Efficient Free Fatty Acid Reduction in Palm Oil Mill Effluent (POME) for Biodiesel Production: Challenges and Optimization Strategies

Abstract

The increasing demand for fossil fuels has led the oil industry to explore biodiesel as a renewable alternative, which is crucial for advancing planetary health. Biodiesel offers environmental benefits and shares similar properties with petroleum diesel, making it a promising substitute. However, Palm Oil Mill Effluent (POME), containing sludge palm oil (SPO), presents challenges due to its high free fatty acid (FFA) content. This study proposes novel optimization strategies to reduce FFAs in SPO and improve biodiesel yield. A combination of base neutralization, esterification, and transesterification processes was employed. Neutralization with sodium hydroxide (NaOH) at concentrations ranging from 0.1% to 0.5% w/w was followed by esterification using sulfuric acid (H₂SO₄) with varying methanol-to-oil ratios. The optimal FFA reduction of 2.26% was achieved at a 6:1 methanol ratio. Transesterification with a 7:1 methanol-to-oil ratio yielded the highest biodiesel output of 71.25%. The biodiesel met ASTM standards, with a calorific value of 40.01 MJ/kg, a flash point of 180.5 °C, and a density of 0.86 g/cm³. Economic analysis estimates an annual net profit of USD 244,901,600, demonstrating that this approach provides a financially viable solution while advancing planetary health by reducing dependency on fossil fuels, mitigating climate change, and supporting sustainable fuel production.

1. Introduction

The oil palm (Elaeis guineensis) is a rapidly growing tropical perennial tree, widely cultivated for its oil, which is primarily used in food, cosmetics, and biofuel production [1,2]. As the demand for palm oil increases globally, the palm oil industry has expanded, driven by its applications in various sectors. However, processing palm oil generates considerable amounts of waste, including both dry and wet waste, which can be utilized for biofuel production [3]. These byproducts present an opportunity for addressing environmental concerns while meeting energy demands.

Environmental factors such as annual rainfall (more than 1800 mm), temperatures between 20 °C and 40 °C, and more than 5 h of daily sunlight make Sri Lanka suitable for oil palm cultivation [4]. In Sri Lanka, oil palm cultivation began in 1968 with a modest 68 plants in 0.5 ha of the Nakiyadeniya Estate, expanding to 20 hectares. Since then, oil palm cultivation has spread across the southern regions, including Galle, Matara, and Kalutara. The island now has palm oil extraction stations at Nakiyadeniya and Baduraliya [5].

The palm oil extraction process yields crude palm oil (CPO) from the mesocarp, which accounts for about 95% of the total oil produced [6], and palm kernel oil (PKO), which is produced in smaller quantities. However, this process also generates various types of waste, such as empty fruit bunches, fibers, Palm Oil Mill Effluent (POME), boiler ash, and decanter cake [7]. Among these, POME contains sludge palm oil (SPO), which is separated during the treatment of POME [8].

Palm Oil Mill Effluent (POME) is a byproduct of the palm oil extraction process and is known for its high organic content, which can cause significant environmental problems if not properly managed [8,9]. POME contains large amounts of biochemical oxygen demand (BOD) and chemical oxygen demand (COD), which, when released into water bodies without treatment, can lead to oxygen depletion, harming aquatic life. It also contributes to the contamination of soil and water sources with high concentrations of nutrients, leading to eutrophication, which results in algal blooms and the further degradation of aquatic ecosystems [10]. Additionally, POME often contains suspended solids, oils, and fats that contribute to pollution and pose challenges to waste management systems. Improper disposal of POME can cause soil degradation, lead to POME leaching into groundwater, and contribute to greenhouse gas emissions, exacerbating climate change concerns [11]. Effective treatment and sustainable management of POME are essential to mitigate its environmental impact. POME represents a significant challenge to the palm oil industry due to its large volume, environmental impact, high organic content, and the complexity and cost of treatment. Effectively managing and reducing the environmental footprint of POME is crucial for the industry to ensure sustainability, regulatory compliance, and the efficient use of resources. This has led to the exploration of more sustainable methods for POME treatment, including using it as a feedstock for biodiesel production, which can help mitigate some of the waste management challenges.

Biodiesel, produced from renewable sources like vegetable oils and waste oils, offers significant environmental benefits compared to fossil fuels, supporting the goals of planetary health by reducing the ecological impact of energy production [12]. It is biodegradable and non-toxic and produces fewer greenhouse gases, sulfur oxides, and particulate matter, making it an environmentally safer alternative. Biodiesel helps reduce the reliance on fossil fuels, lower carbon emissions, and mitigate climate change, which are essential for the long-term health of the planet and its ecosystems [13]. Additionally, using waste oils such as Palm Oil Mill Effluent (POME) for biodiesel production not only provides a sustainable waste management solution but also contributes to the circular economy by transforming waste into a renewable energy source. By integrating waste reuse with clean energy production, biodiesel plays a pivotal role in promoting cleaner, more sustainable energy systems, thus contributing to the preservation of planetary health and ensuring a balanced relationship between human activity and the earth’s resources [14].

