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Korean Society for Biotechnology and Bioengineering Journal 2023; 38(3): 194-202

Published online September 30, 2023 https://doi.org/10.7841/ksbbj.2023.38.3.194

Copyright © Korean Society for Biotechnology and Bioengineering.

Enhancing Biodiesel Production from Microalgae and Sewage Sludge Lipids by Adding Waste Coffee Ground Lipids

Young Wook Go1,2 and Sung Ho Yeom1,*

1Department of Biochemical Engineering, Gangneung-Wonju National University, Gangneung 25457, Korea
2Department of Chemical Engineering, Hanyang University, Seoul 04763, Korea

Correspondence to:Tel: +82-33-640-2406
E-mail: shyeom@gwnu.ac.kr

Received: March 23, 2023; Revised: August 16, 2023; Accepted: August 27, 2023

The free fatty acid (FFA) content in the feedstock lipids of microalgae (MA), sewage sludge (SS), and waste coffee grounds (WCGs) were 7.6%, 38.5%, and 1.7%, respectively. For a single feedstock lipid, the maximum biodiesel conversion was about 70% for MA lipids using acid or alkaline catalysts, 29.6% for SS lipids using acid catalysts, and 93.5% for WCG lipids using alkaline catalysts. For a mixture of MA and SS lipids, the maximum biodiesel conversion was only 38.6% in the experimental range tested. In contrast, for mixtures of WCG and MA lipids and WCG and SS lipids, where WCG lipids accounted for three-fourth of the mixtures, the biodiesel conversions using an alkaline catalyst were 89.5% and 70.0%, respectively. When MA, SS, and WCG lipids were mixed in equal proportions, 71.8% biodiesel conversion was obtained with 3.0 wt% of an alkaline catalyst. This study shows that WCG lipids, when used with lipids having high FFA content, are crucial for improving the conversion of lipids into biodiesel. In addition, by manipulating the proportion of WCG lipids in the mixture, we were able to use an alkaline catalyst, which has several advantages over an acid catalyst, to produce biodiesel at a relatively high biodiesel conversion.

Keywords: biodiesel, microalgae, sewage sludge, waste coffee grounds, lipid mixture, mixing ratio, catalyst type.

Biodiesel is a renewable, carbon-neutral, biodegradable, and non-toxic fuel, which can be used in diesel cars without requiring engine modification [1-3]. Since biodiesel production has increased significantly in recent years, non-edible and sustainable feedstocks, such as microalgae (MA) [4,5], sewage sludge (SS) [6,7], and waste coffee grounds (WCGs) [8,9], have been explored for biodiesel production.

MA can synthesize organic matter through photosynthesis using carbon dioxide (a major greenhouse gas), water, and solar energy [10]. In addition, the productivity of the microalgal biomass is known to be 50 times higher than that of switchgrass, one of the fastest-growing plants [4,10,11]. However, Korea has a relatively high population density, and almost 70% of the land is mountainous. Moreover, since Korea has four distinct seasons, the time available for cultivating microalgae using open systems (such as raceway ponds) is restricted. In addition, closed photobioreactor systems are not economical for the cultivation of microalgae for biodiesel production [12].

SS could be used as a promising substitute for conventional biodiesel feedstocks because it has two unique advantages: (1) it is widely and consistently available, and (2) it contains considerable amounts of organic compounds, i.e., the total lipids extracted from sludge can reach 12-15% of the dry weight of the material [7,13,14]. The annual production of SS in Korea was 4 million tons in 2013 and is expected to increase to 5.4 million tons by 2025 [15,16]; these amounts are equivalent to 116 thousand tons and 157 thousand tons of dried SS, respectively, assuming a moisture content of 97.1% [16]. If all the SS generated in Korea could be used to produce biodiesel, it could yield 8,200 tons of biodiesel by 2025, assuming a 14.5% lipid content, 92.9% lipid extraction efficiency, and 39.0% biodiesel yield [7].

