Sustainable Production of Medium-Chain Fatty Acids from Fresh Leachates in the District of Abidjan: Study of the Feasibility of the Process and Environmental Benefits

Abstract

Leachate management remains a major environmental challenge, especially in rapidly urbanizing cities of developing countries. Traditionally considered toxic and useless, it is a sustainable organic resource with the potential for high-value biochemical production through bioprocessing. This study investigated the characteristics of fresh leachates from three solid waste transfer stations (SWTS) in the Abidjan district, Côte d’Ivoire, and assessed their potential as substrates for medium-chain fatty acid (MCFA) production via microbial chain elongation. The MCFA synthesis was carried out in anaerobic bioreactors operated under methanogenesis inhibition conditions. The leachates from Bingerville, Abobo-Dokui, and Yopougon exhibited acidic and high organic content, particularly volatile fatty acids (VFAs), key precursors for MCFA synthesis. High concentrations of microbial communities associated with chain elongation were observed, including Clostridium (sulphite-reducing), Lactobacillus, Bacillus, and Pseudomonas (greater than 5 log10 CFU/mL). MCFA production ranged from 5 to 10 g/L, mainly C6, C7, and C8, with compositional variation depending on the SWTS. Notably, leachates from higher-income areas demonstrated higher MCFA productivity compared to those from lower-income areas. These findings highlight the potential of fresh SWTS leachates in the Abidjan district for sustainable MCFA production, paving the way for industrial applications.

1. Introduction

Waste management is a major challenge for rapidly expanding cities, particularly in developing countries. In Côte d’Ivoire, the district of Abidjan is experiencing strong demographic growth and rapid urbanization, leading to a significant increase in waste production and the need for sustainable management strategies [1]. Solid waste transfer stations (SWTS) play a crucial role in this system by temporarily storing and compacting waste before its transfer to treatment or landfill sites. However, these facilities generate large quantities of fresh leachate, an effluent rich in biodegradable organic compounds, nutrients, and toxic pollutants such as heavy metals and aromatic hydrocarbons [2]. Improper management of these leachates results in severe environmental consequences, including the contamination of soil, groundwater, and aquatic ecosystems [3].

Conventional approaches to treat municipal solid waste leachate include physicochemical processes such as coagulation–flocculation, activated carbon adsorption, advanced oxidation, and some biological processes such as lagooning and aerobic treatment. Nonetheless, these methods remain costly, energy-consuming, and often ineffective when faced with the complexity of leachates [4,5]. In view of the high concentrations of organic matter in fresh leachate, in particular leachate out from Ivorian waste known to be highly rich in organic compound [6], anaerobic digestion (AD) emerges as a promising sustainable treatment solution for converting this misplaced resource into valuable products such as medium-chain fatty acids (MCFAs) [7].

MCFAs, made up of 6 to 12 carbon atoms, are high-value compounds used in a variety of industries, from cosmetics to the production of biofuels and environmentally friendly lubricants [8]. Their production through biological pathways, in particular a microbial process called reverse beta-oxidation or chain elongation in a non-sterile environment, is gaining interest as a sustainable alternative to fossil-derived sources. Moreover, in a non-sterile environment, no costly sterilization step is required [9]. The process of producing MCFAs entails the elongation of intermediate biochemicals that are generated in the anaerobic digestion process using bacteria such as Clostridium kluyveri [10,11]. These intermediate biochemicals include acetate, propionate, and other volatile fatty acids (VFAs). The length of the VFA carbon chain is extended to six, seven, or even eight carbon atoms by adding a second fermentation stage with the addition of ethanol [9]. Ethanol has an indispensable dual function. It functions as an electron donor, thereby maintaining the redox balance necessary for bacterial metabolic activity, and is also transformed into acetyl-CoA, a key precursor for carbon chain lengthening [12]. The molecule acetyl-CoA, which is derived from ethanol, is added to a carboxylate to lengthen its carbon chain by two carbons at a time. This elongation renders MCFAs more hydrophobic than VFAs, significantly reducing their solubility in water [10].

