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. 2025 Aug 29;10(36):41270–41282. doi: 10.1021/acsomega.5c04105

Implementation of a Lab-Scale Hybrid Landfill Bioreactor to Enhance Municipal Solid Waste Biogas Production

Khanyisile D T Lepota †, Kasturie Premlall †, Major M Mabuza ‡,*
PMCID: PMC12444535  PMID: 40978388

Abstract

Landfilling is one of the primary techniques utilized globally to manage municipal solid waste (MSW), particularly in developing nations. Unlike existing conventional methods, hybrid bioreactor landfilling technology is a promising sustainable approach for maximizing biogas production from MSW. This study aimed to evaluate the application of hybrid bioreactor technology to enhance biogas production from the organic fraction of MSW. Over a 7-month period, two columns were utilized to simulate typical (Br-1) and hybrid (Br-2) landfill bioreactors while monitoring the quality of leachate and biogas generated. Results showed contaminant reduction in leachate with the pH increasing to 8 in Br-2, while Br-1 remained acidic below 7. Physicochemical parameter reductions ranged from 33 to 98%, while total organic carbon and carbon oxygen demand (TOC/COD) ratio varied between 0.04 and 0.61. Analyzed heavy metals reductions ranged from 3 to 88%. Biogas generated was 8.75 L with methane (CH4) below 50% in Br-1, while Br-2 produced 21.5 L with CH4 exceeding 60%. The final CH4 concentration reached 67% in Br-2 and 52% in Br-1. Partial aeration and diluted leachate recirculation enhanced biodegradation in Br-2 while the leachate was treated in situ. The high CH4 content and 41-fold greater biogas yield in Br-2 showed the economic and ecological viability of this method for biogas-to-energy projects.

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1. Introduction

A viable renewable energy source to reduce fossil fuel dependence and mitigate global warming is biogas from municipal solid waste (MSW) biodegradation. The Republic of South Africa generates approximately 12.7 million tons of MSW annually, with biodegradable waste accounting for over 50% of this total. Over 90% of MSW generated in the Republic of South Africa is disposed of in open dumps and landfills, where landfilling is the primary method. However, the high biodegradable content in landfills produces gaseous and liquid emissions, posing environmental and public health risks. Therefore, utilizing innovative techniques to process large amounts of MSW can serve as a sustainable approach to reducing landfill pollution.

Bioreactor technology offers an eco-friendly solution designed to minimize the waste stabilization period and overcome conventional landfill limitations , by optimizing in situ conditions. Operated as aerobic, anaerobic, or hybrid systems, hybrid bioreactors are beneficial, particularly for enhancing biogas production rates and yields. These systems begin with a semi-anaerobic phase to enhance biogas production during the anaerobic phase. Biodegradation is improved through technologies, such as leachate recirculation, aeration, and/or water addition, and other combined in situ treatments to adjust pH, humidity, and redox conditions. Maintaining adequate humidity is crucial in bioreactor landfilling, as it supports microbial life, nutrient transfer, and metabolic processes. ,

Bioreactor landfills are integral to sustainable waste management and green energy, with the goal of achieving sustainability and reducing emissions. Despite their importance, the implementation of bioreactor landfills poses challenges in both developed and developing countries. Though there is sufficient information on various methods to enhance bioreactor landfill operations such as micro aeration and leachate recirculation for the enhancement of microorganisms’ growth, biogas generation, waste degradation efficiency, and stabilization, , there is a gap in understanding the effects of combining partial aeration with the recirculation of diluted leachate into the landfilled waste, which could influence the degradation process and biogas production particularly on municipal organic waste degradation. This study, therefore, makes a significant contribution to advancing fundamental knowledge in this area and addressing the existing research gap.

Recirculation of leachate in bioreactor landfilling has been employed to significantly increase waste humidity and redistribute nutrients and microbes within the landfill. ,, This process enhances waste stabilization, improves final leachate quality (levels of contaminants are reduced, and the leachate is safe for disposal), and boosts the biogas generation rate and yield. , However, monitoring and regulation of recirculated leachate are essential to assess its quality and determine if pretreatment is necessary to reduce its toxicity. The recirculation of toxic leachate may limit methanogenesis by inducing acidic conditions (pH < 6), which are detrimental to biogas production. During the initial degradation stages, hydrolytic acidification takes place where hydrolysis converts large molecules into smaller ones, while acidification breaks this further down into organic acids; however, hydrolytic acidification has limitations, including a low hydrolysis rate, toxin release, and instability. The process of aeration, on the other hand, overcomes these limitations by introducing a micro-oxygen atmosphere within waste bodies, promoting hydrolytic acidification, a rate-limiting step that expedites the methanogenic stage, enhancing the diversity and activity of facultative bacteria while increasing biodegradability. , While in situ nitrification and denitrification of the leachate can be promoted with a suitable degree of aeration, prolonged aeration inhibits methanogenesis.

The significance of this study lies in the utilization of the organic fraction of municipal solid waste (OFMSW) for biogas generation while enhancing its quality and quantity through partial aeration and diluted leachate recirculation, aligning with circular economy and bioeconomy principles. The objective is to employ hybrid bioreactor technology combining the advantages of aeration and leachate recirculation to enhance biogas production from municipal solid waste. This research investigated the effects of partial aeration and diluted leachate recirculation on OFMSW degradation, considering leachate quality, biogas generation, and waste stabilization. The efficacy of the hybrid approach was compared to that of a typical landfill environment using two pilot-scale landfill bioreactors operated at ambient temperature.

2. Materials and Methods

2.1. Municipal Solid Waste Feedstock

The MSW used in the study was obtained from the Hatherley landfill site in the City of Tshwane, Republic of South Africa. The waste was manually segregated according to its type, then shredded using an organic waste shredder (Stalgast 650022, Poland) into pieces smaller than 50 mm in order to facilitate the flow of water, nutrients, and microorganisms inside the waste matrix during the experiments. The waste composition used in the study included kitchen (80%), yard and wood (15%), and paper and cardboard (5%), which were considered the organic fraction of the MSW. Before the waste was fed into the bioreactor columns, it was mixed until a homogeneous mixture was reached and divided into two parts. In each component, 5 L of deionized water was added to simulate natural biodegradation.

