et al. [84], the raw material used for producing biodiesel should present a large amount of long-chain monounsaturated fatty acids (MUFAs) and a small amount of polyunsaturated fatty acids (PUFAs) containing ≥4 double bonds.Biodiesel produced through the transesterification of these types of fatty acids show numerous advantages over petro-diesel such as low emissions of CO, CO 2 , hydrocarbons, and unwanted particles.However, such biodiesel has several problems including low cetane number, poor cold flow properties, high viscosity, and low oxidative stability [92,93].In contrast, long chain and saturated fatty acids give a high cetane number value that increases along with the fatty acids chain length and saturation.Hence, biodiesel derived from raw materials with a high amount of saturated fatty acids demonstrates better cold start and flow properties, while reducing NOx exhaust emissions [85,94].

Historical Perspective
Among all existing raw materials, palm oil presents a high amount of palmitic and oleic acids that have been well known as the most suitable sources for biodiesel production.Owing to the geographical factor, each region currently prefers to utilise different sources such as soybean oil in the USA, Argentina, and Brazil, rapeseed oil in EU, and palm oil in most countries in Asia [95,96].Several studies have highlighted the economic feasibility of palm oil as a primary raw material.For instance, Yusuf et al. [96] reported that the price of palm-biodiesel produced in Malaysia is still competitively priced in the EU.Meanwhile, Shahbazi et al. [97] revealed that the price of palm oil originated from Malaysia can compete with the domestic cultivated oil crops price in the Middle East.Furthermore, palm oil possesses significant advantages due to its high oil content [98,99] with low market price as tabulated in Table 8, abundant resources and high production capacity, accounted for one-third of the total vegetable oil production worldwide, and needs a minimal plantation area compared to other oil crops [96][97][98][99][100][101].In addition, palm oil is a type of perennial crop unlike other oil sources, and its production is continuous throughout the year [102][103][104].Moreover, palm oil is the most versatile vegetable oil due to its numerous uses in various industries [105].Palm oil originated from West Africa; but, since the late of 20th century, most palm oils plants are planted in Southeast Asia.In the middle of the 15th century, palm oil was utilised as a food source by European explorers to West Africa.During the British industrial revolution in the 18th century, the demand for palm oil increased for candle-making and as a lubricant for machines [105,107].Two main products of its fruits are kernel oil from the kernel within the nut and oil from the outer mesocarp [99,103].According to Demirbas [108], the fatty acid content is the major difference between both products.The oil from outer mesocarp is mainly rich in palmitic and oleic acids (about 50% saturated fat) that are very beneficial for biodiesel production, whereas palm kernel oil is rich in lauric acid (more than 89% saturated fat).
Most of the main producers of palm oil such as Malaysia and Indonesia (who supply approximately 80-85% of global capacity) have extensively developed several processes to convert palm oil, its by-products and mill effluent into biodiesel [99,103,104,[109][110][111].For instance, in Malaysia, the Malaysian Palm Oil Board (MPOB) is launched since the 1980s and becomes the forefront of palm-biodiesel research and development.The MPOB has successfully established several methods to produce methyl esters for biodiesel from crude palm oil (CPO) and its by-products [99].Additionally, palm-biodiesel has become more attractive since, based on the current practices in the Malaysian palm oil industry, the use of palm-biodiesel can typically contribute to GHG emissions saving of 50-70% compared to petro-diesel [112].

