LeachateRecirculation for EnhancingMethaneGenerationwithin Field Site in China

State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan 430071, China School of Architecture & Civil Engineering, Shenyang University of Technology, Shenyang 110870, China Hubei Key Laboratory of Contaminated Clay Science & Engineering, Wuhan 430071, China University of Chinese Academy of Sciences, Beijing 100000, China Wuhan Environmental Sanitation and Science Institute, Wuhan, China School of Civil Engineering and Architecture, Hubei University of Arts and Science, Xiangyang 441053, China


Introduction
Leachate recirculation is one of the most effective methods to increase the conversion of organic matter and enhance the stability of bioprocesses in bioreactor landfills [1,2].e microbe degradation rate increases when the content of the organic solution increases; this condition leads to high methane production and collection recovery.e increase is largely attributed to gas reuse, such as increase in power and reduction of emissions, particularly in large-scale landfills.erefore, leachate recirculation experiments must be conducted in the field to evaluate methane production and provide evidence for the liquid injection process.
e decomposition of municipal solid waste can be enhanced by leachate recirculation in anaerobic environments [3,4]. is method is widely recognized.However, the pore structure and composition exhibit a heterogeneous distribution in landfills because of the difference in sample scale and cause a significant variation in recirculation volume and loading between laboratory scale and field scale [5][6][7][8].
erefore, several studies have assessed the effect of leachate recirculation in pilot-scale experiments [9][10][11].e methane generation rate increased from 15% to 700% by leachate recirculation in waste body.Jiang set up a pilot-scale column (2.5 m × 3 m × 4 m) to complete a recirculation test with a highly organic waste sample.e quantity index and rate of injection of liquid volume to waste volume were established to evaluate the capacity to enhance degradation [12].Benbelkacem reported that gas generation under recirculation loading is realistic with actual landfill samples in a 1.2 m 3 bioreactor.Water percolation was found to be an important factor to accelerate the degradation of solid waste.Some researchers found the leachate recirculation could accelerate VFA degradation rate [13].
A full-scale test for long-term monitoring is essential to further analyze the contribution of recirculation to gas emission in an actual landfill (Table 1) [12][13][14][15][16]. e methane content of landfills is often obtained to determine the biodegradation level.However, the report of recirculation in the field was little.
e methane concentration was increased from 45% (before recirculation) to 55% (after recirculation) by vertical wells in Odayeri Sanitary Landfill [15].Benson analyzed the five recirculation landfills in North America and found the definitive relation between the increase in gas production and collection wells distribution.Chung found that the methane concentration was increased from 30% (before recirculation) to 70% with injecting the leachate treated by SBR in an aged landfill.e injection time was continued to more than half a year.Recirculation loading was applied in the range of 0.1% to 1.3% per month.e contribution of leachate to methane production in the injection site increased from 1.5 to 2.2 times than in the noninjection site [13,17,18].ese results are related to the operating mode, such as wasteage, injection volume, and gas collection system.Di Maria et al. found that leachate recirculation in mechanically sorted organic fraction (MSOF) of municipal solid waste could achieve higher landfill gas recovery by developing a full-scale bioreactor [19].
However, no study has reported on the enhancement of gas collection by leachate recirculation in China, particularly in long-term tests in the field.
e unified recirculation method may be unable to increase the methane production in Chinese regions because of the difference in waste composition and the landfill mode.A field experiment on leachate recirculation could provide useful information for the evaluation of methane collection and stability in local landfills.
erefore, a field experiment on recirculation was conducted in this study at a landfill in Hubei Province, China.Continued recirculation (CR) and descending recirculation (DR) were implemented to investigate the effect of liquid injection on methane increase in the operation state.

Materials and Methods
2.1.Landfill Site.H.J.W. Landfill is located in Hubei Province, China.Eight hundred tons of waste is removed daily from this landfill.e power plant was built in 2001.Municipal solid waste has been disposed to an area of 185,000 m 2 for the past 11 years.e average depth is 14 m.e amount of leachate produced per day is 120 m 3 to 150 m 3 .
e physical constituents of the landfill are presented in Table 2.

