The acceleration degradation processes of different aged refuses with the forced aeration for landfill reclamation

Forced aeration is one of the promising ways to accelerate landfill reclamation, and understanding the relation between aeration rates and waste properties is the prerequisite to implementing forced aeration under the target of energy saving and carbon reduction. In this work, landfill reclamation processes with forced aeration were simulated using aged refuses (ARs) of 1, 4, 7, 10, and 13 disposal years, and the potential of field application was also investigated based on a field project, to identify the degradation rate of organic components, the O2 consumption efficiency and their correlations to microbes. It was found that the removal rate of organic matter declined from 20.3% (AR1) to 12.6% (AR13), and that biodegradable matter (BDM) decreased from 5.2% to 2.4% at the set aeration rate of 0.12 L O2/kg waste (Dry Matter, DM)/day. A linear relationship between the degradation rate constant (K) of BDM and disposal age (x) was established: K = − 0.0002193x + 0.0091 (R2 = 0.854), suggesting that BDM might be a suitable indicator to reflect the stabilization of ARs. The cellulose/lignin ratio decrease rate for AR1 (18.3%) was much higher than that for AR13 (3.1%), while the corresponding humic-acid/fulvic-acid ratio increased from 1.44 to 2.16. The dominant bacteria shifted from Corynebacterium (9.2%), Acinetobacter (6.6%), and Fermentimonas (6.5%), genes related to the decompose of biodegradable organics, to Stenotrophomonas (10.2%) and Clostridiales (3.7%), which were associated with humification. The aeration efficiencies of lab-scale tests were in the range of 5.4–11.8 g BDM/L O2 for ARs with disposal ages of 1–13 years, and in situ landfill reclamation, ARs with disposal ages of 10–18 years were around 1.9–8.8 g BDM/L O2, as the disposal age decreased. The increased discrepancy was observed in ARs at the lab-scale and field scale, indicating that the forced aeration rate should be adjusted based on ARs and the unit compartment combined, to reduce the operation cost.


Introduction
Landfills were still the most commonly used method of waste disposal in China, with 0.17 billion tons in 2019 [1], and more than 8 billion tons of waste contained in 27 thousand old dumping sites and 1955 landfills registered in the Ministry of Construction and Ministry of Ecological Environment [2]. To reduce secondary pollution and reuse these lands with the rapid urbanization process, landfill reclamation has been implemented to accelerate the landfill 1 3 stabilization process, including aeration [3], leachate circulation [4], mining [5], etc. Among which forced aeration was considered as an indispensable method to transform traditional anaerobic landfills into a biologically stable state [6], while the aeration process was complex due to the heterogeneous of waste and the disposal years, especially for wastes that are rich in organic matters and moisture, and have commercial value [2].
Forced aeration was one of the possible manual interventions for the acceleration of landfill stabilization [7], and has been applied in many landfills, such as Kuhstedt Landfill [8] and Milmersdorf Landfill [9] in Germany, Mannersdorf Landfill [10] in Austria, and Columbia Landfill and Live Oak landfill [11] in the United States. Aerobic remediation was also implemented at Heishitou Landfill in 2009 [12]. The operational processes varied greatly and mainly relied on their operation experiences, since the disposal years, the waste composition and even the landfill locations, all of which will influence the selection of the aeration volume, the layout of the aeration pipes. The Modena landfill with 20 years closure was treated for 6 months at an aeration rate of 0.02 L O 2 /kg waste (Dry Matter, DM)/day in Italy, and the respiration index of the waste was only 33% of the initial value [3]. An aeration rate of 0.08 L O 2 /kg waste (DM)/day was applied to both Black Stone Landfill and Jinkou Landfill in China, with an average 12 and 18 years old waste, and the organic matter (OM) content decreased from 18.5% to 6.2% in the previous after 2 years' operation, while that in the Jinkou Landfill decreased from 12.4% to 9.1% after 1-year aeration [13]. It is necessary to establish the relationship between the landfill properties, the aeration conditions, and the disposal ages for the aeration process.
The degradation processes can be seen as large-scale composting for landfill aeration, to create an aerobic environment in a heap through aeration [7,14], and some fresh wastes or waste models were simulated to reflect the aerobic aeration in various countries [15], including the aeration frequency [16], aeration rate [17], leachate re-circulation frequency [18], compaction density [19], and exogenous aerobic bacteria [20]. There is no doubt that all of these results were different or even conflicting, and there were still some gaps for practical landfill reclamation projects, since almost all of the aeration should be implemented in landfills disposed of for several years, instead of fresh waste. The wastes of different ages (called aged refuses here, ARs) should be used to simulate landfill reclamation to identify the potential operation conditions for aerobic remediation of landfills. Meanwhile, the generation parameters of the landfill stabilization process, such as leachate properties, landfill gas composition, and waste compositions have been used to reflect the degradation of fresh waste [21]. Some special index should be employed for the ARs, which contain more humus-like matters.
In this work, ARs with 1, 4, 7, 10, and 13 years were collected from a working landfill, which has been well recorded during the landfill stabilization process. The landfill remediation was simulated with the forced aeration rate under the same operation conditions, and the variations in AR compositions were analyzed to identify the degradation process. The primary purpose of this study was to quantify the acceleration of the stabilization process using forced aeration and to investigate the degradation mechanism through analysis of the properties of the ARs, including humus content, cellulose/lignin ratio, and the evolution of microorganisms. A comparison was made between the aeration efficiency of the lab-scale and field-scale landfills to guide the selection of potential aeration rates.