Benefits and Characteristics of Palm-Biodiesel
Together with MPOB, Mercedes-Benz, and Cycle & Carriage in June 1990 until July 1995, Choo et al. [109] recorded a comprehensive field investigation using palm-biodiesel as a diesel fuel on 30 Mercedes-Benz buses with OF 1313 chassis and OM 352 engines.Each bus managed to cover ranges of up to 300,000 to 351,000 km.Their study found that the OF 1313 with OM 352 engines could be operated well with neat or blended palm-biodiesel although the engines are designed for petro-diesel (no modification required).This applies to the long-term engine operation and engine performance, which can be translated to other direct-injection engine modules.Besides, they found that the engines studied were observed with smooth and no knocking sound when starting.Within the mileage recommended by the manufacturer, the engine oil was seen in good condition, suggesting its practical usability.Moreover, much cleaner exhaust emissions were recorded with normal carbon build-up in the engine nozzles and comparable fuel consumption over petro-diesel.On top of that, the palm-biodiesel did not produce explosive vapour due to the higher flash point.Nevertheless, it demonstrated a reaction with the binding material in cement floors apart from attacking hoses and seals, which are of low-grade rubber and plastic products.
Various studies have been conducted to compare the biodiesel production from palm oil with other types of raw materials.Likozar and Levec [113] examined the transesterification of various oils into biodiesel.They discovered that soybean oil achieved the highest overall FAME conversion rate during the mass transfer-controlled stage due to high initial diglyceride content.However, as far as the chemical equilibrium was concerned, the highest final conversion was determined when palm oil was used, which is related to the larger extent of triglyceride transesterification.Other than that, palm oil is also known as a good raw material as the biodiesel has the same properties as regular petro-diesel [114].Additionally, Talukder et al. [115] applied a two-step esterification process using CPO for producing biodiesel and discovered 98% biodiesel yield within two hours reaction period.
Based on Table 9, biodiesel produced from palm oil displayed better properties compared to other types of biodiesel especially regarding cetane number and iodine value [48,84,86,94,102,116,117]. One of the most important properties of fuel is cetane number.High cetane number implies a short ignition delay that can affect the quality of combustion [94].Higher cetane number represents a significant advantage especially concerning clean emissions and engine performance.Palm-biodiesel having a high cetane number is crucial to ensure the biodiesel-fuelled engines will operate smoothly with less noise [118].Several other important properties include kinematic viscosity (that affects the flow, spray, atomization, and combustion process), iodine value (indicates the degree of saturation, thus affecting the melting points, oxidative stability, and storage quality), saponification number (represents the amount of fatty acids that can promote the formation of soap), and higher heating value (depends on the iodine value and saponification number).Nursal et al. [119] observed the performance of engine and characteristics of exhaust gas emissions for three types of biodiesel derived from CPO, waste cooking oil (WCO), and Jatropha curcas oil (JCO) in a marine auxiliary diesel engine.The study utilised 5% (v/v) blending ratio with 0%, 50%, and 90% load conditions throughout the engine speeds of 800, 1200, 1600, and 2000 rpm.Their study found that the overall engine performance of palm-biodiesel was enhanced, the brake thermal efficiency was slightly improved, and the brake specific fuel consumption and exhaust gas emissions were reduced compared to petro-diesel.In addition, the reduction of CO, CO 2 , and HC by palm-biodiesel as well as slight increment in CO 2 , NOx, and HC by JCO-biodiesel were observed in the study.
Furthermore, Vieira da Silva et al. [120] carried out another study and recorded a significant decrease in NOx emissions from palm-biodiesel compared to WCO-biodiesel, with lower or similar emissions to petro-diesel when blended with 20% and 50% of palm-biodiesel.Moreover, it was seen by Abu-Hamdeh and Alnefaie [121] that petro-diesel gave higher emissions compared to that of B10, B30, and B50 at various torques and constant engine speed.Meanwhile, in a reduced steady-state emissions test, Ng et al. [122] performed a study using constant speed and load values that represent on-road driving condition and reported a 5% decrease in NOx emissions for B100.Sharon et al. [123] also reported that palm-biodiesel had presented better results compared to other types of biodiesel as shown in Table 10 [43,124,125].Besides, it was observed by Choo et al. [109] that palm-biodiesel displayed very good storage properties with slight degradation in the fuel characteristics.However, the fuel colour changed from orange to light yellow after storing for more than six months.The change in colour can be explained by the breakdown of carotenes in the methyl esters.Moreover, it was depicted that palm-biodiesel presented high flash point, indicating good properties for storage and transportation with a minimum product yield of 96.5%, which complied with the European Standard on Biodiesel (EN14214).Furthermore, a patented technology was reported in 2002 to solve the palm-biodiesel pour point problem (the temperature of +15 • C means the product can only be used in tropical countries), thus palm-biodiesel became a more versatile product.As a result, the palm-biodiesel produced with low pour points (winter grade with a temperature from −21 to 0 • C) successfully met the seasonal requirements and can now be utilised by temperate countries users.Additionally, Choo et al. [126] added that low pour point palm-biodiesel demonstrated comparable fuel properties to petro-diesel, besides having good low-temperature flow characteristics.
In short, to be used as a raw material for producing biodiesel, the targeted vegetable oils must be available at a competitive price.It is apparent that most biodiesel producers agree about the significant potential of palm oil to meet this criterion with its broad applications.In fact, palm oil has been noted as the most price efficient oil in its production cost among other vegetable oils and can be easily supplied by replanting the plant with the minimum land requirement, fertiliser, water, and pesticide [111,127].Furthermore, the productivity, efficiency, and yield factors of the palm oil have made it preferable than other oils and fats [103,111].When industrialists are considering vegetable oils as renewable sources for biodiesel production, palm oil will be prominent among other types of vegetable oil to meet the expanding demand for greener and cleaner energy.Various studies have been conducted using different raw materials and synthesis routes to evaluate the sustainability of palm-biodiesel by comparing it with other types of biodiesel.For example, Kurnia et al. [128] carried out a sustainability analysis on biodiesel and suggested that more studies should be done with innovative ideas to enhance the properties of biodiesel so that environmental and social impacts could be minimised besides making it more compatible with the existing diesel engines.

