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The impact of compaction and leachate recirculation on waste degradation in simulated landfills
Bioresource Technology 211 (2016) 72–79
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
The impact of compaction and leachate recirculation on waste degradation in simulated landfills Jae Hac Ko, Fan Yang, Qiyong Xu ⇑ Key Laboratory for Eco-efficient Polysilicate Materials, School of Environment and Energy, Peking University Shenzhen Graduate School, Guangdong 518055, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Waste compaction could result in
compaction
compaction r ecirculation
5 4 CH4 (L/day)
either inhibiting methanogens or promoting methane production. Compaction reduced pore space and increased the contact surface area. Determining timing for compaction in wet waste is critical to control MSW degradation.
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Article history: Received 20 January 2016 Received in revised form 10 March 2016 Accepted 13 March 2016 Available online 15 March 2016 Keywords: MSW Waste compaction Methane production Substrate accessibility Volatile fatty acids
a b s t r a c t This study investigated the impact of compaction and leachate recirculation on anaerobic degradation of municipal solid waste (MSW) at different methane formation phases. Two stainless steel lysimeters, C1 and C2, were constructed by equipping a hydraulic cylinder to apply pressure load (42 kPs) on the MSW. When MSW started to produce methane, C1 was compacted, but C2 was compacted when the methane production rate declined from the peak generation rate. Methane production of C1was inhibited by the compaction and resulted in producing a total of 106 L methane (44 L/kg VS). However, the compaction in C2 promoted MSW degradation resulting in producing a total of 298 L methane (125L/kg VS). The concentrations of volatile fatty acids and chemical oxygen demand showed temporary increases, when pressure load was applied. It was considered that the increased substrate accessibility within MSW by compaction could cause either the inhibition or the enhancement of methane production, depending the tolerability of methanogens on the acidic inhibition. Leachate recirculation also gave positive effects on methane generation from wet waste in the decelerated methanogenic phase by increasing mass transfer and the concentrations of volatile fatty acids. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Waste placement, compaction, and/or leachate recirculation are common practices in a bioreactor landfill during or after landfilling municipal solid waste (MSW). These practices change the geotech⇑ Corresponding author at: E118, School of Environment and Energy, Shenzhen Graduate School of Peking University, University Town, Xili, Nanshan District, Shenzhen 518055, China. E-mail address: [email protected] (Q. Xu). http://dx.doi.org/10.1016/j.biortech.2016.03.070 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.
nical properties of landfilled MSW and can affect the degradation of MSW. Due to the compressibility of MSW, landfilled MSW is densified by compaction. The densification of MSW can lead to increase biomass density as the result of void space reduction. Waste placements cause an increase in self-weight stress on landfilled MSW. Consequently, the void ratio of MSW is reduced and the unit weight of MSW increases with increasing waste depth (Chen et al., 2009; Machado et al., 2010; Stoltz et al., 2010). Chen et al. (2009) observed that the void ratio of the waste samples
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Fig. 1. Schematic diagram of a lysimeter. (a) Manual hand pump with a hydraulic pressure gauge, (b) hydraulic hose, (c) hydraulic cylinder, (d) metal frame, (e) extendable shaft, (f) gas collection ports, (g) stainless steel column, (h) compression plate, (i) leachate collection port, and (j) leachate recirculation port.
Table 1 The initial property of each MSW component in simulated reactors.
*
Component
Percent (%) w/w
Wet weight (kg)
Water content (%, w/w)
Total solid (kg)
Volatile solid*(%)
Food waste Paper Plastic Glass Metal Soil Total
57 9 10 3 1 20 100
5.13 0.81 0.90 0.27 0.09 1.80 9.00
77.3 5.3 0.2 0.0 0.0 16.7 –
1.17 0.77 0.90 0.27 0.09 1.50 4.70
73.8 76.1 99.9 0.0 2.8 3.0 –
Dry weight basis.
Table 2 Summary of operation conditions of C1 and C2. Bioreactor
Aerobic pretreatment
Anaerobic operation without compaction
Anaerobic operation with compaction but without leachate recirculation
Anaerobic operation with compaction and leachate recirculation
C1
27 day Aeration gradually reduced (aeration 2 or one times/d, 2 h/time, 600 mL/min) Leachate was recirculated
28–36 day Not compacted No leachate recirculation
37–113 day Pressure load (42 kPa) was applied when CH% reached over 30%
114–158 day Leachate recirculated every second day 500 mL of water was added Compacted
C2
32 day Aeration gradually reduced (aeration 2 or one times/d, 2 h/time, 600 mL/min) Leachate was recirculated
33–64 day Not compacted No leachate recirculation
65–113 day Pressure load (42 kPa) was applied when CH% was over 60%
114–158 day Leachate recirculated every second day No additional water Compacted
decreased from above 3 to around 1 with increasing depth of waste from 0 to about 35 m. Landfilled MSW has heterogeneous nature in both geotechnical properties and the distribution of microbial communities. Staley et al. (2011) observed the presence of large
spatial differences in refuse pH, moisture content, and volatile fatty acids (VFAs) concentrations in a refuse lysimeter at the initiation of methanogenesis. They hypothesized that methanogenic sites might exist in very small scales before methanogenesis.
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Spatially-separated seed and substrate regions in dry-solid digestion were hypothesized and addressed in other studies (Kalyuzhnyi et al., 2000; Martin, 2000; Martin et al., 2003a,b). Because the production of methane from anaerobic degradation relies on the syntrophic interactions of different microorganisms, the balance among different microbial communities is critical. The substrate transportation among different microorganisms is affected by the concentration gradient, the contact surface area, and the diffusion distance (Stams et al., 2012). Intermicrobial distance between acetogens and methanogens also decreases with increasing biomass (Stams et al., 2012). Substrate (acidic products) flux from acidic sites to methanogenic sites can be improved by reducing gas-filled pore space, increasing contact surface area, and decreasing diffusion distance. Ko et al. (2015) showed that MSW compaction could enhance MSW decomposition in the decelerated methane generation phase by promoting substrate accessibility and increasing contact surface area among MSW particles. Leachate recirculation in bioreactor landfills was proposed initially to provide additional moisture to dry waste and to enhance the degradation of MSW. However, the moisture addition is not necessary for MSW containing sufficient moisture (holding moisture more than field capacity). Both positive and negative effects of leachate recirculation for wet waste on methanogenic decompo-
sition have been observed (Hao et al., 2008; Benbelkacem et al., 2010; Shahriari et al., 2012; Xu et al., 2015; Huang et al., 2016). Leachate recirculation may play important roles in improving the rate of hydrolysis and acidogenesis and redistributing nutrients. Xu et al. (2014b) examined the impact of leachate recirculation on food waste hydrolysis using leachate in leach bed rectors and hypothesized that increasing leachate recirculation could enhance the opportunity for hydrolytic microorganisms to contact with the solid surfaces. Consequently, enzyme activates could increase and promote organic leaching (high VFAs and dissolve chemical oxygen demend (COD). Degueurce et al. (2016) confirmed no seeding effect of methanogens by the leachate recirculation in anaerobic digestion of cattle manure. So, leachate recirculation in wet waste could be more influential for hydrolysis and acidogenesis than for methanogenesis. Both MSW compaction and leachate recirculation can affect the distribution and change of VFA levels in leachate. The densification of MSW by compaction can cause a sudden change of VFA level in leachate (Ko et al., 2015). The inhibitory effect of VFAs or low pH caused by VFA accumulation on anaerobic digestion has been well documented (Russell and Diez-Gonzalez, 1997; Chen et al., 2008). However, the tolerability of methanogens for high VFA concentrations can vary with microbial communities’ conditions within
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Fig. 3. Methane generation rate and biogas composition of C1 (a) and C2 (b).