AR samples
AR samples were collected from different cell compartments with disposal ages of 1, 4, 7, 10, and 13 years in Laogang Landfill, which is located in the Pudong District of Shanghai, China. The quantity and property of AR, the placement time, and the location of corresponding compartments were well recorded by the landfill operator. AR samples were collected at 0-3, 3-6, and 6-9 m, and foreign objects of large size, such as stones, glass, and plastics, were manually removed. After thoroughly mixing samples from different depths, 5 kg of typical samples were collected according to the quarter method. After natural air drying, the samples that passed through a 60-mesh sieve after grinding were collected. The characterization of AR with different disposal times is shown in Table 1. The location of the sampling sites, the sampling process and the collected ARs samples are shown in Fig. 1.

Lab-scale lysimeters
The schematic diagram of ARs sample pretreatment and batch experiments is shown in Fig. 2. The batch experiment of 3 parallel controlled tests was carried out in 500 mL triangle flasks with a 250 g sample (dry weight), with the initial moisture content of 50% (wet weight, ww) set. The temperature and aeration rate were kept at 30 °C and 0.12 L O 2 / kg waste (DM)/day in a water bath. The gas was humidified through the scrubber to reduce the evaporation loss. To facilitate controlled aeration, we utilized microporous aerator and regulated aeration intensity through a gas flow meter. Prior to the experiment, we conducted a pre-test to determine the optimal aeration intensity for intermittent aeration. The aeration process consisted of 3-h intervals, followed by 3 h of rest, with a 6-h cycle. A total of 9 samplings 1 3 were carried out on days 0, 1, 3, 5, 8, 12, 17, 24, and 35. The ARs in the triangle flask were evenly mixed before each sampling. The leachate was collected at a designed leaching process with a sample and distilled water at a ratio of 1:10 (weight/volume, dry weight) in a horizontal shaker at 110 ± 10 rpm, and 25 °C for 8 h, left for 16 h and then filtered through a 0.45 μm filter.