Biodiesel production involves several key steps to convert oils or fats into fatty acid methyl esters (FAMEs) [15]. The process begins with the selection and preparation of the feedstock, typically including vegetable oils or animal fats, which are filtered to remove impurities. If the feedstock contains high levels of free fatty acids (FFAs), esterification is required before the main transesterification process. Esterification involves reacting FFAs with methanol in the presence of an acid catalyst, such as sulfuric acid, to produce methyl esters and water [16]. This reduces the FFAs to a suitable level for the next step, ensuring efficient biodiesel production.

The primary step in biodiesel production is transesterification, where triglycerides in the feedstock react with methanol, using a base catalyst (such as sodium hydroxide), to produce biodiesel (methyl esters) and glycerol [17]. This reaction takes place under controlled temperature and pressure conditions. After the transesterification process, the biodiesel is separated from glycerol, and the resulting product undergoes purification. The biodiesel is washed with water to remove impurities and dried to eliminate residual moisture. Finally, quality control tests ensure the biodiesel meets industry standards, such as ASTM D6751 [18] or EN 14214 [19], before it is ready for use [20].

Testing the parameters of biodiesel is crucial to ensure its quality and compliance with industry standards. Key parameters such as viscosity, density, acid value, and cetane number are assessed to determine the fuel’s performance and suitability for use in engines. Viscosity and density are measured to ensure proper flow and combustion characteristics [21,22], while the acid value indicates the level of impurities and free fatty acids in the biodiesel. The cetane number is tested to evaluate the ignition quality of the biodiesel, which impacts engine efficiency and emissions. Additionally, other tests such as those used to determine the flash point, cloud point, and oxidative stability are performed to ensure that the biodiesel is safe, stable, and meets regulations such as the ASTM D6751 or EN 14214 standards [23].

Palm Oil Mill Effluent (POME) can be a valuable feedstock for biodiesel production, offering an eco-friendly solution to both waste management and renewable energy generation. POME, rich in free fatty acids (FFAs), can be processed through esterification to convert these acids into biodiesel [14]. For biodiesel made from palm oil (FAME), the CFPP is particularly important as it can be higher compared to biodiesel from other feedstocks. However, the fulfillment of CFPP requirements can be optimized through blending or by modifying the production process [24].

Optimizing free fatty acid (FFA) reduction is crucial in biodiesel production because high FFA levels can hinder the transesterification process, leading to lower biodiesel yields and compromised fuel quality [25]. By reducing FFAs through methods like alkaline treatment and esterification, the biodiesel production process becomes more efficient, maximizing the yield from the feedstock and improving overall fuel quality [26]. This optimization is essential for ensuring a higher conversion rate, better catalyst utilization, and a more cost-effective and sustainable biodiesel production process.

This research study focuses on the feasibility of using alkaline treatment for free fatty acid (FFA) reduction and esterification to optimize conditions for maximum FFA reduction in the production of biodiesel. The process begins with the use of an alkaline catalyst, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH), to reduce the free fatty acid content of the feedstock [27], which is essential for enhancing the efficiency of the subsequent transesterification reaction. Since high levels of FFAs can inhibit the transesterification process, reducing these acids is critical for improving biodiesel yield and quality. The study explores various reaction conditions, including the concentration of the alkaline catalyst, reaction time, and temperature, to identify the optimal parameters that lead to the maximum reduction in FFAs.

Additionally, esterification is applied to further reduce FFAs [28], especially in cases where the FFA content is still high after the alkaline treatment. By reacting FFAs with methanol in the presence of an acid catalyst, such as sulfuric acid, the esterification process converts the FFAs into methyl esters, improving the feedstock’s suitability for biodiesel production.

This study aims to optimize the esterification process by investigating the ideal conditions, such as the methanol-to-oil ratio, temperature, and catalyst concentration, to achieve the highest reduction in the FFA content. The results of this research will provide valuable insights into the most efficient methods for reducing FFAs and improving biodiesel production, contributing to more sustainable and cost-effective biodiesel production methods.

2. Methodology

This section outlines the procedures followed for sample collection, oil extraction, and the subsequent analysis of the sludge palm oil used for biodiesel production. The methodology includes the preparation and treatment of sludge palm oil and its characterization, and the experimental steps for esterification and transesterification are shown in Figure 1.

2.1. Sample Collection

Biodiesel production from sludge palm oil serves as an effective solution for managing the waste byproducts of oil palm mills. The sludge palm oil used in this study was sourced from a palm oil mill in Sri Lanka that processes crude palm oil. The oil extraction process uses large quantities of water, resulting in the generation of Palm Oil Mill Effluent (POME), which contains small amounts of oil, commonly referred to as sludge palm oil.

In the initial stage of POME treatment, wastewater is stored in tanks where sludge palm oil and Palm Oil Mill Effluent are separated by their density differences. As sludge palm oil has a lower density than the effluent, it floats to the top. A typical palm oil mill generates 250 m³ of effluent daily, with approximately 1% of this being sludge palm oil.

2.2. Oil Extraction from Sludge Palm Oil

The sludge palm oil extracted from the wastewater treatment unit contains a mixture of oil, water, and solid impurities. To purify the oil, the centrifugation method is employed to remove the water and solid particles.

