Introduction

Landfill site closure leachate (LSCL) refers to the leachate produced after a domestic waste landfill is stopped and the site is closed and covered. Thus, the LSCL is generally weakly alkaline, the chemical oxygen demand (COD) and biochemical oxygen demand (BOD) gradually decrease, the ammonia nitrogen (NH₃-N) concentration significantly increases, and the carbon–nitrogen ratio (BOD/COD) value decreases, indicating that it has poor biodegradability1,2, and have a low heavy metal content3,4,5. To meet the requirements of national emission standards, it is generally necessary to use membrane treatment technology for in-depth treatment6. According to the"Communique on China’s Urban Construction Status in 2022"issued by the Ministry of Housing and Urban–Rural Development, China’s urban solid waste harmless treatment capacity is 110.94 million tons/day, of which the incineration treatment capacity accounts for 72.53%. As waste incineration has become the dominant method in China’s economically developed regions, many coastal provinces and cities have already achieved zero raw waste to landfill. Thus, in the landfill leachate produced annually in China, the proportion of LSCL is expected to be greatly increase.

The dry base contents of carbohydrates, crude protein and crude fat in kitchen waste in China can reach 75%7, and the waste is rich in various nutrients. Therefore, kitchen waste is generally treated by biological fermentation8,9. However, owing to the high moisture content of kitchen waste, its perishability and its ability to breed bacterial toxins8,10, anaerobic digestion (AD) technology is often chosen to treat kitchen waste and recycle biogas11,12,13. AD is the most widely used biological technology for the treatment of urban biomass waste. Anaerobic digestion systems can efficiently degrade organic components in kitchen waste and generate biogas through the joint action of facultative and obligate anaerobic microorganisms. We can obtain a biogas yield of 200 ~ 300 m3 per ton of urban biomass waste14,15. Since the four-population theory was proposed by Zeikus at the first International Conference on Anaerobic Digestion in 1979, the four-stage theory of AD has gradually been accepted by relevant scholars and has become the basic theory leading AD research. The microbial communities involved in AD are large and diverse, the succession process is complex and changeable, and the material transformation mechanism is not completely clear. However, these different views suggest that the distribution and population density of microbial communities play key roles in AD process16,17.

A domestic waste landfill is a large anaerobic digestion bioreactor, that is rich in various anaerobic microorganisms that have been domesticated for a long time18,19,20. Liu et al.21 carried out a literature review and analyzed the microbial communities in landfills. According to some other researchers, microorganisms in landfills can be divided into three categories: hydrolytic acidifying bacteria, hydrogen-and acetic acid-producing bacteria, and methane-producing bacteria, each of which includes many different species22,23 among the methane-producing bacteria, the dominant species is Methanococcus24,25,26. Some others24,27 suggested that the transformation of various substances in landfills does not occur only at a specific time but rather throughout the entire process of waste stabilization. However, the dominant microbial species are different in each specific stage, forming a complex and diverse microbial community28,29. Landfill leachate is bound to carry a rich anaerobic microbial community, including bacteria that can promote kitchen waste humification and humus degradation.

Current research indicates that during the combined anaerobic digestion process of kitchen waste and leachate, through mechanisms such as alleviating acid inhibition, optimizing nutrient balance, increasing methane production rate, promoting the initiation of digestion reactions, and reducing the impact of toxic substances, a significant synergistic effect has been achieved, thereby effectively enhancing the stability and treatment efficiency of the system30,31. However, the influence of leachate from different eras on the microbial activity and gas production efficiency in the combined anaerobic digestion process varies. Fresh leachate is rich in high-concentration volatile fatty acids (VFAs) and easily degradable organic matter. When mixed with food waste, it rapidly initiates the hydrolysis and acidification stages32. It also contains trace elements such as Fe, Co, and Ni, which effectively address nutrient deficiencies when food waste is digested alone33. Mid-stage leachate contains some degradable organic matter and humus, requiring adjustment of the C/N ratio to optimize methanogenic bacterial activity34. Mature leachate contains high ammonia nitrogen concentration, which neutralizes acids produced during the acidification stage and enhances system buffering capacity. Humus acts as an electron acceptor to promote microbial extracellular respiration, accelerate acetic acid production, and enhance methanogenic bacterial metabolism35. Furthermore, old leachate carries numerous hydrolytic and methanogenic bacteria, aiding in system startup time reduction36. Therefore, further investigation into the synergistic effect mechanisms of combined anaerobic digestion of kitchen waste and aged leachate is necessary to optimize process parameters and improve treatment efficiency.