2.2. Experimental Design and Operation

The lab-scale biodegradation experiments were carried out in two (2) landfill-simulated bioreactors (Br-1 and Br-2). The bioreactor columns were constructed by using Plexiglas of 5.0 mm thickness with a holding capacity of 96 L. The effective filling height for the bioreactor columns was 0.51 m. Columns were equipped with gas sampling ports, a leachate outlet port at the bottom, a leachate reservoir tank, and a water/leachate addition port on top, as presented in Figure . Sampling ports were situated on the side of the column, upper, middle, and lower, to enable even sampling. A leachate discharge port and collection tank were set up below the columns to enable the natural discharge of leachate under the action of gravity. A valve was installed to control the leachate outflow. Generated leachate flowed into the leachate reservoir tank and underwent pH adjustment with deionized water; the leachate was diluted to alleviate the acidity and maintain neutrality without significantly altering its composition before being reintroduced into the Br-2 column. The pH of the recirculated leachate was maintained at pH 7. After dilution, the leachate was recirculated back to Br-2 through a peristaltic pump. A 30 mm thick pea gravel layer was positioned at the surface of each reactor as a draining film, preventing blockage of pipelines. To maintain humidity in both columns, Br-1 was humidified with water, while recirculation of diluted leachate was practiced on the hybrid operation of Br-2. On both operations, humidification (moisture addition) was provided at a rate of 1.0–1.5 L/day. To prevent anaerobic pockets and provide enough oxygen, partial aeration was implemented in the Br-2 operation on days 29 and 60 after the tests started. This involved turning the waste manually for 5–10 min using a hand garden shovel.

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Schematic of the simulated landfill bioreactors (Br) with leachate recirculation.

2.3. Laboratory Analysis

To identify and correlate the levels of waste decomposition stages in the bioreactor columns, generated leachate and gas samples were monitored twice a week. Parameters analyzed included pH and oxidation–reduction potential (ORP), using an electronic multimeter (OHAUS, Starter 3100M). Carbon oxygen demand (COD) was calculated using the dichromate oxidation and photometry method in a spectrophotometer (Aqua Lytic, AL800). Total organic carbon (TOC) was calculated using a nondispersive infrared (NDIR) analyzer (Shimadzu, TOC-L CPH). Total nitrogen (TN) was calculated using the persulfate digestion method and a spectrophotometer (Aqua Lytic, AL800). Heavy metals (HM) according to USEPA Method 3005A and a spectrometer (Agilent Technologies, 55 AA atomic adsorption spectrometer). Biogas daily production was determined using the water displacement method, while the gas composition was analyzed using a portable biogas analyzer (MCA 100 BIO P, ETG Risorse e Tecnologia, Italy). To ensure the accuracy of the findings, each experimental analysis was carried out three times, and the average of those results is given as a specific leachate parameter value. The standard deviation of the experimental analysis was computed and averaged at ±0.03 for each leachate parameter analyzed, thereby implying good probability results.

2.4. Microbial Composition Studies

Leachate samples collected during weeks 15 and 30 of the experiments were analyzed for microbial composition. Wide-mouth HDPE sterile sample bottles (Thermo Scientific Nalgene) of 500 mL were used for collecting leachate samples for microbiological analyses. After sampling, the samples were kept in a cooler box and directly transported to the laboratory for analysis. The leachate samples were sequenced using shotgun metagenomics (MGI DNBSEQ-G400, MGI Tech, China), and the Kaiju V1.9 method was adopted for taxonomic classifications. Briefly, the Kaiju method classifies individual metagenomic reads by using a reference database comprising the annotated protein-coding genes of a set of microbial genomes. After completion of the sequencing process, the sequencing reads are translated into amino acid sequences, which are then split into fragments at stop codons. The fragments are sorted by length. The sorted list of fragments is then searched against the reference protein database using a modified backward search on a memory-efficient implementation of the Burrows–Wheeler transform (BWT). This finds the maximum exact matches (MEMs), optionally allowing mismatches in the protein alignment.

2.5. Waste Characterization

An attenuated total reflectance-Fourier transformed infrared (ATR-FTIR) spectroscopy (PerkinElmer, Spectrum TWO Lita) was utilized for assessing the decomposition of the waste materials through determining the functional groups present or absent in the decomposed waste. , The analyses were performed on the fresh organic waste sample before the commencement of experiments and on the final decomposed waste retrieved at the end of the experiments. The analytical technique used powdered, dry materials; therefore, the samples were prepared by air-drying for 4 days at room temperature, before being homogenized in a laboratory sample mill (Laboratory-Instruments, SM-450L) and sieved to a particle size passing a 75 μm sieve in preparation for the analysis. A 12 mg sample was used to evaluate the FTIR spectra under ambient conditions in the mid-infrared area at a wavenumber range of 500–4000 cm–1 using the transmission mode. The resolution was set to 4 cm–1, and 16 scans were recorded, averaged, and corrected against ambient air as background.

3. Results and Discussion

3.1. Variations in Leachate Quality

3.1.1. Variation of pH

The variation of the pH profile over time for the simulated bioreactors is provided in Figure (a). The first eight (8) weeks of the testing represent the initial adjustment and transition of MSW degradation from aerobic to anaerobic phases. During this period, the observed pH levels in the young leachate of both bioreactor columns were acidic, with the pH ranging between 4.84 and 5.24. This was associated with the fermenting microorganisms carrying out hydrolysis and acidification (conversion of organic matter and hydrolysis products into various organic acids, alcohols, ammonia, etc.) of organic waste, producing volatile organic acids as byproducts, which led to the accumulation of carboxylic acids and a reduction in pH. While there were similarities in the pH trends observed in Br-1 and Br-2 columns, the pH in Br-2 increased faster than the pH in the Br-1 bioreactor; it reached a maximum pH of 8.20 in 100 days, providing an ideal environment for bacteria and methanogens’ survival. The partial aeration carried out in the column may have contributed to the growth of aerobic bacteria that may immediately consume the organic acids in the system and neutralize the pH of the leachate, which could explain the change in Br-2. The aeration in the upper layer of the hybrid bioreactor served as a buffer layer and contributed to an elevation in leachate pH, as suggested by Xu et al. In contrast, the circulation of the diluted leachate might have provided an ideal environment for the growth of aerobic microbes.