appropriate pH range (Franke-Whittle et al., 2014). Because VFAs are key substrates (or intermediates) and the capacity of methanogens withstanding VFA inhibition varies, understanding the impact of MSW compaction and leachate recirculation on VFA levels and methane formation is important. To find the optimal conditions of anaerobic conversion of MSW to methane, both the negative and positive effects of MSW compaction and leachate recirculation should be considered. In this study, the impact of compaction in different methanogenic phases on methane production was explored using aerobically pretreated MSW. In addition, the impact of leachate recirculation on the degradation of the compacted wet MSW was examined. Biogas production and leachate quality variation of simulated landfills were measured under anaerobic operations, including without compaction, with compaction, and leachate recirculation with compaction. 2. Methods and materials 2.1. Designed lysimeter Two stainless steel lysimeters were constructed as shown in Fig. 1. Each lysimeter was composed of a stainless steel cylinder (height 75 cm, inner diameter 18 cm), a compression plate (perforated stainless steel plate (ID < 18 cm), a fixed cap (Acrylic plate), and a compression unit (a hydraulic cylinder and a hand pump with a pressure gauge). The fixed cap had a hole at the center of the cap holding two O-rings to prevent leaking when a shaft moved through the hole. A lysimeter was placed in a fixed metal frame equipped with a compression unit. Overburden pressure was applied using a piece of shaft attached on the compression plate. An additional piece of shaft was connected to the existing shaft when needed to extend the length of the pieces of shaft. Two ports were located at the bottom of the lysimeter: a leachate collection port (at the lower part of leachate collection layer) and a gas collection port (at the upper part of leachate collection layer). Another two ports were added on the fix cap: a port is for leachate recirculation and the other port is gas collection.
2.2. Preparation and characterization of MSW Fabricated MSW was composed of food waste, shredded paper, film plastic, glass, metal and soil on wet weight basis. Each component of waste was collected and reduced its size less than 50 mm as needed. All components of MSW were well mixed before packing a lysimeter. Table 1 shows the composition of fabricated MSW and the initial property of each component. A total of 9.00 kg fabricated MSW was packed in each lysimeter. After packing every 3.0 kg of MSW, a flexible plastic net was placed to separate the waste into bottom, middle, and top layers. 2.3. Lysimeter operation Before the anaerobic operation of each lysimeter started, the packed MSW was treated by adding air to lower the formation of acidic products at the initial phase of anaerobic degradation. The aerobic pretreatment was performed based on our previous study results (Xu et al., 2014a). The aeration was conducted intermittently using a compressor with a flow rate of 600 mL/h. When pH was stabilized over 6, the aeration was stopped. The anaerobic operation was conducted without leachate recirculation. The compaction of anaerobically-decomposing MSW was performed by loading stress (42 kPa) at different methane production phases. The compaction was performed when the methane concentration of biogas exceeded over 30% (day 37) in C1 and when methane production decreased after the peak methane generation rate (day 65) in C2. Leachate recirculation of each lysimeter was started at day 113 until the end of the experiment. Due to insufficient leachate generation for the recirculation in C1, a total of 500 mL water was added to increase the moisture level of C1, but no additional water was added into C2. The detailed operation conditions are presented in Table 2. Leachate samples were collected from leachate sampling ports and analyzed for pH, chemical oxygen demand (COD), total volatile fatty acids (tVFA), and selected volatile fatty acids. tVFA was measured by a spectrophotometry (SHIMADZU, UV-2600, Japan) at the
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stopping aeration
daily methane generation rate (L/day)
6
contribution of each operational event on the methane production was determined.
leachate recirculation
compaction
5
4
measured methane generation rate (L/day)
2.5. Characterization of MSW
peak 1 peak 2 peak 3
The dry unit weight of MSW was calculated using the volume and the dry mass of MSW. The volume of waste was estimated using the linear displacement of the compression plate in each lysimeter. The dry mass of MSW during the decomposition was estimated by adjusting the initial dry mass of MSW using cumulative mass loss. The mass loss during the decomposition was calculated using the sum of carbon removed as a form of biogas and the mass of total solid (TS) in leachate removed. At the end of the experiment, decomposed MSW was collected from each lysimeter by pushing with a compression plate to minimize the destruction of MSW. The decomposed MSW was split in the three layers using the marked plastic nets during packing MSW. A total of eight MSW samples were collected from each layer. Gravimetric water content (w/w) and tVFA concentration (in pore leachate) of waste sample were measured. To obtain a sufficient volume of liquid for tVFA measurement, a total of 40 mL distilled water was added into a 20 g-MSW sample. The tVFA concentration of pore leachate in each sample was calculated back with the dilution ratio using the amounts of moisture and added water.
3
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day Fig. 4. Gaussian plots of methane production rates of the period of non-compacted, and compacted, leachate-recirculated conditions.
wavelength of 500 nm (Siedlecka et al., 2008). Selected VFAs (acetic acid, propionic acid, butyric acid (the sum of n-and i-butyric acids), and valeric acid (the sum of n- and i-valeric acids)) were measured on a gas chromatograph (Agilent, 7890A, United States) equipped with flame ionization detector. The gas volume and composition was measured daily. Biogas generated from each lysimeter was collected from the gas ports (located at the bottom and the top of the lysimeter) using gas sampling bags. Measured gas volume data were converted at a standard condition (Null °C at 1 atm). The composition of the gas was analyzed for methane and carbon dioxide using a gas chromatograph (FULI GC9790, Hangzhou, China) equipped with a thermal conductivity detector. 2.4. Gaussian model to fit methane generation rate The Gaussian model (Eq. (1)) was applied to quantify the methane productions of MSW in the periods of anaerobic operation without compaction, anaerobic operation with compaction, and anaerobic operation with compaction and leachate recirculation, separately. It was assumed that methane gas production rates of each event were the normal distribution in each period of anaerobic operation. A good agreement between the Gaussian plots and the biogas generation rates of anaerobic bioreactors has been reported by others (Lo et al., 2010).
