Bioresource Technology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Methane production in simulated hybrid bioreactor landfill Qiyong Xu a, Xiao Jin a, Zeyu Ma a, Huchun Tao b, Jae Hac Ko b,⇑ a b
Key Laboratory for Eco-efficient Polysilicate Materials, School of Environment and Energy, Peking University Shenzhen Graduate School, Guangdong 518055, China Key Laboratory for Heavy Metal Pollution Control and Reutilization, School of Environment and Energy, Peking University Shenzhen Graduate School, Guangdong 518055, China
h i g h l i g h t s Temporary aeration into the hybrid bioreactor improved leachate quality. pH adjustment by the aeration boosted the formation of methane generation phase. The hybrid bioreactor was economically feasible by the methane quality improvement.
a r t i c l e
i n f o
Article history: Received 3 December 2013 Received in revised form 5 March 2014 Accepted 8 March 2014 Available online xxxx Keywords: Bioreactor landfill Aeration Leachate recirculation Methane production
a b s t r a c t The aim of this work was to study a hybrid bioreactor landfill technology for landfill methane production from municipal solid waste. Two laboratory-scale columns were operated for about ten months to simulate an anaerobic and a hybrid landfill bioreactor, respectively. Leachate was recirculated into each column but aeration was conducted in the hybrid bioreactor during the first stage. Results showed that leachate pH in the anaerobic bioreactor maintained below 6.5, while in the hybrid bioreactor quickly increased from 5.6 to 7.0 due to the aeration. The temporary aeration resulted in lowering COD and BOD5 in the leachate. The volume of methane collected from the hybrid bioreactor was 400 times greater than that of the anaerobic bioreactor. Also, the methane production rate of the hybrid bioreactor was improved within a short period of time. After about 10 months’ operation, the total methane production in the hybrid bioreactor was 212 L (16 L/kgwaste). Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Landfilling is the most common method of municipal solid waste (MSW) disposal around the world, especially in developing countries (Giusti, 2009; Mor et al., 2006). In China, landfill has been the dominant way for MSW management because it is a cost-effective MSW management option compared with other options and can treat mixed MSW without separation (Chen et al., 2010; Zhang et al., 2010). According to statistical data, in 2010, the amount of MSW disposal was 131 million tons, of which 77% was landfilled (China Statistical Yearbook, 2012). However, it is estimated that the recovery rate of landfill gas generated from traditional landfills is less than 20% (Zhang et al., 2010). One of the main reasons is the relatively slow MSW degradation in traditional landfills (Xu and Ge, 2011). MSW degradation rate and methane production from MSW are mainly influenced by microorganisms, moisture conditions, and other inhibitory factors. One of the inhibitory factors is the imbalance between acidogenesis and methanogenesis, ⇑ Corresponding author. Tel.: +86 755 2603 3289. E-mail address: [email protected] (J.H. Ko).
which is often caused by the rapid hydrolysis of readily biodegradable materials in the beginning of anaerobic digestion. MSW composition in China is dominated by food waste, typically more than 50% by weight. In anaerobic conditions, the excess hydrolysis of easily biodegradable food waste can cause the accumulation of acids resulting in the decrease of pH. So, it may delay the formation of stable methane production phases in Chinese MSW landfills. In some cases, it took several years to start methane generation (O’Keefe and Chynoweth, 2000). Bioreactor landfill technology has been developed to accelerate the biodegradation of the biodegradable fractions and to enhance the stabilization of landfilled waste. The key step to create a landfill bioreactor is water addition or leachate recirculation to increase moisture content of MSW (Townsend et al., 1996; Reinhart et al., 2002; Benson et al., 2007; He et al., 2007; Kumar et al., 2011; Bilgili et al., 2012). In general, there are four types of landfill bioreactors: aerobic, anaerobic, facultative and hybrid systems, equipped with different operating schemes to achieve optimal results (Berge et al., 2005). Among the different types of landfill bioreactors, hybrid bioreactor landfill technology is of great promise (Long et al., 2009a,b). Combination of both aerobic and anaerobic
http://dx.doi.org/10.1016/j.biortech.2014.03.036 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Xu, Q., et al. Methane production in simulated hybrid bioreactor landfill. Bioresour. Technol. (2014), http://dx.doi.org/ 10.1016/j.biortech.2014.03.036
2
Q. Xu et al. / Bioresource Technology xxx (2014) xxx–xxx
conditions is involved in hybrid landfill bioreactors. Aerobic conditions can facilitate degradation of organic matter and rapidly improve leachate quality (Nikolaou et al., 2010). In addition, anaerobic operation can produce methane which can be used for biogas-to-energy projects. Research has shown that operating hybrid systems served for in situ nitrogen removal (Shao et al., 2008). However, few researchers have investigated the effect of hybrid landfill on methane generation. This research investigated a hybrid bioreactor landfill technology using leachate recirculation and temporary aeration to a part of a simulated waste landfill (the upper layer of the compacted waste). The performance of the simulated hybrid bioreactor landfill was compared to the bioreactor with simple leachate recirculation.