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Quality of generated leachate during the operation: (a) pH and (b) oxidation–reduction potential (ORP).

The Br-2 pH levels then fluctuated within the range of 7.93–8.20; meanwhile, in Br-1, the pH levels dropped and remained acidic, ranging between 6.62 and 7.05, as illustrated in Figure (a). The variations in pH were mainly caused by the production of large amounts of volatile organic acids, which can cause the pH of the leachate to decrease, demonstrated by the Br-1 column, whereas Br-2 remained on the methanation phase because the buffering capacity of the reaction system was enhanced by the alkalinity of the diluted leachate recirculated in the column, which might have assisted in further decreasing the concentration of volatile fatty acids (VFA) in the system. The findings revealed that in the OFMSW biodegradation process, the acid phase could be reduced by partial aeration and recirculation of diluted leachate.

3.1.2. Oxidation–Reduction Potential

The oxidation–reduction potential (ORP) was also evaluated in this investigation to determine the mechanism of waste degradation. High ORP indicates aerobic conditions, which causes accelerated degradation of OFMSW, and the optimum ORP required for methanogenesis ranges from −100 to −300 mV. The ORP results of this investigation are shown in Figure (b), and due to the sensitivity of ORP sampling and analysis, the ORP tests were done immediately after sampling. During the first 7 weeks of the landfilling experiments, the ORP values were high, reaching a maximum of 120 mV for Br-1 and 132 mV for Br-2, respectively. A similar rise under aerobic conditions was observed by Pellera et al., indicating that the degradation was in the hydrolysis stage. After the available oxygen (naturally inherent and partially aerated oxygen) was depleted on both bioreactor columns, there was a sudden drop in ORP values, indicating that the degradation shifted from the acid stage to the methanogenesis stage. After 96 days, the ORP decreased below −160 mV in the Br-2 column, while the Br-1 column reached this value after 133 days. The negative ORP values indicated that the degradation was in the anaerobic phase, and reduced substances like methane and ammonia were also produced from the OFMSW degradation process. − After 140 days, the ORP values remained admissibly constant together with the pH in the Br-2 column, at about −190 mV and 8.0, respectively, indicative of persistent strong anaerobic conditions.

3.1.3. Total Nitrogen

The total nitrogen (TN) concentration variations in the leachate over operational time are presented in Figure (a). Analysis of TN concentrations began 60 days after the experimental research began. The TN concentration of the leachate samples after 60 days of conducting the investigation was 395 mg/L for Br-1 and 750 mg/L for Br-2. The high concentrations of TN observed during week 8 in both bioreactor columns might be attributed to aerobically decomposed proteins available in the organic waste into ammonia and nitrogenous substances, which are soluble in leachate, leading to high TN concentrations. The concentration was higher in Br-2 compared to that in Br-1 due to the additional oxygen introduced by partial aeration, therefore maintaining an aerobic environment suitable for the nitrification process and substrates reintroduced by the diluted leachate recirculation, promoting decomposition of the organic waste, and releasing additional TN. As conditions transitioned from aerobic to anaerobic based on ORP analysis, a reduction was then observed in both bioreactors, which is attributed to the nitrification–denitrification reaction, which led to a decrease in TN concentrations while a significant yield of biogas was observed. Toward the end of the investigation, the reduction of TN content progressively decelerated due to the lack of oxygen and substrates.

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Generated leachate quality during the operation: (a) total nitrogen (TN) and (b) carbon oxygen demand (COD).

The findings indicate a lower corresponding TN removal efficiency in Br-1 (90%) compared to that in Br-2 (97%). This can be attributed to the addition of water without any supplementary substrates, which led to a weak simultaneous occurrence of nitrification/denitrification processes, as the substrates were washed out with the leachate without replacement. This occurrence suggests weak nitrification and denitrification and therefore restricts TN removal. In Br-2, the corresponding TN removal efficiency was high, reaching 97% after 210 days of operation, demonstrating that partial aeration and the diluted leachate recirculation operation approach improved TN concentration removal. It has been suggested that nitrogen uptake in the microbial organic matter could play a significant role in nitrogen transformation processes during aeration, , while recirculating the diluted leachate assisted in improving the interaction opportunity between the microbes and nutrients. ,

3.1.4. Carbon Oxygen Demand

The COD variations in the leachate with time are listed in Figure (b). Within the first 8 weeks of the study, the COD increased from beginning levels of 6893 to 29,850 mg/L for Br-1 and from 7673 to 27,990 mg/L for Br-2. This was attributed to the rapid release and hydrolysis of complex MSW organics to the leachate, as well as the buildup of carboxylic acid and organic acids. These MSW organics are broken down into gases or simpler chemicals such as organic acids and ammonia, while the inert elements are leached out and settle at the bottom of the reactor. It has been noted that there is an inverse relationship between the pH and COD values. Higher concentrations of COD were found in the acidogenic phase but declined when pH rose, as a result of organic matter degradation removing the high concentrations of COD in the process. The COD concentration in the leachate, therefore, declined as a result of the methanogenic environments, ending the investigations at 8750 mg/L for Br-1 and 351 mg/L for Br-2.

The unique conditions of aerobic, anoxic, and anaerobic in Br-2 may have enhanced the diversity of microbial communities, and the combined action of some microbial processes may have contributed to the higher elimination of COD in the leachate. These results revealed that Br-2 had greater microbial activity than Br-1, which led to significantly higher organic decomposition efficiency in Br-2 than in Br-1. Venkatesh et al. inferred the degradability of organics to be associated with the decline in COD levels. In both bioreactor columns, the corresponding reduction efficiency (RE) of COD was 71% for Br-1 and 98% for Br-2, indicating the biodegradation of simple molecules such as VFA into CO2, CH4, and other pollutants with the hybrid bioreactor column having the greatest percent reduction.

