Bioresource Technology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Effective anaerobic biodegradation of municipal solid waste fresh leachate using a novel pilot-scale reactor: Comparison under different seeding granular sludge Jinghuan Luo, Jizhi Zhou, Guangren Qian, Jianyong Liu ⇑ School of Environmental and Chemical Engineering, Shanghai University, 333 Nanchen Road, Shanghai 200444, PR China
h i g h l i g h t s A novel IIEC reactor was developed with integration of EGSB and IC construction. 3
The pilot-scale (10–15 m /d) reactor could effectively treat MSW leachate. 3
The OLR was 23.0–40.5 kg COD/m d, much higher than others’ results. Granular sludge derived from leachate-like-wastewater performed better.
a r t i c l e
i n f o
Article history: Received 23 December 2013 Received in revised form 25 March 2014 Accepted 26 March 2014 Available online xxxx Keywords: IIEC reactor MSW leachate Biodegradation Methane production Granular sludge
a b s t r a c t A novel integrated internal and external circulation (IIEC) reactor was developed for anaerobic biodegradation of municipal solid waste (MSW) fresh leachate with chemical oxygen demand (COD) between 40,000 and 60,000 mg/l. The pilot-scale IIEC reactor was inoculated with two kinds of granular sludge from paper mill (SPM) and from citric acid factory (SCF), respectively. The bio-treating capacity in contaminant removal and biogas production performed much superior to others’ results, principally attributed to appropriate configuration modification. Compared to SCF, much higher organic loading rate (40.5 vs 23.0 kg COD/m3 d) and COD removal efficiency (>80% vs 60–75%) were achieved for the reactor with SPM. For methane production, 11.77 or 6 m3STP/m3 d of rate and 66–85% of content were observed with SPM or SCF, respectively. Due to better sludge concentrations and methanogenic activity, these findings indicate the anaerobic reactor could effectively bio-treat MSW leachate for methane generation, especially inoculated with granular sludge derived from leachate-like-wastewater. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Continuing industrial and commercial growth, increasingly affluent lifestyles, coupled with accelerated product obsolescence and ubiquitous wastefulness tendency, all contribute to the drastically increasing generation of municipal solid waste (MSW) (Ahmed and Lan, 2012). Accompanying with staggering amounts of MSW, leachate, always characteristically rich in organic species, ammonium, heavy metals and other contaminants (Renou et al., 2008), has also become one of the most concerned issues in solid waste management. If simply discharged, MSW leachate will often severely cause destruction to its receiving medium (Sanphoti et al., 2006; Oman and Junestedt, 2008). Thus the effective treatment of MSW leachate has been drawing more and more attentions world⇑ Corresponding author. Tel.: +86 21 66137748; fax: +86 21 66137761. E-mail address: [email protected] (J. Liu).
wide, particularly for the fresh leachate generated during the early stage of solid wastes dumping or fermentation. Among various alternatives, anaerobic digestion not only removes most pollutants without massive sludge production, but also produces bio-methane for renewable energy (Zhou et al., 2007; Trzcinski and Stuckey, 2010), serving as a cost-effective technology for the treatment of such high-strength organic wastewater. Several anaerobic processes, such as up-flow anaerobic sludge blanket (UASB) reactor (Peng et al., 2008; Ye et al., 2011) and anaerobic moving-bed biofilm reactor (MBBR) (Chen et al., 2008), have been demonstrated effective (chemical oxygen demand (COD) removal efficiency >80% and average methane yield >0.3 m3/kg CODremoved under organic loading rates (OLRs) between 10 and 20 kg COD/m3 d) when treating MSW leachate with COD concentrations higher than 10,000 mg/l. Furthermore, under pilot-scale studies, Turan et al. (2005) and Zayen et al. (2010) also achieved similar favorable results when employing anaerobic