Therefore, in this work, LSCL was used as the inoculum stock solution, and kitchen waste was used as the digestion material to conduct a sequential batch co-anaerobic digestion experiment, and study the inoculation function of LSCL and the biological transformation process of kitchen waste. This study aimed to investigate the gas production performance of the anaerobic digestion system in the absence of additional inoculum, as well as the potential of co-anaerobic digestion liquid (referred to as co-digestion slurry) to rapidly transform into humus. Additionally, microbial diversity within the co-digestion slurry was analyzed. This research is expected to provide new insights into the joint utilization of landfill site leachate (LSCL) and kitchen waste.

Materials and methods

Anaerobic digestion of raw materials

The LSCL used in the experiment was taken from the Erfei Shan landfill in Wuhan, which has been closed for 10 years. Kitchen waste was collected from the student cafeteria of Huazhong University of Science and Technology. The main components of the kitchen waste were rice, vegetables and meat. After impurities were removed, the kitchen waste was pulped and refrigerated for later use. The main characteristic parameters of the raw anaerobic digestion materials are shown in Table 1.

Table 1 Parameters of the digestion materials.
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Experimental methods

The anaerobic codigestion device uses a sequencing batch reactor, and the total mass of digestion raw materials and digestion stock loaded into the reactor is 1500 g. The anaerobic codigestion experiment consists of four identical batch reactors, which are numbered OL0, OL1, OL2 and OL3 according to the different initial load rates of kitchen waste.

As shown in Table 1, the solid content of kitchen waste is 37.63%. After pulping the kitchen waste, an appropriate amount of landfill site leachate (LSCL) was added at initial loading rates of 0%, 1%, 2%, and 3% (% w/w, based on the dry mass ratio of kitchen waste). The mixture was then further pulped and stirred before being injected into the reactor. Subsequently, nitrogen gas was introduced to purge the system and maintain an anaerobic environment within the reactor. During the digestion process, the liquid in the reactor was regularly agitated, and precautions were taken to prevent air ingress during sampling. Table 2 summarizes the details of the initial loading rates, the quantities of kitchen waste added, and the volumes of leachate used. Additionally, Table 2 outlines the experimental numbers, conceptual design for the four groups of experiments, and the sample numbers for three batches of biological biogas slurry collected at different time points during the experimental process.

Table 2 Experimental design.
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The anaerobic digestion experiment was a medium-temperature experiment (35 ℃), and the temperature error was not greater than 1℃. As the anaerobic digestion experiment began, the amount of gas produced during the digestion process was recorded daily. The pH, DOC and other parameters were measured on an irregular basis. The pH value was determined by pH meter (PHS-3C, Shanghai ShengAoHua Environment Protection Technology Co., Ltd).

The experiment was divided into two stages. The first stage was the denitrification stage of kitchen waste, which lasted for 1 month (1–31 days). The second phase lasted for 4 months (32–152 days). On the 31 st and 152nd days of the experiment, the biogas slurry samples in the system were sampled and analyzed via three-dimensional fluorescence spectroscopy, and the biogas slurry samples after the anaerobic digestion experiment were analyzed via microbial diversity identification. The sample numbers are shown in Table 2.