Leachate Recirculation and Gas Collection System.
e leachate was injected into the landfill by a horizontal well.Two horizontal wells and one vertical well were designed in the test site (Figure 1).e vertical well was placed in the middle of the two horizontal wells and was used to pump gas from the landfill to the power plant.e horizontal well was placed 3 m below the cover.e influence radius of the vertical well is estimated to be 22 m [20].e influence area of the horizontal well for liquid filling should be lower than that of the vertical well for gas pumping to increase the efficiency of methane collection within the site.e depth of the vertical well is 10 m, which is more than 70% of its total height.Barlaz et al. suggested that the evaluation of the gas collection efficiency should consider gas production and collection over the operation life.In the test process, the vertical well was operated at a continuous state by negative pressure for pumping gases, during the whole recirculation.A high-density polyethylene (HDPE) geotechnical liner and 30 cm of soil were paved onto the surface of the cells in March 2012.Geotextile and HDPE were used in the bottom system.

Test Procedures.
e vertical well began operating 2 m after the cover operation.Methane concentration was determined with a spectrum analyzer.e injected leachate achieved from Section 2.2 in H.J.W. landfill was pretreated by NaOH to adjust the pH.
e recirculation test was implemented at three sites in the landfill.Cell 1 is the nonrecirculation (NR) site.Cells 2 and 3 are the continued recirculation (CR) and descending recirculation (DR) sites, respectively.e leachate was continuously injected in cell 2. However, in order to compare the effect of different recirculation modes on the acceleration degradation, the DR was selected in cell 3 in small recirculation loading.e leachate injection volume in cell 2 and cell 3 is shown in Figures 5 and  6, respectively.All tests were operated under anaerobic condition and in vertical well to collect landfill gas.e characteristics of the field-site configuration are shown in Table 3. e abovementioned tests were conducted to investigate the contribution of gas increase by recirculation.
e recirculation loading could be written as follows: where V l is the volume of the leachate injected into the landfill, V s is the volume of the site, and t is the time across the recirculation.

Measurement of Landfill Gas and Leachate in Cells.
e methane content in the extraction well was measured in real time with an infrared gas analyzer.e biogas flow rate was measured with an electromagnetic flow meter.e leachate sample was extracted from the drainage pipe 3 times every month.COD, BOD, and pH were recorded during 10 months.

Methane Concentration and Biogas Flow Rate.
e average methane concentration was approximately 51.2% during the landfill operation (Figure 2).e biogas flow rate of the pumping well was 16.2 m 3 /h.e biogas flow rate was attenuated during operation according to the degradation property of the waste.e high concentration of the injected leachate could be responsible for its variation in the duration.e COD of leachate empty into cell 2 was from 5800 to 9500 mg/L during recirculation (Figure 5).e average flow rate in cell 2 was 2.2 times than that in cell 1, which was 36.1 m 3 /h.
Figure 4 shows the methane concentration and biogas flow rate in cell 3. e biogas flow rate increased significantly in the initial phase, which operated with recirculation in the first two months.e biogas flow rate and methane concentration increased to 49 m 3 /h and 60% in the third month, respectively.When the liquid injection was ceased, the flow rate obviously decreased in the later months.e average flow rate and methane concentration decreased to 19.1 m 3 /h and 51.0% in the nonrecirculation phase, respectively.e average injection volume of the leachate introduced in the CR mode in cell 2 was about 13.0 m 3 /month.e leachate recirculation rate was approximately 0.048% of the waste volume per month (Figure 5).e average injection volume was 13.6 m 3 during the first two months.In the sixth month, injection volume was 4.5 m 3 .During the ninth to eleventh month, the injection volume was further decreased, to 8.1 m 3 (Figure 6).
Figure 7 presents the pH of bleeding liquid in the three cells.e pH in the recirculation cell was higher than that in the nonrecirculation cell.e pH increased from 5.7 to 6.8 in the recirculation cell.When the recirculation was ceased, the pH became slightly less than that during recirculation initialization in cell 2.
An obvious COD trend was exhibited.COD in the three cells decreased with time (Figure 8).e COD in cell 2 was higher than that in cell 1 in the second month.e COD in cell 3 was higher than that in cell 1 during leachate recirculation.
BOD/COD decreased in the three cells as the organic matter decomposed (Figures 9-11).e decision coefficient for cell 1, cell 2, and cell 3 was 0.29, 0.34, and 0.22, respectively.
ere was obvious discreteness of the fitting results because the leachate sampling was inhomogeneous in field site.e fitting results show that the decreasing of BOD/COD was cell 2 > cell 3 > cell 1.It was suggest that the leachate recirculation with the low loading could accelerate the biodegradation of municipal solid waste.e leachate recirculation to enhance waste degradation was also found by Ozkaya (2004) and Jiang (2007) in higher recirculation loading.

