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New parameters to determine the optimum pretreatment for improving the biomethanization performance
Chemical Engineering Journal 198–199 (2012) 81–86
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New parameters to determine the optimum pretreatment for improving the biomethanization performance L.A. Fdez.-Güelfo a,⇑, C. Álvarez-Gallego a, D. Sales Márquez b, L.I. Romero García a a b
Department of Chemical Engineering and Food Technology, Faculty of Science, University of Cádiz, 11510 Puerto Real, Cádiz, Spain Department of Environmental Technologies, Faculty of Sea and Environmental Sciences, University of Cádiz, 11510 Puerto Real, Cádiz, Spain
h i g h l i g h t s " The biological pretreatment with mature compost improves the OFMSW anaerobic digestion. " The total methane production is increased up to 37% over the control. " The hydrolytic stage of the anaerobic digestion process is faster by means of application of pretreatments. " New parameters (tMAX and tOPT) have been defined to select the optimum pretreatment. " New parameters as new user-friendly tools have been used to characterize the biomethanization performance.
a r t i c l e
i n f o
Article history: Received 11 April 2012 Received in revised form 21 May 2012 Accepted 22 May 2012 Available online 29 May 2012 Keywords: Anaerobic digestion Biomethanization OFMSW Pretreatments Parameters Performance
a b s t r a c t Thermochemical and biological pretreatments have been applied to the organic fraction of the municipal solid wastes (OFMSW) coming from an industrial 30 mm-trommel placed in a full-scale mechanical–biological treatment (MBT) plant in order to study their effects on the organic matter solubilization yield and the biomethanization process. To compare the effect of theses pre-treatments on anaerobic biodegradability of pretreated OFMSW, a series of batch experiments were carried out. The accumulated methane production and the temporary evolutions of the volatile fatty acids (VFA) have been used to define two new parameters (tMAX and tOPT) in order to compare the effect of the pre-treatments on anaerobic digestion of pretreated OFMSW. From the point of view of the biomethanization, the results indicate that the optimum pretreatment is the ‘‘Precomposting’’, since it presents the lower tOPT (12 days) and in only 15 days of operation (tMAX = 15) the reactor achieves the maximum accumulated methane production, 37.5 l vs. 7 l of the control digester for the same operation time. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The organic fraction of municipal solid waste (OFMSW) is a substrate that may contain many lignocellulosic and fatty materials. These difficulty biodegradable organic matters may involve limitations process on anaerobic digestion (AD) yield. The AD processes are usually developed through four main stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis. Generally, the hydrolysis is considered the rate-limiting step of anaerobic digestion of solid wastes [1]. Hence, the hydrolysis stage is decisive for the OFMSW biodegradation and it may determine the overall rate of the process.
⇑ Corresponding author. Tel.: +34 956016379. E-mail address: [email protected] (L.A. Fdez.-Güelfo). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.05.077
To avoid the above aspects, in the literature have been reported many studies in which thermochemical and biological pre-treatments are applied in order to pre-hydrolyze the waste and improve the organic matter solubilization to the liquid phase during the hydrolysis stage of the AD [2–13]. However, the literature about the applications of pretreatments on industrial solid wastes with high particle size, as the organic fraction of municipal solid wastes (OFMSW) coming from a fullscale mechanical–biological treatment (MBT) plant used in this study, is very limited. In fact, only three papers have been found about this topic [14–16]. The above information indicates the relevance of studying different pretreatments in order to increase the biodegradation scope by enhancing the hydrolytic stage of complex solid wastes such as the industrial OFMSW used in this work. For all the stated previously, the present paper has two main goals:
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Fig. 1. Batch stirred tank reactor (Patent no. WO2006/111598A1-World Intellectual Property Organization).