Analysis method

Dissolved organic carbon (DOC) is the total amount of dissolved organic carbon in water and includes various forms of organic matter such as dissolved organic acids, humus, and other organic compounds. After anaerobic digestion, the biogas slurry was first filtered with qualitative filter paper and then filtered through a 0.45μm filter membrane, and the organic matter in the filtrate was dissolved organic matter (DOM). The mass conversion relationship between DOC and DOM depends on the carbon content of the various compounds in the DOM.

In this study, a certain amount of LSCL or anaerobic digested biogas slurry was filtered with qualitative filter paper and then filtered through a 0.45 μm filter membrane. The filtrate samples were retained for subsequent detection. The DOC was determined via the direct determination method (NPOC) via a TOC/TNb N/C 2100 multi-analyzer (German), and the DOM was quantitatively analyzed via three-dimensional fluorescence spectroscopy analysis. Three-dimensional fluorescence spectra (FP-6500, Jasco, Japan) was used to analyze the organic compounds in the filtrate samples, and fluorescence regional integration (FRI) was subsequently performed to analyze these fluorescence spectra. Additionally, before the DOM was analyzed, the DOC content of the sample was diluted to the same level by adding purified water.

The various fluorescent substances in the DOM have different responses at a certain excitation wavelength, and fluorescence peaks are displayed at different positions in the three-dimensional fluorescence spectra37. Using the three-dimensional FRI method, various organic substances in DOM can be quantitatively analyzed38. This analytical method has been widely used to analyze dissolved organic pollutants in various substances39,40,41. The relevant detection method is described in Reference40,41.

As shown in Table 3, the fluorescence spectra were divided into five regions by the region integration method: tyrosine-based proteins (region I), tryptophan-based proteins (region II), fulvic acid (region III), soluble microbial byproducts (region IV) and humic acid (region V)40,41.

Table 3 Classification of characteristic fluorescence peaks and division of the fluorescence spectral regions.
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Following the co-anaerobic digestion experiment, samples of the co-digestion slurry were collected from the reactor for analysis and compared with anaerobic digestion sludge to evaluate the impact of co-anaerobic digestion on the microbial community structure. The co-digestion slurry samples were designated as EC0, EC1, EC2, and EC3, while the sludge samples were labeled as WW. Three independent samples were collected from each experimental reactor and immediately processed for DNA extraction. During the extraction procedure, 100 mL of the digestive fluid was measured, and genomic DNA was isolated using the E.Z.N.A Soil DNA Kit according to the manufacturer’s instructions. The extracted DNA was quantified and its purity assessed by measuring absorbance at 230 nm, 260 nm, and 280 nm using a Nanodrop 2000 spectrophotometer (Nanodrop Technologies, Wilmington, DE). The DNA samples were stored at −80 ºC until further analysis. Once DNA extraction was completed for all samples, they were sent to Meg Biotechnology Co., Ltd. (Shanghai, China) for subsequent analyses. Initially, the samples underwent re-testing to ensure quality control, and the qualified parallel DNA samples were pooled. Subsequently, PCR amplification was performed targeting the V3 and V4 hypervariable regions of the 16S rRNA gene. The bacterial primers used were 338 F (ACTCCTACGGGAGGCAGCA) and 806R (GGACTACHVGGGTWTCTAAT), while the archaeal primers were 344 F and 915R. Detailed experimental methods are provided in references32,33.

Results and analysis

Cumulative gas production

Figure 1 presents the cumulative biogas yield of the four anaerobic codigestion reactors at different time points. The experimental results indicate that, after the initiation of the anaerobic digestion experiment, the three combined anaerobic digestion reactors promptly entered the gas production state.

Fig. 1
figure 1

Gas production of each anaerobic digestion reactor at different times (mL).