Centrifugation Process: This process is repeated multiple times to ensure maximum oil recovery, as a single centrifugation cycle yields a low amount of purified oil. Centrifugation is performed in a Multifuge X1 Pro-MD Centrifuge (manufactured by Thermo Fisher Scientific Inc., Waltham, MA 02451, USA) at 4500 RPM for 15 min. The mixture then separates into two layers based on density, with the upper layer containing purified oil and the lower layer consisting of solid particles.

Oil Collection: The volume of the separated oil was measured, and the fraction of oil recovered was calculated by dividing the volume of the purified oil by the total volume of the initial sample. This fraction provided an indication of the efficiency of the centrifugation process in removing impurities and water from the raw sludge palm oil. The higher the fraction of separable oil, the more effective the centrifugation process in purifying the oil.

2.3. Measurement of Calorific Value of Raw Oil

The calorific value (CV) of the raw sludge palm oil was determined using a CKIC 5E-C5508 Automatic Calorimeter (manufactured by CKIC in Changsha, China) using the ASTM-D5865 [29] standard, which measures the energy released when the oil is completely combusted in an oxygen-rich environment. A small, accurately weighed sample of the raw sludge palm oil (approximately 1 g) was placed in the calorimeter’s combustion chamber. The chamber was then sealed, and the sample was ignited in a controlled environment with oxygen. The heat released during combustion raised the temperature of the surrounding water, and this change in temperature was used to calculate the calorific value of the oil. The CV provides insights into the energy content of the oil, which is a crucial parameter for biodiesel production.

2.4. Analysis of Sludge Palm Oil Free Fatty Acid Content

Preparation of NaOH Solution: To prepare the sodium hydroxide (NaOH) solution, a 0.1 M concentration was selected based on industrial practices and the literature [30,31]; 4.0 g of analytical-grade NaOH with a purity of ≥98% was dissolved in 500 mL of distilled water in a beaker, stirred, and then transferred to a 1 L volumetric flask. The volume was increased to 1 L with distilled water, and the solution was mixed thoroughly.

Sample Preparation and Titration: A 5 g sample of sludge palm oil was placed in a clean titration flask. To the flask, 20 mL of methanol was added, and the mixture was thoroughly stirred to dissolve the sample. The solution was then heated at 60 °C for 5 min to ensure complete dissolution of the free fatty acids. After heating, 3 drops of phenolphthalein indicator were added to the solution [30,32]. The FFA content was then measured by titration with the prepared 0.1 M NaOH solution. Titration continued until the solution changed from colorless to faint pink, indicating the neutralization of the free fatty acids.

2.5. Alkaline Neutralization Procedure

Preparation of NaOH Solutions: Sodium hydroxide solutions of varying concentrations (0.1%, 0.2%, 0.3%, 0.4%, and 0.5%) were prepared by dissolving NaOH in water. For example, to prepare a 0.1% NaOH solution, 0.1 g of NaOH was dissolved in 100 mL of water.

Sample Preparation and Titration: The sludge palm oil was heated to 60 °C and mixed with different concentrations of NaOH solution. After the reaction, the final amount of oil was measured (

Table 1. Sodium hydroxide solutions of varying concentrations.

Parameter	0.1% NaOH	0.2% NaOH	0.3% NaOH	0.4% NaOH	0.5% NaOH
NaOH Concentration	0.10%	0.20%	0.30%	0.40%	0.50%
NaOH-to-Oil Ratio	1:3	1:3	1:3	1:3	1:3
Reaction Time	30 and 120 min	30 and 120 min	30 and 120 min	30 and 120 min	30 and 120 min
Temperature	60 °C	60 °C	60 °C	60 °C	60 °C

). The FFA content was assessed by titrating the mixture, as described in Section 2.4.

2.6. Esterification

The esterification reaction was carried out using methanol-to-oil molar ratios of 3:1, 6:1, and 9:1. To each mixture, sulfuric acid was added as a catalyst at 1–2% by weight of the oil to facilitate the reaction. An amount of 60 mL of sludge palm oil was mixed with the respective amount of methanol and the catalyst, and the mixture was then heated to 55 °C and stirred for 1 h in a water bath. This reaction resulted in the formation of an ester layer (biodiesel) and a water layer containing glycerol and byproducts. After the reaction, the mixture was transferred to a separation funnel, where the layers were allowed to settle. The upper layer, consisting of the esterified oil (biodiesel), was carefully separated and measured. The lower layer, containing water and methanol, was also separated and weighed. It is important to note that the unreacted oil would likely remain in the upper layer but in a non-esterified form. Therefore, any oil that did not react would still be present in this layer, potentially affecting the final yield of biodiesel [33]. The final FFA content was determined after the reaction to evaluate the reduction in FFA from the initial oil. For each methanol-to-oil ratio, the ester and water yields were recorded, and the FFA content was reduced from 8.60% in the raw oil to 1.69% after esterification with a 9:1 methanol-to-oil ratio.