3.1.5. Total Organic Carbon

Total organic carbon (TOC) was measured during weeks 15, 20, and 25 of operation, as shown in Table . The initial TOC recorded in week 15 was between 303.2 and 332.5 mg/L, while the TOC concentration recorded in week 25 was 461.1 mg/L for Br-1 and 221.9 mg/L for Br-2. The content of TOC tends to decline with the degree of waste stabilization/maturity, which was positively observed in Br-2. The opposite was observed with Br-1, where the variability showed a decreasing trend between weeks 15 and 20, but in week 25, it increased, indicating that no clear improvement in biodegradation was achieved during week 25. This could be an indication that there were fractions of organic compounds in the waste that were not adequately subjected to biodegradation. The corresponding removal efficiency for Br-2 was 33% in week 25, demonstrating that partial aeration and recirculation of diluted leachate enhanced dissolution and biodegradation of solid-phase organics. The low TOC reduction observed in both bioreactor columns might be due to the presence of complex organic mixtures such as lignin, synthetic materials, and humic substances, forming a major part of the TOC available in the OFMSW studied. Due to the prevalent anaerobic landfilling conditions, only 30–40% of the total organic content (TOC) in MSW is decomposable. Even though the TOC was slightly reduced, it actually does suggest the possibility of humidification to promote stabilization of landfilled organic waste.

1. Total Organic Carbon Content Measured during Weeks 15, 20, and 25 of Operation.
parameter Br-1 Br-2
time (weeks) 15 20 25 15 20 25
TOC (mg/L) 332.50 238.10 461.10 303.20 237.40 221.90
COD (mg/L) 29,200 21,230 10,550 25,130 2870 365
TOC/COD ratio 0.01 0.01 0.04 0.01 0.08 0.61
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3.1.6. Ratio of TOC/COD

The TOC/COD ratio was utilized to describe the maturity and stabilization of waste in the bioreactor columns. The TOC/COD findings from this study yielded an increasing trend as the landfilling proceeded, with the peak ratio reached in week 25. Table shows that the average TOC/COD ratios for Br-1 and Br-2 columns were 0.011 and 0.012, respectively, during week 15. This can correspond to the fact that the strongest decrease of the readily decomposable organics occurs during the initial phases of landfilling, while the hardly biodegradable inorganic fraction had no significant impact. During week 25, both columns experienced an increased TOC/COD ratio, signifying that more organic compounds were being degraded. However, the Br-1 column ratio (0.04) was less compared to the Br-2 column (0.61) ratio when the investigations were on week 25, even less than the 0.5 ratio of a stabilized leachate. The TOC/COD ratio in Br-2 increases during week 25 of the experimentation, which is attributable to the significant reduction in COD. During partial aeration, the availability of oxygen in the landfilled waste is enhanced, thereby promoting aerobic processes and reducing the concentration of oxygen-demanding pollutants. , Conversely, the recirculation of leachate facilitates waste degradation, leading to the removal of various pollutants, including COD. , The increased ratio suggested that Br-2 was already in the stable methanogenesis stage, with the majority of organic carbons decomposed, while Br-1 was still experiencing the acidic formation stage and methanogenesis stage simultaneously. Furthermore, the ratio of Br-1 might represent young landfill leachate, while Br-2, with a ratio of 0.61, represented a stabilized landfill leachate. ,

3.1.7. Heavy Metals

The concentration of heavy metals (HMs) is regarded as one of the most crucial factors in optimizing the production of biogas from landfills, which can exist in various forms in the leachate based on the pH, waste matrix complexation, and biodegradation phase. Trace elements are required by microorganisms to promote enzymatic activities, coprecipitation, and chemical reactions during OFMSW biodegradation, though the presence of substantial amounts retards microbial activity, biogas generation, and landfilled waste stabilization processes. The concentration of heavy metals in leachate not only influenced biogas production in landfills but also raised concerns regarding the potential contamination of the soil environment and the associated detrimental effects. The analyzed HMs in the investigation are presented in Table . The concentration of the HMs was higher in week 15 of landfilling than in week 25, and this was associated with leachate having high concentrations of HMs in the hydrolysis and acidification phases of landfilling, since more HMs dissolve in low pH as a result of high content of organic acids.

2. Heavy Metals Measured during Weeks 15 and 25 of the Investigation and the Recorded Percent Reduction .
parameter Br-1 Br-2
time (weeks) 15 25 RE (%) 15 25 RE (%)
heavy metals (mg/L)            
Ni 0.06 ± 0.01 0.06 ± 0.01 7 0.11 ± 0.0 0.10 ± 0.0 11
Cu 0.04 ± 0.0 0.04 ± 0.0 10 0.08 ± 0.0 0.07 ± 0.0 17
Cr 0.03 ± 0.01 0.02 ± 0.0 28 0.07 ± 0.01 0.06 ± 0.01 3
Co 0.12 ± 0.01 0.04 ± 0.0 67 0.15 ± 0.0 0.06 ± 0.0 59
Pb 0.29 ± 0.01 0.11 ± 0.0 61 0.35 ± 0.01 0.16 ± 0.0 54
Zn 0.54 ± 0.01 0.48 ± 0.0 10 1.14 ± 0.01 0.32 ± 0.0 72
Mn 0.64 ± 0.0 0.49 ± 0.0 24 1.01 ± 0.0 0.36 ± 0.0 65
Fe 15.64 ± 0.01 13.98 ± 0.0 11 60.92 ± 0.6 7.47 ± 0.01 88
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a

RE: reduction efficiency.

The concentrations of the HMs on both occasions of analysis were below 1.2 mg/L, except for Fe, with a high concentration of 60.92 mg/L. The concentrations of HMs can be arranged in their descending order: Fe > Zn > Mn > Pb > Co > Ni > Cu > Cr. During the methanogenesis stage in week 25, the concentration of the HMs declined for both bioreactors due to the high pH, which improves sorption and precipitation of the HMs, while the low content of the dissolved organic material may complex the cations. The concentrations of HMs were reduced on both bioreactor columns, and their average percent reduction ranged between 88 and 3%. The phenomenon is associated with the fact that most metal ions are needed for structural and/or catalytic functions by microbes and can function as stimulating agents in biodegradation processes, with a consequent increase in methane generation. Trace metals such as Zn, Ni, and Cu are nutrients and are consequently exploited by bacterial metabolism. Furthermore, the decrease in HMs may possibly be linked to the detoxification of heavy metals by biogenic hydrogen sulfide present during the early phases of landfilling, which will raise the production of biogas. Due to the lower toxicity levels of the OFMSW utilized in this experiment, it can safely be stated that the selected composition of OFMSW can be used to produce biogas.