2. Methods 2.1. Materials Each waste component was collected from Shenzhen University Town (Shenzhen, China) and was synthesized to represent the
typical MSW composition in Shenzhen. Collected waste components were screened and shredded to reduce particle size to less than 5 cm. The initial moisture content of the synthesized waste was 49.7%. Moisture content, volatile solid (VS), and total solid (TS) of each component are listed in Table 1. The total volatile solid of biodegradable components in each bioreactor was about 3.4 kg. 2.2. Experiment set-up Two laboratory-scale columns were constructed to simulate an anaerobic landfill bioreactor and hybrid landfill bioreactor. Fig. 1 shows the structure of the laboratory bioreactors. The bioreactor was constructed using 20-cm-diameter polyacrylic plastic pipe with a total height of 100 cm. A 5-cm thick gravel layer was placed at the bottom of each column as a drainage layer. A total 13.2 kg of synthesized MSW was loaded into each column. The bulk density of the compacted waste was 600 kg/m3. A layer of gravel was placed on the top of the loaded waste to facilitate the even distribution of recirculated leachate. In the hybrid bioreactor, a 10-cm thick aeration layer (gravel) was placed at the middle of the waste layer. To channel air into the aeration layer, a 0.5-cm diameter of
Table 1 Composition of the MSW in simulated bioreactors. Waste component
Food waste Inert Paper Plastics Glass Metal Total * **
% By wet weight Mass
Moisture content
Volatile solid (VS)
Total solid (TS)
VS/TS*
55.0 20.0 10.0 10.0 4.5 0.5 100
80.9 21.7 8.5 0.4 – 0.01 49.7
18.6 –** 7.0 – – –
19.1 78.3
0.96 – 0.83 – – –
99.6 100 99.9
The ratio of volatile solid to total solid. Not measured.
Fig. 1. Schematic of bioreactor landfills (a) anaerobic bioreactor and (b) hybrid bioreactor.
Please cite this article in press as: Xu, Q., et al. Methane production in simulated hybrid bioreactor landfill. Bioresour. Technol. (2014), http://dx.doi.org/ 10.1016/j.biortech.2014.03.036
Q. Xu et al. / Bioresource Technology xxx (2014) xxx–xxx
3
polyacrylic plastic tube was installed at the center of the upper waste layer in the hybrid bioreactor (Fig. 1b). A leachate injection port and a gas collection port were installed on the top of each column. A leachate collection port was located at the bottom of each column for leachate collection. At the beginning of the experiment, 1.5 L DI water was added into each bioreactor landfill to adjust the moisture content of the waste and promote the leachate production. Both columns were operated in a thermostatic room (25 ± 5 °C) for ten months. Leachate collected from each column was recirculated with a flow rate of 500 ml/day. For the hybrid bioreactor, air was injected for 4 h/day with the air flow rate of 30 L/h. The aeration stopped when the pH value of leachate achieved 7.0 (day 72) and then the hybrid bioreactor was operated under anaerobic conditions for landfill gas generation. 