Prospects and Directions of Palm-Biodiesel
Palm-biodiesel seems to be prospering, with enormous interest in other countries.Ashnani et al. [127] and Johansson [129] found that palm-biodiesel can secure the energy supply, preserve the environment, and develop the rural regions particularly the palm oil producers such as Malaysia, Indonesia, Colombia, and Thailand by not relying on petro-diesel.Besides, producing palm-biodiesel can give social advantages that include creating new job opportunities, accelerating social development, and improving living standards for the community that work in palm tree farms.

Environmental Impact
The increment interest and use of palm-biodiesel have resulted in increasing concern on environmental impact of palm tree cultivation as well as a food versus fuel dilemma [130,131].Moreover, the expansion of new palm tree farms to meet recent demand has triggered several issues that have been presented by many studies.Mukherjee and Sovacool [104] stated that, although the expansion of palm tree plantation is linked with deforestation and diminishes the biological habitat, the specific forestry impact from increased palm-biodiesel production remains inconclusive.This is because the spread of palm tree plantations is mainly due to the increasing food and industry demands.Nevertheless, Ashnani et al. [127] and Tan et al. [132] argued that the purpose of expanding new palm tree farms is to substitute the old plantations for a better economic revenue and not to destroy the biodiversity of the forest.Tan et al. [132] further added that sustainable development in palm tree plantation and production in the country could be strengthened if environmental-friendly measures are taken including the prohibition of burning operation, conservation of wildlife, minimisation and utilisation of waste, and integration of post management system.Kiss et al. [98] also highlighted that palm tree plantation at deforested areas at the Brazilian Amazon not only causes no damage to the plant diversity but also helps in recovering the degraded areas and offers new alternative plantation zones.
Furthermore, palm tree farms are the artificial green forests with timber and fibre which also function like the native forests but in different ways.It was discovered that palm tree farms could act as a more efficient carbon sink than rainforests, which indicates an area of dry mass that can absorb harmful GHG.Every hectare of palm tree farm assimilates up to 64.5 tonnes of CO 2 annually compared to only 42.2 tonnes for the original rainforest [127].Palm tree farms also assimilate 44 tonnes of dry matter/ha/year compared to 25.7 tonnes by the original rainforest [99].Moreover, a mitigation plan had been drawn to ensure carbon balance and GHG emissions due to deforestation for palm tree farms, which include increasing the use of organic fertilisers, planting on degraded land or with low biomass accumulation, and producing biochar at the time of replanting [133].
Palm oil had also been certified with a crop-specific sustainable certification standard by the Roundtable on Sustainable Palm Oil (RSPO) organisation [98].One of the most important RSPO criteria is "no primary forests or areas which contain significant concentrations of biodiversity (for example endangered species) or fragile ecosystems, or areas which are fundamental to meet the basic or traditional cultural needs of local communities (high conservation value areas) can be cleared" [134].Alternatively, clearing more land for palm tree farms can be avoided by cloning enhanced breeds of palm tree, which have a high oil yield and short maturity periods [37].Thus, all these initiatives will ensure that the negative impacts due to the expansion of palm tree farms can be minimised.