Field-scale landfills
Based on the lab-scale results, field-scale aeration was performed in a landfill of about 19.65 hm 2 , located in a city in Zhejiang Province, China. About 2.38 × 10 4 m 3 of municipal solid waste was landfilled at the site between 8 and 21 years, with a bulk density of about 650 kg/m 3 and average moisture of 40.3% (based on wet weight of waste). The site has an average depth of approximately 12 m, and the maximum depth reaches 15 m. A low-pressure aeration system was employed for active aeration and tail gas extraction, with 394 gas wells set up. The wells are strategically placed in an encrypted configuration, with an insertion depth up to 10 m and a well spacing of 20 m. The pumping and injection wells are arranged in an equilateral triangle pattern to optimize the efficiency of the aeration process. Based on the design specifications, the well is anticipated to have a radius of influence of approximately 12 m. Four roots fans with a flow rate of 50 m 3 /min were used to aerate the landfill intermittently for 3 h cycles. The total aeration rate was approximately 10.8 L O 2 /kg waste (DM). S1, S2, and S3, with disposal ages of about 18, 14, and 10 years (Fig. 3),  were selected as field monitoring points, and AR 18-f , AR 14-f and AR 10-f at these three points were collected and analyzed before and after aeration for 9 months.

Physical and chemical properties analysis
Biodegradable matter (BDM) content of the AR was determined by oxidation with potassium dichromate at 25 °C under strong acid conditions and conversion based on the amount of oxidizer consumed [22]. Cellulose and lignin were determined by Fan's detergent fiber analysis method [23]. pH and oxidation-reduction potential (ORP) were measured by a portable pH meter (Five Go, METTLER TOLEDO, Schwerzenbach, Switzerland). The water content was determined using the method described in "Sampling and Analysis Methods for Domestic Waste" [24]. The AR extract processes were described in our previous works [25]. The total organic carbon (TOC) was evaluated by a Multi N/C 3100 Analyzer (Analyti Jena, Jena, Germany). All index analyses were performed three times, and then the average value was taken.

Humus extraction and excitation-emission-matrix (EEM) spectra
Humus was extracted and determined. AR samples were extracted with 0.1 mol/L Na 4 P 2 O 7 and 0.1 mol/L NaOH at a ratio of 1:20 (weight/volume, dry weight) [26]. After standing for 24 h, the extract was centrifuged at a speed of 10,000 rpm and then filtered by a membrane filter (0.45 μm). One part of the mixed combined supernatants was used to determine the content of soluble humus (total humic acid (HA) and fulvic acid (FA) content) and EEM spectroscopy. Another part was adjusted to pH 1 with 1 mol/L H 2 SO 4 in a water bath at 80 °C. After standing at 25 °C for 24 h, the supernatant containing FA was centrifuged at 10,000 rpm, and the precipitate was collected and dissolved with 0.05 mol/L NaOH to determine the HA content [27]. EEM spectroscopy of the filtrate was measured by spectrophotometer (Model F-7000, Hitachi, Tokyo, Japan). Both the excitation (Ex) and emission (Em) wavelengths were set at 200-600 nm. The slit bandwidths of Ex and Em were set to 10 nm. Spectra were scanned and recorded at a rate of 40 nm/s. The EEM data were analyzed by MAT-LAB R2018b software (MathWorks Inc., Natick, USA) and DOMFluor toolbox to perform PARAFAC model analysis [28], and the number of fluorescence components was obtained through half-analysis. The maximum fluorescence intensity (F max ) output of the model was used to express the relative concentration of a specific fluorescent component in Raman units (R.U.) [29].

Microbial community structure analysis
AR samples at 1, 7 and 13 years were collected and used for microbial community structure analysis, and DNA was extracted with a soil DNA extraction kit (BioTeke, Beijing, China). The DNA solution was stored at − 80 °C before being used. The 338F/806R primers were used for bacteria. DNA was amplified by PCR on ABI GeneAmp ® 9700 (Thermo, Foster City, CA, USA). The characterization of AR extracts was based on previous methods [30]. The sequencing was carried out by Majorbio Company (Shanghai, China). Library preparation and sequencing reactions were performed with TruSeqTM DNA Sample Preparation Kit and Illumina Miseq (Illumina, San Diego, CA, USA).

