ORIGINAL ARTICLE

Single-step extraction–esterification process to produce biodieselfrom palm oil mill effluent (POME) using microwave heating:a circular economy approach to making use of a difficultwaste product

Elis Davies1 & Pauline Deutz1 & Sharif H. Zein1

Received: 1 April 2020 /Revised: 22 June 2020 /Accepted: 1 July 2020# The Author(s) 2020

AbstractThis investigation explores single-step biodiesel synthesis using palm oil mill effluent (POME) as a feedstock. Normallyconsidered a waste product from the extraction process of palm oil, POME treatment is difficult and can cause significantenvironmental pollution if discharged directly into watercourses. Fatty acids (FAs) present in POME were extracted and subse-quently esterified in situ to FA methyl esters (FAME) suitable for use as biodiesel. The process of simultaneous extraction andesterification was performed under microwave irradiation in a sealed vessel, which has been shown to reduce both time ofreaction and energy use considerably in other reactions. The simultaneous extraction and esterification result in an 89% yield ofbiodiesel from the available FAs in the POME solids. The facile separation of biodiesel products from the reaction mixtureaugments the process and here presented is a potential route to making economic use of this difficult feedstock.

Keywords Biodiesel . Palm oil mill effluent . Microwave . Esterification . Circular economy

1 Introduction

The circular economy (CE), whereby resources are kept ineconomic circulation for as long as possible and waste mini-mized, is currently a hotly debated concept in academic andpolicy circles. CE initiatives, such as recovery, recycling andre-use, are seen as promoting carbon emissions reductions,resource security and economic competitiveness [1].Successfully implemented, CE approaches could make a sub-stantial contribution to the United Nations’ SustainableDevelopment Goals, though that is presently largely a matterfor speculation [2, 3]. Cognisant of the wider social and eco-nomic context, this research tackles a specific, currently

environmentally problematic, residue stream of an industryof significance to a number of countries in the Global South.Approaches to the CE, as with the earlier concepts on which itbuilds, need to take into account geographic context industriesand nations start with certain circumstances (e.g., dependenceon particular industries), which may more constructively beadapted rather than abandoned [4].

Since its introduction in the early 1900s fromWest Africa,the oil palm Elaeis guineensis has become the most valuablecrop for countries in South East Asia. By 2007, palm oil hadbecome the most consumed oil globally unseating soybean oil[5, 6]. Oil palm production has attracted much criticism onenvironmental grounds, with attention focusing on the preven-tion of further deforestation. There has also been research andpolicy activity into how to make existing production moreefficient and less polluting. The Malaysia government, forexample, introduced regulations mandating the capture of car-bon emissions from POME that are typically left to microbialaction in ponds open to the atmosphere [7]. Palm oil hashelped lift Malaysia’s economy to the ranks of upper-middleincome countries, and the programme of increasing efficiencyof production is part of the Malaysian government’s drive toreach high-income status (aiming at 2020). An existing

* Elis Daviese.davies@2011.hull.ac.uk

Pauline Deutzp.deutz@hull.ac.uk

Sharif H. Zeins.h.zein@hull.ac.uk

1 University of Hull, Hull, UK

Biomass Conversion and Biorefineryhttps://doi.org/10.1007/s13399-020-00856-1

technology for managing POME is anaerobic digestion,which implies decomposition in a controlled environment(minimizing leakages to air or water) and generation of energyfrom the methane. This paper, however, pursues a higher val-ue and more mobile form of energy, i.e., biodiesel. Pollutionfrom the extraction of palm oil is mainly found in the wastewater that is created; this is primarily the discharge from thesterilization process and separator hydrocyclones known aspalm oil mill effluent (POME) [8, 9]. The harmful character-istics of POME can be summarized by its low pH and excep-tionally high chemical and biological oxygen demand (CODand BOD respectively) that cause significant damage to wa-tercourses if directly discharged. [10–12] During the rapidexpansion of the Malaysian palm oil industry, which has seenthe number of palm oil mills rise sharply from 10 in 1960 to410 in 2008, there has also been a corresponding increase inthe quantity of POME produced [13]. It is estimated that forevery tonne of crude palm oil produced, around 3 t of POMEis also created leading to large volumes requiring treatment.For example, in 2009 43.8 million cubic metres of POMEwasgenerated in Malaysia alone [9, 12, 14, 15].