Food vs. Fuel
Meanwhile, several social movements and non-governmental organisations (NGOs) have claimed that the use of edible oils for biodiesel production as the major factor of the global food market price increments.At present, evidence that biodiesel leads to food price increases is only circumstantial.Instead, many other reasons have been cited to have significantly caused the increment in price such as the law of supply and demand, increased production cost, fast growth of the global population, natural disasters, increased price of other oils (soybean and rapeseed oils), income growth, climate change, and political instability [37,135].Koizumi et al. [136] highlighted that the price changes for edible palm oil in Malaysia and Indonesia caused by an increase in biodiesel demand are weak.Additionally, various studies on advance planting strategies, specialised fertilisers, and advanced crop biotechnology as well as cloning have been proposed to improve the palm oil production in an environmental-friendly and economical approach to guarantee an equal role of palm oil in the food and fuel supply for a sustainable prospect [37].Due to that, the giant producers of palm oil such as Indonesia and Malaysia have agreed to implement a certain limit amount of palm oil resources to be used as biodiesel to ensure that the food supply is not disrupted.The fast growth of palm tree farms in Indonesia and Malaysia is mainly influenced by the expanding demand for industrial and food processes in Asia such as China and India, instead of the need for biodiesel production [135].

The Prospect of Palm Oil By-Products and Mill Effluent for Biodiesel Production
Although the effects of biodiesel on the global food market prices could be considered as non-significant, the focus must be put on non-edible sources rather than the edible sources to ensure the social acceptance of biodiesel.Concerning this, the prospect and direction of palm-biodiesel have been expanded to utilising by-products and mill effluent from the palm oil industry such as waste palm oil (WCO), palm fatty acid distillate (PFAD), sludge oil, fatty acid residue, palm stearin, palm olein, residual oil from empty fruit bunch (EFB), residual oil from palm decanter cake (PDC), residual oil from palm oil mill effluent (POME), residual oil from spent bleaching earth (SBE), and CPO industrial liquid waste.Table 11 summarises various studies on the use of by-products and mill effluent from palm oil industry in producing biodiesel.
The palm oil industry produces various types of wastes in a large quantity either in the liquid or solid forms [99].In fact, only 10% of the total biomass obtained from palm oil farm is converted into edible oil, while the other 90% are disposed-off as waste materials [128,137,138].In specific locations such as Indonesia, the biomass produced from the palm oil industry is seven times higher than that of other timber industries [128,139].For instance, Herjanto and Widana [140] stated that an overall palm oil refining process could produce 73% of olein, 21% of stearin, 5% of PFAD, and 0.5% of effluent.Disposing of those wastes will create serious problems for the environment and community.For example, SBE is usually disposed as a waste by dumping it in landfills without any attempts to recover the oil, thus posing a serious threat to the environment through fire and the release of pollution hazards.Hence, it has been suggested that the oil in SBE is recovered and reused as an alternative energy source, which in turn could reduce the cost associated with refining processes [141,142].
Other than that, POME is one of the major causes of industrial oily wastewater, which is generated as a by-product during palm oil processing.Most of the palm oil factories use conventional ponding systems to treat POME that requires a long treatment period and a large area.Instead of being left in the pond, POME could become an attractive natural source for biodiesel production since the oil concentration in POME ranges between 4000 and 8000 mg/L [143][144][145].Although those wastes can be efficiently treated by available treatment technologies to meet the standard discharge limits, it also can be used as a cheaper raw material for producing biodiesel together with the importance of highlighting the global sustainability challenges.In agreement with the research necessity, various research has been performed using different processing routes to produce biodiesel from palm oil by-products and mill effluent as summarised in Table 11.
The biodiesel yield and characteristics vary depending on the raw materials and methods used.The utilization of palm oil by-products and mill effluent in biodiesel production generates numerous benefits in the biodiesel industry especially in terms of reduction in production cost, maintenance of the biodiesel quality and yield, and environmental protection for a better quality of social life as tabulated in Table 12.These studies have been successfully carried out at the laboratory scale that further proved their various potentials to be applied to the biodiesel industry.Although the palm oil by-products and mill effluent are not currently employed for a large-scale biodiesel production, it is believed that this proposed area of study is useful for the sustainable biodiesel production as the results of these studies could open new opportunities in finding various alternative raw materials through an oil recovery from low-cost and available sources.
Apart from that, wastes from palm oil industry not only can be directly utilised as a feedstocks for producing biodiesel but also for other purposes.For example, Lam and Lee [146] utilised POME as a cost-efficient nutrient medium for microalgae cultivation which could produce biodiesel and bioethanol.Besides, lipid from microalgae has been recognised as a good feedstock for biodiesel production as the yield produced is approximately 100 times higher than those produced from a hectar of oilseeds.Furthermore, various studies have been done to utilise the solid wastes like EFB into efficient solid catalysts that are suitable for producing biodiesel [147].Table 12.Benefits of utilization palm oil by-products and mill effluent in biodiesel production.