"
2 # t t0 y ¼ a exp 0:5 b
ð1Þ
where, y is the biogas production rate (L/kg/d) at time t (day), t is the time (day) over the digestion period, a (L/kg/d) and b (day) are constants and t0 is the time (day) where the peak (maximal) biogas production rates occurred. First, Gaussian plot was determined using methane generation rate data during the anaerobic operation period without compaction. To obtain the methane generation rates enhanced by the compaction, the measured methane generation rates of the compacted period were subtracted by the estimated methane generation rates from the first Gaussian plot. The obtained methane generation rates in the period of anaerobic operation with compaction were used to determine the second Gaussian plot. In turn, the methane generation rates of the leachate recirculation period was estimated in the same manner and used for the third Gaussian plot. Using the three peaks of methane generation rates, the
3. Results and discussion 3.1. Change of dry unit weight of MSW The change of unit weight of MSW in each lysimeter during the experiment is shown in Fig. 2. Initial dry unit weight of C1 and C2 was 2.71 kN/m3 and 2.78 kN/m3, respectively. By compaction, dry unit weight of C1 and C2 increased to 6.7 and 6.2 kN/m3, respectively, resulting in densifying MSW more than doubling. The dry unit weight of C1 increased suddenly after pressure load was applied and then increased slowly between day 50 to 90 (Fig. 2 (a)). The dry unit weight of C2 increased slowly and continuously until the end of the experiment, after the sudden increase of the dry unit weight by compaction (Fig. 2(b)). The different patterns of dry unit of C1 and C2 could be attributed to the different moisture levels and MSW degradation rates over time in C1 and C2. At the end of the experiment, the dry unit weight of C1 and C2 reached 7.4 and 7.3 kN/m3, respectively. Obviously, the compaction was the dominant cause for the change of MSW unit weight. The degradation of MSW also was responsible for the slow increase of unit weight under the pressure loaded conditions. The biomass of C1 and C2 would be densified by the increase of dry unit of MSW after pressure load was applied. The range of dry unit weight measured in this study is in the ranges of unit weight reported in field studies. However, it should be cautioned to compare the dry unit weight results of C1 and C2 with landfill field data, because large pore spaces and large pieces of MSW are likely present in a landfill. 3.2. Biogas production Fig. 3 shows the daily methane generation rates and the gas compositions of C1 and C2. MSW compaction showed completely different influences on the anaerobic degradation depending on the time of the compaction. The compaction of MSW at the beginning of methane production phase in C1 (relatively small population of methanogens) inhibited methane generation considerably. The methane generation rate of C1 decreased from 0.31 L/day to less than 0.03 L/day by the compaction. The adverse effect of the
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Fig. 5. Concentrations of selected VFAs, tVFA, COD, and pH of C1 (a) and C2 (b).
compaction on methane production in C1 was lasted over a month. Even though the methane production restarted to increase in C1 from day 66, daily methane generation rates were still relatively small ranging between one and two liters per day. The cumulative volume of methane generated from C1 was 106 L (44 L/kg VS) at the end of the experiment. In contrast to C1, the MSW compaction of C2 in relatively mature methane production phase (after a peak generation rate of methane) greatly enhanced the daily methane generation rates. The peak methane generation rate after the compaction was greater than that before the compaction. In the anaer-
obic operation without compaction, the methane generation rate of C2 reached the peak of 4.0 L/day at day 45 and then decreased to 2.5 L/day. After being compacted, C2 showed the peak methane generation of 5.3 L/day at day 72. These results indicated that MSW densification in the decelerated methanogenic phase could promote the methanogenic decomposition of MSW. Primarily, the compaction reduced the porosity of MSW and caused to increase the contact surface area among waste particles. Consequently, the increase of contact surface area could enhance the substrate accessibility of microorganisms. The enhancement of substrate
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accessibility aided to promote methane generation in the decelerated methanogenic phase. These double-faced effects of MSW compaction on methane production showed the importance of compaction timing. The cumulative volume of methane from C2 was 298 L (125 L/kg VS) at the end of the experiment. Leachate recirculation also improved the methane generation rates in the decelerated methane production phase. Both C1 and C2, methane production rate to some extent increased after leachate recirculation began at day 114 (Fig. 3). The increase of methane generation in C2 was not because of the change of water content because the water content of C2 maintained constant by recirculating leachate. In addition, the seeding effect of leachate recirculation in the decelerated methanogenic phase may not be considerable (Degueurce et al., 2016). However, leachate recirculation could promote the substrate transport by adding the advective transportation of substrates in the static conditions where leachate movement through waste mass was limited and substrate transport was mainly governed by the diffusion. To quantify the volume of methane in different operational conditions, the methane generation rates of C2 separated in three Gaussian plots as shown Fig. 4. Using those plots, the total methane volume of the 1st, 2nd, and 3rd peak in the period of each event was estimated to be 111, 150, and 34 L, respectively. The
ascending limb (the dotted curve of each plot) of each plot before an event began was not included for the total volume estimation. It was estimated that about 51% methane produced from C2 was led by anaerobic operation with MSW compaction. Note that the time of compaction is an important factor which determines the magnitude of the impact on methane generation. 3.3. Leachate quality and VFAs Fig. 5 shows concentrations of the selected VFAs, tVFA, COD and pH in leachate of C1 and C2. Among the selected VAFs (acetic acids, propionic acid, butyric acid, and valeric acid), acetic and butyric acids were dominant. Jiang et al. (2013) found acetic and butyric acids were main VFAs on acidogenesis of food waste. The levels of butyric acid were greater than those of acetic acid because acetic acid could be consumed by methanogens. In C1, VAF concentrations increased during the aeration and after stopping the aeration. With compaction, VFA levels somewhat fluctuated and showed the highest VFA peak concentration. During this period, COD levels were maintained around 60 g/L and pH remained around 6. All VFA concentrations in leachate of C1 started to reduce quickly with producing methane. With the methane production, tVFA and COD decreased and pH increased to around 8. These results suggested