Recent studies, mainly conducted in Europe, have demonstrated the potential of leachate as a substrate for MCFA production, with yields ranging from 0.74 to 12.8 g/L [8,10,13,14]. In the Sub-Saharan context, research remains limited, except for a study [15] on the district of Abidjan’s leachates, which reported the production of hexanoic acid (C6), heptanoic (C7) and octanoic acid (C8) and unexpected production of C9 and C10, whereas only the synthesis of AGCM from C6 to C8 was possible [8,14], highlighting the specificity of these leachates. However, these were based on a single SWTS (Abobo-Dokui), while Abidjan hosts several SWTS spread across different municipalities with stark socioeconomic disparities. Indeed, according to [16], there are several inequalities in the standard of living from one municipality to another. Municipalities on the outskirts of Abidjan have the highest incidence of poverty, affecting up to 92% of households, while some, located in the heart of Abidjan, have only 15% [16].

Since the composition of waste is influenced by socioeconomic, demographic, and consumption habits [17,18], a broader study including multiple SWTS, particularly targeting those located in areas with marked socioeconomic disparities, is essential to establish a more accurate profile of Abidjan’s leachate for MCFA production. This research aims to characterize fresh leachates from various SWTS in the district of Abidjan and evaluate their potential as substrates for microbial chain elongation.

2. Materials and Methods

2.1. Description of the Sampling Areas

The district of Abidjan, located in Côte d’Ivoire, covers an area of 2.119 km² and comprises 13 municipalities with a population stands at 6,321,017 inhabitants [19]. This study focuses on three SWTSs from different municipalities of this district (Figure 1):

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The SWTS of Abobo-Dokui: located between the municipalities of Abobo and Cocody. It collects waste that comes largely from Abobo-Dokui and partially from Cocody;

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The SWTS of Bingerville: from the municipality of Bingerville, it nevertheless manages waste from the municipality of Cocody;

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The SWTS of Yopougon: located between the municipalities of Abobo and Yopougon, receives waste from the municipalities of Abobo and Anyama.

These SWTS were selected according to the income level of the municipality to which they belong. [16] grouped the 13 communes of Abidjan into 7 groups, based on the level of poverty per percentage (

Table 1. Municipalities of Abidjan grouped by their level of poverty [16].

Group	Municipalities	Poverty Level (%)
Group 1	Plateau	15%
Group 2	Cocody	25%
Group 3	Yopougon Treichville Marcory	30 to 35%
Group 4	Koumassi Attécoubé Adjamé Abobo	40 to 45%
Group 5	Port-Bouët Bingerville	45 to 50%
Group 6	Songon	60 to 70%
Group 7	Anyama	Up to 80%

). Based on this classification, the SWTS of Yopougon, Abobo-Dokui, and Bingerville would correspond, respectively, to low-, medium-, and high-income areas.

2.2. Leachates Collection

According to [15], the characteristics of Abidjan’s leachate vary greatly between dry and rainy seasons, strongly influencing the production of MCFAs. The highest MCFA concentrations were observed in leachates collected during the rainy season. Therefore, over a two-year period (2023 to 2024), sampling was conducted weekly during the major rainy season between May and July from the SWTS of Abobo-Dokui (coordinated 5°22′43.94″ N 4°0′22.84″ W), Yopougon (5°24′31.1″ N 4°05′32.3″ W) and Bingerville (5°21′03″ N 3°51′34″ W). During this period, 24 samples were taken in duplicate from each site. Each sample consisted of 50 mL of fresh leachate, collected directly from the tanks of waste collection trucks using a long-handled sampler (delt19575, DeltaLab, Barcelona, Spain) fitted with a sterile container at the extremity. Samples were collected in sterile bottles, transported in a cooler, and analyzed within 48 h of collection. The fresh leachates collected from the different SWTS were designated as LAD (Abobo-Dokui), LBG (Bingerville), and LYO (Yopougon).