Production cost
The utilization of PFAD shows around 20% to 30% lower in production cost as compared to the refined vegetable oil in the conventional processing route [168].
The raw materials are readily available, abundant in quantity, and cheaper price.These results support the potential and competitiveness of the proposed raw materials in the industry.
Product yield and quality The production yield obtained a higher rate than 60% from the raw materials with high fatty acids content.
The high amount of FFAs could be reduced through esterification process without major modification, in which they have successfully reduced 93% of FFAs content to less than 2% [149].
Biodiesel produced showed similar properties with palm-biodiesel and follow the international standards (ASTM D6751 and EN 14214).
Environment and social Waste utilization will improve the environment quality.
It also can reduce the releasing and emissions of hazardous materials from the waste.These practices will provide better living quality and standard for the people.
While producing the biodiesel from palm oil by-products and mill effluent is able to give many benefits to various parties, more studies are yet to be conducted to: (i) design an efficient conversion method to produce biodiesel with high yield and quality using minimum production cost and impacts on the environment; (ii) develop a comprehensive strategy to connect and transport the wastes from their location and production facilities; (iii) conduct a detailed study and simulation using computational approach to understand the reaction possibilities and optimise its production rate; (iv) perform a complete field test using biodiesel produced from various types of palm oil by-products and mill effluent to determine their performance and effects to the engine; and (v) implement the improvement gained from the sustainability studies (using life cycle analysis) to create an affordable, environmental-friendly, and profitable biodiesel production using palm oil by-products and mill effluent.

Sustainability and Feasibility of Palm-Biodiesel Production
The World Commission on Environment and Development (WCDE) defined sustainable development as the "development that meets the needs of the present without compromising the ability of future generations to meet their own needs".In 1992, the United Nation (UN) highlighted the significance of sustainable development and declared that "the right to development must be fulfilled to equitably meet developmental and environmental needs of present and future generations".Meanwhile, in 2002, the World Summit on Sustainable Development established the target of sustainable development as the "access to reliable, affordable, economically viable, socially acceptable, and environmentally sound energy services and resources" [169,170].
Therefore, the application of sustainable energy usage involves a wise utilisation of energy produced by clean technologies and derived from renewable sources, leading to no damage to the environment and society.In short, sustainable palm-biodiesel production comprises legal and economic viable, appropriate to the environment, and is beneficial for the development of society and community.

Economic Viewpoint
From the economic perspective, the utilisation of bioenergy like biodiesel may not be as economically attractive as using conventional energy (petro-diesel).However, this should not prevent the widespread use of biodiesel as the concerns about depletion of the fossil resources and environmental protection must be prioritised.The continuous expansion of the biodiesel industries has been acknowledged and promoted by renewable fuel policies including subsidies, incentives, mandates, and taxes in various countries such as the USA, EU, Malaysia, Indonesia, and Brazil.Moreover, the global biofuel market has been estimated to approximately grow at 5.4% CAGR (compound annual growth rate) during the year of 2017 to 2024 while the capacity of biodiesel production has been expected to achieve 12 billion gallons by 2020.In 2013, the Environmental Protection Agency (EPA) set the Renewable Volume Obligation (RVO) in the yearly rulemaking under a congressional mandate that termed for the nation to utilise 36 billion gallons of renewable fuels in transportation by 2022 [171,172].
Other than that, the use of biodiesel in many other applications besides a petro-diesel substitute for transportation can enhance its economic viability.For instance, palm-biodiesel can be employed as a heating fuel in commercial and domestic boilers.It also can be used as a solvent for oleochemical industries as biodiesel exhibits better solvency power than that of petroleum-based solvent.However, the economic potential of palm-biodiesel will only be viable if the price of its primary raw material (CPO) remained at a competitive level or when sufficient subsidies are applied.Thus, it is crucial to study the utilisation of palm oil by-products and mill effluent in biodiesel production to achieve and sustain economic viability.Currently, biomass sources including palm oil by-products and mill effluent are gaining huge attention for supplying the world's energy demand and have been predicted to share by one half of the total world energy demand by 2050 [173].