However, due to the oil content present in POME, there isemerging interest in the utilization of this material as a feedstockfor biodiesel production. Biodiesel is a synthetic fuel commonlydefined as the alkyl esters of long chain FAs sourced from or-ganic biomass such as vegetable oils or animal fats [16].Biodiesel produced from waste products has become a focus ofresearch in contemporary times. This approach offers several keyadvantages; chief among which is the reduction in competitionfor raw materials, e.g., food [7, 17–19]. In addition, waste prod-ucts are generally abundant, cheap and potentially difficult todispose of currently; therefore, any potential use is advantageousto both the producer of the waste and the environment as a whole[20, 21]. Even when the considering the low oil content ofPOME, the potential biodiesel yield could be significant. Theuse of POME as a biodiesel feedstock is a relatively new areawith few publications at the time of writing, mainly concerningbiological conversion methods [22, 23].

The difficulties of using POME include the relatively low oilconcentration in addition to the complex mix of surface-activeparticles as well as the variable composition of the POME frommill to mill. One potential avenue for the treatment of POME isthe field of microwave chemistry. Since the seminal paper in1986 by Gedye et al. microwave processing has been success-fully applied to many organic chemical reactions, with reports ofhigh yields and very short processing times the primary advan-tages [24, 25]. As well as numerous applications in the field oforganic chemistry, microwave technology has been successfullyapplied to process of solvent extraction, known as microwaveassisted extraction (MAE). First explored by Ganzler et al. MAEmakes use of the direct heating of the sample/solvent with mi-crowave radiation to enhance extraction yields and lower bothextraction times and energy usage [26–28]. The single step,

single vessel extraction and esterification reactions being per-formed in this investigation represent a novel integrated methodfor the direct conversion of POME to biodiesel. By combiningthe advantages microwave technology has in both material ex-traction and organic reactions to the difficult feedstock that isPOME, this investigation has increased the economic viabilityof waste treatment, thus incentivizing the removal of pollutingFAs from the waste stream.

2 Methods and materials

Raw POME was sourced from Langkon Palm Oil Mill inSabah, Malaysia, and stored in sealed containers for transpor-tation and storage. Methanol, heptane, magnesium sulphateand sulphuric acid were obtained from VWR chemicals.

For the following experiments raw POME was used.Samples used for experimentation were taken from an agitatedsample, representing freshly discharged, unsettled POME.

2.1 Extraction and esterification of fatty acids presentin POME

About 15 ml of POME was centrifuged at RCF 3.0 (4400RPM) in an Eppendorf 5702 centrifuge for 30 min. The su-pernatant was put aside, and the pellet was transferred to atemperature-controlled microwave vessel. The organic sol-vent and acid catalyst were then added; the vessel was sealedand placed in a Milestone Ethos EX microwave. Microwaveheating was performed for 15 min at a constant temperature of150 °C. After the heating period the system was allowed tocool to room temperature. Once cooled the reaction mixturewas transferred to a separating funnel and the organic layerwas removed. The organic layer was diluted 1:1 with BSTFA+ TMCS (99:1) derivatization agent, and concentrations ofFAs and methyl esters (MEs) were analysed by GC-MS per-formed on a Hewlett Packard 6890 Plus fitted with a ThamesRestek Rxi-17Sil MS moderate polarity column.

2.2 Determination of the initial oil content of thePOME sample

For the purpose of the evaluation of microwave reactions, theoil content of the raw POME sample was initially determined.A modified extraction procedure as set out in ISO standard9377-2 was performed in addition to GC-MS quantification ofthe extracted materials. Briefly, 50 ml of POMEwas acidifiedto pH 2, sealed in a vessel and stirred with the heptane extrac-tion solvent for 30 min. Afterwards, magnesium sulphate wasadded and the organic layer was removed. A sample of theorganic layer was diluted 1:1 with BSTFA + TMCS (99:1)derivatization agent and concentrations of FAs quantified byGC-MS as above.

Biomass Conv. Bioref.

2.3 Water content of POME solids

An Ohaus MB23 moisture content determination apparatuswas used to accurately quantify the water content in the cen-trifuged POME used for biodiesel production experiments. Asample of centrifuged POME (around 1.0 g) was placed onto apre-weighed aluminium disc and placed onto the cradle of theMB23. The apparatus was activated, and the device trackedthe mass of the sample as the temperature is increased by aninfra-red heating element present above the sample disc. Afterno additional change in temperature is recorded, the systemreports the difference in mass between the initial and finalreading as a percentage of the initial mass. This is equivalentto the mass lost as water vapour from the sample.