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In the OL0 reactor, LSCL was digested anaerobically alone, and only 2055 mL of biogas was produced on Day 32. The OL1 reactor and OL2 reactor have similar biogas production trends. In the OL1 reactor, the anaerobic digestion reactor began to produce gas on the first day of the experiment, and the amount of gas produced reached 15,930 ml, which was 95% of the cumulative biogas yield. In the OL2 reactor, the amount of gas production reached 20,730 ml on the first day of the experiment, accounting for 90% of the cumulative biogas yield.

However, in the OL3 reactor, biogas production is different from that in the other reactors. With an initial loading rate of 3%, 23,500 ml of gas was produced within 7 days of the experiment, accounting for 44% of the cumulative biogas yield. Owing to rapid hydrolysis and acidification, the growth and reproduction of methanogenic bacteria are subsequently inhibited, and gas production in the system stops. After nearly three months of recovery, the system entered the second peak of gas production on the 96th day, producing approximately 20,000 mL of biogas within another 7 days, accounting for approximately 38% of the total gas produced.

Additionally, the methane content in the biogas produced by the three combined anaerobic digestion reactors ranges from 45 to 65%, while the carbon dioxide content is between 23 and 50%. However, the maximum methane content in the LSCL was digested anaerobically alone is only 45%. Combined anaerobic digestion not only increases the production of biogas, but also helps to enhance the methane content in the biogas. The experimental results show that when the load rate of kitchen waste is less than or equal to 2%, the experimental system can efficiently degrade kitchen waste and essentially complete the biogas production process within 24 h. When the load rate is greater than or equal to 3%, the anaerobic system might enter an intermittent biogas production state, and unstable phenomena may occur in the system.

At present, to improve the biogas yield per unit volume of anaerobic digestion reactor, researchers usually increase the solid hold-up rate. However, an increase in the solid hold-up rate accelerates the acidification of the system and even leads to the collapse of the system. Therefore, increasing the biogas production rate of kitchen waste with a low solid content to improve the biogas yield rate per unit volume of anaerobic digestion may be a more stable and effective method.

pH

Figure 2 shows the pH curves of the different anaerobic digestion reactors. As shown in the figure, each experimental system presented acidification to different extents. The degree of acidification is positively proportional to the initial load rate. The higher the loading rate is, the more severe the acidification of the anaerobic digestion system becomes. In the OL1 reactor, when the initial loading rate was 1%, the lowest pH (6.65) was observed on the 8th day, and acidification was not obvious. In the OL2 reactor, when the initial loading rate was 2%, the lowest pH (5.35) was observed on the 18th day, with obvious acidification. However, it recovered to 6.74 on the 24th day, and thereafter, the pH fluctuated around 7. When the initial loading rate was 3%, in the OL3 reactor, acidification was the most obvious, and the pH decreased to 4.97 on the 5th day of the experiment and then recovered slowly. The acidification of the system affected the methanation process, resulting in intermittent gas production in the OL3 reactor.

Fig. 2
figure 2

Changes in pH with time under different initial load conditions.

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During the period from the 85th to the 150th day of the experiment, the pH of each digestive system exhibited distinct fluctuations to varying degrees, ranging from 7 to 9. Then, the pH of the system began to stabilize, and the mixture was finally concentrated in the range of 7.8–8.1, which was close to the pH value of the LSCL. Methanogens require an absolutely alkaline environment, and the optimal pH range is 7.3- 8.6.

As shown in Fig. 2, during the third to fifth months of the experiment, the pH fluctuated greatly. The fluctuation of pH is actually a process in which the microbial community of the system is repeatedly transformed from imbalanced to equilibrium, and it is also a process in which acid-producing bacteria and methanogens alternately become the dominant microbial community42,43. When acid-producing bacteria became dominant, the pH of the system decreased, and as methanogens were dominant, the pH of the system increased44,45.

Dissolved organic carbon (DOC)

Figure 3 shows the DOC values of LSCL and different biogas slurry samples collected from four anaerobic codigestion reactors at different nodes simultaneously.