Coffee is produced and traded in large volumes internationally; the world’s coffee consumption reached 10.03 billion kg in 2021/22, 3.3% more than the previous year [17]. In Korea, 204 million kg of coffee was consumed in 2021/22, as compared to 167 million kg in 2018/19, representing an annual growth of 6.90% [17]. The Korean government estimated that 163 million kg of dried WCGs was discharged in 2021/22, assuming the mass of WCGs to be 79.8% of that of green coffee beans [18]. Coffee lipids comprise of 80-95% glyceride, which is readily converted to fatty acid methyl esters by transesterification [8,9].

Although these three sustainable feedstocks have been successfully used for biodiesel production in the laboratory or on a pilot scale, the available amount of each feedstock is minor as compared to the conventional feedstocks widely used for biodiesel production, such as soybean oil, palm oil, and rapeseed oil. Therefore, the use of a mixture of these feedstocks is necessary to meet the increasing requirement for biodiesel. In particular, SS and WCGs are consistently generated wastes that need to be disposed of, and valorization of these materials is beneficial to the environment as well as energy economics. It should be noted that each feedstock yields different compositions of fatty acid methyl esters, resulting in different physical properties of biodiesel, including heating value, viscosity, cetane number, flash point, cloud point, and pour point. Therefore, desirable physical properties of biodiesel according to seasons and geographical location could be obtained by controlling the mixing ratio of the individual feedstocks. Since the lipids from the mixture of feedstock as well as from each individual feedstock have different composition and free fatty acid (FFA) contents, reaction conditions such as the catalyst type, methanol loading, reaction temperature, and reaction time should be different as well. Therefore, reaction conditions depending on the individual type of feedstock, the type of feedstocks to be mixed and the mixing ratio of feedstocks should be optimized.

In this study, we produced biodiesel using a mixture of lipids from these sustainable feedstocks (MA, SS, and WCGs), as well as lipids from each feedstock individually. The effect of catalyst type and concentration on biodiesel production was investigated, and characteristics of the biodiesel produced using a lipid mixture were evaluated.

2.1. Feedstock of biodiesel and chemicals for lipid extraction

A large amount of the marine microalga Nannochloropsis sp. was donated for this study by a company that cultivates this microalga using tubular photobioreactors with a working volume of 50 L at the Marine Biology Center for Research and Education, Gangneung-Wonju National University in Korea. The microalgae were harvested and dried under sunlight for 5 d. The dried microalgae were ground and dried again in an oven at 70oC for 24 h, before their use in the experiments. SS was donated by a wastewater treatment facility located in the city of Gangneung, which discharged the sludge after mixing the thickened primary and secondary SS [7]. The SS was dried in a fume hood for 4 d, and the dried SS was ground and dried again in an oven at 105°C for 24 h, prior to their use in the experiments [7]. WCGs were collected from a café located at Gangneung-Wonju National University and dried at 105oC for 12 h before being used [9]. The photos of the three types of feedstocks and lipids extracted from these feedstocks for biodiesel production are presented in Fig. 1 and Fig. 2, respectively. Chloroform (99% purity, Wako, Japan), methanol (99% purity, Showa, Japan), and n-hexane (96% purity, Showa, Japan) were used as the lipid extraction solvents. Methanol and n-hexane were also used as an acyl donor and a co-solvent, respectively, for the transesterification of lipids during biodiesel production. Sulfuric acid (99% purity, Showa, Japan) and sodium hydroxide (98% purity, Daejung, Japan) were used as acid and alkaline catalysts, respectively, for the transesterification of the lipids with methanol. All the chemicals were of analytical grade. The standard materials for gas chromatography (GC) analysis (methyl palmitate, methyl oleate, methyl linoleate, methyl stearate, and methyl heptadecanoate) were purchased from Sigma–Aldrich, USA.

Figure 1. Three types of feedstock used in this study for biodiesel production. Five grams of each feedstock is placed in a dish: (from left) microalgae, waste sewage sludge, and waste coffee grounds.

Figure 2. Three types of lipids extracted from microalgae, waste sewage sludge, and waste coffee grounds (from left).