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that VFA producing processes were limited and VFAs were consumed for the production of methane. It was considered that a large amount of organic matter in C1 was preserved without being decomposed because the production of methane in C1 was far less than that in C2. In C2, each VFA showed a peak concentration during the anaerobic operation without compaction. Simultaneously, the methane generation rate of C2 showed a peak during the anaerobic operation without compaction. After the compaction in C2, VFAs increased to some degree and then decreased until the leachate recirculation started. C2 showed another peak concentration of VFAs after the leachate recirculation started. In contrast to VFAs of C1, the production of VFAs of C2 increased when MSW was compacted and leachate recirculation. The compaction and leachate recirculation in C2 aided to produce more VFAs than conditions without compaction and leachate recirculation. Consequently, methane production increased by applying the compaction and recirculating leachate. The acidic inhibition for methanogens by high VFA concentrations in leachate was not observed in C2. Franke-Whittle et al. (2014) also concluded high concentrations of VFA had no significant effects on methanogenic communities when the microbial communities could endure the changes in VFA concentrations. After applying stress and leachate recirculation in C2, the peaks of tVFA and COD concentration appeared. The increases of tVFA and COD may indicate that MSW compaction and leachate recirculation could improve the contact surface area for extracellular enzyme activities to increase organic solubility (Xu et al., 2014b). 3.4. Characteristics of degraded MSW tVFA and water content of waste samples collected at the end of the experiment are presented in Fig. 6. The distribution of water content in C1 and C2 did not vary among sampled layers, likely, because of leachate recirculation. The concentrations of tVFA in pore leachate in C1 and C2 varied from few thousands to few ten thousand part per million. Half of MSW samples collected in three layers of C1 showed VFA concentrations over 20,000 mg/L. The range and the median values of tVFA in the bottom layer of C1 were greater than those of the top layer. This result indicates that the lower section of MSW in C1 underwent the greater inhibition by the compaction. It was thought that moisture level would be high at the bottom layer of MSW when MSW was compacted in C1 because of the movement of gravitational water. Consequently, substrate accessibility could enhance in the lower layer of MSW and a large part MSW of the lower layer could ensile in the acidic conditions. In contrast, the difference of the range of tVFA concentrations among MSW layers of C2 was relatively small. Also, tVFA in the solid samples of C2 was lower than those of C1. A half of MSW samples collected in the three layers of C2 showed VFA concentrations below 10,000 mg/L. Comparing C1 with C2, it was suggested that the compaction of MSW in the earlier phase of methane production increased the heterogeneity of VFA levels in different depths and caused the different levels of inhibitory effects on methanogens. For the survival of methanogens in waste mass against the attack of acids, either sites of methanogenesis are protected or methanogen populations are able to withstand with high VFA concentrations. In C2, methanogens resisted the elevated VFA concentrations by compaction. 4. Conclusion It was concluded that the densification of degrading MSW by applying pressure load could result in either inhibiting or promoting methane production. Methanogenesis was inhibited by MSW compaction (42 kPs) in early methanogic phase (C1) resulting in
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producing 44 L/kg VS in the period of the experiment. However, MSW compaction in decelerated methanogenic phase (C2) promoted MSW degradation resulting in producing 125L/kg VS. Leachate recirculation also enhanced hydrolysis and acidogenesis as the increases of VFAs indicated. Therefore, determining timing for compaction and leachate recirculation in wet waste was critical to control the anaerobic degradation of MSW. Acknowledgements This research was supported by the Shenzhen government of China with Grant No. JCYJ20150616145013931 and CXZZ20151117141320317. References Benbelkacem, H., Bayard, R., Abdelhay, A., Zhang, Y., Gourdon, R., 2010. Effect of leachate injection modes on municipal solid waste degradation in anaerobic bioreactor. Bioresour. Technol. 101, 5206–5212. Chen, Y., Cheng, J.J., Creamer, K.S., 2008. Inhibition of anaerobic digestion process: a review. Bioresour. Technol. 99, 4044–4064. Chen, Y.M., Zhan, T.L.T., Wei, H.Y., Ke, H., 2009. Aging and compressibility of municipal solid wastes. Waste Manage. 29, 86–95. Degueurce, A., Tomas, N., Le Roux, S., Martinez, J., Peu, P., 2016. Biotic and abiotic roles of leachate recirculation in batch mode solid-state anaerobic digestion of cattle manure. Bioresour. Technol. 200, 388–395. Franke-Whittle, I.H., Walter, A., Ebner, C., Insam, H., 2014. Investigation into the effect of high concentrations of volatile fatty acids in anaerobic digestion on methanogenic communities. Waste Manage. 34, 2080–2089. Hao, Y.-J., Wu, W.-X., Wu, S.-W., Sun, H., Chen, Y.-X., 2008. Municipal solid waste decomposition under oversaturated condition in comparison with leachate recirculation. Process Biochem. 43, 108–112. Huang, W., Wang, Z., Guo, Q., Wang, H., Zhou, Y., Ng, W.J., 2016. Pilot-scale landfill with leachate recirculation for enhanced stabilization. Biochem. Eng. J. 105, 437–445. Jiang, J., Zhang, Y., Li, K., Wang, Q., Gong, C., Li, M., 2013. Volatile fatty acids production from food waste: effects of pH, temperature, and organic loading rate. Bioresour. Technol. 143, 525–530. Kalyuzhnyi, S., Veeken, A., Hamelers, B., 2000. Two-particle model of anaerobic solid state fermentation. Water Sci. Technol. 41, 43–50. Ko, J.H., Li, M., Yang, F., Xu, Q., 2015. Impact of MSW compression on methane generation in decelerated methanogenic phase. Bioresour. Technol. 192, 540– 546. Lo, H.M., Kurniawan, T.A., Sillanpää, M.E.T., Pai, T.Y., Chiang, C.F., Chao, K.P., Liu, M. H., Chuang, S.H., Banks, C.J., Wang, S.C., Lin, K.C., Lin, C.Y., Liu, W.F., Cheng, P.H., Chen, C.K., Chiu, H.Y., Wu, H.Y., 2010. Modeling biogas production from organic fraction of MSW co-digested with MSWI ashes in anaerobic bioreactors. Bioresour. Technol. 101, 6329–6335. Machado, S.L., Karimpour-Fard, M., Shariatmadari, N., Carvalho, M.F., Nascimento, J. C.F.D., 2010. Evaluation of the geotechnical properties of MSW in two Brazilian landfills. Waste Manage. 30, 2579–2591. Martin, D., 2000. A novel mathematical model of solid-state digestion. Biotechnol. Lett. 22, 91–94. Martin, D.J., Potts, L.G.A., Heslop, V.A., 2003a. Reaction mechanisms in solid-state anaerobic digestion. Process Saf. Environ. 81, 171–179. Martin, D.J., Potts, L.G.A., Heslop, V.A., 2003b. Reaction mechanisms in solid-state anaerobic digestion – II. The significance of seeding. Process Saf. Environ. 81, 180–188. Russell, J.B., Diez-Gonzalez, F., 1997. The effects of fermentation acids on bacterial growth. Adv. Microb. Physiol. 39, 205–234 (Poole, R.K., Academic Press). Shahriari, H., Warith, M., Hamoda, M., Kennedy, K.J., 2012. Effect of leachate recirculation on mesophilic anaerobic digestion of food waste. Waste Manage. 32, 400–403. Staley, B.F., de Los Reyes, F.L., Barlaz, M.A., 2011. Effect of spatial differences in microbial activity, pH, and substrate levels on methanogenesis initiation in refuse 3rd. Appl. Environ. Microbiol. 77, 2381–2391. Stams, A.J.M., Worm, P., Sousa, D.Z., Alves, M.M., Plugge, C.M., 2012. Syntrophic Degradation of Fatty Acids by Methanogenic Communities. Microbial Technologies in Advanced Biofuels Production. Springer, Hallenbeck P.C. New York, US. Stoltz, G., Gourc, J.-P., Oxarango, L., 2010. Characterisation of the physicomechanical parameters of MSW. Waste Manage. 30, 1439–1449. Xu, Q., Jin, X., Ma, Z., Tao, H., Ko, J.H., 2014a. Methane production in simulated hybrid bioreactor landfill. Bioresour. Technol. 168, 92–96. Xu, Q., Tian, Y., Wang, S., Ko, J.H., 2015. A comparative study of leachate quality and biogas generation in simulated anaerobic and hybrid bioreactors. Waste Manage. 41, 94–100. Xu, S.Y., Karthikeyan, O.P., Selvam, A., Wong, J.W.C., 2014b. Microbial community distribution and extracellular enzyme activities in leach bed reactor treating food waste: effect of different leachate recirculation practices. Bioresour. Technol. 168, 41–48.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
The impact of compaction and leachate recirculation on waste degradation in simulated landfills Jae Hac Ko, Fan Yang, Qiyong Xu ⇑ Key Laboratory for Eco-efficient Polysilicate Materials, School of Environment and Energy, Peking University Shenzhen Graduate School, Guangdong 518055, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Waste compaction could result in
compaction
compaction r ecirculation
5 4 CH4 (L/day)
either inhibiting methanogens or promoting methane production. Compaction reduced pore space and increased the contact surface area. Determining timing for compaction in wet waste is critical to control MSW degradation.