About the second goal, in many cases classical operational and control parameters to check the performance of biomethanization process as the specific methane production, accumulated methane production or kinetic parameters not provide unambiguous information to determine the optimum pretreatment. For these reasons, two new parameters (as new user-friendly tools) to evaluate the performance/efficiency of pretreatments on the biomethanization, without resorting to the kinetic characterization of the process, are defined in this work. Below, these new parameters are presented to the scientific community:
if tMAX is low, it represents that the substrate biodegradation and, hence, the biomethanization process occurs rapidly. In this case the effect of the pretreatment will be better compared to the opposite cases (high tMAX). 2. Kinetics of batch anaerobic processes is well established. In general, in any microbiological process, the microorganisms require a lag phase to adapt to substrate and subsequently, both the acceleration phase and the exponential growth phase take place. However, in anaerobic processes the complexity of microbiota leads to a concatenation of the phases for the different microorganism groups and hence, in most of the cases, a spontaneous separation of stages can be observed. Taking into account the above mentioned, VFA evolution in the system shows an initial increase as consequence of the hydrolysis and acidogenesis phases, followed of a stationary stage in which the microorganisms are adapting to these new conditions. Later, acetoclastic population degrades the VFA to methane causing a continuous decreasing in VFA concentration. Simultaneously, the methane production is related initially to the hydrogenotrophic microorganisms in the hydrolysis and acidogenesis phases and, later, to the VFA degradation in the methanogenic phase by means of acetoclastic microorganisms. In this sense, VFA degradation is considered the main route for methane production. As a consequence, the maximum removal rate of VFA and the maximum rate for methane production are located in the methanogenic phase of the process developed by acetoclastic microorganisms. The operation time necessary for both maximum rates (maximum values of the slopes) match, it is a specific parameter that can be denominated ‘‘tOPT’’. Obviously, lower tOPT values imply that the system reaches the methanogenic phase faster and, hence, the pre-treatment is more effective.
1. ‘‘tMAX’’ is defined as the operation time necessary to reach the accumulated methane production value from which the accumulated methane production is increased by less than 5%. Thus,
The value of both parameters (tMAX and tOPT) must be determined graphically from the normalized curves of cumulative methane production and temporal evolution of VFA.
Table 1 Configuration and operational conditions in batch pretreatment tests. Reactor
Pretreatment
Conditions
1 2
– 2.5% (v/v)
5
Control Biological – mature compost Biological – sludge (WWTP) Biological – Aspergillus awamori Thermochemical
6
Thermochemical
3 4
2.5% (v/v) 2.5% (v/v) 180 °C - 5 bar - 3 g/L of NaOH (inert atmosphere with N2) 180 °C - 5 bar - 3 g/L of NaOH (oxidizing atmosphere with synthetic air)
To examine different pretreatments applied to OFMSW to enhance the hydrolysis stage, in a previous stage to the drythermophilic AD process, in order to increase the methane production and the organic matter (expressed in terms of volatile fatty acids) removal rate. To define new parameters in order to determine the optimum pretreatment among the tested in this work.
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(a) The cumulative methane production curve must be normalized (NAMPi) by dividing each daily value of accumulated methane production (AMPi) by the accumulated methane production value at the end of the assay (maximum accumulated methane production, AMPMAX).
NAMPi ¼
AMPi AMPMAX
(a) On the other hand, the VFA curve must be normalized (NVAFi) by dividing each daily value of VFA (VFAi) by the maximum VFA concentration determined along the assay (VFAMAX). This maximum VFA concentration is associated, generally, at the end of hydrolytic stage and its value is usually higher than the VFA value at the beginning of the assay.
NVFAi ¼
VFAi VFAMAX