http://dx.doi.org/10.1016/j.biortech.2014.03.141 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Luo, J., et al. Effective anaerobic biodegradation of municipal solid waste fresh leachate using a novel pilot-scale reactor: Comparison under different seeding granular sludge. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.141
2
J. Luo et al. / Bioresource Technology xxx (2014) xxx–xxx
fluidized bed reactor (AFBR) and anaerobic membrane bioreactor (AnMBR), respectively. Recently, the third generation of anaerobic reactors (e.g., expanded granular sludge bed (EGSB) reactor or internal circulation (IC) reactor with an elevated height/diameter ratio and a recirculation unit for much higher up-flow velocity), modified from the former UASB construction, have received more concerns. Higher OLR of 20 kg COD/m3 d and COD removal efficiency of 90% were reported with bench-scale EGSB reactor treating fresh leachate up to 70,000 mg COD/l (Liu et al., 2010; Dang et al., 2013). However, there have hardly been studies using this kind of pilot-scale reactor to treat MSW leachate, although some on-site UASB reactors fed with only low-concentration leachate (2 mm 1–2 mm 0.5–1 mm 0.1–0.5 mm >2 mm 1–2 mm 0.5–1 mm 0.1–0.5 mm
SPM
SCF
30.1 59.9 4.9 5.1 159 108 64 28 116,478 86,844 74.6 38
7.2 89.0 1.0 0.9 135 91 34 20 57,050 53,683 94.1 50
Please cite this article in press as: Luo, J., et al. Effective anaerobic biodegradation of municipal solid waste fresh leachate using a novel pilot-scale reactor: Comparison under different seeding granular sludge. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.141
J. Luo et al. / Bioresource Technology xxx (2014) xxx–xxx
3
Fig. 1. Schematic diagram of the IIEC bioreactor employed to treat MSW leachate: (1) Leachate pool, (2) Pump, (3) Liquid flowmeter, (4) Buffer unit, (5) pH meter, (6) Thermostat & heating resistor, (7) Reacting unit, (8) Gas–liquid–solid separator, (9) Internal circulator, (10) External circulator, (11) pH & thermo meter, (12) Gas–liquid separator, (13) Water-sealed tank, (14) Gas flowmeter, (15) Drain.
using a GC9800 gas chromatography with a stainless steel column (1 m * 3 mm) packed with TD-01 and a thermal conductivity detector. The diameter distribution by wet sieving method, settling velocity by gravimetric method and SMA by fermentation method with serum bottle for granular sludge were all detailed in our previous work (Liu et al., 2011). 3. Results and discussion 3.1. Physicochemical characteristics of the seeding sludge Table 1 presents the main characteristics of two kinds of anaerobic seeding granular sludge sampled from a paper mill and a citric acid factory. The diameter of the seeding sludge both dominated in the range of >1 mm, accounting for 90.0% of SPM and 96.2% of SCF, respectively. But SPM > 2 mm occupied 30.1%, much more than that of SCF with only 7.2%. Proportional to the particle diameter, the settling velocity of two kinds of granules increased from 20 to 159 m/h, each with satisfactory settling property. Besides, SPM performed higher settling velocity under the identical particle size when compared to SCF. Liu et al. (2006) have also found that larger granule behaved higher mean settling velocities in 4–140 m/h and thus considerable sludge washout was avoided even applied high hydraulic loads. Based on their experimental results and available literature data, they further developed a theoretical model regarding the settling process of the granular sludge, as shown in function (1).
h i0:714 1:6 ut ¼ 0:781 dp ðqp qÞ=q0:4 l0:6
ð1Þ
where ut the settling velocity of the granule (m/h), dp the granule diameter by wet sieving (mm), qp the density of the granule (kg/m3), q the density of the wastewater (kg/m3), and l the fluid viscosity (Pa s). As also shown in the table, the SS and VSS concentrations for SPM were much higher than SCF yet with the lower VSS/SS ratio (i.e., 74.6% vs 94.1%), similar to the findings when comparing the sludge from paper mill with distillery (Pevere et al., 2007; van Hullebusch et al., 2007). It is also notable that the SMA of SPM was only 38 ml CH4/g VSS d, worse than SCF of 50 ml CH4/g VSS d.