2.3. Sample collection and analysis Leachate was collected for sampling from the leachate collection port on a weekly basis, and leachate samples were analyzed for pH (Sartorius, PB-10, German), COD, BOD5, and ammonia– nitrogen. COD was measured by the fast digestion-spectrophotometric method (the Environmental Protection Industry Standard of the People’s Republic of China, HJ/T 399-2007). BOD5 was measured using dilution and seeding method (HJ 505-2009). Ammonia–nitrogen was analyzed by Nessler’s reagent colorimetric method (HJ 535-2009). Blanks, replicates, and calibration check samples were performed as appropriate. Gas generated from each column was collected by connecting a Tedlar bag to the gas port of the column. The volume of the collected gas was measured using a gas tight syringe (100 ml) at room temperature (25 ± 5 °C). The Tedlar bag was connected to the syringe via a three-port valve. The measured gas volume was adjusted for 0 °C. CH4 was analyzed by gas chromatograph (GC-9790, FULI, China) equipped with a Porapak Q column analytical system (3 m by 3 mm) connected to a thermal conductivity detector (TCD). The operational temperatures of injector, column and TCD were 50, 70 and 100 °C, respectively. The applied current was 110 mA. 3. Results and discussion 3.1. Leachate pH Fig. 2a presents the change of leachate pH during the experiment. The pH values in both bioreactors dropped to about 5.5 in the first three weeks, which indicated that the accumulation of organic acids occurred in the anaerobic bioreactor and the hybrid bioreactor. Leachate pH of the anaerobic bioreactor slowly increased and reached about 6.5 at the end of the study. In the hybrid bioreactor, however, pH quickly increased from 5.5 to over 7.0 within the first 50 days (Fig. 2a). After stopping aeration at pH 7 on day 72, pH of the leachate slightly increased and then fluctuated around 7.7. Aerobic microorganisms might grow in the hybrid bioreactor during the aeration process. The aerobes could quickly consume organic acids in the recirculated leachate and neutralize the pH of leachate. In this way, the upper aeration layer probably functioned as a buffer layer and helped increase leachate pH in the hybrid bioreactor. The results indicated that aeration could shorten the acid phase in the waste biodegradation process. Similar observations are also found in elsewhere (Erses et al., 2008; Sang et al., 2009).
Fig. 2. The change of leachate (a) pH and (b) ammonia–nitrogen concentrations over time.
3000 mg/L at the end of the experiment. The variation of ammonia–nitrogen in leachate of the hybrid bioreactor showed a similar trend with that of the anaerobic bioreactor. During the aeration process, ammonia–nitrogen concentration in the hybrid bioreactor increased rapidly from 160 mg/L to near 2500 mg/L. After aeration ceased, ammonia–nitrogen concentration kept increasing slightly and fluctuated around 3000 mg/L at the end of the study. Overall, the ammonia–nitrogen concentrations of the hybrid bioreactor were slightly greater than those of the anaerobic bioreactor. Under anaerobic conditions, ammonia–nitrogen in leachate tends to accumulate due to the lack of an ammonia degradation pathway (Long et al., 2009b). Besides, leachate recirculation can increase the ammonification rate, resulting in rapid accumulation of high levels of ammonia–nitrogen. For the hybrid bioreactor, it is considered that the aeration into the hybrid bioreactor facilitated the biodegradation of waste resulting in more ammonia–nitrogen dissolved in leachate. However, due to the relatively low air flow rate during aeration, the dissolution rate of ammonia–nitrogen into leachate was lower than the conversion rate of ammonia– nitrogen via nitrification or volatilization (Price et al., 2003; He and Shen, 2006). The results indicated that the aeration with the low air flow rate was not enough to reduce the level of ammonia.