2.7. Transesterification

Transesterification of sludge palm oil was carried out to convert triglycerides into biodiesel, using methanol as the alcohol and sodium hydroxide (NaOH) as the catalyst. The experiment was conducted with different methanol-to-oil molar ratios of 6:1, 7:1, and 8:1. For each reaction, 100 mL of oil reacted with a calculated amount of methanol (25 mL for the 6:1 ratio, 28.88 mL for the 7:1 ratio, and 26.2 mL for the 8:1 ratio) and 1 g of NaOH for the 6:1 and 7:1 ratios and 0.8 g of NaOH for the 8:1 ratio. The amount of NaOH was kept constant for the 6:1 and 7:1 ratios as these were closer in terms of the methanol content. For the 8:1 ratio, the reduced NaOH amount was chosen based on previous studies [34] suggesting that higher methanol quantities could reduce the need for excess catalyst. The NaOH was dissolved in methanol to form a sodium methoxide solution, which was added to the oil. The mixture was then stirred at 300 RPM and heated at 60 °C for 1 h in a water bath to allow the transesterification reaction to occur. After the reaction, the mixture was transferred to a separation funnel, where the biodiesel (upper layer) and glycerol (lower layer) were allowed to separate. The biodiesel was carefully collected and measured before being washed with warm water at 50 °C to remove residual methanol, soap, and glycerol traces. After washing, the biodiesel was vacuum-dried at 50 °C to remove any remaining water.

The final biodiesel yield was calculated based on the weight and volume of the biodiesel collected, with the highest biodiesel yield being observed at the 8:1 methanol-to-oil ratio, which yielded 71.25% biodiesel after washing and drying. Glycerol yields were also recorded, with the amount of glycerol decreasing as the methanol-to-oil ratio increased, indicating higher conversion efficiency at higher methanol ratios.

2.8. Blended Biodiesel Preparation

Blended biodiesel samples were manufactured at varying biodiesel concentrations, including B30 (30% biodiesel, 70% diesel), B20 (20% biodiesel, 80% diesel), B10 (10% biodiesel, 90% diesel), and B05 (5% biodiesel, 95% diesel). The blends were prepared by volume to ensure accurate and reproducible mixing of biodiesel and diesel. These blends were created to assess the impact of a varying biodiesel content on the fuel properties and performance characteristics. The biodiesel used in the blends adhered to ASTM D6751 standards, ensuring it met the required quality specifications for compatibility with conventional diesel engines. Fuel properties such as the viscosity, density, and flash point were characterized for each blend to ensure they fell within the acceptable range for engine operation.

2.9. Biodiesel Property Analysis Procedure

The following instruments and standards were used to characterize the properties of biodiesel derived from Palm Oil Mill Effluent (POME), produced using a standard transesterification procedure followed by blending.

2.9.1. Calorific Value (CV) Determination

The calorific value of the POME-derived biodiesel was measured using a CKIC 5E-C5508 Automatic Calorimeter (manufactured by CKIC in Changsha, China) following the ASTM D5865 standard [29]. A known mass (approximately 0.5 g) of the biodiesel sample was placed into a sealed oxygen bomb along with a known quantity of oxygen. The sample was ignited electrically, and the heat released from the combustion was absorbed by a known mass of water surrounding the bomb. The rise in water temperature was measured and used to calculate the calorific value of the biodiesel, expressed in MJ/kg.

2.9.2. Flash Point Measurement

The flash point of the biodiesel was determined using the PMA 500 flash point tester (manufactured by Anton Paar in Graz, Austria) in accordance with the ASTM D93B [35] standard. Approximately 50 mL of biodiesel was placed in the tester’s cup, and the sample was heated at a controlled rate while a small flame was periodically applied to the surface. The flash point was defined as the temperature at which the vapors from the biodiesel ignited momentarily. The test was conducted in a closed cup to prevent vapor loss, ensuring the accuracy of the results. The flash point was recorded in degrees Celsius.

2.9.3. Density and Viscosity Measurements

The density and viscosity of the biodiesel were measured using the SMV 3001 Automatic Kinetic Viscometer (manufactured by Anton Paar in Graz, Austria) in compliance with ASTM D396 [36]. For density measurement, the viscometer was calibrated, and the biodiesel sample was placed in the instrument’s sample chamber. The instrument calculated the density at a specified temperature (typically 15 °C or 20 °C) [37]. The viscosity of the biodiesel was determined at 40 °C, where the biodiesel sample was introduced into the viscometer, and the instrument measured its dynamic viscosity in mm²/s. The results were then used to assess the fuel’s flow characteristics, which are crucial for engine performance.

2.9.4. Thermal Conductivity Measurement

The thermal conductivity of the POME-derived biodiesel was measured using the LAMBDA Thermal Conductivity Meter (manufactured by LAMBDA Instruments, CH-6340 Baar Switzerland) in compliance with ASTM D7896-19 [38]. Thermal conductivity was measured across a temperature range of 30 °C to 55 °C using the Hot Wire Method, where the biodiesel sample was placed in direct contact with the thermal probe of the meter and a constant heat source was applied to the probe. The resulting temperature changes in the sample were monitored. The thermal conductivity (expressed in W/m·K) was calculated based on the temperature gradient and heat transfer rate through the biodiesel at each temperature point. This range was selected to simulate typical operational conditions and to assess how the biodiesel responds to heat during combustion. The instrument was calibrated using standard reference materials (ASTM E1952-21 [39]) with known thermal conductivities to ensure accuracy.