2.3. Assessment of the Leachate Quality

Leachate characterization was essential to determine their composition and suitability for the chain elongation process. Therefore, pH and conductivity were measured using a multi-parameter probe (Electro Photometric Multi-parameter HI2829, Hanna Instruments, Vöhringen, Germany). In addition, Chemical Oxygen Demand (COD) and Total Organic Acids (TOA) were analyzed using LCK 514 and LCK 365 test tubes (Hach Lange, Berlin, Germany), respectively, in combination with a HACH Lange DR 2800 photometer (Hach Lange, Berlin, Germany) for measurement. K, P, Fe, and some heavy metals (Cd, Cr, Cu, Pb, Zn) were also determined with an ICP-OES (Agilent 5800 ICP-OES, Waldbronn, Germany).

2.4. Microbial Characterization of Fresh Leachate

The microbiological characterization of the samples was carried out using standard cultural techniques on specific agars and following ISO 13720, ISO/TS 11059, and ISO 16266 standard. Only microorganisms known or suspected to be involved in MCFA production were concerned in this study. These included the Clostridium sulfite-reducing, the genera Lactobacillus, Bacillus, and Pseudomonas [5,15,20,21].

The anaerobic sulfite-reducing bacteria, Clostridium, were selectively isolated on liquid medium (TSN, Merck, Darmstadt, Germany) in anaerobic culture tubes, with the addition of paraffin oil to ensure the anaerobic condition. The cultures were then incubated for 18 to 24 h at 46 ± 1 °C. Lactobacillus counts were determined by culturing on Man Rogosa Sharpe medium (MRS, Merck), followed by incubation at 37 °C for 48 h. Bacillus strains were isolated after sporulation treatment at 80 °C for 10 min, and purifications were achieved by repeated streaking of the same clone on Trypticase-Soy Agar (TSA, Biorad, Feldkirchen, Germany). Bacillus isolated were confirmed through Gram staining, as well as oxidase and catalase tests. Pseudomonas spp. was detected and enumerated using Pseudomonas Agar Base, a selective medium.

Standard culture techniques carried out microbiological characterization of the samples on specific agars. The counting of petri dishes containing between 15 and 150 colonies was used to calculate the microbial loads according to the following formula:

N = Σ Colonies/(V(n1 + 0.n2)d)

(1)

N: Microbial load (CFU/g);

Σ: colonies: Sum of colonies counted on petri plates;

V: Volume of inoculum inoculated (mL);

d: Smallest considered dilution;

n1: Number of petri plates at the first dilution considered;

n2: Number of petri plates at the second dilution considered.

2.5. Setup of Bioreactor and Operation of Chain Elongation

Once the collected leachates had been characterized, the chain elongation process was initiated. However, before starting, the pH of the sample was adjusted between 5.5 and 6.5 using HCl or NaOH to create an optimal condition. Methanogenic bacteria and those involved in chain elongation use the same substrates (anaerobic digestion intermediates, acetate, ethanol, etc.) for their metabolism. Consequently, this step is crucial for the inhibition of methanogenic bacteria, whose activity is optimal at neutral pH [11]. Subsequently, batch reactors were set up in duplicate, each containing 1 L of leachate from the three SWTS, operating under anaerobic conditions at ambient temperature (32–35 °C) (Figure 2) [15] and controlled pH system as described [8,15]. To stimulate the elongation of VFAs into MCFAs, 1% (v/v) ethanol was added as an electron donor to each bioreactor [8,15]. Approximately one week is required for microorganisms (e.g., Clostridium kluyveri) to utilize the ethanol. This step is called ethanol maturation [8]. The entire process lasted five (5) weeks, with weekly sampling of 15 mL of leachate for analysis.