Data analysis
First-order kinetics were developed to describe the degradation process of MSW in landfills, and BDM was used as an indicator for AR biodegradation.
BDM content was simulated using the following equation: where, BDM t refer to BDM content after t days reaction; BDM 0 is the initial value; K is the degradation rate constant; t is the aeration period.

Biodegradation kinetics of AR with different disposal ages
Variations in the properties of the AR samples over time are shown in Fig. 4. The initial pH values were 7.93-8.24, and then dropped by 0.2-0.3 after aeration, meaning that the pH range was within an optimal period for aeration [31]. A significant difference in pH was observed between AR 13 and the other samples. pH in AR 13 increased significantly from 7.86 to 8.59 after 5 days of aeration and then remained almost stable. The pH in the other samples declined until 12 days of operation, since more organic matters were presented in young ARs. The decline of pH in ARs was mainly due to the generation of organic acids and the conversion of NH 4 + into NO 3 − during the aeration process [32], as shown in equation [33]. Similar results were observed in ORP. A lower OPR range of − 30 mV to − 60 mV was observed in young ARs with high organic matter content, which resulted in active degradation reactions with forced aeration.
The relationship between BDM content in AR and the disposal age x was as follows:

Variations in the AR components
Variations between AR properties, including volatile solids (VS), cellulose/lignin (C/L) and humic acid/fulvic acid (HA/ FA), and the aeration time are shown in Fig. 5. VS content decreased around 5.9% in AR 1 , from 29.0 to 23.1%, and only 2.9% of VS was degraded in AR 13 , meaning that most of the organic content in old ARs were non-BDMs. The cellulose and lignin contents were around 3.8%-5.1% and 7.2%-8.4%, respectively, which belongs to hard degradable organic matters. C/L ratio decreased around 3.1% as the aeration time was extended, from 49.5% to 46.4% in AR 13 , while more than 10% was found in those in AR [1][2][3][4][5][6][7] , and the C/L of all ARs remained between 44 and 48% (with an average value of 46%), meaning that C/L could be employed as an indicator to assess the degradation potential of ARs. With regard to the HA/FA ratio, AR at 1, 7 and 13 years increased from 1.27, 0.73 and 0.15 to 1.44, 1.55 and 2.16, respectively, meaning that more HA was generated as the aeration time increased [35].
EEM spectra of ARs were employed to identify humus properties, as shown in Fig. 5d. Ex/Em = 320/420 (proteinlike substances) decreased by 2.4% and 8.25% for AR 7 and AR 13 , respectively, while Ex/Em of 290/440 (humiclike substances) increased by 2.3% and 3.2%, respectively. The protein-like and humic-like substances had a gradient relationship with the disposal ages, suggesting that during the aeration process, the overall composition of young ARs maintains a dynamic balance due to the rapid degradation of organic matter. In ARs with more disposal ages, most organic substances tend to be converted to HA.

Characteristics of leachate pollutants
Leachates collected and their properties are presented in Fig. 6. Chemical oxygen demand (COD) showed a rapid decline from 2280 mg/L to 127 mg/L after 35 days of aeration in AR 13 , and those of AR 1-7 showed a similar slow decline, with an average decline of 677 mg/L. More substances could leach from the ARs, since COD concentration increased greatly in the first 4 days, and then decreased in the following days, indicating that more dissolved matters of the organic substance were presented in ARs after aeration. TOC/COD was around 35%-42%, and then decreased slightly to 36.6%, 36.5%, 32.6%, 23.1% and 21.4%, (6) BDM t = BDM 0 e (−0.0002193x+0.0091)×t 1 3 respectively, suggesting that most biodegradable components were degraded.
TN showed a declining trend under the test periods, decreasing by 236-368 mg/L, and the removal rates of AR 1-13 were 50.4%, 62.6%, 76.8%, 83.1% and 91.3%, respectively. For NH 3 -N/TN of AR 1-10 , it increased first in the first days and the largest for AR 7 , 70.7%, since more NH 3 -N was generated in the initial period. Although fluctuations occurred in the following days, NH 3 -N continued to decrease due to the substantial decrease in TN. The NH 3 -N/ TN of AR 13 continued to drop from 27.0 to 6.6%, the NH 3 -N concentration dropped from 109 mg/L to 2 mg/L, and the degradation rate was 97.9%, indicating that aeration can thoroughly remove NH 3 -H from old ARs.