2.10. Cost Benefit and Sensitive Analysis of POME Management vs. Biodiesel Production

This cost–benefit analysis compares the costs and potential revenues of managing Palm Oil Mill Effluent (POME) through traditional disposal methods versus converting it into biodiesel. The analysis includes a breakdown of treatment costs, biodiesel production costs, and the potential revenue from biodiesel sales.

A sensitivity analysis was performed by varying key input parameters from their base case values. The following parameters were selected for the analysis:

• POME Treatment Cost: A cost of USD 0.50 per m³ of POME was assumed in the base case, with variations of ±10%, ±20%, and ±30%.
• Biodiesel Selling Price: The base price for biodiesel was assumed to be USD 2.00 per liter, with variations of ±10%, ±20%, and ±30%.
• Biodiesel Production Cost per Liter: The base production cost was assumed to be USD 1.50 per liter, with variations of ±10%, ±20%, and ±30%.

For each parameter, the corresponding annual net profit was recalculated, holding all other parameters constant.

3. Results and Discussion

3.1. Centrifugation of Sludge Palm Oil

The centrifugation process was used to purify the raw sludge palm oil by separating water and solid impurities from the oil. Centrifugation was performed at different speeds, and the amount of oil recovered was measured. The experiment data is given in

Table 2. Centrifugation results of sludge palm oil at different speeds and times.

Centrifugation Speed	Centrifugation Time	Initial Oil Weight (g)	Final Oil Weight (g)	Oil Recovery (w/w%)
4500 RPM	15 min	50	48.2	96.40%

.

The results show that increasing the centrifugation speed and time improved oil recovery. At a speed of 4500 RPM for 15 min, the oil recovery rate was 96.4%.

3.2. Initial FFA Content of Raw Sludge Palm Oil

The free fatty acid (FFA) content of the raw sludge palm oil was measured before any treatment to assess its initial quality. The FFA content was found to be 10.09%, which is high and indicates the need for further purification.

3.3. Alkaline Neutralization Results

The sludge palm oil was treated with various concentrations of sodium hydroxide (NaOH) to reduce the FFA content. The treatment was carried out at different NaOH concentrations (0.1%, 0.2%, 0.3%, 0.4%, and 0.5%) and reaction times (30 min and 120 min) as shown in

Table 3. Effect of NaOH treatment on FFA reduction in sludge palm oil.

NaOH Concentration	Reaction Time	Initial FFA (%)	Final FFA (%)	Final Oil Weight (%)	Soap Weight (%)
0.10%	30 min	10.09%	8.84%	84.64%	14.71%
0.10%	120 min	10.09%	7.91%	84.41%	15.35%
0.20%	30 min	10.09%	7.58%	83.37%	16.62%
0.20%	120 min	10.09%	7.04%	81.87%	18.12%
0.30%	30 min	10.09%	5.03%	77.70%	22.29%
0.30%	120 min	10.09%	3.86%	71.44%	28.55%
0.40%	30 min	10.09%	3.87%	69.34%	30.65%
0.40%	120 min	10.09%	3.62%	68.75%	31.24%
0.50%	30 min	10.09%	3.53%	66.85%	33.14%
0.50%	120 min	10.09%	3.48%	64.89%	35.10%

. After the treatment, the FFA content was measured.

Increasing the NaOH concentration and the reaction time resulted in a greater reduction in FFA. The highest reduction in FFA (to 3.48%) was achieved with 0.5% NaOH after 120 min. This also came with a trade-off of increased soap formation (35.10%). So, achieving a low FFA and minimizing soap production is a balancing act. Lower NaOH concentrations (0.10% or 0.20%) show reduced soap formation with insufficient FFA reduction for biodiesel production.

3.4. Transesterification of Alkaline Neutralized Sludge Palm Oil

Despite replicating the transesterification process three times, we were unable to successfully obtain biodiesel from the oil samples. After alkaline neutralization, which reduced the free fatty acid (FFA) content to 3.48%, the reaction was carried out following soap removal via filtration, using a 6:1 molar ratio of methanol to oil. However, each attempt resulted in the mixture solidifying, with only a thin layer of liquid, preventing the proper separation of biodiesel and glycerol.

Several factors can contribute to the failure of the transesterification process. The residual FFA level of 3.48%, though lower, was still relatively high for alkaline-catalyzed transesterification. Alkaline catalysts are most effective when the FFA content is below 1–2%. Higher FFA content can lead to soap formation, which causes emulsification and impedes phase separation, thus preventing the formation of biodiesel.

Additionally, the excess methanol used in the reaction (6:1 molar ratio) may have hindered phase separation. If the temperature was not maintained within the optimal range (55–65 °C), this could have further reduced the efficiency of the reaction, causing incomplete conversion and resulting in the solidification of the mixture.