Fig. 3
figure 3

DOC of LSCL and different biogas slurry samples.

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As shown in Fig. 3, the DOC concentration in the LSCL barely changed, whereas that in the combined digestive mixture increased significantly after the first month of anaerobic digestion, the content of humic acid in the digestion liquid could reach more than 20 times that of the original LSCL after one month of combined anaerobic digestion. The DOC value of EA1 from OL1 was 2314 mg/L, whereas that of EA3 from OL3 reached 6407 mg/L. The higher the initial kitchen waste loading rate is, the higher the DOC value. During the 152 days of anaerobic digestion, the concentration of dissolved organic matter in the three anaerobic codigestion reactors decreased significantly, with the DOC decreasing to approximately 300 mg/L, approaching the value of the initial landfill leachate (279 mg/L).

The analysis results indicate that in the anaerobic codigestion system, the higher the initial loading rate is, the greater the rate of DOC decreases. After 5 months of combined digestion, the DOC in all the samples was approximately equal to that of the leachate from anaerobic mono-digestion, suggesting that the biological transformation of organic matter in anaerobic codigestion was accomplished and that the kitchen waste was completely degraded.

3DEEM fluorescence spectroscopy

The three-dimensional fluorescence spectral regional integration method (3DEEM) enables the acquisition of relative quantitative results of DOM components through regional division and integration, which is conducive to comparing the composition and concentration of DOM in different samples. However, 3DEEM merely offers relative quantitative results for DOM components. For absolute quantitative analyses, other approaches (such as high-performance liquid chromatography, gas chromatography, and parallel factor analysis, etc.) are needed for calibration and verification.

Figure 4 presents the DEEM values of the different samples on the 31 st day and 152nd day of the anaerobic digestion experiment. The figure shows that in the DEEM of the anaerobic mono-digestion leachate in OL0 on the 31 st day, the integrated fluorescence intensity indicated that humic substances (fulvic acid + humic acid) accounted for 94%; for the combined digestate with initial loads of 1%, 2%, and 3% on the 31 st day, the proportions of humic substances were 90.8%, 90.5%, and 90.1%, respectively.

Fig. 4
figure 4

Integral values from DEEM for the different batches of biological biogas slurry samples (EA0, EA1, EA2, and EA3 are the biogas slurry samples collected on the 31 st day of the anaerobic digestion experiment; EB0, EB1, EB2, and EB3 are the biogas slurry samples collected on the 152nd day of the anaerobic digestion experiment).

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On the 152nd day of anaerobic codigestion, the integrated fluorescence intensity determined via DEEM indicated that humus accounted for 92.8% of the anaerobic mono-digestion leachate in OL0, and in the combined digestive fluid with initial loads of 1%, 2% and 3%, the humus proportions were 92.1%, 93.1% and 92.5%, respectively.

On the basis of the results shown in Fig. 3, a thorough analysis revealed that there was a significant decrease in DOM during the experimental period from Day 31 to Day 152. Moreover, after 5 months of combined anaerobic digestion, the DOC content in the biogas slurry was similar to that of the leachate of anaerobic digestion alone (288 mg/L), indicating that the kitchen waste added to the reactor was fully degraded and digested during the combined anaerobic digestion process.

Microbial diversity analysis of leachate combined with anaerobic digesters

To understand the inoculation effect of the leachate, the leachate in the reactor was sampled and analyzed after the anaerobic digestion experiment and compared with the anaerobic digestion sludge to analyze the effect of combined anaerobic digestion on the microbial community. The results of the analysis are shown in Fig. 5 and Fig. 6.

Fig. 5
figure 5

Distribution of bacterial communities at the phylum level.

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Fig. 6
figure 6

Distribution of archaea at the class and genus levels.

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Distribution of bacterial communities at the phylum level and the genus level

Figure 5 shows that the library coverage of bacteria reached more than 99%, indicating that most of the bacteria in the system were detected. In addition, both bacterial richness and diversity were greater than those of Archaea, which is consistent with related studies46,47.