3.1.8. Variations in Microbial Communities

The microbial community analyses of leachate samples from Br-1 and Br-2 revealed Bacteroidota (53–64%), Firmicutes (13–19%), and Proteobacteria (23–31%) as the top three dominant phyla during the landfilling experiments. The results support those reported by Wang et al. that Firmicutes, Bacteroidota, and Proteobacteria are the most common phyla in the landfill. Bacteroidota was the most prevalent bacterial phylum, followed by Proteobacteria and Firmicutes. In landfills, Proteobacteria transform sugars into fatty acids and monosaccharides, while Bacteroides and Firmicutes decompose cellulose and starch polysaccharides. Bacteroidota breaks down cellulose into cellobiose and glucose, while Firmicutes secrete hydrolytic enzymes such as protease, lipase, and cellulase to break down lipids, proteins, and cellulose. Firmicutes are considered to multiply to a maximum level in the early stage of landfilling, providing adequate carbohydrate and monosaccharide substrates for Proteobacteria to thrive and proliferate. As a result, low Firmicute abundance was reported at W15 and W30 of this investigation, which corresponded to the middle and final stages of the landfilling studies.

To further understand the impact of partial aeration and diluted leachate recirculation on the population, Figure (a) shows the distribution of microorganisms at the genus level in the two columns. Genus or archaea, less than 1% of the total sequences, and unclassified genus or archaea were classified as others.

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Analysis of bacterial and archaeal community structures of Br-1 and Br-2 at (a) the genus level and (b) archaea levels during weeks 15 (W15) and 30 (W30) of experiments.

The Firmicutes phylum had the Clostridium bacteria being the most dominant genus in the Br-1 column, with a relative abundance of 74%, while Br-2 had 25% during W15. Clostridium genera metabolize ethanol and lactic acid to produce hexanoic and butanoic acid. Given the low relative abundance in Br-2, the landfill environment was beneficial for the methanogenic phase because it significantly reduced the buildup of butyric and caproic acid concentrations. Lactobacillus, Bacillus, and Butyrivibrio genera were also observed at this stage. Lactobacillus (45–13%), a Gram-positive bacterium, is adept at metabolizing carbohydrates to produce acid, synthesizing heteropolysaccharides and glucan, and undergoing sugar fermentation to produce lactic acid. , Butyric acid-producing bacteria, Butyrivibrio (<7%) and Bacillus (<10%), both of which are hydrolytic bacteria, were low in both columns. The relative abundance of Bacillus in Br-2 was 10% (W15), and partial aeration and diluted leachate recirculation created a favorable environment for these facultative bacteria to metabolize the short-chain fatty acids, facilitating the methanogenic phase. At the termination of the landfilling, Bacillus decreased to <2%, while Clostridium were more enriched and maintained a relative abundance of 34%.

The most varied bacterial phylum, Proteobacteria, has been demonstrated to be significant in nutrient cycling, which has dominance of Rhodobacter, Acetobacter, Pseudomonas, and Acinetobacter in this study. Rhodobacter (66%), the most dominant genus in this phylum with Br-2 having the highest abundance, is a facultative bacterium responsible for the production of hydrogen gas from carbon sources. The genus also has the ability to utilize nitrogen compounds, contributing to the removal of ammonia and nitrate, which leads to the significant decrease of total nitrogen in the leachate observed in Section . The hydrolytic bacteria, Pseudomonas, had a relative abundance of 31% in Br-2 and 11% in Br-1. Elevated concentrations of Pseudomonas are advantageous for producing hydrogen gas from acetic acid, as they facilitate rapid degradation of acetic acid, a process that was enhanced by the hybrid approach in Br-2. The relative abundance of the Acetobacter genus and Acinetobacter genus was relatively low because both genera are aerobic bacteria. At W30, the Rhodobacter genus was slightly decreased (51%), while Pseudomonas exhibited an increase (37%) for both columns, with Br-2 showing a significant change.

The dominant genera, as the top 4 in overall abundance of Bacteroidota, were Bacteroides, Prevotella, Proteiniphilum, and Petrimonas. On W15, Bacteroides, which belongs to Gram-negative, obligate anaerobic bacteria, with a relative abundance of 43–33% was the most dominant genus. Bacteroides are well-known for their potent capacity to break down macromolecular organic matter into acetate and succinate, which are primarily the main contributors in all organic load situations during anaerobic digestion. At the termination of the experiments, the relative abundance decreased in Br-2, reaching 28%, whereas no significant change was observed in Br-1. Prevotella (32–27%) and Proteiniphilum (26–24%) were also the dominant genera in both columns, with Br-2 experiencing the highest community of both genera. Prevotella, which belongs to Gram-negative bacteria, degrades starch and hemicellulose to produce sugars and further converts them into volatile fatty acids, while Proteiniphilum, a facultative anaerobic bacterium, has been proven to have the function of hydrolysis and acidification. At the end of the landfilling, the richness of Prevotella and Proteiniphilum decreased significantly in the Br-2 column than in the Br-1 column.

The diversity indexes of the archaea increased in Br-2, while a slight increase was observed in Br-1, which indicated that the archaeal diversity was negatively affected by the acidic environment that prevailed in the column. The rationale for this finding might be because archaea, particularly methanogens, are obligate anaerobic microbiota, and the presence of an acidic environment may impede their proliferation. As presented in Figure (b), the dominant methanogens in both columns were Methanosarcina, Methanobacterium, Methanoculleus, and Methanobrevibacter. However, the relative abundance varied with the landfilling time. Methanosarcina were abundant in both columns during the landfilling process due to their metabolic diversity and capacity to efficiently use diverse substrates included in the waste. The methanogens are essential for the generation of methane in landfills because they are extremely versatile and thrive in anaerobic conditions. The presence of Methanobacterium during W15 was caused by the totally anaerobic atmosphere on both columns, which was appropriate for their development. It was also observed that the diversity of Methanoculleus was obvious in both columns, and these methanogens together with Methanobacterium are known to be hydrogenotrophic. Furthermore, the presence of Methanobrevibacter (a bacterium that can only use CO2 as a substrate) in both columns (W15) revealed that CO2 gas was the dominant component at that stage.