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Article history: Received 20 January 2016 Received in revised form 10 March 2016 Accepted 13 March 2016 Available online 15 March 2016 Keywords: MSW Waste compaction Methane production Substrate accessibility Volatile fatty acids
a b s t r a c t This study investigated the impact of compaction and leachate recirculation on anaerobic degradation of municipal solid waste (MSW) at different methane formation phases. Two stainless steel lysimeters, C1 and C2, were constructed by equipping a hydraulic cylinder to apply pressure load (42 kPs) on the MSW. When MSW started to produce methane, C1 was compacted, but C2 was compacted when the methane production rate declined from the peak generation rate. Methane production of C1was inhibited by the compaction and resulted in producing a total of 106 L methane (44 L/kg VS). However, the compaction in C2 promoted MSW degradation resulting in producing a total of 298 L methane (125L/kg VS). The concentrations of volatile fatty acids and chemical oxygen demand showed temporary increases, when pressure load was applied. It was considered that the increased substrate accessibility within MSW by compaction could cause either the inhibition or the enhancement of methane production, depending the tolerability of methanogens on the acidic inhibition. Leachate recirculation also gave positive effects on methane generation from wet waste in the decelerated methanogenic phase by increasing mass transfer and the concentrations of volatile fatty acids. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Waste placement, compaction, and/or leachate recirculation are common practices in a bioreactor landfill during or after landfilling municipal solid waste (MSW). These practices change the geotech⇑ Corresponding author at: E118, School of Environment and Energy, Shenzhen Graduate School of Peking University, University Town, Xili, Nanshan District, Shenzhen 518055, China. E-mail address: [email protected] (Q. Xu). http://dx.doi.org/10.1016/j.biortech.2016.03.070 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.
nical properties of landfilled MSW and can affect the degradation of MSW. Due to the compressibility of MSW, landfilled MSW is densified by compaction. The densification of MSW can lead to increase biomass density as the result of void space reduction. Waste placements cause an increase in self-weight stress on landfilled MSW. Consequently, the void ratio of MSW is reduced and the unit weight of MSW increases with increasing waste depth (Chen et al., 2009; Machado et al., 2010; Stoltz et al., 2010). Chen et al. (2009) observed that the void ratio of the waste samples
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Fig. 1. Schematic diagram of a lysimeter. (a) Manual hand pump with a hydraulic pressure gauge, (b) hydraulic hose, (c) hydraulic cylinder, (d) metal frame, (e) extendable shaft, (f) gas collection ports, (g) stainless steel column, (h) compression plate, (i) leachate collection port, and (j) leachate recirculation port.
Table 1 The initial property of each MSW component in simulated reactors.
*
Component
Percent (%) w/w
Wet weight (kg)
Water content (%, w/w)
Total solid (kg)
Volatile solid*(%)
Food waste Paper Plastic Glass Metal Soil Total
57 9 10 3 1 20 100
5.13 0.81 0.90 0.27 0.09 1.80 9.00
77.3 5.3 0.2 0.0 0.0 16.7 –
1.17 0.77 0.90 0.27 0.09 1.50 4.70
73.8 76.1 99.9 0.0 2.8 3.0 –
Dry weight basis.
Table 2 Summary of operation conditions of C1 and C2. Bioreactor
Aerobic pretreatment
Anaerobic operation without compaction
Anaerobic operation with compaction but without leachate recirculation
Anaerobic operation with compaction and leachate recirculation
C1
27 day Aeration gradually reduced (aeration 2 or one times/d, 2 h/time, 600 mL/min) Leachate was recirculated
28–36 day Not compacted No leachate recirculation
37–113 day Pressure load (42 kPa) was applied when CH% reached over 30%
114–158 day Leachate recirculated every second day 500 mL of water was added Compacted
C2
32 day Aeration gradually reduced (aeration 2 or one times/d, 2 h/time, 600 mL/min) Leachate was recirculated
33–64 day Not compacted No leachate recirculation
65–113 day Pressure load (42 kPa) was applied when CH% was over 60%
114–158 day Leachate recirculated every second day No additional water Compacted
decreased from above 3 to around 1 with increasing depth of waste from 0 to about 35 m. Landfilled MSW has heterogeneous nature in both geotechnical properties and the distribution of microbial communities. Staley et al. (2011) observed the presence of large
spatial differences in refuse pH, moisture content, and volatile fatty acids (VFAs) concentrations in a refuse lysimeter at the initiation of methanogenesis. They hypothesized that methanogenic sites might exist in very small scales before methanogenesis.
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8
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(b) 6
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0 0
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day Fig. 2. Change of MSW unit weight in C1 (a) and C2 (b).
Spatially-separated seed and substrate regions in dry-solid digestion were hypothesized and addressed in other studies (Kalyuzhnyi et al., 2000; Martin, 2000; Martin et al., 2003a,b). Because the production of methane from anaerobic degradation relies on the syntrophic interactions of different microorganisms, the balance among different microbial communities is critical. The substrate transportation among different microorganisms is affected by the concentration gradient, the contact surface area, and the diffusion distance (Stams et al., 2012). Intermicrobial distance between acetogens and methanogens also decreases with increasing biomass (Stams et al., 2012). Substrate (acidic products) flux from acidic sites to methanogenic sites can be improved by reducing gas-filled pore space, increasing contact surface area, and decreasing diffusion distance. Ko et al. (2015) showed that MSW compaction could enhance MSW decomposition in the decelerated methane generation phase by promoting substrate accessibility and increasing contact surface area among MSW particles. Leachate recirculation in bioreactor landfills was proposed initially to provide additional moisture to dry waste and to enhance the degradation of MSW. However, the moisture addition is not necessary for MSW containing sufficient moisture (holding moisture more than field capacity). Both positive and negative effects of leachate recirculation for wet waste on methanogenic decompo-
sition have been observed (Hao et al., 2008; Benbelkacem et al., 2010; Shahriari et al., 2012; Xu et al., 2015; Huang et al., 2016). Leachate recirculation may play important roles in improving the rate of hydrolysis and acidogenesis and redistributing nutrients. Xu et al. (2014b) examined the impact of leachate recirculation on food waste hydrolysis using leachate in leach bed rectors and hypothesized that increasing leachate recirculation could enhance the opportunity for hydrolytic microorganisms to contact with the solid surfaces. Consequently, enzyme activates could increase and promote organic leaching (high VFAs and dissolve chemical oxygen demend (COD). Degueurce et al. (2016) confirmed no seeding effect of methanogens by the leachate recirculation in anaerobic digestion of cattle manure. So, leachate recirculation in wet waste could be more influential for hydrolysis and acidogenesis than for methanogenesis. Both MSW compaction and leachate recirculation can affect the distribution and change of VFA levels in leachate. The densification of MSW by compaction can cause a sudden change of VFA level in leachate (Ko et al., 2015). The inhibitory effect of VFAs or low pH caused by VFA accumulation on anaerobic digestion has been well documented (Russell and Diez-Gonzalez, 1997; Chen et al., 2008). However, the tolerability of methanogens for high VFA concentrations can vary with microbial communities’ conditions within
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Fig. 3. Methane generation rate and biogas composition of C1 (a) and C2 (b).