2.6. MCFA Measurement

The MCFA content of the samples was determined using a gas chromatograph (GC) coupled to a flame ionization detector (FID) (Agilent 7890B, Waldbronn, Germany) and a liquid injection autosampler (PAL3 CTC Analytics AG, Zwingen, Switzerland). Prior to GC-FID analysis, substrate samples were diluted 1:10 with Milli-Q water, and the pH was adjusted between 1.5 and 2.0 by the addition of 1 N HCl solution. Subsequently, diluted samples were filtered through a microfilter (0.45 μm polyethersulfone, VWR International, Radnor, PA, USA). The column employed in this study was TG WAXMS-A (30 m; i.d. 0.32 mm; thickness: 0.50 μm; stationary phase: polyethylene glycol; Thermo Scientifics, Dreieich, Germany). For each measurement, 0.5 μL of sample was injected into a split/splitless injector (Agilent, Waldbronn, Germany) heated to 260 °C and analyzed at a split ratio of 1:10. The GC oven programmed was set as follows: initial temperature of 60 °C for 2 min; 10 °C/min to 80 °C; 21 °C/min to 145 °C; 6.1 °C/min to 205 °C; and finally 205 °C for 10 min.

3. Results and Discussion

3.1. Physicochemical Characteristics of the Fresh Leachates from the SWTS in the District of Abidjan

The results of the physicochemical characteristics of fresh leachates from the SWTS of Abobo-Dokui (LAD), Bingerville (LBG), and Yopougon (LYO) are presented in

Table 2. Physicochemical characteristics of fresh leachates from SWTS in the district of Abidjan.

Parameter	Unit	LAD	LBG	LYO
pH	-	4.5 ± 0.7 a	5.1 ± 0.5 a	4.6 ± 0.3 a
Conductivity	mS/cm	17.0 ± 2.3 a	41.9 ± 1.3 b	18.7 ± 1.5 a
COD	g·L−1	94.1 ± 5.2 a	114.5 ± 14.2 b	76.7 ± 2.3 c
TOA	g·L−1	21.4 ± 4.7 a	36.6 ± 0.8 b	16.1 ± 0.2 c
TFA	g·L−1	13 ± 1.3 b	33.7 ± 2.4 a	6.4 ± 0.7 c
VFA	g·L−1	11.9 ± 0.8 b (91.3% TFA)	31.1 ± 1.9 a (92% TFA)	5.8 ± 0.6 c (91% TFA)
MCFA	g·L−1	1.1 ± 0.6 (8.7% TFA)	2.6 ± 0.3 (8% TFA)	0.6 ± 0.2 (9% TFA)
K	mg·L−1	2 400 ± 480 b	3 665 ± 325 a	1 735 ± 176 b
P	mg·L−1	305 ± 27 a	477 ± 38 a	136 ± 45 b
Fe	mg·L−1	280 ± 19 b	506 ± 23 a	422 ± 18 a
Zn	mg·L−1	<1	<1	<1
Cd	mg·L−1	<1	<1	<1
Cr	mg·L−1	<1	<1	<1
Cu	mg·L−1	<1	<1	<1
Pb	mg·L−1	<1	<1	<1

Values are means ± standard deviation (n = 3). Different letters within a row indicate significant differences (p < 0.05) according to ANOVA followed by Tukey’s HSD test. COD (Chemical Oxygen Demand); TOA (Total Organic Acids); TFA (Total Fatty Acids); VFA (volatile fatty acids); MCFA (medium-chain fatty acids).