The variations of methanogenic bacteria between Br-1 and Br-2 at the termination of the landfilling experiments (W30) are shown in Figure (b). It was observed that all methanogenic bacteria in Br-2, except for Methanosarcina, were significantly higher than those in Br-1. The results show that Br-2 experienced a greater level of degradation and a more rapid stabilization process. As the landfill process advanced, the differences in microbial composition at both the genus and archaea levels became more evident in the hybrid landfilling method, indicating that this approach modified the microbial community structure during the landfill process. The acetate and hydrophilic methanogens were enriched by the hybrid approach. A comparison of the microbial composition between the two landfill methods revealed that the hybrid method had improved ability to degrade organic matter, aligning with the observed changes in leachate characteristics.

3.2. Waste Characterization

The degradation of the waste was analyzed using the FTIR analytical technique. To observe the transformation of the organic functions, fresh and decomposed wastes were evaluated. The obtained spectra were interpreted with reference to the spectra assignments of refs − . The FTIR spectra of the samples from both columns are reported in Figure , while the assignment of the infrared adsorption bands is reported in Table .

5.

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FTIR spectra of waste samples from fresh waste (FW) and Br-1 and Br-2 columns.

3. Interpretation of the FTIR Bands and Assignments Observed for the Waste Samples .

      sample ID
IR absorbance intensity
wavenumber (cm–1) assignment functional group or component FW Br-1 Br-2
3400–3200 O–H stretching, N–H stretching hydroxyl groups and amide A in proteins m, b a a
2930–2920 C–H stretching of CH2 fatty acids and aliphatic methylene m a a
2860–2830 C–H stretching of CH2 fatty acids and alkanes w a a
1750 CO aldehyde, ketone, carboxylic acids, and esters a b, w b
1650–1630 CC stretching, CO aromatics and amide I m b b
1545–1540 CH2, CH3, C–O, and N–H stretching in plane amine w a a
1440–1410 C–H asymmetric deformation carbohydrates and lignin m w b
1390–1380 N–O stretching nitrate in solid waste samples a a m, w
1375–1370 C–H of CH2 and CH3 cellulosic band w w b
1320–1315 C–N stretching aromatic amines w a a
1250–900 C–O–C, C–O, C–O–P polysaccharides and phosphodiesters s b a
1240–1230 C–O, C–N stretching carboxylic acids and amide III w a a
1030–1020 C–O and Si–O stretching, Si–O–Si polysaccharides, clay minerals, silica s b a
950–940 C–H bending aromatic w a a
890–870 C–O out of plane carbonate and polysaccharides w s m
650–630 S–O bends inorganic sulfates s b b
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a

s: strong; b: broad; m: medium; w: weak; a: absent.

The main adsorption bands in the fresh waste (FW) at 3268 cm–1 were assigned to −OH groups and C–H bonds of the type of phenol, alcohol, or carboxyl −OH and the hydrogen vibration of amide N–H functions. Medium and weak bands at 2920 and 2850 cm–1 were attributed to the stretching of C–H bonds in fatty acids and aliphatic and alkane structures, respectively. A medium band at 1645 cm–1 is assigned to CC stretching and CO, while the weak peak at 1540 cm–1 is due to amine groups. A medium band at 1413 cm–1 due to C–H asymmetric deformation was assigned to carbohydrates and lignin. A band at 1240 cm–1 is generally linked to the absorbance of ethers, aromatic esters, and the N–H of amides, and a most intensive broad band appearing at 1030 cm–1 due to carbohydrates, aromatic ethers, and polysaccharides was also observed.

At the termination of the landfilling experiments, samples of degraded waste were collected from both columns and analyzed to understand the transformation of the organic functions. The first significant change observed from both samples of degraded waste was the absence of peaks at 3268, 2920, and 2850 cm–1 from both samples, indicating a decrease of the aliphatic fractions in the humic structure, and could be attributed to the degradation of lipids and carbohydrates in the waste samples. , Microbial activity during degradation reduces the intensity of CO group bands observed at 1680–1640 cm–1, which disappeared in the Br-2 sample but presented a broad peak in the Br-1 column sample. The disappearance of the peak at 1740 cm–1 on both column samples due to CO vibration of aldehydes, ketones, esters, and carboxylic acids signified their complete degradation. The occurrence of lignin characteristic bands at 1460–1440 cm–1 was observed with a weak peak in Br-1, while a broad peak observed in Br-2 was evident for degraded lignocellulosic components. The intensities of bands at 1030–1020 cm–1 could indicate mineralization from effective biodegradation, which may result in an increase in inorganic compounds and typically attributed to the decomposition of carbohydrates or polysaccharides on both columns.

The characteristic band of nitrate observed in the 1380–1390 cm–1 region was an obvious difference in the spectra of the Br-2 sample (Figure ) compared to the Br-1 sample. The band is usually detected exclusively at a later stage of degradation when the material is well degraded, indicating the state of decomposition at which nitrogen from degraded components is oxidized. , The band position is stable and greatly reproducible at 1384 cm–1. A high content of nitrogen in the humic fractions could be an indicator of microbial enrichment. As microbes die during the degradation process, their nitrogen becomes accessible to the living organisms, and microbial use of cellulose may intensify with time. A broad weak occurrence of an aromatic amines band at 1320 cm–1, which indicates immaturity, was slightly observed in Br-1 but not observed in Br-2. This was because it decreases during biodegradation and disappears when the material has reached stability. Traces of polysulfide and alkyl halides were responsible for the smaller peaks between 870 and 500 cm–1.

3.3. Biogas Composition and Yield

3.3.1. Concentration of Carbon Dioxide

In the biodegradation of OFMSW, biogas is generated, with CH4 and CO2 being the main gases produced in which these gases can be used to describe the stages that occur during the biodegradation process. Variations in the concentration of these gases in both bioreactor columns during the landfilling investigation are listed in Figure . The results show that in both bioreactor columns, CO2 was generated, and a pattern of initially rising was observed until it reached a maximum concentration and then modestly declined until the end of the investigation period. The CO2 production of each bioreactor peaked after 120 days (Br-1) and 90 days (Br-2) of the landfilling experiments, with CO2 concentrations reaching an average maximum of 61.81 and 58.93%, respectively. The concentration then started to decline in both bioreactor columns, with the lowest concentration being observed in Br-2 by the end of the investigation reaching an average of 32%. In Br-1, a decline was observed until the concentration became relatively stable at an average of 40%, and the stable condition persisted until the investigation was terminated.