2.4 Statistical optimization methods

The experimental condition matrix was designed using anal-ysis of variance (ANOVA) software Design Expert 11 usingresponse surface methodology (RSM) coupled with centralcomposite design (CCD) to form a of this experimental designstrategy that enables the embodiment of advantages ofperforming a specific number of experiments from otherwiseperforming unnecessary experiments, whilst fully consideringthe multivariate nature of any novel process parameters (fac-tors), as well as consideration of the definite interactions be-tween parameters (factors: volume of methanol, H2SO4 con-centration, stirring), and their impact on the response yield(ME and FA concentrations).

3 Results and discussion

3.1 Initial fatty acid concentration in POME

The principle components identified in the solvent extractionof raw untreated POME were FAs, principally palmitic acidand to a lesser extent oleic acids and isomers thereof. Traceamounts of other FAs were detected as expected. For easycomparison the concentration of palmitic acid was used toevaluate the extraction efficiency and the esterification yield

in this investigation. The concentration of palmitic acid in theraw pome sample was found to be 1.16 mg/ml. All extractionsand reactions will be compared with this reference value forthe purpose of determining yield and conversion to biodiesel.Figure 1 represents the make-up of oil extracted from rawPOME according to the ISO method detailed in the previoussection. The first and largest peak present at a retention time(RT) of 3.1 min is palmitic acid, unsurprisingly the most abun-dant component of POME. The peak at RT 3.65 min is theinternal standard, docosane, used for accurate quantitation.The series of peaks between 4.1 and 4.4 min RT arethe various C18 fatty acids, including primarily oleicacid itself (cis-9-octadecenoic acid) as well as linoleicacid (9,12-octadecadienoic acid) as well as the saturatedstearic acid (octadecanoic acid). The solvent extractionproduces a remarkably clean extraction profile with onlyfatty acids being extracted.

3.2 Water content of POME solids

The water content of the POME sample was determined ThePOME analysed was centrifuged to be representative of thePOME used in the biodiesel production experiments. The wa-ter content of the centrifuged POME was found to be 97.9 ±0.3%. Even after centrifugation, the water content of the cen-trifuged POME solids is still high, with almost the entire massof the sample being water. Despite this, the appearance of thePOME is that of a fibrous paste. The temperature of the waterdetermination apparatus was set at a mild 120 °C in order toprevent the volatilization of any fatty acids or other organicsthat could detrimentally affect the accuracy of the water con-tent determination.

3.3 Comparison between microwave and benchheating

In order to assess the rate enhancement possible whenworking with microwave heating system, 3 identical reac-tion mixtures containing pure palmitic acid were heatedwith methanol and a H2SO4 catalyst. The first reactionwas performed under reflux heating, whereas the second

Fig. 1 Chromatogram ofderivatized POME components asextracted according to theprocedure above and analysed byGC-MS

Biomass Conv. Bioref.

and third were performed in a sealed microwave vessel at100 and 150 °C respectively. Figure 2 shows the FAMEconcentration after only 1 min of microwave heating issignificantly higher than the 30-min FAME concentrationfrom standard bench heating. The microwave heating wasconducted at two temperatures in order to show the effecttemperature has upon the esterification reaction takingplace. As the reaction mixture contains 10 ml of bothmethanol and heptane, this approaches the azeotropic ra-tio resulting in a boiling point of the benchtop reaction ofaround 59.1 °C at atmospheric pressure. The modest in-crease of reaction temperature used in the 100 °C micro-wave reaction gives a significantly higher concentrationof FAME; the 150 °C reaction further increased the con-centration of FAME produced in the 15-min reaction win-dow. The additional 50 °C in reaction temperature be-tween the two microwave reactions has a large effect onthe resultant FAME concentration. This may be explainedby the increase in pressure imparted upon the reaction bythe heating of the solvent mixture far in excess of theazeotropic boiling point. As shown Fig. 2 as well as in-creasing the rate of extraction/esterification, the highertemperature microwave reactions also increased the totalamount of FAME after the 15-min heating period. Toinvestigate how the increased temperature (and micro-wave power) can increase not only the rate but more im-portantly the yield of FAME through increased extraction