. This leachate exhibited acidic pH values below 5, likely due to the predominant action of hydrolytic and acidogenic bacteria in the initial stages of waste degradation, leading to the accumulation of organic acids in the leachate produced [11]. Ref. [15] revealed a dominance of the genus Lactobacillus in the young leachate from Abobo-Dokui, known for its strong acidification capacity through the production of various organic acids such as lactic and acetic acid. For the conductivity, the values ranged from 17 to 41.9 mS/cm, consistent with previous findings [22], which reported conductivity values between 10 and 37.9 mS/cm in leachates from the Akouedo landfill in Abidjan. LAD and LYO displayed similar conductivity (17.0 ± 2.3 and 18.7 ± 1.5 mS/cm, respectively), whereas LBG showed a significantly higher conductivity (41.9 ± 1.3 mS/cm). A high conductivity value indicates an elevated amount of dissolved salts, nutrients, or metals [23]. The major metals detected in these fresh leachates were K, P, and Fe, in line with finding reported by [7,24]. Additionally, trace amounts of highly toxic metals such as Cd, Cu, Cr, and Pb were also detected in all fresh leachate samples, as reported by [7].

The organic matter content of these fresh leachates was assessed through COD and TOA measurements [8]. The results revealed a substantial organic load. LBG exhibited the highest COD concentration (114.5 ± 14.2 g/L), nearly double that of LYO (76.7 ± 2.3 g/L), while LAD recorded 94.1 ± 5.2 g/L. These high COD values suggest a significant presence of high organic compounds in the waste streams feeding the SWTS, particularly in Bingerville. Additionally, a high concentration of organic acids was observed, as previously reported by [8,13,15]. LBG still had the highest TOA concentration, with a value of 36.6 ± 0.8 g/L, more than twice that of LYO (16.1 ± 0.2 g/L), whereas LAD recorded 21.4 ± 4.7 g/L. The high concentration of organic acids supports the predominant action of hydrolytic and acidogenic bacteria, which degrade fresh organic matter into alcohols, hydrogen, CO₂, lactate, and VFAs such as acetic acid (C2), propionic acid (C3), butyric acid (C4) and valeric acid (C5) [25]. Regarding the fatty acid content, LBG exhibited the highest concentration (33.7 g/L), more than two times that of LAD (13 g/L) and five times that of LYO (6.4 g/L). However, VFAs accounted for more than 90% of the Total Fatty Acids (TFAs) in leachates from all SWTS, while MCFAs represent less than 10%. The low MCFA concentration may be attributed to the limited or absent activity of MCFA-producing bacteria (e.g., Clostridium kluyveri), which require the presence of electron donors such as ethanol, methanol or lactic acid in appropriate proportions [5,10].

These results indicate that LAD, LBG, and LYO possess a high potential for energy recovery and MCFA production through chain elongation due to their high organic potential. However, LBG appears to be the most suitable substrate. This could be due to its origin and the composition of the waste from which it is derived. Indeed, LBG, LAD, and LYO come from SWTS, which receive waste from high-, medium-, and low-income areas, respectively [16]. Ref. [18] established a correlation between household income level and waste composition of the district of Abidjan, revealing that high-income areas generate more organic matter, especially fermentable waste (53%), compared to low-income areas (49.69%). These findings align with the observation made in this study, reinforcing the influence of socioeconomic factors on waste composition and, consequently, on leachate characteristics.

3.2. Microbial Quality of the Fresh Leachates

Table 3. Microbiological characteristics of fresh leachates from SWTS in the district of Abidjan.

Microorganisms	Unit	LAD	LBG	LYO
Clostridium	log10 CFU/mL	6.06 ± 0.36 a	6.20 ± 0.52 a	5.86 ± 0.53 a
Lactobacillus	log10 CFU/mL	7.14 ± 1.29 a	7.36 ± 2.15 a	7.25 ± 2.11 a
Bacillus	log10 CFU/mL	7.09 ± 0.13 a	7.11 ± 0.01 a	7.31 ± 0.09 a
Pseudomonas	log10 CFU/mL	7.64 ± 0.25 a	7.83 ± 0.48 a	7.68 ± 0.57 a

Comparisons were made between the values of leachate parameters collected from different sites. Values are means ± standard deviation (n = 3). Different letters within a row indicate significant differences (p < 0.05) according to ANOVA followed by Tukey’s HSD test.

shows the load of the microorganisms known or suspected to be involved in the production of MCFAs isolated from the fresh leachates of the SWTS of Abobo-Dokui, Bingerville, and Yopougon.