3.3. Dissolved organic matter 3.2. Ammonia–nitrogen As shown in Fig. 2b, ammonia–nitrogen concentration in the anaerobic bioreactor accumulated and reached more than
The concentrations of COD and BOD5 monitored in the two bioreactors are shown in Fig. 3a and b. In each bioreactor, the leachate showed high concentrations of COD (>60,000 mg/L) and BOD5
Please cite this article in press as: Xu, Q., et al. Methane production in simulated hybrid bioreactor landfill. Bioresour. Technol. (2014), http://dx.doi.org/ 10.1016/j.biortech.2014.03.036
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Q. Xu et al. / Bioresource Technology xxx (2014) xxx–xxx
(>50,000 mg/L) at the beginning of the operation. The high concentrations of COD and BOD5 indicated that the waste had dissolved organic matter from the beginning of the experiments. In the anaerobic bioreactor, the COD concentrations fluctuated between 60,000 mg/L and 80,000 mg/L and the BOD5 concentrations also fluctuated between 50,000 mg/L and 60,000 mg/L (Fig. 3a). In contrast to the anaerobic bioreactor, the COD and BOD5 concentrations of the hybrid bioreactor decreased quickly during the aeration period (Fig. 3b). During aeration, the COD and BOD5 concentrations reduced by 65% and 80%, respectively. After ceasing aeration, the COD and BOD5 concentrations continued decreasing to 7100 mg/ L and 1900 mg/L at the end of the experiment. The BOD5/COD ratio is an important parameter to reflect the biodegradability of leachate and an indirect indicator of the 100000
(a)
COD BOD5
COD, BOD5 (mg/L)
80000
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stability of landfill bioreactors (Alvarez-Vazquez et al., 2004; Kjeldsen et al., 2002). As illustrated in Fig. 3c, the BOD5/COD ratio of the anaerobic bioreactor remained stable at about 0.7 during the experiment. However, the ratio of the hybrid bioreactor gradually decreased from initial value of 0.8 to about 0.3 at the end of the experiment. MSW in the hybrid bioreactor was stabilized more rapidly than that in the anaerobic bioreactor. The decreasing BOD5/COD ratio in the aeration period indicated that temporary aeration in the hybrid bioreactor enhanced MSW stabilization with comparison to the anaerobic bioreactor. 3.4. Landfill gas generation Landfill gas concentrations and methane production rates of the two bioreactors are presented in Fig. 4. The highest methane concentration measured in the biogas of the anaerobic bioreactor was 20% on day 43, then the methane concentration gradually went down. The methane production rate per waste mass (wet weight) in the anaerobic bioreactor was as low as 2 mL/day-kgwaste over the ten months of the experiment. The anaerobic bioreactor could not reach a stable methane production phase during that period. This result could largely be attributed to the long period of acid phase as shown in Fig. 2a. In the hybrid bioreactor, after pH reached 7.0 on day 72, air injection was stopped and the system was switched to an anaerobic bioreactor. The methane concentration of gas sharply increased to over 50% within a short time (9 days) and remained around 68% throughout the anaerobic operations. As shown in Fig. 4b, the
0 0
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Day Fig. 3. The change of COD and BOD5 of leachate (a) anaerobic bioreactor, (b) hybrid bioreactor, and (c) BOD5/COD ratio.
Fig. 4. Methane production from simulated bioreactors. (a) Methane concentration and (b) methane production rate.
Please cite this article in press as: Xu, Q., et al. Methane production in simulated hybrid bioreactor landfill. Bioresour. Technol. (2014), http://dx.doi.org/ 10.1016/j.biortech.2014.03.036
Q. Xu et al. / Bioresource Technology xxx (2014) xxx–xxx
methane production rate also increased quickly after aeration ceased and peaked on day 95 (219 mL/day-kgwaste). The rapid methane production in the hybrid bioreactor was attributed to aeration in the upper layer, which reduced leachate acid concentration. After aeration ceased, methanogenic conditions were quickly developed in the reactor with leachate recirculation, increasing the methane generation rate. The total volume of methane produced in the hybrid landfill bioreactor was over 400 times greater than that in the anaerobic bioreactor. For biogas-to-energy projects, not only the quantity but also the quality of the biogas is important. Typically, the minimum methane content required of the biogas is greater than 40% to be technically or economically feasible. Overall, 212 L of methane was generated from the hybrid bioreactor, and 97% of collected methane was found in biogas with methane concentrations above 50%. Also, the peak methane production completed within a short period of time. Of the total methane collected from the hybrid bioreactor, about 90% was collected from day 80 to day 200. From day 200 to day 300, the volume of methane generated comprised only 7% of the total methane volume. However, the anaerobic bioreactor generated only 0.5 L methane in total as a form of low quality biogas (methane
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Methane production in simulated hybrid bioreactor landfill Qiyong Xu a, Xiao Jin a, Zeyu Ma a, Huchun Tao b, Jae Hac Ko b,⇑ a b
Key Laboratory for Eco-efficient Polysilicate Materials, School of Environment and Energy, Peking University Shenzhen Graduate School, Guangdong 518055, China Key Laboratory for Heavy Metal Pollution Control and Reutilization, School of Environment and Energy, Peking University Shenzhen Graduate School, Guangdong 518055, China
h i g h l i g h t s Temporary aeration into the hybrid bioreactor improved leachate quality. pH adjustment by the aeration boosted the formation of methane generation phase. The hybrid bioreactor was economically feasible by the methane quality improvement.