2. Materials and methods A series of experiments were carried out to compare the effect of different pretreatments on the anaerobic degradation of OFMSW and determine their influence on the scope of the biomethanization process. The methodology of the anaerobic digestion tests of the pretreated OFMSW were developed according to Fdez.-Güelfo et al. [15]. As experimental equip, a battery of batch 2-L stirred tank reactors with a temperature control system (Fig. 1, Patent no. WO/2006/ 111598 – World Intellectual Property Organization) was used. The experimental design was defined according to these authors and it is detailed in Table 1. The initial and final physicochemical characterization for each reactor is summarized in Table 2. To check the solubilization yield of the pretreatments and the scope of the biomethanization process, the following analytical determinations were used: total solids (TS), volatile solids (VS), alkalinity, pH, dissolved organic carbon (DOC), ammonium, volatile fatty acids (VFA), biogas composition and volume. All the parameters were analyzed once a day and the determinations were performed according to Fdez.-Güelfo et al. [15]. 3. Results and discussion To develop this section, two new parameters, related to the rate of the AD process, have been defined in the ‘‘Introduction’’ section
in order to facilitate the discussion of the results. This additional information may help to determine, from the point of view of the improving of the waste biomethanization, the best pretreatment in cases in which classical performance parameters are not sufficiently concise. On the basis of the previous definitions, the following interpretation may be developed:
3.1. Discussion based on new parameter tMAX In Fig. 2, as can be seen from the shape of the normalized-accumulated methane production curves, when the waste is pretreated no step-wise gas productions were detected. However, when the OFMSW is non-pretreated (control digester), step-wise gas generation is clearly observed [17]. According to Fdez.-Güelfo et al. [15], the methane production rate is conditioned by the oxidation state of the anaerobically-biodegraded organic matter. Thus, equals amount of CH4 and CO2 are generated from the carbohydrates biodegradation and higher CH4 proportion are achieved from the lipids digestion. Therefore, if the shape of the curves of Fig. 2 is taken into account, it may be stated that the application of pretreatments may partially pre-hydrolyze the different organic matter fractions of the OFMSW and, consequently, these prehydrolyzed fractions may be converted to methane faster and more efficiently. As can be seen in Table 3, when classical parameters (specific methane production expressed as LCH4/gVSr, maximum accumulated methane production or kinetic parameters) of the biomethanization performance are compared, it is very difficult to determine the best pretreatment. In addition, this situation may be aggravated if the specific methane production is expressed in terms of other forms of organic matter (VFA, DOC or COD). To avoid this situation of uncertainty, a classification of the pretreatments on the basis of the tMAX values (from lowest to highest) may be reported: Mature compost (15 days); fungus Aspergillus awamori (20 days); Sludge from WWTP (23 days); Thermochemical NaOH– Air (28 days); Thermochemical NaOH–N2 (higher than 31 days, characteristic of the control reactor). As can be seen in the above classification, the best pretreatment is the Precomposting since it presents the lower tMAX (15 days vs. 31 days for the control reactor) and the higher accumulated methane production. Furthermore, the Precomposting reactor is the only one that presents an accumulated methane production clearly superior to the Control reactor, 37.5 l vs. 27.3 l respectively. Thus, the total methane generation has been increased to 37.36%. This
Table 2 Initial and final physicochemical characterization for each reactor. Control
Sludge from WWTP
Aspergillus awamori
Thermochemical (NaOH–N2)
Thermochemical (NaOH–Air)
Analytical parameters
Initial
Final
Initial
Final
Initial
Final
Initial
Final
Initial
Final
Initial
Final
Total solids (g/g) Volatile solids (g/g) Fixed solids (g/g) Dissolved organic carbon (mg/g) Dissolved inorganic carbon (mg/g) Alkalinity (gCaCO3/g) Ammonium (gNH3-N/g)
0.195 0.116 0.079 12.4
0.092 0.045 0.046 7.69
0.218 0.111 0.106 12.6
0.014 0.070 0.072 5.68
0.200 0.112 0.088 13.2
0.078 0.038 0.039 7.69
0.209 0.115 0.093 13.1
0.1183 0.062 0.056 5.43
0.250 0.133 0.116 13.8
0.0944 0.052 0.041 11.0
0.210 0.112 0.102 14.6
0.046 0.016 0.029 5.66
0.35
4.29
0.24
1.05
0.22
4.59
0.23
1.04
0.24
3.34
0.24
2.36
0.13 0.17101
0.28 0.14101
0.13 0.17101
0.26 0.12101
0.13 0.17101
0.27 0.13101
0.12 0.17101
0.28 0.13101
0.12 0.16101
0.32 0.13101
0.12 0.16101
0.26 0.11101
20 <10 <10 <10 <10
315 24 78 26 533
40 <10 <10 <10 <10
237 26 87 29 566
34 <10 <10 <10 <10
180 20 78 34 416
26 <10 <10 <10 <10
587 43 148 55 1005
51 341 <10 <10 636
430 83 104 39 790
42 <10 <10 <10 <10
Main volatile fatty acids -VFA- (mg/L)a Acetic 342 Propionic 27 Butyric 92 Caproic 30 Total VFA (mgAcH/ L) 595 a
Mature compost
Isobutyric, Isovaleric, Valeric, Isocaproic and Heptanoic acids were not detected during the experiment.