of FA, SEM images of the POME solids before and aftermicrowave treatment were taken. A marked increase inporosity demonstrated the appearance of holes in thePOME solid structure. Figure 3 shows the structure ofthe POME solids under × 200 magnification. The imageshows the flaky nature of the substrate. Figure 4 showstwo images of POME solids taken after the microwavereaction. The appearance of pores, holes and tears in theflakes of the POME solids could be responsible for theincreased quantity of oil extracted from the POME solidsin the microwave reactions by increasing the surface areaof the solid as well as by breaking down the physicalstructure that could be retaining oils. The temperature ofthe microwave-heated reactions is controlled through themeans of a PID loop that controls the microwave powerdelivered by the magnetron to the samples based on thesetpoint temperature and the current sample temperature.In the case of the 150 °C microwave samples, not onlywas the temperature achieved by the microwave 50 °Chotter than the 100 °C reaction, the average incident pow-er used to achieve and maintain this temperature wouldhave been higher too. In addition, the reported tempera-ture by the microwave control software can only refer tothe bulk temperature inside of the vessel (due to the proxymeasurement of temperature via a thermowell). Therefore,a value for the temperature of the individual constituentsof the reaction vessel cannot be determined. It is a known

0

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2.5

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m,tcudorpni

noitartnecnocleseidoiB

Reac�on �me, minutes.Bench hea�ng Microwae hea�ng at 100°C Microwave hea�ng at 150°C

Fig. 2 FAME concentration ofreactions comparing microwaveand bench heating

Fig. 3 SEM image of POMEsolids showing new holes andtears formed during microwavetreatment at 150 °C (mag. × 1000)

Biomass Conv. Bioref.

phenomenon that solid particles in microwaves may ex-perience super-heating and form “hot-spots” as a functionof very strong microwave absorbance in a localized area,and this could be used to explain how the modest in-crease in bulk microwave reaction temperature (from100 to 150 °C) could result in a significantly higheryield of oils liberated from the solid structure [29].

The higher incident power delivered by the microwavecontrol software in the case of the hotter reaction could leadto an increased level of porosity developing as a result of thebreakdown of the organic POME solids as a function of theincreased level of superheating.

3.4 CCD model and RSM analysis

A central composite design (CCD) model was used to esti-mate the impact of methanol, H2SO4 concentration and stir-ring on the yield of biodiesel and the quantity of fatty acidsextracted from POME. In this initial design, the temperatureand time of reaction was fixed at 150 °C and 15 min. Thereason for this was to more accurately attribute the effects ofmethanol to oil ratio and acid concentration to the yield ofbiodiesel. The factors and their minimum and maximumvalues for the optimization are presented in Table 1.

In this investigation, both the concentration of biodiesel inthe final product and the concentration of fatty acid were stud-ied. The purpose of this is to investigate simultaneously theperformance of fatty acid extraction from POME aswell as thesecondary step of esterification. This is done by back-calcu-lating the concentration of fatty acid using the presentconcentration of the methyl ester and adding this to thevalue retuned from GC-MS of the unreacted (residual)fatty acid. The methanol to heptane volume ratio wasvaried between 0.1 and 0.5, with a fixed 10-ml volumeof heptane being used for the extraction.

Table 2 shows the 28 reactions for the RSM study incor-porating a CCD design. The effect of stirring, sulphuric acidconcentration and methanol present in the reaction was inves-tigated in order to study the extraction of FAs from the cen-trifuged POMEmatrix and the subsequent esterification of theextracted FAs to biodiesel. The experimental variables andresponses from the CCD are given in Table 2. The highest 3concentrations of FAME in the trial were from runs utilizingthe maximum values for methanol content and H2SO4 con-centration. In some runs there was an unquantifiable concen-tration of FAME in the sample post-reaction. The runsexhibiting zero FAME were almost exclusively characterizedby the low H2SO4 concentration stressing the importance ofacid in the esterification component of the extraction-esterification process occurring in the microwave vessel.This trend is also observed in similar extraction/reaction ex-periments [30, 31]. In terms of the fatty acid concentration,relating to the extraction component of the extraction-esterification process, the highest concentrations of FA werealso found from runs utilizing the maximum values formethanol:heptane volume ratio and H2SO4 concentration sug-gesting a role for both the acid and methanol in the initialextraction of the fatty acid from the POME matrix.