a r t i c l e
i n f o
Article history: Received 3 December 2013 Received in revised form 5 March 2014 Accepted 8 March 2014 Available online xxxx Keywords: Bioreactor landfill Aeration Leachate recirculation Methane production
a b s t r a c t The aim of this work was to study a hybrid bioreactor landfill technology for landfill methane production from municipal solid waste. Two laboratory-scale columns were operated for about ten months to simulate an anaerobic and a hybrid landfill bioreactor, respectively. Leachate was recirculated into each column but aeration was conducted in the hybrid bioreactor during the first stage. Results showed that leachate pH in the anaerobic bioreactor maintained below 6.5, while in the hybrid bioreactor quickly increased from 5.6 to 7.0 due to the aeration. The temporary aeration resulted in lowering COD and BOD5 in the leachate. The volume of methane collected from the hybrid bioreactor was 400 times greater than that of the anaerobic bioreactor. Also, the methane production rate of the hybrid bioreactor was improved within a short period of time. After about 10 months’ operation, the total methane production in the hybrid bioreactor was 212 L (16 L/kgwaste). Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Landfilling is the most common method of municipal solid waste (MSW) disposal around the world, especially in developing countries (Giusti, 2009; Mor et al., 2006). In China, landfill has been the dominant way for MSW management because it is a cost-effective MSW management option compared with other options and can treat mixed MSW without separation (Chen et al., 2010; Zhang et al., 2010). According to statistical data, in 2010, the amount of MSW disposal was 131 million tons, of which 77% was landfilled (China Statistical Yearbook, 2012). However, it is estimated that the recovery rate of landfill gas generated from traditional landfills is less than 20% (Zhang et al., 2010). One of the main reasons is the relatively slow MSW degradation in traditional landfills (Xu and Ge, 2011). MSW degradation rate and methane production from MSW are mainly influenced by microorganisms, moisture conditions, and other inhibitory factors. One of the inhibitory factors is the imbalance between acidogenesis and methanogenesis, ⇑ Corresponding author. Tel.: +86 755 2603 3289. E-mail address: [email protected] (J.H. Ko).
which is often caused by the rapid hydrolysis of readily biodegradable materials in the beginning of anaerobic digestion. MSW composition in China is dominated by food waste, typically more than 50% by weight. In anaerobic conditions, the excess hydrolysis of easily biodegradable food waste can cause the accumulation of acids resulting in the decrease of pH. So, it may delay the formation of stable methane production phases in Chinese MSW landfills. In some cases, it took several years to start methane generation (O’Keefe and Chynoweth, 2000). Bioreactor landfill technology has been developed to accelerate the biodegradation of the biodegradable fractions and to enhance the stabilization of landfilled waste. The key step to create a landfill bioreactor is water addition or leachate recirculation to increase moisture content of MSW (Townsend et al., 1996; Reinhart et al., 2002; Benson et al., 2007; He et al., 2007; Kumar et al., 2011; Bilgili et al., 2012). In general, there are four types of landfill bioreactors: aerobic, anaerobic, facultative and hybrid systems, equipped with different operating schemes to achieve optimal results (Berge et al., 2005). Among the different types of landfill bioreactors, hybrid bioreactor landfill technology is of great promise (Long et al., 2009a,b). Combination of both aerobic and anaerobic
http://dx.doi.org/10.1016/j.biortech.2014.03.036 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Xu, Q., et al. Methane production in simulated hybrid bioreactor landfill. Bioresour. Technol. (2014), http://dx.doi.org/ 10.1016/j.biortech.2014.03.036
2
Q. Xu et al. / Bioresource Technology xxx (2014) xxx–xxx
conditions is involved in hybrid landfill bioreactors. Aerobic conditions can facilitate degradation of organic matter and rapidly improve leachate quality (Nikolaou et al., 2010). In addition, anaerobic operation can produce methane which can be used for biogas-to-energy projects. Research has shown that operating hybrid systems served for in situ nitrogen removal (Shao et al., 2008). However, few researchers have investigated the effect of hybrid landfill on methane generation. This research investigated a hybrid bioreactor landfill technology using leachate recirculation and temporary aeration to a part of a simulated waste landfill (the upper layer of the compacted waste). The performance of the simulated hybrid bioreactor landfill was compared to the bioreactor with simple leachate recirculation.