3.5. Esterification of Sludge Palm Oil

Esterification was conducted at different methanol-to-oil molar ratios (3:1, 6:1, and 9:1) to further reduce the FFA content and produce biodiesel. The esterification results in

Table 4. Esterification results for different methanol-to-oil ratios.

Methanol-to-Oil Ratio	Methanol (mL)	Oil (mL)	Ester Yield (w/w)	Water Yield (w/w)	Initial FFA Content (%)	Final FFA Content (%)
3:1	7.42 mL	60 mL	94.22%	5.77%	10.09%	8.60%
6:1	14.83 mL	60 mL	86.34%	13.65%	10.09%	2.26%
9:1	22.24 mL	60 mL	85.71%	14.28%	10.09%	1.69%

show considerable reduction in the FFA content after treatment.

As the methanol-to-oil ratio increased, the FFA content was considerably reduced. The 9:1 ratio produced the most considerable reduction, with the FFA content dropping to 1.69%. The biodiesel production process from acid-neutralized POME to biodiesel production is shown in Figure 2.

3.6. Cost Comparison of Alkaline Neutralization and Esterification

The cost comparison between alkaline neutralization and esterification reveals distinct differences in terms of both chemical usage and overall process efficiency. A detailed comparison is given in

Table 5. Cost–benefit analysis of FFA reduction (alkaline neutralization vs. esterification).

Parameter	Alkaline Neutralization	Esterification
Initial FFA Content	10.09%	10.09%
Method for FFA Reduction	NaOH treatment (analytical grade, ≥98%)	Methanol (analytical grade, ≥99%) and H2SO4 (reagent grade, ≥98%) for esterification
Best FFA Reduction Achieved	0.5% NaOH for 120 min	9:1 methanol-to-oil ratio
FFA Reduction Efficiency	FFA reduced to 3.48%	FFA content reduced to 1.69%
Amount of Chemicals	0.005 kg of NaOH per liter of oil at 0.5% concentration	0.13 L of methanol per liter of oil at 9:1 ratio
Cost per Liter of Chemicals	USD 0.002 per liter of oil (0.005 kg × USD 400 per ton of NaOH)	USD 0.065 per liter of oil (0.13 L × USD 500 per ton of methanol)
		USD 0.002 per liter of oil (0.01 kg × USD 200 per ton of H2SO4)
Energy Costs	USD 0.01 to USD 0.02 per liter for heating	USD 0.02 to USD 0.03 per liter for heating
Total Estimated Chemical and Energy Cost	USD 0.012 per liter	USD 0.087 per liter
Processing Time	30 min to 120 min	Varies, but generally longer than alkaline neutralization
Required Additional Processes	Requires esterification or transesterification to produce biodiesel	Can be followed by transesterification for biodiesel
Net Benefit of FFA Reduction	Lower initial cost but requires further processing to achieve biodiesel	More effective in reducing FFA, improving biodiesel yield

.

From an industrial standpoint, alkaline neutralization is a cost-effective method for initial FFA reduction, with a low cost of USD 0.012 per liter. However, it only reduces the FFA content to 3.48%, requiring additional processes like esterification and transesterification, which increase overall costs and time. On the other hand, esterification is more expensive at USD 0.087 per liter but provides considerable FFA reduction with a content of 1.69%, making it more efficient for producing high-quality biodiesel. While it requires more energy and a longer processing time, it reduces the need for further processing, making it more suitable for industries focused on higher biodiesel yields and better product quality. In conclusion, alkaline neutralization is ideal for cost-effective pretreatment, while esterification offers better long-term efficiency and quality, making it more beneficial for industries prioritizing higher biodiesel yield and sustainability.

3.7. Transesterification of Esterified Sludge Palm Oil

Transesterification reactions were performed using different methanol-to-oil molar ratios (6:1, 7:1, and 8:1), with sodium hydroxide (NaOH) as the catalyst. The biodiesel yield and glycerol byproduct were measured for each reaction, as shown in

Table 6. Transesterification results for different methanol-to-oil ratios.

Methanol-to-Oil Ratio	Oil (mL)	Methanol (mL)	NaOH (g)	Biodiesel Yield (g)	Glycerol Yield (g)	Biodiesel Yield (w/w%)
6:1	100.00	25.00	1.00	54.45	39.42	58.00
7:1	100.00	28.88	1.00	63.42	27.18	70.00
8:1	80.00	26.20	0.80	61.53	24.82	71.25

.

Higher methanol-to-oil ratios (6:1, 7:1, and 8:1) led to increased biodiesel yields, with the 7:1 ratio producing the highest yield at 70%. The biodiesel yield per gram of catalyst also increased with higher ratios, but the 8:1 ratio showed diminishing returns. The glycerol yield did not follow a consistent pattern, with the highest glycerol yield being observed at a 6:1 ratio, but a decrease was observed at the 8:1 ratio. Overall, higher methanol ratios generally improve biodiesel production efficiency, but excess methanol may lead to diminishing benefits. The 7:1 ratio appeared to offer the best balance in terms of biodiesel yield. The produced biodiesel samples are given in Figure 3.