The fit statistics as shown in Table 3 above can be used toevaluate the applicability of the model for both the FA extrac-tion and the subsequent esterification to create FAME. A highR2 close to 1.00 for both models show that the fit of is goodwhen compared with the experimental data. The modest dif-ference between the predicted R2 and adjusted R2 could beused to indicate a block effect. However, the large valueretuned for adequate precision means that the model is signif-icantly representative of the data and can be used for theintended purpose to navigate the design space.

Fig. 4 SEM image of POME solid pre-microwave treatment (mag. ×200)

Table 1 Factors and ranges of experimental conditions used for the optimization investigation

Factor Name Units Type Minimum Maximum Mean Std. Dev.

A Methanol:Heptane vol ratio Ratio Numeric 0.1000 0.5000 0.2929 0.1585

B H2SO4 concentration Molar Numeric 0.0010 0.0400 0.0198 0.0155

C Stirring % Numeric 20.00 100.00 58.57 31.71

Biomass Conv. Bioref.

3.5 Fatty acid extraction performance using RSM

Table 4 is an excerpt from the ANOVA performed on therelationship between the model variables and the FA concen-tration and is used to determine the significance, or lack ofsignificance of each model term. An F value for the entiremodel of 40.62 implies significance, with a 0.01% chance thisvalue being due to noise. P values lower than 0.05 indicatemodel terms that are significant to the model. In this case,methanol quantity, A, and H2SO4 concentration, B, aredeemed to be significant. Stirring, C, with a P value of0.1998 is not deemed to have a significant effect upon thefatty acid concentration.

The 3D surface in Fig. 5 shows the concentration ofextracted FA at all levels of H2SO4 concentration and meth-anol. As can be seen, FA concentration peaks when both thelevels of H2SO4 concentration and methanol are at theirmaximum. The peak value of 1.05 mg/ml represents a90.5% yield of all available oil in POME when comparedwith the ISO method used for determination. This high val-ue is obtained after only 15 min of extraction at 150 °Cdegrees and using only the centrifuged POME solids as

opposed to the raw POME that the reference extractionwas performed on.

The extraction of the FAs from the centrifuged POMEsolids was found to be dependent on both the concentra-tion of H2SO4 and the volume of methanol present. Asshown, even with all factors at their lowest the extractedFA concentration is still above 0.75 mg/ml, over 64% ofthe oil extracted according to the reference ISO method;this must be attributed to the action of the high tempera-ture of the microwave and the heptane that is also present

Table 2 Reaction variables andresponses Factor 1 Factor 2 Factor 3 Response 1 Response 2

Experimental conditions Result

Run A: Volumeof methanol

B:H2SO4

concentrationC: Stirring FAME

concentrationFA concentration(equivalent)

ml Molar % mg/ml mg/ml

1 5 0.021 60 1.01 1.012 5 0.040 20 1.09 1.053 3 0.021 60 0.22 0.904 3 0.021 60 0.20 0.865 3 0.021 60 0.20 0.876 1 0.021 60 0.00 0.797 5 0.040 20 1.07 1.048 5 0.001 20 0.00 0.689 3 0.040 60 0.45 0.9510 3 0.001 60 0.00 0.8111 1 0.001 20 0.00 0.7912 3 0.021 60 0.21 0.8813 5 0.001 20 0.00 0.6714 1 0.040 100 0.08 0.8315 1 0.001 20 0.00 0.7716 1 0.040 100 0.07 0.8317 3 0.021 60 0.20 0.8918 1 0.001 100 0.00 0.7419 5 0.001 100 0.00 0.6820 3 0.021 60 0.21 0.8821 1 0.001 100 0.00 0.8122 5 0.001 100 0.09 0.7423 3 0.021 100 0.37 0.8624 3 0.021 60 0.21 0.9125 1 0.040 20 0.08 0.7826 1 0.040 20 0.05 0.8027 5 0.040 100 1.08 1.0528 3 0.021 20 0.65 0.91

Table 3 Fit statistics for the model

Parameter FA concentration FAME yield

Std. Dev. 0.0239 0.0991

Mean 0.8486 0.2693

C.V. (%) 2.82 36.81

R2 0.9742 0.9616

Adjusted R2 0.9502 0.9259

Predicted R2 − 0.1485 − 5.9939Adequate precision 22.5356 17.6275

Biomass Conv. Bioref.

in the vessel. The methanol content of the reaction has thestrongest effect on the FA concentration suggesting somesolubility of palmitic acid in the methanol or perhaps themethanol-heptane azeotrope that would be formed in re-actions containing the 5 ml of methanol. The next mostsignificant factor was the H2SO4 concentration.