Evolutions of microbial community
The bacterial community at the genus level is shown in Fig. 7. After 35 days of aeration, the dominant species were Corynebacterium 9.2%, Acinetobacter 6.6%, and Fermentimonas 6.5%, in AR 1 , followed by proteolytic bacteria (Proteiniclasticum 10.7%, Proteiniphilum 3.2%), and anaerobic humic bacteria (Stenotrophomonas 10.2%, Clostridiales 3.7%, etc.) were the main contents in AR 13 . With regard to AR 7 , the proportion of Aquabacterium occupied 9.8%, which could be used to degrade complex organic compounds, such as gasoline and diesel [36]. The degradation substances in ARs converted from the readily degradable organics in young ARs [37] to humus-like organics in old ARs [38,39]. The aerobic denitrifying bacteria Diaphorobacter was around 3.5% in AR 7 , meaning that the denitrification process was still active [40].

Aeration efficiency comparation in field-scale landfill reclamation
BDM contents of AR 18-f , AR 14-f and AR 10-f decreased from 10.1%, 12.3% and 16.3% to 8.1%, 7.3% and 7.1% after  18-f , AR 14-f and AR 10-f should be around 6.4%, 7.1%, and 8.8%, respectively, under the same aeration rate, meaning that the oxygen consumption efficiency for ARs decreased significantly with increasing disposal age at both field scales, from 8.8 (AR 10-f ) to 1.90 (AR 18-f ) g BDM/L O 2 . Taking S1 as an example, the practical value of BDM of AR 18-f was 1.7% higher than the predicted value, which occupied around 26.6% higher, the difference between field scale and labscale was due to the difference in the operation conditions, especially for the landfill unit compartments, and lab-scale lysimeters created a more favorable environment to promote the degradation of refractory ARs in the disposal of overaged AR [41]. For ARs with a lower disposal age, predictions of BDM deviate relatively less from practical. The practical value of BDM of AR 14-f was in good agreement with the predicted value, with an error of 2.8%, and the error of AR 10-f was 18.1%, indicating that the BDM degradation model could be a promising method to optimize the aeration parameters according to the disposal age of the AR.

Conclusions
ARs with different ages were collected and employed for landfill aeration to identify the relationship between AR properties and forced aeration. The pH was in the suitable aeration range, and the degradation of BDM in different stable stages was 5.2%, 4.7%, 4.4%, 4.0% and 2.4%, at 0.12 L O 2 /kg waste (DM)/day with the intermittent aeration for 3 h cycles on 35 aeration days. The BDM degradation rate constants decreased from 8.72 × 10 -3 (R 2 = 0.858) in AR of 1 year to 5.80 × 10 -3 (R 2 = 0.915) in that of 13 years. With increasing disposal year, the ratios of C/L decreased 1% and finally stabilized at around 46%, while HA/FA gradually increased, indicating that the degradation target of AR changes from ready biodegradation to complex organic matter. The increase in HA/FA of AR with 13 disposal years was the most, from 0.15 to 2.16, and that of Stenotrophomonas, and Clostridiales (humus related) of 13.9%, indicating that in this stabilization stage, aerobic treatment mainly promotes the formation of humic and enhanced humification. The degradation rate of ARs in the field landfill was lower than that of the lab-scale landfill, especially for the ARs with longer disposal ages, indicating that the aerobic frequency and volume should be adjusted based on the disposal ages and BDM contents together for landfill reclamation.