3.8. Biodiesel Property Analysis

The fuel properties of various biodiesel blends (B100, B05, B10, B20, and B30) were determined and are summarized in Table 6. The properties tested include the calorific value, flash point, kinematic and dynamic viscosities, and density at different temperatures (30 °C and 40 °C).

3.8.1. Calorific Value

The calorific value of the biodiesel blends ranged from 40.21 MJ/kg for B100 to 46.240 MJ/kg for B05, with intermediate values for the other blends (

Table 7. CVs and flash points of biodiesel blends.

Property	Biodiesel (B100)	Biodiesel (B05)	Biodiesel (B10)	Biodiesel (B20)	Biodiesel (B30)
Calorific Value (MJ/kg)	40.21	46.24	46.025	45.187	44.708
Flash Point (°C)	180.5	79.5	79.5	82.5	82.5

). The calorific value generally decreased as the biodiesel concentration increased, consistent with the lower energy content of the biodiesel compared to conventional diesel.

3.8.2. Flash Point

The flash point was considerably higher for B100 (180.5 °C) compared to the biodiesel blends containing lower concentrations of biodiesel. For B05, the flash point dropped to 79.5 °C, indicating that lower biodiesel concentrations reduce the volatility and increase the flammability of the fuel. A summary is given in Table 7.

3.8.3. Viscosity

The kinematic viscosity of the biodiesel blends at various temperatures was measured for all blends, as shown in

Table 8. Density and kinematic viscosity values of biodiesel blends at different temperatures.

Temperature (°C)	20	25	30	35	40
B05 Density (g/cm3)	0.8423	0.8386	0.8351	0.8316	0.8282
B05 Kinematic Viscosity (mm2/s)	6.9603	6.0685	5.3230	4.7026	4.1589
B10 Density (g/cm3)	0.8437	0.8399	0.8364	0.8330	0.8295
B10 Kinematic Viscosity (mm2/s)	6.8030	5.9257	5.1933	4.5892	4.0556
B20 Density (g/cm3)	0.8471	0.8433	0.8398	0.8363	0.8328
B20 Kinematic Viscosity (mm2/s)	6.8376	5.9569	5.2238	4.616	4.0814
B30 Density (g/cm3)	0.8472	0.8434	0.8399	0.8363	0.8328
B30 Kinematic Viscosity (mm2/s)	6.5822	5.7548	5.0615	4.4872	3.9788

. As the biodiesel content in the blends increased, the kinematic viscosity also increased. For instance, at 40 °C, the kinematic viscosity of B05 was 4.1589 mm²/s, while for B30, it was 3.9788 mm²/s. This trend aligns with expectations, as higher biodiesel concentrations typically lead to lower viscosity. However, in this case, the viscosity of B05 is slightly higher than that of B30, which might suggest other influencing factors such as the specific properties of the biodiesel blend or minor experimental variations. This trend was consistent across all measured temperatures, where the viscosity decreased with an increasing temperature for all blends. The B30 blend consistently exhibited the lowest viscosity values, whereas B05 had the highest viscosity across all temperatures.

3.8.4. Density

The density of the biodiesel blends at different temperatures was also measured. At 20 °C, the density values for the biodiesel blends ranged from 0.8423 g/cm³ for B05 to 0.84726 g/cm³ for B30, as shown in Table 8. As the temperature increased, the density of all blends decreased, with the B05 blend showing the lowest density values at each temperature. The trend of decreasing density with increasing temperature was consistent for all blends. Specifically, at 40 °C, the density for B05 was 0.8282 g/cm³, while for B30, it was 0.83289 g/cm³, indicating that blends with higher biodiesel contents had slightly higher densities compared to those with lower biodiesel concentrations.

As the percentage of biodiesel increases (from B05 to B30), the density slightly increases at each temperature. This may be due to the chemical composition of the biodiesel blends, with a higher biodiesel content contributing to slightly higher density values.

All blends show a similar pattern, where the kinematic viscosity decreases with an increasing temperature. This is typical for fluids, as viscosity generally decreases as temperature rises due to a reduced molecular interaction. Additionally, blends with higher biodiesel contents (B30) tend to have slightly lower viscosity values compared to blends with lower contents (B05).

B30 consistently has the lowest kinematic viscosity at all temperatures, which could suggest that a higher biodiesel content reduces the fluid’s resistance to flow. B05, which has the least biodiesel, has the highest viscosity among the blends.

3.8.5. Thermal Conductivity (λ Lambda) of Various Biodiesel Blends

The thermal conductivity (λ lambda) of various biodiesel blends (B10, B05, B20, and B30) was measured at different temperatures (30 °C to 55 °C). The results are summarized in Figure 4 below. As observed, the thermal conductivity values show a slight variation with the temperature change for each blend.

The thermal conductivity of biodiesel blends (B5, B10, B20, and B30) was measured at temperatures ranging from 30 °C to 55 °C. Across all blends, thermal conductivity decreased as temperature increased, which is typical for fluids. B30 exhibited the highest thermal conductivity at lower temperatures (126.16 MW/(mK) at 30 °C) and maintained the highest conductivity across the temperature range, while B5 showed the lowest at 30 °C (122.69 MW/(mK)). The B20 blend had slightly higher thermal conductivity than B5 and B10 at the lower temperatures but showed a steady decrease similar to the other blends.