3.2. OLR and COD removal Fig. 2 shows changes of OLR, influent/effluent COD concentration and COD removal efficiency of the IIEC bioreactor treating MSW leachate when inoculated with SPM and SCF, respectively, during the whole running process. From Fig. 2a, we find that the reactor inoculated with SPM (denoted as RPM) could ultimately achieve an OLR of 40.5 kg COD/m3 d from initial 5.0 kg COD/m3 d, when fed with raw leachate stepwise from 1.7 to 15 m3/d (HRT reduced from 11 to 1 d). While for the reactor inoculated with SCF (denoted as RCF), the OLR only achieved 23.0 kg COD/m3 d under the final influent of 10 m3/d (HRT from 15 to 2 d). Note that here the high-strength raw leachate from the pool was fed directly into the bioreactor system (or the buffer unit) without any external dilution even at its initial start-up stage, unlike other works did (Liu et al., 2010; Dang et al., 2013). In terms of COD removal, the effluent COD concentration from RPM was mainly in the range of 7000–10,000 mg/l, except for days 16–23 somewhat higher than 10,000 mg/l. Thus, the average COD removal efficiency of RPM was above 80% although with raw leachate around 50,000 mg COD/l (Fig. 2b). For RCF, the influent COD dropped gradually from 60,000 to 40,000 mg/l during the running process, yet the effluent inversely increased. At the start-up stage (days 1–9), the effluent COD was all below 10,000 mg/l, after which the concentration even went up to about 19,000 mg/l especially on days 24–26. This resulted in COD removal efficiency mostly ranging between 60% and 75% with a decline tendency, indicating the micro-community from SCF performed poorer activity than SPM when bio-treating high-strength MSW leachate. However, the RCF could still restore 75% of COD removal efficiency (corresponding to 10,000 mg COD/l of effluent) when fed with 10 m3/d of leachate. Obviously, the IIEC reactor inoculated with SPM or SCF was capable of bio-treating raw MSW leachate of 40,000–60,000 mg COD/l as influent without any dilution. The reactor performed much better than the result (only 18 kg COD/m3 d of OLR albeit with 86.7% of COD removal) obtained from a bench-scale EGSB bioreactor under similar conditions (Dang et al., 2013). This improvement is probably in principle attributed to much higher up-flow velocity (integrated internal and external circulation) and much better
Please cite this article in press as: Luo, J., et al. Effective anaerobic biodegradation of municipal solid waste fresh leachate using a novel pilot-scale reactor: Comparison under different seeding granular sludge. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.141
J. Luo et al. / Bioresource Technology xxx (2014) xxx–xxx
RPM
50 40
RCF
a
30 20 10 0
Influent
60000 50000 40000 30000 20000 10000 0
Effluent
Removal
100 90
b
80 RPM
70 60 50 Influent
70000 60000 50000 40000 30000 20000 10000 0
Effluent
Removal
100 90
c
Removal (%)
COD (mg/l)
3
OLR (kg COD/m .d)
4
80 RCF
70 60
0
10
20
30
40
50 50
Time (d) Fig. 2. (a) OLR of RPM and RCF; and COD concentration in influent/effluent and COD removal efficiency of (b) RPM and (c) RCF.
gas–liquid–sludge separation (two-stage triphase separators) for the reactor herein. Thus more efficient hydraulic mixing (e.g., larger sludge-organics contact) and easier biomass accumulation could be achieved. For another, higher OLR and COD removal for MSW leachate treatment could be observed in RPM compared to RCF, suggesting that SPM was more adaptive with respect to such anaerobic process. This may be primarily ascribed to disparate microbial acclimatization of seeding sludge to high-strength and toxic MSW leachate, since they have been cultivated from composition-distinct-wastewater treatment process. Unlike low-toxic citric acid effluent only heavily loaded with organic pollutants (Li et al., 2013), paper mill effluent always unintentionally generates various hazardous compounds (chlorinated phenols, surfactants, biocides, dibenzop-dioxins, etc.) and heavy metals (Zn, Cr, Pb, Hg, etc.) (Chandra and Singh, 2012; Mishra et al., 2013) as the leachate does (Oman and Junestedt, 2008). Hence, SPM is reasonably supposed to have already acclimatized to such severe circumstance and the bioreactor with that leachate-like-wastewater derived inoculum should perform better for MSW leachate treatment. Besides, higher VSS concentration and settling velocity as well as larger particle diameter of SPM (Table 1) resulted in more microbial biomass and easier substance transfer inside the RPM, also contributing to its much better effectiveness in anaerobic process. 3.3. CH4 production CH4 production rate from RPM at the standard temperature and pressure (STP) was monitored, as shown in Fig. 3a. An increase of CH4 production from 1.45 to ca. 11 m3STP/m3 d was recorded with ascending OLR and the maximal rate of 11.77 m3STP/m3 d was observed at the highest OLR of 40.5 kg COD/m3 d. The CH4 content in the produced biogas did not fluctuate largely, mostly in the range of 72–80%, whereas the CO2 was mainly the remaining 20–28% (Fig. 3b). These steady biogas components suggest that the property of microbe inside the bioreactor was relatively stable during the process. We also note that the CH4 content is a bit higher than that (
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Effective anaerobic biodegradation of municipal solid waste fresh leachate using a novel pilot-scale reactor: Comparison under different seeding granular sludge Jinghuan Luo, Jizhi Zhou, Guangren Qian, Jianyong Liu ⇑ School of Environmental and Chemical Engineering, Shanghai University, 333 Nanchen Road, Shanghai 200444, PR China
h i g h l i g h t s A novel IIEC reactor was developed with integration of EGSB and IC construction. 3
The pilot-scale (10–15 m /d) reactor could effectively treat MSW leachate. 3
The OLR was 23.0–40.5 kg COD/m d, much higher than others’ results. Granular sludge derived from leachate-like-wastewater performed better.