2.2. Analysis of free fatty acid content and biodiesel conversion

The lipid contents of MA and SS were measured according to the modified Bligh and Dyer method [4,19,20]. The lipid content of WCGs was determined using the ether extraction method (B-324/435/412, Buchi, Switzerland), based on AOAC [20,21]. The free fatty acid (FFA) content was determined according to the modified AOAC Official Method [20,21]. After 0.7 g of sample was introduced into a 150-mL flask, 10 mL of 95% ethanol and three drops of 1% phenolphthalein solution (1 g of phenolphthalein in 100 mL of ethanol) were added into the flask. Subsequently, this mixture was titrated using 0.13 N sodium hydroxide solution. The FFA content was calculated as a percentage of oleic acid, as instructed in the AOAC Official Method, using equation (1) [8,9,20].

FFAs as oleic acid(%)=alkaline volume (mL)×alkali normalitymass of sample(g)×28.2

Biodiesel conversion was determined using a gas chromatography (GC) (7850A, Agilent, USA) equipped with an autoinjector (G4513A, Agilent, China) to allow for constant volume injection of the sample, an HP-5 column (30 m × 0.32 mm × 0.25 μm film thickness) and a flame ionization detector (FID) [22]. The temperature of the injector and the detector was 250°C and that of the column was elevated from 150 to 250oC at 5oC/min after the oven temperature was initially maintained at 150oC for 2 min. Helium was used as the carrier gas. The GC was calibrated regularly by analyzing a standard solution of known concentration of the standard materials. Each biodiesel component was identified using a GC-Mass Spectrometer (6890 GC/5973i MSD; Agilent, USA), and the biodiesel conversion was quantified using equation (2) [22]:

Biodiesel conversion(%)=ATAC17AC17×V×CM×100

Here, AT and AC17 represent the total peak area of methyl esters and internal standard (methyl heptadecanoate, C17), respectively. V, C, and M represent the volume of the internal standard (mL), the concentration of the internal standard (mg/mL), and the mass of the sample (mg), respectively [9,22].

2.3. Lipid extraction

As per the protocol of previous studies, the lipid extraction solvent used for MA and SS was a mixture of chloroform and methanol [4,7] at a chloroform:methanol ratio of 2 : 1 for MA and 1 : 2 for SS [4,7]. n-Hexane was used as the extraction solvent for the WCGs [9]. The liquid–solid ratio for lipid extraction was 20 mL/g for MA and SS and 5 mL/g for WCGs [7,9]. Lipid extraction was carried out at 25oC in a 1 L flask containing 10 g MA, 10 g SS, or 50 g WCGs, which was placed in a shaking incubator at 150 rpm (SI-600R, Jeio Tech, Korea). After extraction, 100 mL of water was added to the flask for phase separation. The upper, organic layer was separated using a syringe (Kovax-Syringe, Korea Vaccine, Korea) and passed through a filter paper (No.2, 5 μm, Advantec, Japan) using an aspirator pump (VE-11, Jeio Tech, Korea). The extraction solvent was vaporized using a rotary evaporator (R-210, Buchi, Switzerland) to collect the lipids. Crude lipids without posttreatments, such as degumming, bleaching, or decoloring, were used to produce the biodiesel.

2.4. Biodiesel production process

One gram of the lipid was placed in a 30-mL vial along with methanol and a catalyst, at amounts dictated by the experimental design. n-Hexane was additionally added as a co-solvent for the dissolution of lipids and separation of biodiesel following the reaction [7]. The reaction conditions varied with the catalyst type. For an acid catalyst (H2SO4), the reaction conditions included 10 mL methanol/g-lipid, 5 mL-hexane/g-lipid, 60oC, and 12 h, whereas for an alkaline catalyst (NaOH), the reaction conditions were 10 mL methanol/g-lipid, 5 mL-hexane/g-lipid, 60oC, and 6 h of reaction time. Biodiesel production was carried out in a shaking incubator (BS-21, Jeio Tech, Korea) operating at 200 rpm. After the reaction was completed, 2 mL of distilled water was added to the vial, and the reaction mixture was centrifuged at 4000 rpm for 30 min (Gyro 1236MG, Gyrozen, Korea) for phase separation. The upper layer was separated and dried in a fume hood for 24 h to obtain the biodiesel. The biodiesel was analyzed using GC-Mass to determine its composition, and thereby GC to determine biodiesel conversion. All experiments were conducted in triplicate, and data are presented as the mean ± standard deviation.