Overall, these fresh leachates are particularly rich in microorganisms beneficial to chain elongation, with observed loads ranging between 5.5 and 7.8 log 10 (CFU/mL). Moreover, no significant differences in microbial loads were observed between leachates from these different SWTS. Clostridium was the least abundant genus, with loads varying between 5.5 and 6.2 log 10 (CFU/mL), while the genera Pseudomonas, Lactobacillus, and Bacillus exhibited higher loads, exceeding 7 log 10 (CFU/mL). These findings could justify the low concentration of MCFAs (less than 10% of the TFA) compared to VFAs (80% of TFA) from the fresh leachate [Section 3.1]. The e of these genera, which play an important role in the initial stages of anaerobic digestion, favors the presence of VFAs, acetate, hydrogen, and carbon dioxide at the expense of MCFAs. The synthesis of the latter requires particular microorganisms (e.g., Clostridium kluyveri), strict anaerobiosis, and pH adjustment between 5.5 and 6.5 [8]. Consequently, although the Clostridium genus is detected at lower levels, its presence remains highly significant. These findings corroborate [15,26], which identified Clostridium, Lactobacillus, Bacillus, and Pseudomonas as dominant bacterial genera in the chain elongation process.

The results indicated that these leachates have similar microbial potential for MCFA production, reinforcing their suitability as substrates. Indeed, a high presence of these microorganisms in the leachates can have a positive effect on the production of MCFAs, as their direct and indirect involvement has been shown at different levels. The genus Clostridium is the most prevalent in open-culture MCFA production reactors. It is directly involved in the synthesis of MCFAs. Ref. [10] reported that Clostridium spp. constituted more than 50% of the microbiome during the ethanol hydrolysis and oxidation phase. The hydrolysis of ethanol leads to the synthesis of an acetyl-CoA molecule, which associates with a carboxylate to lengthen its carbon chain by two carbons [12]. As for the Bacillus genus, it would act indirectly in the process by producing hydrolytic enzymes such as proteases, amylases, and lipases, which break down the proteins, starches, and lipids present in the waste [27,28]. The glucose resulting from the degradation of starches is then fermented by bacteria, particularly those of the genus Lactobacillus. Indeed, Lactobacillus, which is present in high abundance in these fresh leachates, has the capacity to produce lactic acid by homofermentation or lactic acid, acetic acid, and ethanol by heterofermentation [29,30]. These compounds can then serve as electron donors for the synthesis of MCFAs [5] either by species of this genus or by other groups such as Clostridium, including the MCFA species C. kluyveri [26]. Regarding Pseudomonas, its involvement remains to be clearly elucidated, although [15] reported its strong correlation with MCFAs, especially C9.

3.3. Changes in COD, TOA During Chain Elongation

The evolution of COD and TOA is shown in Figure 3. Both serve as key indicators of bioreactors’ performance during the chain elongation process [31].

In the early stages of the process, COD levels decreased from D0 to D7 in all bioreactors, indicating intense microbial activity. The most significant reduction was observed in LAD (35%), followed by LYO (27%) and LBG (22%). This would indicate a strong activity of acidogenic and acetogenic hydrolytic bacteria, leading to the degradation of organic matter into VFAs and acetate, as well as H₂ and CO₂ [32]. This explains the simultaneous increase in TOA during this same stage Figure 3b, with rises of 66%, 35%, and 22% for LYO, LAD, and LBG, respectively.