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Generalize phases and variations in generated biogas composition on both bioreactor columns with time: (a) Br-1 and (b) Br-2.

The observed changes in CO2 content can be explained as follows: during the initial stages (I and II) of biodegradation, the oxygen content trapped within the waste pile is gradually depleted by aerobic microbes (such as Bacteroides, Acetivibrio, and Clostridium) that consume it, shifting the biodegradation stage from aerobic to anaerobic during the transition phase (II) in Figure (a),(b). At this stage, hydrolysis occurs through hydrolytic and fermenting microbes, producing organic acids, glycerol, and dissolved sugars, with byproducts being CO2 and H2 gases. According to the graphs, high detection of CO2 concentration was observed between the 2nd and 18th week due to the hydrolysis and acid formation stages (II and III) occurring. During this period, the biodegradation process will shift to the acetogenesis stage, where the produced CO2 from the previous stage together with H2 will be reduced into methane (CH4) by hydrophilic methanogen microbes, as indicated by the reduction of the ORP concentration in Figure (b). Additionally, acetogenic bacteria will transform the degradation products of the organic material from the preceding stage into acetic acid, CO2, and H2 during the acetogenesis phase, as well. Acetophilic and hydrophilic methanogen microbes will then transform acetic acid, CO2, and H2 into CH4. These phase changes, as reflected in Figure (a),(b) between the 11th and 30th week, occurred in both bioreactor columns, although concentrations of CO2 gas were still detectable but in a lower range.

With a reduced methanogenic lag time, the hybrid bioreactor column (Br-2) was able to enter the methanogenic phase (IV) earlier than the typical bioreactor (Br-1) column. According to the findings, aerobic microbes could have been encouraged to undergo hydrolytic acidification through partial aeration, which produced a significant amount of CO2. This allowed Br-2 to swiftly experience the acidogenesis and acetogenesis stages of the biodegradation process within the first 10 weeks of the investigation. When Br-2 initially entered the methanogenic phase, its CH4 concentration was lower than its CO2 concentration, which is associated with both hydrolysis and acidic phases producing a vast amount of CO2. The CO2 concentration slowly decreased; as discussed above, CO2 is reduced to CH4 by hydrogenotrophic or hydrogenophilic methanogens in the presence of H2. , In contrast, an anomaly occurred in Br-1 where the column experienced acidic conditions (pH < 7) with relatively high fluctuating CO2 concentrations until the investigation was terminated. This might have indicated that the Br-1 column was still experiencing the hydrolysis, acidogenesis, and acetogenesis stages of organic compound biodegradation, where CO2 was still generated, and a little fraction of the generated CO2 was converted into CH4. Furthermore, this showed that the organic content in Br-1 was still relatively high, with the process of biodegradation proceeding slowly, thereby causing the system to have high acid contents, which might result in high quantities of CO2 and H2 gases. Besides slowing down the conversion of acetic acid, CO2, and H2 to CH4, an acidic environment will also inhibit the activities of methanogenic bacteria, yielding a low generation of CH4 as verified by the Br-1 column.

3.3.2. Methane Concentration

The formation of CH4 gas occurs at the ultimate stage of the OFMSW degradation process, which can also indicate organic waste stabilization. To ascertain how the degradation process proceeded in each bioreactor column, this investigation also monitored the formation of CH4 in each landfilling bioreactor column in addition to CO2. The variations in CH4 generation in the typical and hybrid bioreactor columns can be seen in Figure (a),(b). From the graphs, methane generation in each bioreactor showed a similar pattern, first slowly rising until a maximum is reached and then progressively falling until the termination of the investigation. The maximum methane production was intensive on days 147 (Br-1) and 118 (Br-2), while its highest concentration reached an average of 52.3% for Br-1 and 67.5% for Br-2.

It was also observed that Br-2 entered the methanogenesis stage 33 days earlier than Br-1, in which Br-1 showed a decrease in the amount of CO2 and an increase in the amount of CH4 after 127 days of landfilling, indicating that it had entered the initial methanogenesis stage. When both bioreactor columns first experienced the methanogenic stage, their CH4 concentration was lower than the CO2 concentration, which is associated with both hydrolysis and acidic phases producing a vast amount of CO2. However, the CH4 concentration rose steadily over the course of the landfilling period surpassing the CO2 concentration, signifying that the CO2 is being converted by methanogenic microbes into CH4. Although, after 63 days in the methanogenesis stage, the generation of CH4 in Br-1 became constant (±2%) with little change in the CH4 production rate, while CO2 concentration remained unstable above 40%. While the methanogens may not have had enough time to grow due to being flushed out of the column with leachate removal and replacement with water, it can be assumed that the Br-1 column underwent both the hydrolysis and acid formation stages during the methanogenesis stage. The lengthy interval (14 weeks) between the initial CH4 detection and 50% CH4 concentration also suggested a slow growth rate for methanogens and a significant chance of failure due to elevated organic content.

3.3.3. Biogas Yield

The cumulative yields of the biogas produced by each bioreactor column are listed in Figure . Biogas generation results after 210 days of landfilling studies displayed that partial aeration and leachate recycling practiced in Br-2 had the highest cumulative biogas production, 41 times greater than that of the typical bioreactor column (Br-1), as illustrated in Figure (a),(b). The bioreactor column Br-2 produced 21.52 L at an average rate of 0.12 L/day, which was higher than the cumulative volume of biogas produced in Br-1 of 8.75 L, averaging 0.05 L/day for 7 months. Maximum biogas production in Br-2 occurred mainly between days 91 and 124 of operation, which was after the last partial aeration conducted on day 60 of operation. The rapid biogas production in Br-2 might be attributed to partial aeration in the upper layer of landfilled waste, which decreased the leachate acid concentration quickly, promoting methanogenic environments in Br-2 with recirculation of diluted leachate, increasing the generation rate of methane. The results concurred with findings of Xu et al.