3.1. Characteristics of each sustainable feedstock

The lipid contents of the MA, SS, and WCGs were determined to be 23.1%, 10.6%, and 12.8%, respectively. Though the content of FFAs in WCG lipids was only 1.7%, those in MA and SS lipids were 7.6% and 38.5%, respectively. The composition of fatty acid methyl esters (FAMEs) from each lipid was also analyzed and has been summarized in Table 1. For MA, methyl linoleate (C19:2, 54.6%), methyl palmitate (C17:0, 24.2 %), and methyl hexadecenoate (C17:2, 16.1%) accounted for 94.9% of the FAMEs. For WCGs, methyl linoleate (C19:2, 46.4%), methyl palmitate (C17:0, 34.5%), methyl oleate (C19:1, 8.9%), and methyl stearate (C19:0, 7.1%) comprised 96.9% of the FAMEs. Accordingly, the FAME composition of MA and WCGs was similar; in particular, methyl linoleate occupied approximately half of the FAMEs from MA or WCGs lipids. In contrast, the FAME composition of SS was diverse; that is, methyl palmitate (C17:0, 25.4%), methyl stearate (C19:0, 23.7%), methyl palmitate (C17:1, 15.5%), and methyl oleate (C19:1, 13.8%) were the major constituents; however, 21.6% of the compounds were not identified. Since SS lipids contain various chemicals, including steroids, terpenoids, polyhydroxyalkanoates, hydrocarbons, linear alkyl benzene, polycyclic aromatic hydrocarbons, pharmaceutical chemicals, etc. [23], which might have been designated as unidentified compounds during GC analysis.

Table 1 Comparison of the fatty methyl ester composition in biodiesel produced from microalgae, sewage sludge, and waste coffee ground lipids

FeedstockComponent (%)
C17:0C17:1C17:2C19:0C19:1C19:2Others
Microalgae24.2-16.10.9-54.64.2
Sewage sludge25.415.5-23.713.8-21.6
Waste coffee grounds34.5--7.18.946.42.4


3.2. Biodiesel production using lipids from a single feedstock

Biodiesel was produced using lipids from each feedstock with an acid or alkaline catalyst. An alkaline catalyst has many advantages over acid catalysts, such as a high reaction rate and reduced requirement for methanol [9,24], however, it cannot be applied to lipids with a high FFA content due to the saponification of the FFAs with alkaline ions. Previous studies prescribe an FFA-content limit of 0.5% [25,26], 1.0% [27-29], 2.0% [30], 3.0% [31,32], or 4.0% [33] to avoid saponification during biodiesel production. Since the threshold of FFA content for the possible use of an alkaline catalyst for biodiesel production seems to differ depending on the feedstock, catalyst, and reaction conditions, it should be experimentally determined in each case.

In this study, the effects of catalyst type and concentration on biodiesel conversion were investigated for the lipids from each feedstock. For MA lipids, as the acid catalyst concentration increases, so did the biodiesel conversion; however, for the alkaline catalyst, biodiesel conversion decreases with the increase in catalyst concentration, as shown in Fig. 3(a). The maximum biodiesel conversion was 72.6% at 3 wt% of the acid catalyst and 67.9% at 0.5 wt% of the alkaline catalyst. For SS lipids, the biodiesel conversion was the lowest (24.9%) at 0.5 wt%, which increased to 29.6% at 3 wt% of the acid catalyst. For the alkaline catalyst, the maximum conversion was 8.8% at 0.5 wt%, and the conversion decreased as the alkaline catalyst concentration increased. Since the FFA content of SS was as high as 38.5%, an alkaline catalyst was not suitable for biodiesel production. The low biodiesel conversion, even with the acid catalyst, may be due to the high level of polar lipids and unsaponifiable chemicals, such as hydrocarbon, sterols, and waxes in the extracted SS lipids [23,34,35]. A previous study reported that the amount of unsaponifiable chemicals accounted for 28% of SS lipids [35], which conformed with the value obtained in the present study (21.6%). For the WCG lipids, as the acid catalyst concentration increased from 0.5 wt% to 3.0 wt%, the biodiesel conversion increased from 15.4% to 39.0%. For the WCG lipids, the alkaline catalyst was readily used for biodiesel production, and the highest biodiesel conversion (93.5%) was achieved at 1.0 wt% of the alkaline catalyst concentration. The maximum biodiesel conversions for each lipid type under an acid or alkaline catalyst are summarized in Fig. 4.