appropriate pH range (Franke-Whittle et al., 2014). Because VFAs are key substrates (or intermediates) and the capacity of methanogens withstanding VFA inhibition varies, understanding the impact of MSW compaction and leachate recirculation on VFA levels and methane formation is important. To find the optimal conditions of anaerobic conversion of MSW to methane, both the negative and positive effects of MSW compaction and leachate recirculation should be considered. In this study, the impact of compaction in different methanogenic phases on methane production was explored using aerobically pretreated MSW. In addition, the impact of leachate recirculation on the degradation of the compacted wet MSW was examined. Biogas production and leachate quality variation of simulated landfills were measured under anaerobic operations, including without compaction, with compaction, and leachate recirculation with compaction. 2. Methods and materials 2.1. Designed lysimeter Two stainless steel lysimeters were constructed as shown in Fig. 1. Each lysimeter was composed of a stainless steel cylinder (height 75 cm, inner diameter 18 cm), a compression plate (perforated stainless steel plate (ID < 18 cm), a fixed cap (Acrylic plate), and a compression unit (a hydraulic cylinder and a hand pump with a pressure gauge). The fixed cap had a hole at the center of the cap holding two O-rings to prevent leaking when a shaft moved through the hole. A lysimeter was placed in a fixed metal frame equipped with a compression unit. Overburden pressure was applied using a piece of shaft attached on the compression plate. An additional piece of shaft was connected to the existing shaft when needed to extend the length of the pieces of shaft. Two ports were located at the bottom of the lysimeter: a leachate collection port (at the lower part of leachate collection layer) and a gas collection port (at the upper part of leachate collection layer). Another two ports were added on the fix cap: a port is for leachate recirculation and the other port is gas collection.
2.2. Preparation and characterization of MSW Fabricated MSW was composed of food waste, shredded paper, film plastic, glass, metal and soil on wet weight basis. Each component of waste was collected and reduced its size less than 50 mm as needed. All components of MSW were well mixed before packing a lysimeter. Table 1 shows the composition of fabricated MSW and the initial property of each component. A total of 9.00 kg fabricated MSW was packed in each lysimeter. After packing every 3.0 kg of MSW, a flexible plastic net was placed to separate the waste into bottom, middle, and top layers. 2.3. Lysimeter operation Before the anaerobic operation of each lysimeter started, the packed MSW was treated by adding air to lower the formation of acidic products at the initial phase of anaerobic degradation. The aerobic pretreatment was performed based on our previous study results (Xu et al., 2014a). The aeration was conducted intermittently using a compressor with a flow rate of 600 mL/h. When pH was stabilized over 6, the aeration was stopped. The anaerobic operation was conducted without leachate recirculation. The compaction of anaerobically-decomposing MSW was performed by loading stress (42 kPa) at different methane production phases. The compaction was performed when the methane concentration of biogas exceeded over 30% (day 37) in C1 and when methane production decreased after the peak methane generation rate (day 65) in C2. Leachate recirculation of each lysimeter was started at day 113 until the end of the experiment. Due to insufficient leachate generation for the recirculation in C1, a total of 500 mL water was added to increase the moisture level of C1, but no additional water was added into C2. The detailed operation conditions are presented in Table 2. Leachate samples were collected from leachate sampling ports and analyzed for pH, chemical oxygen demand (COD), total volatile fatty acids (tVFA), and selected volatile fatty acids. tVFA was measured by a spectrophotometry (SHIMADZU, UV-2600, Japan) at the
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stopping aeration
daily methane generation rate (L/day)
6
contribution of each operational event on the methane production was determined.
leachate recirculation
compaction
5
4
measured methane generation rate (L/day)
2.5. Characterization of MSW
peak 1 peak 2 peak 3
The dry unit weight of MSW was calculated using the volume and the dry mass of MSW. The volume of waste was estimated using the linear displacement of the compression plate in each lysimeter. The dry mass of MSW during the decomposition was estimated by adjusting the initial dry mass of MSW using cumulative mass loss. The mass loss during the decomposition was calculated using the sum of carbon removed as a form of biogas and the mass of total solid (TS) in leachate removed. At the end of the experiment, decomposed MSW was collected from each lysimeter by pushing with a compression plate to minimize the destruction of MSW. The decomposed MSW was split in the three layers using the marked plastic nets during packing MSW. A total of eight MSW samples were collected from each layer. Gravimetric water content (w/w) and tVFA concentration (in pore leachate) of waste sample were measured. To obtain a sufficient volume of liquid for tVFA measurement, a total of 40 mL distilled water was added into a 20 g-MSW sample. The tVFA concentration of pore leachate in each sample was calculated back with the dilution ratio using the amounts of moisture and added water.
3
2
1
0 0
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180
day Fig. 4. Gaussian plots of methane production rates of the period of non-compacted, and compacted, leachate-recirculated conditions.
wavelength of 500 nm (Siedlecka et al., 2008). Selected VFAs (acetic acid, propionic acid, butyric acid (the sum of n-and i-butyric acids), and valeric acid (the sum of n- and i-valeric acids)) were measured on a gas chromatograph (Agilent, 7890A, United States) equipped with flame ionization detector. The gas volume and composition was measured daily. Biogas generated from each lysimeter was collected from the gas ports (located at the bottom and the top of the lysimeter) using gas sampling bags. Measured gas volume data were converted at a standard condition (Null °C at 1 atm). The composition of the gas was analyzed for methane and carbon dioxide using a gas chromatograph (FULI GC9790, Hangzhou, China) equipped with a thermal conductivity detector. 2.4. Gaussian model to fit methane generation rate The Gaussian model (Eq. (1)) was applied to quantify the methane productions of MSW in the periods of anaerobic operation without compaction, anaerobic operation with compaction, and anaerobic operation with compaction and leachate recirculation, separately. It was assumed that methane gas production rates of each event were the normal distribution in each period of anaerobic operation. A good agreement between the Gaussian plots and the biogas generation rates of anaerobic bioreactors has been reported by others (Lo et al., 2010).