However, from D7 onwards, COD levels increased in all bioreactors with values exceeding 100 gO₂/L Figure 3b. The largest increase was recorded in LYO with a value of 169 gO₂/L, more than double the initial value (76.7 gO₂/L). The results confirm those of [13], who reported an increase in COD in fresh and young leachates during the chain elongation experiment, with values above 100 gO₂/L. This increase in the COD (more pronounced from D21) could be attributed to the saturation of the medium with AGCM and VFA and secondary products generated [8]. Moreover, these latter have toxic effects, at high concentrations, on the cell membranes of microorganisms involved in chain elongation [12,15], and their decomposition can also lead to additional production of organic carbon [12]. Simultaneously, TOA levels progressively decreased until the end of the process, probably due to the conversion of VFAs into MCFAs [8]. This suggests that the microbial community successfully facilitated chain elongation, with VFAs serving as precursors and ethanol as an electron donor for MCFA production.

3.4. Effect of Ethanol on the Chain Elongation

Figure 4 shows the positive impact of ethanol on MCFA production. In all bioreactors, the addition of ethanol resulted in a clear increase in MCFAs. An inversely proportional evolution between AGCM production and ethanol consumption highlights this observation. As ethanol is consumed, MCFA production increases significantly. Overall, ethanol concentration fell by 73%, 71%, and 51% at the end of the process in LYO, LAD, and LBG, respectively. However, at the beginning of the process, from D0 to D7, a slow consumption of ethanol was observed. This phase corresponds to the maturation phase, i.e., the time required to start observing MCFA production after ethanol addition [8]. The low rate of ethanol consumption during this period could reflect the difficulty or time required for AGCM-producing microorganisms to exclusively dispose of ethanol due to the competitive phenomena elicited by open-culture bioreactors. Afterward, the consumption increased significantly from D7 to D14 and then slightly until the end of the process. At the same time, from D7 onwards, MCFA levels were 27.89% in LAD and 23.68% in LBG, compared to less than 10% at the initial stage (before the addition of ethanol). The production rate then increased to an average of 30.67% in LAD but remained at around 22.80% for LBG by the end of the process. In contrast, MCFA production in LYO was delayed, starting at D14 with a recorded rate of 24.53% before increasing to an average of 28.90% up to D35. This delay could be attributed to differences in microbial adaptation or substrate composition, which may have influenced the onset of the chain elongation process [5,9]. These results corroborate [13,15], who reported an effective MCFA production with the addition of ethanol from a week onwards using fresh and young leachates, confirming the crucial role of ethanol in enhancing MCFA production. The results of Figure 4 can be found in the Supplementary Data Table S1.

3.5. Potential of Fresh Leachates from SWTS in Abidjan for MCFA Production

To assess the potential of fresh leachate from the SWTS (Abobo-Dokui, Bingerville, and Yopougon) for MCFA production, anaerobic bioreactors were operated under methanogenesis inhibition conditions for 5 weeks. The results of the MCFA production are presented in Figure 5.

Overall, MCFA production was effective, reaching from D₇ more than 30% of the TFAs, as mentioned in Section 3.4. This positive response to the chain elongation reaction was obvious since these leachates exhibited their suitability for the process. Moreover, municipal solid wastes from developing countries are mainly composed of organic matter [33]. In particular, in the Abidjan district, organic matter represents more than 50% of the waste [6,15].

In terms of production yield, more than 10 g/L of total MCFAs were obtained from these SWTS leachates. These results are comparable to several studies performing chain elongation using organic waste or leachates as substrates, with production yield ranging from 0.74 to 12.8 g/L [5,8,10,13,14,15]. Among the three sites, LBG and LAD were the most productive, with average total MCFA production of 10.9 and 10.2 g/L, respectively, compared to 5.9 g/L for LYO. Differences in organic compounds composition, particularly VFAs, between these leachates could explain these variations. LBG had a moderately higher VFA concentration than LAD but five times higher than LYO [Section 3.1]. Notably, LBG, LAD, and LYO originated from high-, medium-, and low-income areas, respectively [16].