Figure 3. Effect of catalyst type and concentration on biodiesel conversion. (a): Microalgae lipids, (b): sewage sludge lipids, (c): waste coffee ground lipids. ■: acid catalyst, ▨: alkaline catalyst.

Figure 4. Maximum biodiesel conversion for the lipids derived from various sustainable feedstocks.
■: acid catalyst, ▨: alkaline catalyst

These results imply that WCG lipids are suitable for biodiesel production using an alkaline catalyst, while SS lipids may not be suitable for biodiesel production or may require additional purification. For MA lipids, the high FFA content could be problematic, but approximately 70% biodiesel conversion was obtained under an alkaline catalyst. However, MA lipids are known to contain polar lipids, such as galactolipids and phospholipids, and some other impurities, including carbohydrates, proteins, and pigment [33,36]. Therefore, MA lipids also require some purification to ensure a higher biodiesel conversion. However, the cost of production would be significantly increased when a purification process is added. The development of purification processes is outside of the scope of this study, but will be addressed in a future study.

3.3. Biodiesel production using a mixture of lipids from two types of feedstock

3.3.1. Mixture of MA and SS lipids

Biodiesel is produced using a mixture of MA and SS lipids, in various proportions (1 : 3 to 3 : 1), and the results are shown in Fig. 5(a). The mixing ratio is presented as the proportion of MA lipids (PMAL) in the mixture. For example, when the mixing ratio of MA lipids to SS lipids was 1 : 3, the PMAL was 1/4 or 25%, while the proportion of SS lipids (PSSL) was 3/4 or 75%. The biodiesel conversion using an acid catalyst is higher than that achieved using an alkaline catalyst for all the PMALs, as can be seen in Fig. 5(a). The biodiesel conversion was less than 2% when an alkaline catalyst was used, irrespective of the PMAL. For the acid catalyst, the biodiesel conversion was about 5% at 1/4 of PMAL and increased as the PMAL increased. When the PMAL was 3/4, the biodiesel conversion was the highest (38.6%). Accordingly, although an acid catalyst is relatively applicable for biodiesel production from a mixture of MA and SS lipids, the biodiesel conversion remains low.

Figure 5. Biodiesel production using a mixture of microalgal and sewage sludge lipids. (a) Effect of catalyst type and lipid mixing ratio on biodiesel conversion. ■: acid catalyst, ▨: alkaline catalyst. (b) Estimated free fatty acid (FFA) content and biodiesel conversion and actual biodiesel conversion, for the proportion of microalgal lipids. ■: estimated FFA content, ●: actual biodiesel conversion, ○: estimated biodiesel conversion.

The FFA content of the lipid mixture was estimated using equation (3), where X is the PMAL, which would be a reasonable calculation because no chemical reaction occurs when these lipids are mixed. The biodiesel conversion using an acid catalyst was also approximately estimated by a simple linear arithmetic equation (4).

FFA(%)=7.6X+38.5(1X)=30.9X+38.5 (0X1.0)
Biodiesel conversion (%)=72.6X+29.6(1X)=43.0X+29.6

The FFA content of this mixture was expected to be 15.3–30.8% for the experimental ranges, which is considerably higher than the value at which saponification occurs. Fig. 5(b) shows the estimated FFA content, estimated biodiesel conversion, and actual biodiesel conversion with an acid catalyst according to the PMAL. As the PMAL increased, the FFA content decreased and biodiesel conversion increased. The actual biodiesel conversion was less than 40%, which is much lower than that estimated by equation (4) (40.4 - 61.9%); however, the difference between the estimated and the actual biodiesel conversion decreased as the PMAL increased. This may imply that SS lipids (with a high percentage of impurities and unsaponifiable matter) adversely influence biodiesel production from a mixture of MA and SS lipids, resulting in a lower biodiesel conversion than expected.