a r t i c l e
i n f o
Article history: Received 23 December 2013 Received in revised form 25 March 2014 Accepted 26 March 2014 Available online xxxx Keywords: IIEC reactor MSW leachate Biodegradation Methane production Granular sludge
a b s t r a c t A novel integrated internal and external circulation (IIEC) reactor was developed for anaerobic biodegradation of municipal solid waste (MSW) fresh leachate with chemical oxygen demand (COD) between 40,000 and 60,000 mg/l. The pilot-scale IIEC reactor was inoculated with two kinds of granular sludge from paper mill (SPM) and from citric acid factory (SCF), respectively. The bio-treating capacity in contaminant removal and biogas production performed much superior to others’ results, principally attributed to appropriate configuration modification. Compared to SCF, much higher organic loading rate (40.5 vs 23.0 kg COD/m3 d) and COD removal efficiency (>80% vs 60–75%) were achieved for the reactor with SPM. For methane production, 11.77 or 6 m3STP/m3 d of rate and 66–85% of content were observed with SPM or SCF, respectively. Due to better sludge concentrations and methanogenic activity, these findings indicate the anaerobic reactor could effectively bio-treat MSW leachate for methane generation, especially inoculated with granular sludge derived from leachate-like-wastewater. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Continuing industrial and commercial growth, increasingly affluent lifestyles, coupled with accelerated product obsolescence and ubiquitous wastefulness tendency, all contribute to the drastically increasing generation of municipal solid waste (MSW) (Ahmed and Lan, 2012). Accompanying with staggering amounts of MSW, leachate, always characteristically rich in organic species, ammonium, heavy metals and other contaminants (Renou et al., 2008), has also become one of the most concerned issues in solid waste management. If simply discharged, MSW leachate will often severely cause destruction to its receiving medium (Sanphoti et al., 2006; Oman and Junestedt, 2008). Thus the effective treatment of MSW leachate has been drawing more and more attentions world⇑ Corresponding author. Tel.: +86 21 66137748; fax: +86 21 66137761. E-mail address: [email protected] (J. Liu).
wide, particularly for the fresh leachate generated during the early stage of solid wastes dumping or fermentation. Among various alternatives, anaerobic digestion not only removes most pollutants without massive sludge production, but also produces bio-methane for renewable energy (Zhou et al., 2007; Trzcinski and Stuckey, 2010), serving as a cost-effective technology for the treatment of such high-strength organic wastewater. Several anaerobic processes, such as up-flow anaerobic sludge blanket (UASB) reactor (Peng et al., 2008; Ye et al., 2011) and anaerobic moving-bed biofilm reactor (MBBR) (Chen et al., 2008), have been demonstrated effective (chemical oxygen demand (COD) removal efficiency >80% and average methane yield >0.3 m3/kg CODremoved under organic loading rates (OLRs) between 10 and 20 kg COD/m3 d) when treating MSW leachate with COD concentrations higher than 10,000 mg/l. Furthermore, under pilot-scale studies, Turan et al. (2005) and Zayen et al. (2010) also achieved similar favorable results when employing anaerobic
http://dx.doi.org/10.1016/j.biortech.2014.03.141 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Luo, J., et al. Effective anaerobic biodegradation of municipal solid waste fresh leachate using a novel pilot-scale reactor: Comparison under different seeding granular sludge. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.141
2
J. Luo et al. / Bioresource Technology xxx (2014) xxx–xxx