Whilst this was added ostensibly as an esterificationcatalyst (for the second step of the reaction) the effect ofacid upon the POME solids may act to break down thesolid structures allowing increased penetration of the ex-traction solvents, and subsequently, a higher amount ofFA extracted. The least significant factor for FA extrac-tion is the stirring which has a mildly detrimental effecton FA extraction. Figure 6 is a perturbation plot showingthe effect of the three factors upon the FA concentration.The horizontal dimension of each line represents therange of values used in the experiments. Line A repre-sents effect of the methanol volume on the extraction andranges from 1 to 5 ml, line B represents the effect ofH2SO4 concentration upon the extraction when variedfrom 0.001 to 0.04 mg/ml and line C represents the stir-ring rate used in the microwave reactor as a unitless

value and was adjusted between 20 and 100% of themaximum. As can be seen, A is the steepest gradientshowing that the amount of methanol is most importantto this initial extraction step.

3.6 Biodiesel production performance using RSM

The esterification of the extracted palmitic acid to methyl palmi-tate (FAME)was also analysed throughRSMas the final yield ofbiodiesel from the POME is the most important factor in thisstudy overall. As shown in Table 5, and in common with theFA extraction model, methanol quantity (A) and H2SO4

concentration (B) are deemed to be significant. Stirring, C,with a P value of 0.0656, is not deemed to have a significanteffect upon the FAME concentration. Figure 7 is a 3D surfaceshowing that a high yield of biodiesel is only produced underconditions with the highest amount of acid and methanol.However, under these circumstances the peak yield of biodie-sel when compared with the available FAs in raw POME is89%. Whilst conditions used for the reaction were somewhatharsh and a high amount of methanol used, this can be recov-ered from the reaction mixture in a simple way through

Table 4 ANOVA table for fattyacid extraction (type III—partial) Source Sum of squares df Mean square F value p value

Model 0.3016 13 0.0232 40.62 < 0.0001

A-Methanol:Heptane vol ratio 0.0237 1 0.0237 41.47 < 0.0001

B-H2SO4 concentration 0.0093 1 0.0093 16.35 0.0012

C-Stirring 0.0010 1 0.0010 1.81 0.1998

Fig. 5 Effect of H2SO4

concentration and methanol onthe extracted FA concentration.Colour represents FAMEconcentration from 0 (blue) to1.05 mg/ml (red)

Biomass Conv. Bioref.

distillation. The amount of methanol used could be reduced ifthe POME solids were dried prior to reaction however withonly mild centrifugation this method can achieve a high bio-diesel yield with only a 15-min reaction time

Further study into optimizing the reaction with re-spect to temperature and reaction time could yield evenmore impressive results for the exploitation of this dif-ficult feedstock. The perturbation plot, Fig. 8, shows themagnitude of the contributions of the factors to the finalconcentration of FAME in the product. As shown, themost significant factor to biodiesel concentration is themethanol content in the reaction. This can be explainedby the dynamic equilibrium present in the esterificationreaction between reactants and products as shown inFig. 9. An excess of methanol will help to drive theequilibrium towards the biodiesel, this is especially im-portant due to the residual water present in the centri-fuged POME that will work in favour of the reversereaction. The effect of H2SO4 concentration on the yield

of biodiesel is the second most significant factor in thisseries of reactions. As the esterification catalyst it wouldbe expected that the reaction responds well to an in-crease in acid concentration. In addition, H2SO4 alsoacts to dehydrate the reaction, diminishing the disruptiveeffect of residual water on the dynamic equilibriumformed between the biodiesel product and reactants inthe reaction. As seen with the FA extraction, stirringhad a modest albeit negative effect upon the final bio-diesel concentration. Possible reasons for this could bethat the increased agitation causes mixing of the organicand aqueous layer leading to increased contact betweenFAME molecules and water present in the reaction lead-ing to acid hydrolysis of the product.

3.7 RSM insight in to maximizing biodiesel yield

A useful output from a statistical model is the ability toattribute relative (and actual) contributions from each factorto deconvolute the relationships and possible interactionsbetween factors in order to optimize the reaction conditionstowards minimizing energy use or amount of materials usedfor example. Table 6 is a table of factor coefficients thatestimate the contributions from not only the individual fac-tors but the interactions between factors (compound fac-tors) represented by the multiple letter entries. A positivevalue for the estimated coefficient represents a positive re-lationship between increasing the value of the factor and aresultant increase in response value. For example, factor B,H2SO4 concentration has a positive coefficient, meaningthat an increase in H2SO4 concentration will give a higherbiodiesel yield. Whereas in factor C, stirring has a negativecoefficient, meaning that in order to maximize the response,stirring should be minimized.