"
2 # t t0 y ¼ a exp 0:5 b
ð1Þ
where, y is the biogas production rate (L/kg/d) at time t (day), t is the time (day) over the digestion period, a (L/kg/d) and b (day) are constants and t0 is the time (day) where the peak (maximal) biogas production rates occurred. First, Gaussian plot was determined using methane generation rate data during the anaerobic operation period without compaction. To obtain the methane generation rates enhanced by the compaction, the measured methane generation rates of the compacted period were subtracted by the estimated methane generation rates from the first Gaussian plot. The obtained methane generation rates in the period of anaerobic operation with compaction were used to determine the second Gaussian plot. In turn, the methane generation rates of the leachate recirculation period was estimated in the same manner and used for the third Gaussian plot. Using the three peaks of methane generation rates, the
3. Results and discussion 3.1. Change of dry unit weight of MSW The change of unit weight of MSW in each lysimeter during the experiment is shown in Fig. 2. Initial dry unit weight of C1 and C2 was 2.71 kN/m3 and 2.78 kN/m3, respectively. By compaction, dry unit weight of C1 and C2 increased to 6.7 and 6.2 kN/m3, respectively, resulting in densifying MSW more than doubling. The dry unit weight of C1 increased suddenly after pressure load was applied and then increased slowly between day 50 to 90 (Fig. 2 (a)). The dry unit weight of C2 increased slowly and continuously until the end of the experiment, after the sudden increase of the dry unit weight by compaction (Fig. 2(b)). The different patterns of dry unit of C1 and C2 could be attributed to the different moisture levels and MSW degradation rates over time in C1 and C2. At the end of the experiment, the dry unit weight of C1 and C2 reached 7.4 and 7.3 kN/m3, respectively. Obviously, the compaction was the dominant cause for the change of MSW unit weight. The degradation of MSW also was responsible for the slow increase of unit weight under the pressure loaded conditions. The biomass of C1 and C2 would be densified by the increase of dry unit of MSW after pressure load was applied. The range of dry unit weight measured in this study is in the ranges of unit weight reported in field studies. However, it should be cautioned to compare the dry unit weight results of C1 and C2 with landfill field data, because large pore spaces and large pieces of MSW are likely present in a landfill. 3.2. Biogas production Fig. 3 shows the daily methane generation rates and the gas compositions of C1 and C2. MSW compaction showed completely different influences on the anaerobic degradation depending on the time of the compaction. The compaction of MSW at the beginning of methane production phase in C1 (relatively small population of methanogens) inhibited methane generation considerably. The methane generation rate of C1 decreased from 0.31 L/day to less than 0.03 L/day by the compaction. The adverse effect of the
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Fig. 5. Concentrations of selected VFAs, tVFA, COD, and pH of C1 (a) and C2 (b).
compaction on methane production in C1 was lasted over a month. Even though the methane production restarted to increase in C1 from day 66, daily methane generation rates were still relatively small ranging between one and two liters per day. The cumulative volume of methane generated from C1 was 106 L (44 L/kg VS) at the end of the experiment. In contrast to C1, the MSW compaction of C2 in relatively mature methane production phase (after a peak generation rate of methane) greatly enhanced the daily methane generation rates. The peak methane generation rate after the compaction was greater than that before the compaction. In the anaer-
obic operation without compaction, the methane generation rate of C2 reached the peak of 4.0 L/day at day 45 and then decreased to 2.5 L/day. After being compacted, C2 showed the peak methane generation of 5.3 L/day at day 72. These results indicated that MSW densification in the decelerated methanogenic phase could promote the methanogenic decomposition of MSW. Primarily, the compaction reduced the porosity of MSW and caused to increase the contact surface area among waste particles. Consequently, the increase of contact surface area could enhance the substrate accessibility of microorganisms. The enhancement of substrate
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40000
50000
tVFA in pore leachate (mg/L) Fig. 6. Box plots of the gravimetric water content and pore leachate tVFA of degraded samples. (a) C1 and (b) C2.
accessibility aided to promote methane generation in the decelerated methanogenic phase. These double-faced effects of MSW compaction on methane production showed the importance of compaction timing. The cumulative volume of methane from C2 was 298 L (125 L/kg VS) at the end of the experiment. Leachate recirculation also improved the methane generation rates in the decelerated methane production phase. Both C1 and C2, methane production rate to some extent increased after leachate recirculation began at day 114 (Fig. 3). The increase of methane generation in C2 was not because of the change of water content because the water content of C2 maintained constant by recirculating leachate. In addition, the seeding effect of leachate recirculation in the decelerated methanogenic phase may not be considerable (Degueurce et al., 2016). However, leachate recirculation could promote the substrate transport by adding the advective transportation of substrates in the static conditions where leachate movement through waste mass was limited and substrate transport was mainly governed by the diffusion. To quantify the volume of methane in different operational conditions, the methane generation rates of C2 separated in three Gaussian plots as shown Fig. 4. Using those plots, the total methane volume of the 1st, 2nd, and 3rd peak in the period of each event was estimated to be 111, 150, and 34 L, respectively. The
ascending limb (the dotted curve of each plot) of each plot before an event began was not included for the total volume estimation. It was estimated that about 51% methane produced from C2 was led by anaerobic operation with MSW compaction. Note that the time of compaction is an important factor which determines the magnitude of the impact on methane generation. 3.3. Leachate quality and VFAs Fig. 5 shows concentrations of the selected VFAs, tVFA, COD and pH in leachate of C1 and C2. Among the selected VAFs (acetic acids, propionic acid, butyric acid, and valeric acid), acetic and butyric acids were dominant. Jiang et al. (2013) found acetic and butyric acids were main VFAs on acidogenesis of food waste. The levels of butyric acid were greater than those of acetic acid because acetic acid could be consumed by methanogens. In C1, VAF concentrations increased during the aeration and after stopping the aeration. With compaction, VFA levels somewhat fluctuated and showed the highest VFA peak concentration. During this period, COD levels were maintained around 60 g/L and pH remained around 6. All VFA concentrations in leachate of C1 started to reduce quickly with producing methane. With the methane production, tVFA and COD decreased and pH increased to around 8. These results suggested