2. Methods 2.1. Materials Each waste component was collected from Shenzhen University Town (Shenzhen, China) and was synthesized to represent the
typical MSW composition in Shenzhen. Collected waste components were screened and shredded to reduce particle size to less than 5 cm. The initial moisture content of the synthesized waste was 49.7%. Moisture content, volatile solid (VS), and total solid (TS) of each component are listed in Table 1. The total volatile solid of biodegradable components in each bioreactor was about 3.4 kg. 2.2. Experiment set-up Two laboratory-scale columns were constructed to simulate an anaerobic landfill bioreactor and hybrid landfill bioreactor. Fig. 1 shows the structure of the laboratory bioreactors. The bioreactor was constructed using 20-cm-diameter polyacrylic plastic pipe with a total height of 100 cm. A 5-cm thick gravel layer was placed at the bottom of each column as a drainage layer. A total 13.2 kg of synthesized MSW was loaded into each column. The bulk density of the compacted waste was 600 kg/m3. A layer of gravel was placed on the top of the loaded waste to facilitate the even distribution of recirculated leachate. In the hybrid bioreactor, a 10-cm thick aeration layer (gravel) was placed at the middle of the waste layer. To channel air into the aeration layer, a 0.5-cm diameter of
Table 1 Composition of the MSW in simulated bioreactors. Waste component
Food waste Inert Paper Plastics Glass Metal Total * **
% By wet weight Mass
Moisture content
Volatile solid (VS)
Total solid (TS)
VS/TS*
55.0 20.0 10.0 10.0 4.5 0.5 100
80.9 21.7 8.5 0.4 – 0.01 49.7
18.6 –** 7.0 – – –
19.1 78.3
0.96 – 0.83 – – –
99.6 100 99.9
The ratio of volatile solid to total solid. Not measured.
Fig. 1. Schematic of bioreactor landfills (a) anaerobic bioreactor and (b) hybrid bioreactor.