4 Actual reaction equation

Equation 1 can be used to predict the actual methyl esterconcentration from the model when variables are enteredusing the same units as used in the model.

Fig. 6 Perturbation plot showing the influence of all factors on the FAconcentration

Table 5 ANOVA table forbiodiesel esterification [Type III -Partial]

Source Sum of squares df Mean square F value p value

Model 3.44 13 0.2648 26.96 < 0.0001

A-Methanol:Heptane vol ratio 0.5101 1 0.5101 51.92 < 0.0001

B-H2SO4 concentration 0.1012 1 0.1012 10.31 0.0063

C-Stirring 0.0392 1 0.0392 3.99 0.0656

Biomass Conv. Bioref.

Predicted methyl ester concentration ¼ −0:050599þ 2:51617Að Þ�� 21:66933Bð Þ�� 0:003557Cð Þ þ152:49966ABð Þ− 0:054369 ACð Þ þ 0:006367 BCð Þ− 4:56470A2

� �þ 499:33209B2� �þ 0:000067C2

� �−

0:031617ABCð Þ þ 75:40736A2B� �þ 0:093011A2C

� �− 3220:69398AB2� �

Where : A ¼ Methanol : Heptane volume ratioB ¼ H2SO4 ConcentrationC ¼ Stirring rate

Equation 1 Predicted methyl ester concentration calculated f rom model terms

5 Conclusion

This study has shown the potential for conversion of fattyacids from POME into biodiesel. In keeping with circulareconomy principles, this provides a route to extracting furthervalue from an industrial residue. Once implemented, this ap-proach would provide a financial incentive to mill operators tocapture and manage POME without leakages to the environ-ment. It also offers an additional relatively high value outputin the context of rural economies in a developing country. FAswere extracted and esterified in a single reaction vessel fromcentrifuged POME and converted to biodiesel with a peakyield of 89% at 150 °C and a reaction time of 15 min. Thishigh efficiency reaction could provide a method to obtainlarge quantities of biodiesel from a waste material, reducing

Fig. 7 Effect of H2SO4

concentration and methanol onthe FAME concentration. Colourrepresents FAME concentrationfrom 0 (blue) to 1.09 mg/ml (red)

Fig. 8 Perturbation plot showing the influence of all factors on thebiodiesel concentration

Fig. 9 Equilibrium formed between reactants and products in theproduction of esters

Biomass Conv. Bioref.

the environmental impact of the free FA component ofPOME. Further studies could include the scale-up of thebatch-based microwave technique to a larger flow systemmore suited to the continuous nature of POME production atpalm oil mills.

Funding information The authors received financial support from TheNewton Fund (British Council Institutional Links Grant, with theUniversity of Malaysia Sabah Institutional Links Grant (Application ID172702808) used to obtain representative samples of POME from Sabah,Malaysia.

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Table 6 Factor coefficients forbiodiesel esterification yield Factor Coefficient estimate df Standard error 95% CI low 95% CI high

Intercept 0.2782 1 0.0321 0.2094 0.3470

A-Methanol:Heptane vol ratio 0.5050 1 0.0701 0.3547 0.6553

B-H2SO4 concentration 0.2250 1 0.0701 0.0747 0.3753

C-Stirring − 0.1400 1 0.0701 − 0.2903 0.0103

AB 0.2488 1 0.0263 0.1925 0.3052

AC 0.0063 1 0.0263 − 0.0500 0.0627

BC − 0.0024 1 0.0263 − 0.0588 0.0539

A2 0.1025 1 0.0590 − 0.0241 0.2291

B2 − 0.1775 1 0.0590 − 0.3041 − 0.0509C2 0.1075 1 0.0590 − 0.0191 0.2341

ABC − 0.0049 1 0.0263 − 0.0613 0.0514

A2B 0.0588 1 0.0748 − 0.1017 0.2194

A2C 0.1488 1 0.0748 − 0.0117 0.3094

AB2 − 0.2449 1 0.0748 − 0.4055 − 0.0844

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