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that VFA producing processes were limited and VFAs were consumed for the production of methane. It was considered that a large amount of organic matter in C1 was preserved without being decomposed because the production of methane in C1 was far less than that in C2. In C2, each VFA showed a peak concentration during the anaerobic operation without compaction. Simultaneously, the methane generation rate of C2 showed a peak during the anaerobic operation without compaction. After the compaction in C2, VFAs increased to some degree and then decreased until the leachate recirculation started. C2 showed another peak concentration of VFAs after the leachate recirculation started. In contrast to VFAs of C1, the production of VFAs of C2 increased when MSW was compacted and leachate recirculation. The compaction and leachate recirculation in C2 aided to produce more VFAs than conditions without compaction and leachate recirculation. Consequently, methane production increased by applying the compaction and recirculating leachate. The acidic inhibition for methanogens by high VFA concentrations in leachate was not observed in C2. Franke-Whittle et al. (2014) also concluded high concentrations of VFA had no significant effects on methanogenic communities when the microbial communities could endure the changes in VFA concentrations. After applying stress and leachate recirculation in C2, the peaks of tVFA and COD concentration appeared. The increases of tVFA and COD may indicate that MSW compaction and leachate recirculation could improve the contact surface area for extracellular enzyme activities to increase organic solubility (Xu et al., 2014b). 3.4. Characteristics of degraded MSW tVFA and water content of waste samples collected at the end of the experiment are presented in Fig. 6. The distribution of water content in C1 and C2 did not vary among sampled layers, likely, because of leachate recirculation. The concentrations of tVFA in pore leachate in C1 and C2 varied from few thousands to few ten thousand part per million. Half of MSW samples collected in three layers of C1 showed VFA concentrations over 20,000 mg/L. The range and the median values of tVFA in the bottom layer of C1 were greater than those of the top layer. This result indicates that the lower section of MSW in C1 underwent the greater inhibition by the compaction. It was thought that moisture level would be high at the bottom layer of MSW when MSW was compacted in C1 because of the movement of gravitational water. Consequently, substrate accessibility could enhance in the lower layer of MSW and a large part MSW of the lower layer could ensile in the acidic conditions. In contrast, the difference of the range of tVFA concentrations among MSW layers of C2 was relatively small. Also, tVFA in the solid samples of C2 was lower than those of C1. A half of MSW samples collected in the three layers of C2 showed VFA concentrations below 10,000 mg/L. Comparing C1 with C2, it was suggested that the compaction of MSW in the earlier phase of methane production increased the heterogeneity of VFA levels in different depths and caused the different levels of inhibitory effects on methanogens. For the survival of methanogens in waste mass against the attack of acids, either sites of methanogenesis are protected or methanogen populations are able to withstand with high VFA concentrations. In C2, methanogens resisted the elevated VFA concentrations by compaction. 4. Conclusion It was concluded that the densification of degrading MSW by applying pressure load could result in either inhibiting or promoting methane production. Methanogenesis was inhibited by MSW compaction (42 kPs) in early methanogic phase (C1) resulting in
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producing 44 L/kg VS in the period of the experiment. However, MSW compaction in decelerated methanogenic phase (C2) promoted MSW degradation resulting in producing 125L/kg VS. Leachate recirculation also enhanced hydrolysis and acidogenesis as the increases of VFAs indicated. Therefore, determining timing for compaction and leachate recirculation in wet waste was critical to control the anaerobic degradation of MSW. Acknowledgements This research was supported by the Shenzhen government of China with Grant No. JCYJ20150616145013931 and CXZZ20151117141320317. References Benbelkacem, H., Bayard, R., Abdelhay, A., Zhang, Y., Gourdon, R., 2010. Effect of leachate injection modes on municipal solid waste degradation in anaerobic bioreactor. Bioresour. Technol. 101, 5206–5212. Chen, Y., Cheng, J.J., Creamer, K.S., 2008. Inhibition of anaerobic digestion process: a review. Bioresour. Technol. 99, 4044–4064. Chen, Y.M., Zhan, T.L.T., Wei, H.Y., Ke, H., 2009. Aging and compressibility of municipal solid wastes. Waste Manage. 29, 86–95. Degueurce, A., Tomas, N., Le Roux, S., Martinez, J., Peu, P., 2016. Biotic and abiotic roles of leachate recirculation in batch mode solid-state anaerobic digestion of cattle manure. Bioresour. Technol. 200, 388–395. Franke-Whittle, I.H., Walter, A., Ebner, C., Insam, H., 2014. Investigation into the effect of high concentrations of volatile fatty acids in anaerobic digestion on methanogenic communities. Waste Manage. 34, 2080–2089. Hao, Y.-J., Wu, W.-X., Wu, S.-W., Sun, H., Chen, Y.-X., 2008. Municipal solid waste decomposition under oversaturated condition in comparison with leachate recirculation. Process Biochem. 43, 108–112. Huang, W., Wang, Z., Guo, Q., Wang, H., Zhou, Y., Ng, W.J., 2016. Pilot-scale landfill with leachate recirculation for enhanced stabilization. Biochem. Eng. J. 105, 437–445. Jiang, J., Zhang, Y., Li, K., Wang, Q., Gong, C., Li, M., 2013. Volatile fatty acids production from food waste: effects of pH, temperature, and organic loading rate. Bioresour. Technol. 143, 525–530. Kalyuzhnyi, S., Veeken, A., Hamelers, B., 2000. Two-particle model of anaerobic solid state fermentation. Water Sci. Technol. 41, 43–50. Ko, J.H., Li, M., Yang, F., Xu, Q., 2015. Impact of MSW compression on methane generation in decelerated methanogenic phase. Bioresour. Technol. 192, 540– 546. Lo, H.M., Kurniawan, T.A., Sillanpää, M.E.T., Pai, T.Y., Chiang, C.F., Chao, K.P., Liu, M. H., Chuang, S.H., Banks, C.J., Wang, S.C., Lin, K.C., Lin, C.Y., Liu, W.F., Cheng, P.H., Chen, C.K., Chiu, H.Y., Wu, H.Y., 2010. Modeling biogas production from organic fraction of MSW co-digested with MSWI ashes in anaerobic bioreactors. Bioresour. Technol. 101, 6329–6335. Machado, S.L., Karimpour-Fard, M., Shariatmadari, N., Carvalho, M.F., Nascimento, J. C.F.D., 2010. Evaluation of the geotechnical properties of MSW in two Brazilian landfills. Waste Manage. 30, 2579–2591. Martin, D., 2000. A novel mathematical model of solid-state digestion. Biotechnol. Lett. 22, 91–94. Martin, D.J., Potts, L.G.A., Heslop, V.A., 2003a. Reaction mechanisms in solid-state anaerobic digestion. Process Saf. Environ. 81, 171–179. Martin, D.J., Potts, L.G.A., Heslop, V.A., 2003b. Reaction mechanisms in solid-state anaerobic digestion – II. The significance of seeding. Process Saf. Environ. 81, 180–188. Russell, J.B., Diez-Gonzalez, F., 1997. The effects of fermentation acids on bacterial growth. Adv. Microb. Physiol. 39, 205–234 (Poole, R.K., Academic Press). Shahriari, H., Warith, M., Hamoda, M., Kennedy, K.J., 2012. Effect of leachate recirculation on mesophilic anaerobic digestion of food waste. Waste Manage. 32, 400–403. Staley, B.F., de Los Reyes, F.L., Barlaz, M.A., 2011. Effect of spatial differences in microbial activity, pH, and substrate levels on methanogenesis initiation in refuse 3rd. Appl. Environ. Microbiol. 77, 2381–2391. Stams, A.J.M., Worm, P., Sousa, D.Z., Alves, M.M., Plugge, C.M., 2012. Syntrophic Degradation of Fatty Acids by Methanogenic Communities. Microbial Technologies in Advanced Biofuels Production. Springer, Hallenbeck P.C. New York, US. Stoltz, G., Gourc, J.-P., Oxarango, L., 2010. Characterisation of the physicomechanical parameters of MSW. Waste Manage. 30, 1439–1449. Xu, Q., Jin, X., Ma, Z., Tao, H., Ko, J.H., 2014a. Methane production in simulated hybrid bioreactor landfill. Bioresour. Technol. 168, 92–96. Xu, Q., Tian, Y., Wang, S., Ko, J.H., 2015. A comparative study of leachate quality and biogas generation in simulated anaerobic and hybrid bioreactors. Waste Manage. 41, 94–100. Xu, S.Y., Karthikeyan, O.P., Selvam, A., Wong, J.W.C., 2014b. Microbial community distribution and extracellular enzyme activities in leach bed reactor treating food waste: effect of different leachate recirculation practices. Bioresour. Technol. 168, 41–48.