fluidized bed reactor (AFBR) and anaerobic membrane bioreactor (AnMBR), respectively. Recently, the third generation of anaerobic reactors (e.g., expanded granular sludge bed (EGSB) reactor or internal circulation (IC) reactor with an elevated height/diameter ratio and a recirculation unit for much higher up-flow velocity), modified from the former UASB construction, have received more concerns. Higher OLR of 20 kg COD/m3 d and COD removal efficiency of 90% were reported with bench-scale EGSB reactor treating fresh leachate up to 70,000 mg COD/l (Liu et al., 2010; Dang et al., 2013). However, there have hardly been studies using this kind of pilot-scale reactor to treat MSW leachate, although some on-site UASB reactors fed with only low-concentration leachate (2 mm 1–2 mm 0.5–1 mm 0.1–0.5 mm >2 mm 1–2 mm 0.5–1 mm 0.1–0.5 mm
SPM
SCF
30.1 59.9 4.9 5.1 159 108 64 28 116,478 86,844 74.6 38
7.2 89.0 1.0 0.9 135 91 34 20 57,050 53,683 94.1 50
Please cite this article in press as: Luo, J., et al. Effective anaerobic biodegradation of municipal solid waste fresh leachate using a novel pilot-scale reactor: Comparison under different seeding granular sludge. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.141
J. Luo et al. / Bioresource Technology xxx (2014) xxx–xxx
3
Fig. 1. Schematic diagram of the IIEC bioreactor employed to treat MSW leachate: (1) Leachate pool, (2) Pump, (3) Liquid flowmeter, (4) Buffer unit, (5) pH meter, (6) Thermostat & heating resistor, (7) Reacting unit, (8) Gas–liquid–solid separator, (9) Internal circulator, (10) External circulator, (11) pH & thermo meter, (12) Gas–liquid separator, (13) Water-sealed tank, (14) Gas flowmeter, (15) Drain.
using a GC9800 gas chromatography with a stainless steel column (1 m * 3 mm) packed with TD-01 and a thermal conductivity detector. The diameter distribution by wet sieving method, settling velocity by gravimetric method and SMA by fermentation method with serum bottle for granular sludge were all detailed in our previous work (Liu et al., 2011). 3. Results and discussion 3.1. Physicochemical characteristics of the seeding sludge Table 1 presents the main characteristics of two kinds of anaerobic seeding granular sludge sampled from a paper mill and a citric acid factory. The diameter of the seeding sludge both dominated in the range of >1 mm, accounting for 90.0% of SPM and 96.2% of SCF, respectively. But SPM > 2 mm occupied 30.1%, much more than that of SCF with only 7.2%. Proportional to the particle diameter, the settling velocity of two kinds of granules increased from 20 to 159 m/h, each with satisfactory settling property. Besides, SPM performed higher settling velocity under the identical particle size when compared to SCF. Liu et al. (2006) have also found that larger granule behaved higher mean settling velocities in 4–140 m/h and thus considerable sludge washout was avoided even applied high hydraulic loads. Based on their experimental results and available literature data, they further developed a theoretical model regarding the settling process of the granular sludge, as shown in function (1).
h i0:714 1:6 ut ¼ 0:781 dp ðqp qÞ=q0:4 l0:6
ð1Þ
where ut the settling velocity of the granule (m/h), dp the granule diameter by wet sieving (mm), qp the density of the granule (kg/m3), q the density of the wastewater (kg/m3), and l the fluid viscosity (Pa s). As also shown in the table, the SS and VSS concentrations for SPM were much higher than SCF yet with the lower VSS/SS ratio (i.e., 74.6% vs 94.1%), similar to the findings when comparing the sludge from paper mill with distillery (Pevere et al., 2007; van Hullebusch et al., 2007). It is also notable that the SMA of SPM was only 38 ml CH4/g VSS d, worse than SCF of 50 ml CH4/g VSS d.