Effect of the Influence Area.
e lateral impact zone of the horizontal well can be controlled with injection pressure and waste properties [22,23].A high gas collection rate is achieved when the impact zone of the injection system is limited by the influence area with a pumping well. is design is important to evaluate the possibility of increasing methane production with leachate recirculation.

Scaling Effect.
Recirculation loading from 0.35% per week to 5.3% per week can be achieved in a pilot-scale test [3,11,24].is value is much higher than that in the test   Journal of Chemistry conducted in the eld.e leachate in this study was often injected into the site at a range of 0.1% to 1.3% per month mainly because the amount of liquid lled into the waste was limited.A number of factors, such as high moisture content, large lling area, and heterogeneous waste composition, also contributed to recirculation loading.erefore, the feasibility of recirculation loading should be determined by a eld test.

Methane Production with Recirculation
Loading. e recirculation loading in the H.J.W. land ll (0.05% per month) is much lower than that in Yolo County (1.3% per

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Journal of Chemistry month) and Denton (0.1% per month) land lls as reported by Mehta et al. [18] and Manzur et al. [17], respectively.However, the contribution of methane output is almost  e reason for the frequent injection, which induces organic runoff and decreases methane production, can be eliminated [25].Fresh waste and high leachate concentration account for the buildup of methane output in this study.

Conclusions
Long-term and short-term monitoring tests for leachate recirculation in the field were conducted in this study.e methane concentration and biogas flow rate were recorded during the operation phase of the landfill in NR, CR, and DR modes.High concentration and continuous injection of the leachate had a significant effect on the increase in the methane generation rate.e design of the collection well was also a critical element in the evaluation of the methane output.Determining recirculation loading based on the field scenario is essential to improve methane output.
e influence area of the liquid injection and gas-pumping wells should be considered to provide significant evidence to improve the reliability of quantity analysis in methane generation by leachate recirculation in the field site.

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Journal of Chemistry e methane concentration and biogas flow rate during the recirculation period in cell 2 are shown in Figure 3. e average methane concentration was 53.2% during the first eight months and increased to 58.2% between the eighth and eleventh months.

Figure 1 :
Figure 1: Design of the horizontal and vertical well within landfill.

Figure 2 :
Figure 2: e distribution of the methane concentration and biogas ow rate in cell 1.

Figure 4 :
Figure 3: e distribution of the methane concentration and biogas ow rate in cell 2.

Figure 5 :
Figure 5: e distribution of the leachate injection volume and COD in cell 2.

Figure 6 :Figure 7 :Figure 9 :Figure 10 :Figure 8 :
Figure 6: e distribution of the leachate injection volume and COD in cell 3.

Table 2 :
Summary of waste composition.

Table 3 :
Characteristics of the eld-site con guration.