Figure 5 shows that the abundance and dominance of each bacterial component in the leachate, and the biogas slurry and inoculated sludge significantly differed. If the relative abundance in at least one sample was greater than 10%, the dominant bacterial phyla in the anoxically digested leachate alone (EC0) were Proteobacteria, Bacteroidetes and Firmicutes, their respective relative abundances are 35.46%, 17.32%, and 28.17%.

Compared with the leachate and biogas slurry, the inoculated sludge (WW) had greater bacterial richness and diversity. Proteobacteria, Firmicutes, Bacteroidetes, and Chloroflexi were the dominant bacteria in the inoculated sludge samples.

In the combined anaerobic digestion biogas slurry with a 1% kitchen waste loading rate, Proteobacteria, Bacteroidetes and Firmicutes were the dominant bacterial communities, their respective relative abundances are 58.34%, 12.76%, and 15.91%. Among them, Proteobacteria was significantly enriched, compared with the filtrate subjected only to anaerobic digestion, the value increased by 22.88%, and became the dominant bacterial community in the biogas slurry.

When the loading rate was 2%, Proteobacteria, Firmicutes, Bacteroidetes and Deinococcus Thermus were the dominant bacteria in the biogas slurry, their respective relative abundances are 29.73%, 20.18%, 13.93% and 16.31%.

In the biogas slurry with a loading rate of 3%, the dominant bacteria were Proteobacteria, Firmicutes, Bacteroidetes, Deinococcus-Thermus and Synergistetes, their respective relative abundances are 32.09%, 27.18%, 9.09%, 8.82% and 16.31%. Compared with that in WW, the proportion of Chloroflexi in EC2 and EC3 decreased, whereas that of Synergistetes increased significantly.

The Proteobacteria phylum is extremely diverse in terms of species and genetic diversity, and has great application value in industrial wastewater treatment, soil remediation, and complex pollutant degradation. Proteobacteria bacteria can play a leading role in the degradation of organic pollutants and total nitrogen in wastewater under different hydraulic loads48,49.

Bacteroidetes and Firmicutes are persistent microorganisms that can produce various metabolic enzymes during anaerobic digestion and are involved mainly in the hydrolysis and acidification stages. For example, Bacteroides can ferment dietary fiber to produce acetic acid50, and Petrimonas bacteria can consume sugars, with the main metabolites being acetic acid and propionic acid. Proteiniphilum is a protein or amino acid degrading bacteria51. In addition to hydrolytic acidification, the Clostridia of the phylum Firmicutes are also involved in hydrogen production, acetic acid production and acetic acid oxidation. For example, Syntrophomonas, a representative genus, can interact with hydromethanogens to convert various organic acids to H2 and acetic acid50. The other representative genus, Syntrophaceticus, is an acetic acid-oxidizing bacterium, that can catabolize acetic acid into H2 and CO252.

Related studies have shown that the presence of Synergistetes represents the good acetic acid-consuming performance of the system53,54. The presence of such microorganisms in the activated sludge may explain why, to a certain extent, the addition of activated sludge can activate the system to restart anaerobic digestion after the combined anaerobic digestion system ceases gas production. Especially for the combined anaerobic digestion system with acid inhibition, Synergistetes in the system can consume acetic acid after the addition of activated sludge, thereby helping the system re-enter a more stable gas production stage.

In general, the dominant species detected in sample EC0 from leachate anaerobic digestion alone were consistent with those detected in samples EC1, EC2 and EC3 from combined anaerobic digestion. However, among the dominant species, Chloroflexi appeared to be derived from activated sludge.