3.2. OLR and COD removal Fig. 2 shows changes of OLR, influent/effluent COD concentration and COD removal efficiency of the IIEC bioreactor treating MSW leachate when inoculated with SPM and SCF, respectively, during the whole running process. From Fig. 2a, we find that the reactor inoculated with SPM (denoted as RPM) could ultimately achieve an OLR of 40.5 kg COD/m3 d from initial 5.0 kg COD/m3 d, when fed with raw leachate stepwise from 1.7 to 15 m3/d (HRT reduced from 11 to 1 d). While for the reactor inoculated with SCF (denoted as RCF), the OLR only achieved 23.0 kg COD/m3 d under the final influent of 10 m3/d (HRT from 15 to 2 d). Note that here the high-strength raw leachate from the pool was fed directly into the bioreactor system (or the buffer unit) without any external dilution even at its initial start-up stage, unlike other works did (Liu et al., 2010; Dang et al., 2013). In terms of COD removal, the effluent COD concentration from RPM was mainly in the range of 7000–10,000 mg/l, except for days 16–23 somewhat higher than 10,000 mg/l. Thus, the average COD removal efficiency of RPM was above 80% although with raw leachate around 50,000 mg COD/l (Fig. 2b). For RCF, the influent COD dropped gradually from 60,000 to 40,000 mg/l during the running process, yet the effluent inversely increased. At the start-up stage (days 1–9), the effluent COD was all below 10,000 mg/l, after which the concentration even went up to about 19,000 mg/l especially on days 24–26. This resulted in COD removal efficiency mostly ranging between 60% and 75% with a decline tendency, indicating the micro-community from SCF performed poorer activity than SPM when bio-treating high-strength MSW leachate. However, the RCF could still restore 75% of COD removal efficiency (corresponding to 10,000 mg COD/l of effluent) when fed with 10 m3/d of leachate. Obviously, the IIEC reactor inoculated with SPM or SCF was capable of bio-treating raw MSW leachate of 40,000–60,000 mg COD/l as influent without any dilution. The reactor performed much better than the result (only 18 kg COD/m3 d of OLR albeit with 86.7% of COD removal) obtained from a bench-scale EGSB bioreactor under similar conditions (Dang et al., 2013). This improvement is probably in principle attributed to much higher up-flow velocity (integrated internal and external circulation) and much better
Please cite this article in press as: Luo, J., et al. Effective anaerobic biodegradation of municipal solid waste fresh leachate using a novel pilot-scale reactor: Comparison under different seeding granular sludge. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.141
J. Luo et al. / Bioresource Technology xxx (2014) xxx–xxx
RPM
50 40
RCF
a
30 20 10 0
Influent
60000 50000 40000 30000 20000 10000 0
Effluent
Removal
100 90
b
80 RPM
70 60 50 Influent
70000 60000 50000 40000 30000 20000 10000 0
Effluent
Removal
100 90
c
Removal (%)
COD (mg/l)
3
OLR (kg COD/m .d)
4
80 RCF
70 60
0
10
20
30
40
50 50
Time (d) Fig. 2. (a) OLR of RPM and RCF; and COD concentration in influent/effluent and COD removal efficiency of (b) RPM and (c) RCF.
gas–liquid–sludge separation (two-stage triphase separators) for the reactor herein. Thus more efficient hydraulic mixing (e.g., larger sludge-organics contact) and easier biomass accumulation could be achieved. For another, higher OLR and COD removal for MSW leachate treatment could be observed in RPM compared to RCF, suggesting that SPM was more adaptive with respect to such anaerobic process. This may be primarily ascribed to disparate microbial acclimatization of seeding sludge to high-strength and toxic MSW leachate, since they have been cultivated from composition-distinct-wastewater treatment process. Unlike low-toxic citric acid effluent only heavily loaded with organic pollutants (Li et al., 2013), paper mill effluent always unintentionally generates various hazardous compounds (chlorinated phenols, surfactants, biocides, dibenzop-dioxins, etc.) and heavy metals (Zn, Cr, Pb, Hg, etc.) (Chandra and Singh, 2012; Mishra et al., 2013) as the leachate does (Oman and Junestedt, 2008). Hence, SPM is reasonably supposed to have already acclimatized to such severe circumstance and the bioreactor with that leachate-like-wastewater derived inoculum should perform better for MSW leachate treatment. Besides, higher VSS concentration and settling velocity as well as larger particle diameter of SPM (Table 1) resulted in more microbial biomass and easier substance transfer inside the RPM, also contributing to its much better effectiveness in anaerobic process. 3.3. CH4 production CH4 production rate from RPM at the standard temperature and pressure (STP) was monitored, as shown in Fig. 3a. An increase of CH4 production from 1.45 to ca. 11 m3STP/m3 d was recorded with ascending OLR and the maximal rate of 11.77 m3STP/m3 d was observed at the highest OLR of 40.5 kg COD/m3 d. The CH4 content in the produced biogas did not fluctuate largely, mostly in the range of 72–80%, whereas the CO2 was mainly the remaining 20–28% (Fig. 3b). These steady biogas components suggest that the property of microbe inside the bioreactor was relatively stable during the process. We also note that the CH4 content is a bit higher than that (