Furthermore, the MCFAs produced are mainly composed of caproic acid (C6), heptylic acid (C7), and caprylic acid (C8), as reported by [5,8]. However, the distribution varied across the SWTS. LBG showed a strong affinity for C6 (Figure 5b), with an average concentration of 8.6 g/L and a peak of 9.2 g/L at D14, representing more than 78% of the total MCFAs. The maximum concentrations of C7 and C8 were 2.1 g/L (17%) and 0.6 g/L (5%), respectively. This strong selectivity of C6 in LBG could be related to its VFAs composition, which was initially dominated by even-numbered acids (C2 and C4 accounted for more than 80% of the VFAs). The even-numbered VFAs then evolved significantly, consequently inducing the strong synthesis of C6 to the detriment of C7. In contrast, C7 was the main MCFA in LAD, representing 62% of total production (Figure 5a). The average concentration of C7 was 6.7 g/L, with a peak of 7.2 g/L (D14). C6 and C8 accounted for 32% and 6% of the MCFAs, respectively. LAD exhibited significant production of odd-chain VFAs (C3 and C5), with an average concentration of 4.1 g/L and 3.6 g/L, compared to 1.6 g/L for C4. This could reflect a reduction in the acetic acid or lactic acid to propionic acid. In the propionic pathway, acetic acid or lactic acid could be converted into propionic acid under the action of bacteria such as Propionibacterium [34]. Regarding LYO, C6 and C7 were produced in almost equal proportions, accounting for 43% (2.5 g/L) and 48% (2.9 g/L) of the total MCFAs, respectively (Figure 5c). C8 was the least abundant, representing only 9% (0.6 g/L). The average production of C4 and C5 is 2.7 g/L and 2.6 g/L, respectively. Across all SWTS, C8 production remained low (<1 g/L), with a maximum concentration of 0.8 g/L for LAD and 0.6 g/L for both LBG and LYO. Ref. [11] also observed low C8 production (0.3 g/L) from a batch culture with ethanol, acetic acid, and hydrogen as substrates using mixed cultures. Similarly, ref. [35] reported 0.4 g/L of C8 from organic municipal waste, even via two-stage fermentation and chain elongation reactors. According to [5], high C8 and even capric acid (C10) production are involved with high ethanol availability in the substrate, resulting in high availability of acetyl-CoA. In this context, a higher amount of ethanol would be necessary to reach an important concentration of C8 during chain elongation. As the chain elongation cycle increases, two carbon atoms of carboxylic acids decrease water solubility and increase energy density [36]. Moreover, this lower concentration of C8 could be due to the saturation of the medium, which can inhibit microbial activity [8,15]. Ref. [8] has developed a liquid–liquid extraction method to counter this recurring phenomenon during the elongation chain process. Furthermore, the conversion of C6 to C8 may require a longer period of time, given that this study only lasted five weeks [37].

Overall, the results confirm that fresh leachates from SWTS in Abidjan have significant potential for MCFA production, with variations in yield and composition depending on their organic content. The results of Figure 5 can be found in the Supplementary Data Table S2.

4. Conclusions

The fresh leachates from Abobo-Dokui, Bingerville, and Yopougon SWTS were characterized by an acidic pH, a high organic content, and a high load of natural bacteria known to be involved in chain elongation. These characteristics offer a favorable environment for the chain elongation efficiency for the production of MCFAs.

During the process, substantial amounts of MCFAs (C6 to C8) were generated, confirming the potential of fresh leachate from the SWTS of the Abidjan district as a valuable substrate for chain elongation. This approach opens up promising opportunities for the valorization of leachates in sustainable industrial processes.

Although laboratory results are encouraging, scaling up to an industrial level will require substantial infrastructure investments. Installing large-scale anaerobic reactors to treat leachate could be costly, but long-term economic benefits include reduced leachate management costs and the production of MCFAs, high-value products used in the cosmetics and bioenergy sectors. In addition, government incentives and sustainable waste management policies could make this process economically viable for developing cities. Beyond economic considerations, this approach could significantly reduce greenhouse gas emissions, particularly methane (CH₄), and limit groundwater contamination, sources of public health risks.