Please cite this article in press as: Xu, Q., et al. Methane production in simulated hybrid bioreactor landfill. Bioresour. Technol. (2014), http://dx.doi.org/ 10.1016/j.biortech.2014.03.036
Q. Xu et al. / Bioresource Technology xxx (2014) xxx–xxx
3
polyacrylic plastic tube was installed at the center of the upper waste layer in the hybrid bioreactor (Fig. 1b). A leachate injection port and a gas collection port were installed on the top of each column. A leachate collection port was located at the bottom of each column for leachate collection. At the beginning of the experiment, 1.5 L DI water was added into each bioreactor landfill to adjust the moisture content of the waste and promote the leachate production. Both columns were operated in a thermostatic room (25 ± 5 °C) for ten months. Leachate collected from each column was recirculated with a flow rate of 500 ml/day. For the hybrid bioreactor, air was injected for 4 h/day with the air flow rate of 30 L/h. The aeration stopped when the pH value of leachate achieved 7.0 (day 72) and then the hybrid bioreactor was operated under anaerobic conditions for landfill gas generation. 2.3. Sample collection and analysis Leachate was collected for sampling from the leachate collection port on a weekly basis, and leachate samples were analyzed for pH (Sartorius, PB-10, German), COD, BOD5, and ammonia– nitrogen. COD was measured by the fast digestion-spectrophotometric method (the Environmental Protection Industry Standard of the People’s Republic of China, HJ/T 399-2007). BOD5 was measured using dilution and seeding method (HJ 505-2009). Ammonia–nitrogen was analyzed by Nessler’s reagent colorimetric method (HJ 535-2009). Blanks, replicates, and calibration check samples were performed as appropriate. Gas generated from each column was collected by connecting a Tedlar bag to the gas port of the column. The volume of the collected gas was measured using a gas tight syringe (100 ml) at room temperature (25 ± 5 °C). The Tedlar bag was connected to the syringe via a three-port valve. The measured gas volume was adjusted for 0 °C. CH4 was analyzed by gas chromatograph (GC-9790, FULI, China) equipped with a Porapak Q column analytical system (3 m by 3 mm) connected to a thermal conductivity detector (TCD). The operational temperatures of injector, column and TCD were 50, 70 and 100 °C, respectively. The applied current was 110 mA. 3. Results and discussion 3.1. Leachate pH Fig. 2a presents the change of leachate pH during the experiment. The pH values in both bioreactors dropped to about 5.5 in the first three weeks, which indicated that the accumulation of organic acids occurred in the anaerobic bioreactor and the hybrid bioreactor. Leachate pH of the anaerobic bioreactor slowly increased and reached about 6.5 at the end of the study. In the hybrid bioreactor, however, pH quickly increased from 5.5 to over 7.0 within the first 50 days (Fig. 2a). After stopping aeration at pH 7 on day 72, pH of the leachate slightly increased and then fluctuated around 7.7. Aerobic microorganisms might grow in the hybrid bioreactor during the aeration process. The aerobes could quickly consume organic acids in the recirculated leachate and neutralize the pH of leachate. In this way, the upper aeration layer probably functioned as a buffer layer and helped increase leachate pH in the hybrid bioreactor. The results indicated that aeration could shorten the acid phase in the waste biodegradation process. Similar observations are also found in elsewhere (Erses et al., 2008; Sang et al., 2009).
Fig. 2. The change of leachate (a) pH and (b) ammonia–nitrogen concentrations over time.
3000 mg/L at the end of the experiment. The variation of ammonia–nitrogen in leachate of the hybrid bioreactor showed a similar trend with that of the anaerobic bioreactor. During the aeration process, ammonia–nitrogen concentration in the hybrid bioreactor increased rapidly from 160 mg/L to near 2500 mg/L. After aeration ceased, ammonia–nitrogen concentration kept increasing slightly and fluctuated around 3000 mg/L at the end of the study. Overall, the ammonia–nitrogen concentrations of the hybrid bioreactor were slightly greater than those of the anaerobic bioreactor. Under anaerobic conditions, ammonia–nitrogen in leachate tends to accumulate due to the lack of an ammonia degradation pathway (Long et al., 2009b). Besides, leachate recirculation can increase the ammonification rate, resulting in rapid accumulation of high levels of ammonia–nitrogen. For the hybrid bioreactor, it is considered that the aeration into the hybrid bioreactor facilitated the biodegradation of waste resulting in more ammonia–nitrogen dissolved in leachate. However, due to the relatively low air flow rate during aeration, the dissolution rate of ammonia–nitrogen into leachate was lower than the conversion rate of ammonia– nitrogen via nitrification or volatilization (Price et al., 2003; He and Shen, 2006). The results indicated that the aeration with the low air flow rate was not enough to reduce the level of ammonia.