Environmental Preservation
In terms of environmental preservation, the major concern on the use of petro-diesel is in the serious rising level of GHG emissions, mainly CO 2 , that is correlated to global warming.Several studies have found that although CO 2 was emitted during the use of biodiesel, the net emissions of CO 2 is still estimated to be zero since it is derived from plants that require CO 2 for their growth.As highlighted by the Department of Agricultural Energy USA, the life cycle of CO 2 could be shortened by 78% using biodiesel owing to the low emissions of CO and hydrocarbons to the atmosphere.Furthermore, the utilisation of palm-biodiesel demonstrated the lowest emissions of CO 2 compared to petro-diesel and other types of biodiesel due to the lesser land, fertilisers, pesticides, and machinery used during palm tree plantation [98,108].
In addition, it has been revealed that palm trees show a very high photosynthetic rate with the ability to absorb up to 10 times more CO 2 and emit 8 to10 times more O 2 per hectare annually compared to other crops cultivated in temperate countries [37,174].Besides, the utilisation of palm oil by-products and mill effluent in producing biodiesel could also give a positive waste management effect on the environment.Finally, the biodiesel produced from palm oil, by-products, and mill effluent is preferred over the petro-diesel owing to its environmental advantages [141].

Social Impact
A well-planned development of palm-biodiesel industry is undoubtedly beneficial to the society and community especially in the rural areas involved with the plantation of palm trees.New palm tree plantation requires an intensive ground work and a large number of workers that will offer huge opportunities and alternative income for the community [98].As Mukherjee and Sovacool [104] also added, in some countries like Malaysia and Indonesia, palm tree plantations are some of the major sources of employment and job providers.
Additionally, the progress and fast growth for palm-biodiesel demand and production will increase crop yields, thus discovering new applications for the agricultural products and intensifying the economic and revenue growth [99].Consequently, new markets opportunities can be developed whether local or overseas with more employment chances created, which could provide better living quality and standards for the public.Looking at a broader benefit, it has been expected by the industry players that the palm oil-related sectors have benefitted around six million peoples around the world with many of them have been relieved from poverty [99,135,175].

Conclusions
The depletion of fossil resources, increased petroleum crude oil price, and awareness of environmental protection have led to increased studies and development efforts to find renewable and environmental-friendly alternative energy sources.Additionally, the current high demand for renewable energies has caused a high production of biodiesel and developed the fastest growing industry worldwide.Nonetheless, the booming of biodiesel has led to speculations and concerns on the relationship between palm-biodiesel with food supply and tropical deforestation.However, if suitable practices are employed, those speculations can be avoided, and palm-biodiesel can 'come clean'.Thus, the historical development of palm-biodiesel seemed attractive.
Palm-biodiesel has successfully existed as an efficient energy source despite being relegated for many years due to many issues and speculations.Other milestones related to the palm-biodiesel industry are the reduction of environmental impact, an increment of job opportunities, enhancement of energy security, and maximisation of waste utilisation.Besides, palm-biodiesel has been noted as a viable and practical alternative or additive to petro-diesel through its excellent characteristics such as clean-burning, nontoxicity, renewability, sustainability, and acceptability.On top of that, it also has benefits including its cheaper cost and more positive carbon benefits than other major biodiesel sources [176].These can consequently lead to the increase in prospects for the palm-biodiesel industry.
To summarise all this, the future trends of palm-biodiesel will likely move towards the balance between market demands with the community and consumer perceptions.The exploration of alternative sources for green biodiesel production is still ongoing.This study had proposed palm oil by-products and mill effluent as the future raw materials for biodiesel mainly due to their cheaper cost, availability, abundance, environmental friendliness as well as the minimum impact on food security.Few technologies have been developed with more improvements that are yet to be proposed.Several key challenges involved will offer new research potentiality to improve the product quality and solve other problems especially related to the environment.A comprehensive study is still needed to be carried out for foreseeable future.
. The C 16 and C 18 fatty acids are the major components used for producing biodiesel.Palmitic and stearic acids are some examples of saturated fatty acids, while oleic, linoleic, and linolenic acids are the examples of unsaturated fatty acids.

Table 2 .
Properties of biodiesel

Table 4 .
The

Table 5 .
The USA average fuel price.

Table 6 .
Important parameters for transesterification/esterification process for producing biodiesel.

Table 8 .
Oil content and price for various raw materials.

Table 11 .
Production of biodiesel from palm oil by-products and mill effluent.