3.3.2. Mixture of WCG and MA lipids

Biodiesel is produced using a mixture of WCG and MA lipids at various mixing ratios, and the results are shown in Fig. 6(a). Although the biodiesel conversion increased with the increase in the proportion of WCG lipids (PWCL) under an acid catalyst, this increase was minor, from 2.2% at 1/4 PWCL to 9.1% at 3/4 PWCL. In contrast, biodiesel conversion under an alkaline catalyst increased markedly when the PWCL was 1/2 or more. As the PWCL increased from 1/3 to 1/2 and 3/4, the biodiesel conversion increased from < 3% to 77.1% and 89.5%, respectively. Therefore, it can be concluded that an alkaline catalyst is suitable for biodiesel production from a mixture of MA and WCG lipids. As described above, the FFA content of the lipid mixture and the biodiesel conversion under an alkaline catalyst was estimated using linear arithmetic equations (5) and (6), respectively, where X is the PWCL.

Figure 6. Biodiesel production using a mixture of waste coffee ground and microalgal lipids. (a) Effect of catalyst type and lipid mixing ratio on biodiesel conversion. ■: acid catalyst, ▨: alkaline catalyst. (b) Estimated free fatty acid (FFA) content and biodiesel conversion and actual biodiesel conversion, for the proportion of waste coffee ground lipids. ■: estimated FFA content, ●: actual biodiesel conversion, ○: estimated biodiesel conversion.

FFA(%)=1.7X+7.6(1X)=5.9X+7.6 (01.0)
Biodiesel conversion (%) =93.5X+67.9(1X)=25.9X+67.9 (01.0)

The FFA content estimated by equation (5) ranged from 3.1 to 6.1%, which is slightly higher than the threshold FFA value to avoid saponification by an alkaline catalyst [31-33]. Fig. 6(b) shows that the actual biodiesel conversion is similar to the estimated value when the PWCL is higher than 1/2, at which the FFA content is estimated to be 4.7%. Accordingly, a mixture of WCG and MA lipids would be a good choice for biodiesel production, if the PWCL is at least 50%.

3.3.3. Mixture of WCG and SS lipids

Biodiesel is produced using a mixture of WCG and SS lipids at various mixing ratios, and the results are presented in Fig. 7 (a). With an acid catalyst, the biodiesel conversion was slightly influenced by the PWCL, i.e., 20.6% at 1/4 of the PWCL and 27.7% at 3/4 of the PWCL. Contrastingly, with an alkaline catalyst, the biodiesel conversion generally increased with an increase in the PWCL, and it markedly increased when the PWCL was 1/2 and higher. The biodiesel conversion was 4.7% at 1/3 of the PWCL, which increased to 60.6% and 70% at PWCL of 1/2 and 3/4, respectively. Accordingly, an alkaline catalyst was found to be suitable for biodiesel production from a mixture of WCG and SS lipids.

Figure 7. Biodiesel production using a mixture of waste coffee ground and sewage sludge lipids. (a) Effect of catalyst type and lipid mixing ratio on biodiesel conversion. ■: acid catalyst, ▨: alkaline catalyst. (b) Estimated FFA content and biodiesel conversion and actual biodiesel conversion, for the proportion of waste coffee ground lipids. ■: estimated FFA content, ●: actual biodiesel conversion, ○: estimated biodiesel conversion.

The FFA content of the lipid mixture and biodiesel conversion under an alkaline catalyst was estimated using linear arithmetic equations (7) and (8), where X is the PWCL.