3.3. Dissolved organic matter 3.2. Ammonia–nitrogen As shown in Fig. 2b, ammonia–nitrogen concentration in the anaerobic bioreactor accumulated and reached more than
The concentrations of COD and BOD5 monitored in the two bioreactors are shown in Fig. 3a and b. In each bioreactor, the leachate showed high concentrations of COD (>60,000 mg/L) and BOD5
Please cite this article in press as: Xu, Q., et al. Methane production in simulated hybrid bioreactor landfill. Bioresour. Technol. (2014), http://dx.doi.org/ 10.1016/j.biortech.2014.03.036
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Q. Xu et al. / Bioresource Technology xxx (2014) xxx–xxx
(>50,000 mg/L) at the beginning of the operation. The high concentrations of COD and BOD5 indicated that the waste had dissolved organic matter from the beginning of the experiments. In the anaerobic bioreactor, the COD concentrations fluctuated between 60,000 mg/L and 80,000 mg/L and the BOD5 concentrations also fluctuated between 50,000 mg/L and 60,000 mg/L (Fig. 3a). In contrast to the anaerobic bioreactor, the COD and BOD5 concentrations of the hybrid bioreactor decreased quickly during the aeration period (Fig. 3b). During aeration, the COD and BOD5 concentrations reduced by 65% and 80%, respectively. After ceasing aeration, the COD and BOD5 concentrations continued decreasing to 7100 mg/ L and 1900 mg/L at the end of the experiment. The BOD5/COD ratio is an important parameter to reflect the biodegradability of leachate and an indirect indicator of the 100000
(a)
COD BOD5
COD, BOD5 (mg/L)
80000
60000
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stability of landfill bioreactors (Alvarez-Vazquez et al., 2004; Kjeldsen et al., 2002). As illustrated in Fig. 3c, the BOD5/COD ratio of the anaerobic bioreactor remained stable at about 0.7 during the experiment. However, the ratio of the hybrid bioreactor gradually decreased from initial value of 0.8 to about 0.3 at the end of the experiment. MSW in the hybrid bioreactor was stabilized more rapidly than that in the anaerobic bioreactor. The decreasing BOD5/COD ratio in the aeration period indicated that temporary aeration in the hybrid bioreactor enhanced MSW stabilization with comparison to the anaerobic bioreactor. 3.4. Landfill gas generation Landfill gas concentrations and methane production rates of the two bioreactors are presented in Fig. 4. The highest methane concentration measured in the biogas of the anaerobic bioreactor was 20% on day 43, then the methane concentration gradually went down. The methane production rate per waste mass (wet weight) in the anaerobic bioreactor was as low as 2 mL/day-kgwaste over the ten months of the experiment. The anaerobic bioreactor could not reach a stable methane production phase during that period. This result could largely be attributed to the long period of acid phase as shown in Fig. 2a. In the hybrid bioreactor, after pH reached 7.0 on day 72, air injection was stopped and the system was switched to an anaerobic bioreactor. The methane concentration of gas sharply increased to over 50% within a short time (9 days) and remained around 68% throughout the anaerobic operations. As shown in Fig. 4b, the
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Fig. 4. Methane production from simulated bioreactors. (a) Methane concentration and (b) methane production rate.
Please cite this article in press as: Xu, Q., et al. Methane production in simulated hybrid bioreactor landfill. Bioresour. Technol. (2014), http://dx.doi.org/ 10.1016/j.biortech.2014.03.036
Q. Xu et al. / Bioresource Technology xxx (2014) xxx–xxx
methane production rate also increased quickly after aeration ceased and peaked on day 95 (219 mL/day-kgwaste). The rapid methane production in the hybrid bioreactor was attributed to aeration in the upper layer, which reduced leachate acid concentration. After aeration ceased, methanogenic conditions were quickly developed in the reactor with leachate recirculation, increasing the methane generation rate. The total volume of methane produced in the hybrid landfill bioreactor was over 400 times greater than that in the anaerobic bioreactor. For biogas-to-energy projects, not only the quantity but also the quality of the biogas is important. Typically, the minimum methane content required of the biogas is greater than 40% to be technically or economically feasible. Overall, 212 L of methane was generated from the hybrid bioreactor, and 97% of collected methane was found in biogas with methane concentrations above 50%. Also, the peak methane production completed within a short period of time. Of the total methane collected from the hybrid bioreactor, about 90% was collected from day 80 to day 200. From day 200 to day 300, the volume of methane generated comprised only 7% of the total methane volume. However, the anaerobic bioreactor generated only 0.5 L methane in total as a form of low quality biogas (methane