At higher temperatures, the differences in thermal conductivity between the blends became minimal, indicating that temperature has a greater impact on conductivity than the biodiesel concentration. In conclusion, a higher biodiesel content resulted in better heat transfer properties at lower temperatures, though the effect diminished as the temperature rose.

3.9. Cost–Benefit Analysis of POME Management vs. Biodiesel Production

This study compares the cost and feasibility of biodiesel production from Palm Oil Mill Effluent (POME) with traditional POME treatment. With an oil recovery efficiency of 96.40%, the study achieved a daily recovery of 240.92 m³ of oil, which was converted into 168,640 liters of biodiesel daily through esterification and transesterification with optimized methanol-to-oil ratios of 9:1 and 7:1, respectively. A detailed cost–benefit analysis is shown in

Table 9. Cost–benefit analysis of POME management vs. biodiesel production.

Parameter	POME Management	Biodiesel Production (Esterification + Transesterification)
POME Volume (Daily)	250 m3	250 m3
Oil Recovery Efficiency	N/A	96.40% of POME recovered as oil
Oil Extracted (Daily)	N/A	240.92 m3 (96.40% of 250 m3)
Biodiesel Yield	N/A	70% of extracted oil converted into biodiesel
Biodiesel Produced (Daily)	N/A	168.64 m3 (or 168,640 L)
Cost of POME Treatment (Per m3)	USD 0.50	N/A
Total Cost of POME Treatment (Daily)	USD 125	N/A
Total Cost of POME Treatment (Annual)	USD 45,625	N/A
Cost of Biodiesel Production (per liter)	N/A	USD 1.50 per liter
Daily Production Cost for Biodiesel	N/A	USD 252,960 (168,640 L × USD 1.50 per liter)
Annual Production Cost for Biodiesel	N/A	USD 92,378,400 (252,960 × 365 days)
Market Price of Biodiesel	N/A	USD 2.00 per liter
Daily Revenue	N/A	USD 337,280 (168,640 L × USD 2.00 per liter)
Annual Revenue	N/A	USD 337,280,000 (337,280 × 365 days)
Net Profit/Loss	−USD 45,625	USD 244,901,600 (revenue—production cost)
Environmental Benefits	None	Reduces environmental impact by converting waste into fuel
Byproducts	N/A	Glycerol, which can be used or sold

.

The sensitivity analysis highlights the varying impacts of key parameters on biodiesel production’s net profit. The POME treatment cost, while important, has a relatively minor effect on profitability, with a 30% decrease increasing the net profit by USD 32,000 and a 30% increase resulting in a USD 42,000 reduction in net profit. In contrast, the biodiesel selling price proves to be highly sensitive, with a 10% decrease reducing the net profit by approximately USD 32 million, while a 10% increase boosts profit by a similar amount. The biodiesel production cost per liter also has a significant impact on profitability. A 30% reduction in production costs leads to a USD 33 million increase in net profit, while a 30% increase in production costs results in a USD 33 million decrease. Overall, the biodiesel selling price and production costs are the most influential factors, while POME treatment costs have a smaller, albeit still important, effect on net profit.

This study has significant implications for both the palm oil industry and the renewable energy sector. By converting Palm Oil Mill Effluent (POME) into biodiesel, the palm oil industry can reduce the environmental impact, create a new revenue stream, and improve waste management. For the renewable energy sector, using waste oils like POME for biodiesel production offers a sustainable alternative to fossil fuels, contributing to reduced carbon emissions and energy security. Overall, this process has the potential to enhance sustainability in the palm oil industry and accelerate the transition to cleaner energy sources.

4. Conclusions

In conclusion, this study successfully optimized the esterification and transesterification processes for biodiesel production from Palm Oil Mill Effluent (POME), with a focus on reducing the free fatty acid (FFA) content and maximizing biodiesel yield. The centrifugation and NaOH treatment effectively improved oil recovery and reduced FFA levels. However, transesterification was unsuccessful without further esterification to lower the residual FFA content. The highest biodiesel yield of 71.25% was achieved with an 8:1 methanol-to-oil ratio, with the optimal ratio for maximum efficiency being 7:1. The process demonstrated high oil recovery (96.4%) and produced 168,640 L of biodiesel daily, generating significant revenue with an annual net profit of USD 244,901,600. The cost–benefit analysis confirmed the economic viability, with minimal POME treatment costs and substantial revenue from biodiesel production. A sensitivity analysis highlighted that the biodiesel selling price and production costs have the greatest impact on profitability, with a 10% change in the selling price affecting profit by USD 32 million. Environmental benefits include waste-to-fuel conversion and glycerol byproduct utilization, promoting eco-friendly energy solutions and contributing to environmental sustainability by reducing waste and supporting renewable energy. Despite these successes, limitations remain in further reducing the FFA levels and optimizing pretreatment methods. Future research should focus on refining FFA reduction techniques, exploring alternative catalysts, and improving overall biodiesel production efficiency and sustainability, further enhancing its contribution to a greener future.