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Cumulative and daily volumes of generated biogas on both bioreactor columns with time: (a) biogas in Br-1 and (b) biogas in Br-2.

The maximum yields of CH4 in the biogas generated are demonstrated in Figure (a),(b). The total volume of CH4 produced in Br-2 accounted for 61% of the total biogas produced. Biogas-to-energy plants require both the amount and quality of the biogas, and to be technically or commercially viable, the biogas must normally contain more than 40% methane. Methane yielded from Br-2 was 13 L in total, with biogas containing methane concentrations above 50% accounting for 77% of the methane collected. Approximately 89% of the total methane recovered from Br-2 was generated between days 90 and 180. Methane produced between days 180 and 210 accounted for only 8% of the overall methane volume. In contrast to the 61% obtained in Br-2, only 47% of methane (4.2 L of methane) was recovered from the 8.75 L of biogas produced in Br-1, which was a type of low-quality biogas. More than half of the total biogas volume collected in Br-1 was composed primarily of CO2 gas. As a result of the high methane content in the biogas collected in Br-2, it demonstrated more economical and technical viability.

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Cumulative volumes of generated biogas and methane on both bioreactor columns with time: (a) in Br-1 and (b) in Br-2.

3.4. Mechanism of the Hybrid Approach on Biogas Generation

In biogas production from municipal solid waste (MSW), partial aeration during initial landfilling promotes facultative aerobic/anaerobic microorganisms. These microorganisms decompose complex organic matter through hydrolysis and convert it through acidogenesis into volatile fatty acids with limited oxygen. The study reported a high abundance of facultative microbes in Br-2, where partial aeration was implemented, while their abundance was low in Br-1. This method encouraged bacteria to secrete extracellular enzymes, enhancing the breakdown of lignocellulosic biomass and complex materials for subsequent landfilling stages, while the diluted leachate, a landfill byproduct rich in organic matter and nutrients, gave the microorganisms enough humidity and natural buffer capacity to get through the acid phase. This allowed nutrients and associated hydrolytic enzymes to freely circulate, which accelerated the onset of the methanogenesis stage and enhanced the generation of biogas in the bioreactor. However, the entire approach of partial aeration and diluted leachate recirculation enhanced the biodegradation process and generation of CH4, which improved the stability of the landfilled waste while providing in situ treatment of the generated leachate.

4. Conclusions and Future Work

Simulated landfill reactor columns were operated for 210 days at ambient temperature as typical and hybrid bioreactors with partial aeration and diluted leachate recirculation to provide an understanding of OFMSW degradation characteristics and to study the mechanism associated with the enhancement of methane generation efficiency. The key findings from the study are summarized as follows.

  • (1)

    In this experiment, the feasibility of introducing humidity only by deionized water addition seemed to be unsuitable for methane generation enhancement due to the high concentration of leachate contaminants released, indicating inhibitory conditions in the system for biogas generation. The typical bioreactor (only water addition) was on the acidic phase throughout the investigation, with COD above 8000 mg/L, TOC < 400 mg/L, and high heavy metals. Therefore, during the hydrolysis and acidogenesis stages, flushing and diluting of high organic acid concentration are essential for providing a favorable environment for organisms. Adding or reutilizing diluted leachate in landfilling can more quickly provide a suitable environment with lower organic acids and a higher pH for methanogens.

  • (2)

    As demonstrated by the substantial decrease in leachate composition, the increased proportion of OFMSW biodegradation, and a shorter time needed for waste degradation and stabilization, the hybrid bioreactor approach had a significant impact on methanogenesis onset acceleration, enhancement of biogas production, and organic stabilization. In contrast, the typical bioreactor operated column had an acidic phase that lasted long, delaying the onset of methanogenesis, which was reached after 124 days from the commencement of the investigation. It also generated low volumes of biogas with a cumulative volume of 8.75 L compared to the 21.52 L generated by the hybrid bioreactor (Br-2), with methanogenesis phase onset occurring 33 days earlier.

  • (3)

    Partial aeration introduced oxygen in the process, enhancing aerobic bacteria growth, whereas diluted leachate recirculation reintroduced the discharged essential nutrients and substrates with leachate, neutralizing the high concentration of leachate contaminants and enhancing microbial diversity. However, optimal environmental parameters for methanogens are also essential for methanogenesis to be efficient, particularly pH, which should be kept between 7 and 8 to establish a favorable environment for methanogenesis. This will help avoid conditions like in Br-1, where the methanogens were exposed to an inhibitory environment (pH < 7) throughout the investigation, resulting in low biogas yield; hence, the leachate pH needs to be regulated during methane generation enrichment. On the other hand, the study on microbial communities reveals that the abundance of microorganisms in landfills increases with landfill time, particularly in Br-2, where partial aeration and diluted leachate recirculation occurred. This abundant bacterial community enhances the system efficiency in treating landfills through interpopulation coordination, promoting landfill system stability.

  • (4)

    From this study, it is suggested that the integrated approach of partial aeration and recirculation of diluted leachate is an efficient way to maximize the level of landfilled MSW stabilization and enhance the quality and yield of biogas generated. The composition of OFMSW utilized in the study can be exploited for biogas production, though it is recommended that a small amount of an inoculum can be added in the hybrid bioreactor column to promote microbial activity and development while minimizing salinization effects. This could reduce the content of CO2 while increasing the concentration of methane in the biogas to achieve a better yield of methane.

Acknowledgments

The authors are very grateful to the Climate Change, Water Security and Disaster Management Research Niche Area Centre from Tshwane University of Technology for their unwavering support during this study. The authors would like to thank the Agricultural Research Council of South Africa for conducting the study on microbial communities.

The data will be made available on request due to privacy restrictions.

K.P.: conceptualization, supervision, project administration, resources, funding acquisition, and writingreview and editing. M.M.: supervision, funding acquisition, and writingreview and editing. K.L.: methodology, investigation, data curation, writingoriginal draft preparation, and writingreviewing and editing. All authors have read and agreed to publish the version of the manuscript.

The study was funded by the National Research Foundation (NRF) of South Africa (Grant No. TTK2204224344).

The authors declare no competing financial interest.

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