FFA(%)=1.7X+38.5(1X)=36.8X+38.5 (01.0)
Biodiesel conversion (%) =93.5+23.1(1X)=70.4X+23.1 (01.0)

The FFA content estimated by equation (7) ranges from 10.9% to 29.3%, which is much higher than the threshold value for avoiding saponification. However, Fig. 7(b) shows that the actual biodiesel conversion is very similar to the estimated value when the PWCL was 1/2 and higher, and it reached 70.1% at a PWCL of 3/4. Thus, an alkaline catalyst can be readily used for biodiesel production from a mixture of WCG and SS lipids, provided the PWCL is 1/2 or more. The reason for the relatively high biodiesel conversion despite the high FFA content may be the large amounts of methanol (10 mL) and hexane (5 mL) that were added, which diluted the FFA content in the reaction mixture, thereby mitigating saponification and facilitating biodiesel production.

3.4. Mixture of MA, SS, and WCG lipids

Biodiesel was produced using a mixture of lipids derived from all three types of feedstock. Since the lipids from WCGs were found to have a positive effect on biodiesel conversion, it was considered that the PWCL should be as high as possible. However, the generated amount of WCGs is much smaller than that of SS in Korea. Moreover, the lipid content of WCGs is approximately half of that of MA. These factors limit the use of high PWCL. Therefore, we set the mixing ratio of the three lipids as 1 : 1 : 1 and then investigated the effects of catalyst type and concentration on biodiesel conversion. As shown in Fig. 8, the alkaline catalyst yields a higher biodiesel conversion than the acid catalyst throughout the experimental range of 0.5-3.0 wt% catalyst concentration. The biodiesel conversion steadily increased with an increase in the catalyst concentration; however, the increase was minor for an increase in the alkaline catalyst from 2.0 wt% (68.2%) to 3.0 wt% (71.8%). Interestingly, a relatively high biodiesel conversion was achieved even at a high FFA content estimated at 15.9%. As stated above, the large volume of methanol and hexane may have diluted the FFAs in the lipids, facilitating biodiesel production. The resultant GC chart is presented in Fig. 9 where each component of biodiesel is identified. Biodiesel has a slightly larger energy density compared to gasoline, by 5%, but is smaller than fossil diesel by 7% [37]. Accordingly, it can be said that biodiesel, gasoline, and fossil diesel have similar energy densities. Since the heating value of biodiesel is 119,550-127,960 Btu/gal, corresponding to 38.09-40.8 kJ/g or 9.10-9.74 kcal/g, abiodiesel conversion of 71.8% implies that 27.3-29.3 kJ/g or 6.5-7.0 kcal/g of energy can be produced from the lipid mixture [37].

Figure 8. Effect of catalyst type and concentration on biodiesel conversion using a mixture composed of equal amounts of microalgae, sewage sludge, and waste coffee ground lipids. ●: acid catalyst, ○: alkaline catalyst.

Figure 9. GC chart of biodiesel from the mixture of an equal portion of lipids from microalgae, waste sewage sludge, and waste coffee grounds.

In this study, the MA, SS, and WCGs contained 23.1%, 10.6%, and 12.8% of lipids, respectively. The FFA contents were 7.5%, 38.5%, and 2.1% for MA, SS, and WCG lipids, respectively. The maximum biodiesel conversion was approximately 65% for MA lipids alone, with either an acid or alkaline catalyst; 29.3% for SS lipids alone, with an acid catalyst; and 93.5% for WCG lipids alone, with an alkaline catalyst. The maximum biodiesel conversion from lipid mixtures was observed to be 40% for MA + SS lipids, with an acid catalyst at a PMAL of 3/4; 90% for MA + WCG lipids, with an alkaline catalyst at a PWCL of 3/4; and > 60% for WCG + SS lipids, with an alkaline catalyst at a PWCL higher than 1/2. When MA, SS, and WCG lipids were mixed in equal proportions, the biodiesel conversion was approximately 70% at 3 wt% of an alkaline catalyst. This study showed that WCG lipids play an important role in achieving a high biodiesel conversion from a lipid mixture. Moreover, by manipulating the proportion of WCG lipids in the mixture, we could use an alkaline catalyst. An alkaline catalyst many advantages over an acid catalyst, including a higher biodiesel conversion rate. Finally, this study suggests that it is possible to economically produce biodiesel at a larger scale using various types of sustainable feedstock.

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Korea (2018R1D 1A1B07051113 and 2022R1F1A1072189). The authors greatly appreciate this support.

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