Distribution of archaeal communities at the phylum level and the genus level

Figure 6 shows that in the samples after anaerobic digestion of leachate alone (EC0), the archaeal sample library coverage was less than 15%, and the archaeal community not covered by the archaeal sample library (Archaca-unclassified) accounted for 85%, indicating that most of the microorganisms in the system were not detected. However, the library coverage of the combined anaerobic digestion liquid samples EC1, EC2, EC3 and the archaeal sample (WW) inoculated with sludge were all above 95%, indicating that most of the microorganisms in the samples had been detected.

Methanobacterium, Methanosaeta, Methanosarcina, and Methanoculleus were the dominant species in samples EC1, EC2, and EC3. In EC1 these dominant species respective relative abundances are 22.76%, 16.51%, 58.07% and 0.82%; in EC2 these dominant species respective relative abundances are 30.84%, 19.09%, 6.85% and 33.19%; in EC3 these dominant species respective relative abundances are 38.19%, 40.09%, 13.51% and 1.03%. These four methanogens were the four methanogenic phyla covered by the archaeal library in the EC0 archaeal sample. However, their relative abundances are all relatively low, their respective relative abundances are 2.73%, 1.05%, 8.94% and 0.57%. The reason might be that the introduction of kitchen waste has altered the microbial community structure, thereby activating these archaea via co-metabolic processes and increasing their relative abundance.

Methanobacterium and Methanosaeta were the most dominant microbial communities in WW from the activated sludge archaeal samples. The results revealed that leachate was the main source of methanogenic bacteria in the combined anaerobic digestion system.

The methanogenic genera found in landfill dumps include Methanosarcinales, the hydrotroph Methanobacteriles, Methanomicrobiales, Methanoccocales and the mixed troph Methanosarcinales55,56,57. The results also showed that Archaea such as methanoophthalmia, methanophytes, methanophytes and halobacteria were detected in the landfill leachate58. Methanogens are hydrogen-producing methanogenic bacteria belonging to the order Methanogenomicales. Methanalga, included in the order Methanalga, are mixotrophic methanogens. Therefore, the combined anaerobic digestion of landfill leachate and kitchen waste does not require the addition of inoculum.

The results of the microbial diversity analysis revealed that the dominant species in the combined anaerobic digestion system were essentially consistent with the dominant microbial community in the leachate and were only partially consistent with the dominant species in the seeded sludge.

There were abundant anaerobic microorganisms in the closed field leachate, which has a good inoculation effect. Without addition of any other inoculum, the combined anaerobic digestion system of kitchen waste and leachate can quickly enter the methanogenic state.

Conclusion

(1) Without the addition of other inoculants, the anaerobic codigestion system of LSCL and kitchen waste can quickly reach peak gas production. When the initial loading rates of kitchen waste are 1% and 2%, the gas production of the anaerobic codigestion system on the first day can reach 95% and 90% of the total gas production, respectively. This shows that the combination of LSCL and kitchen waste can greatly improve the gas production rate of an anaerobic digestion reactor by increasing the gas production rate.

(2) Combined anaerobic digestion accelerated the putrefaction process of kitchen waste. According to the measured value of DOC and the calculated relative fluorescence volume of each region, the content of humic acid in the digestion liquid could reach more than 20 times that of the original LSCL after one month of combined anaerobic digestion.

(3) The dissolved organic matter (including humus) formed by the transformation of kitchen waste was essentially degraded. In the second stage of the experiment, the DOC in the anaerobic codigestion biogas slurry decreased to 284 mg/L, 322 mg/L and 340 mg/L, respectively, and the humus in the digested biogas slurry was greatly degraded to the same level (288 mg/L) as that in the leachate. The results revealed that humus respiration occurred in the combined anaerobic digestion reactor and that most of the humus formed by the transformation of kitchen waste was ultimately degraded.

(4) There are abundant anaerobic microorganisms in the leachate of the sealed field, which has a good inoculation effect. Without the addition of any other inoculum, the combined anaerobic digestion system of LSCL and kitchen waste can quickly enter the methanogenic state.