1,0
0,9
0,9
Total VFA Accumulated
0,8
tMAX=15 days tOPT=12 days
0,7
0,7
0,6
0,6
0,5
0,5
0,4
0,4
0,3
0,3
0,2
0,2
0,1
0,1
0,0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
0,0
0,9 0,8 0,7 0,6
tMAX=23days tOPT=18 days
0,9 0,8 0,7
0,6
0,6
0,5
0,5
0,4
0,4
0,3
0,3
0,2
0,2
0,1
0,1
0,0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
0,0
Normalized-total VFA (mgAcH/ L)
Total VFA Accumulated
1,0
Normalized-accumulated of CH4 (L)
Normalized-total VFA (mgAcH/ L)
0,9 0,7
0,8
0,7
0,7
tMAX=31days
0,6
0,6
tOPT=23 days
0,5
0,4
0,4
0,3
0,3
0,2
0,2
0,1
0,1
0,0
0,0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
Time (days)
Normalized-total VFA (mgAcH/ L)
0,9
Normalized-accumulated of CH4 (L)
Normalized-total VFA (mgAcH/ L)
Total VFA Accumulated
1,0
0,8
0,5
0,7 0,6 0,5 0,4
0,3
0,3
0,2
0,2
0,1
0,1
0,0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
0,0
1,0 0,9
Thermochemical (NaOH-Air) tMAX=28days Total VFA Accumulated
0,8 0,7
1,0 0,9 0,8 0,7
0,6
tOPT=16 days
0,5
0,6 0,5
0,4
0,4
0,3
0,3
0,2
0,2
0,1
0,1
0,0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
0,0
Time (days)
Control
0,9
0,8
tMAX=20 days tOPT=14 days
0,4
Time (days)
1,0
0,9
Time (days)
Sludge from WWTP
0,8
1,0
Total VFA Accumulated
0,5
Time (days)
1,0
Aspergillus awamori
Normalized-accumulated of CH4 (L)
0,8
1,0
Normalized-accumulated of CH4 (L)
Mature compost
1,0
Thermochemical (NaOH-N2) Total VFA Accumulated
0,9 0,8
0,9 0,8
0,7 0,6
1,0
0,7
tOPT=22 days
0,6
0,5
0,5
0,4
0,4
0,3
0,3
0,2
0,2
0,1
0,1
0,0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
Normalized-accumulated of CH 4
1,0
Normalized-total VFA (mgAcH/ L)
L.A. Fdez.-Güelfo et al. / Chemical Engineering Journal 198–199 (2012) 81–86
Normalized-accumulated of CH4 (L)
Normalized-total VFA (mgAcH/ L)
84
0,0
Time (days)
Fig. 2. Normalized-temporary evolutions of the accumulated methane production and total VFA expressed as liters of CH4 and mgAcH/L respectively.
fact may be checked in Table 3 if the kinetic parameters reported by Fdez.-Güelfo et al. [15] are considered. As can be seen, the maximum specific growth rates of the microorganisms are obtained when mature compost is used as biological pretreatment, 0.540 and 0.309 days1 for the ‘‘Product-Generation’’ and ‘‘SubstrateConsumption’’ kinetic adjustment respectively. As it is reported by Fdez.-Güelfo et al. (2011c), most of the research works are focused on pretreatments and anaerobic biodegradability of pretreated sewage sludge (liquid waste with small
particle size) with increments of methane production between 11% and 100%. The higher increment of methane production achieved in this study by means of Precomposting has been 37.36%. According to these authors, this value could be highlighted since the OFMSW is a semi-solid waste (20% of TS concentration) with a high particle size (30 mm) and, hence, the increments of methane generation may be limited by mass transfer phenomena on the solid organic fractions. On the other hand, it must be noted that a high fraction of the organic matter contained on this OFMSW
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L.A. Fdez.-Güelfo et al. / Chemical Engineering Journal 198–199 (2012) 81–86 Table 3 New parameters and classical parameters values to characterize the biomethanization performance.
New parameters tOPT (days) tMAX (days) Maximum accumulated methane production (L) Average specific methane production (LCH4/gVSr) b lmax (day–1) Product – generation fitting Substrate – consumption fitting a b
Control
Mature compost
Sludge from WWTP
Aspergillus awamori
Thermochemical (NaOH–N2)
Thermochemical (NaOH– Air)
23 31 27.35
12 15 37.50
18 23 27.85
14 20 21.85
22 – 28.05a
16 28 29.88
0.225
0.308
0.215
0.404
0.210
0.130
0.178 0.118
0.540 0.309
0.401 0.227
0.413 0.289
0.117 0.116
0.201 0.161
This value is the accumulated methane production after 34 days but, in this case, the maximum value has not been reached during the experimentation time. These data are reported by Fdez.-Güelfo et al. [15].
may be considered as refractory (low biodegradability), since the VS/TS ratio is 0.5 and, hence, lower than to the limit of 0.7 established by other authors for the organic matter could be considered easily-biodegradable [18]. The Thermochemical (NaOH–N2) reactor is the only one that it has presented a worse performance than the control digester. In fact, this reactor is not able to reach its maximum accumulated methane production in 34 days of operation time. It may be due to the high VFA concentration (1005 mgAcH/L) at the beginning of the assay which may cause partial inhibition of the methanogenesis. In Fig. 2, it can be seen that there is a latency time, for the start-up of the methanogenesis, until the sixth day that it does not occur in the other reactors. Therefore, it can be stated that the biodegradability of the waste is slowed when the release of VFA during pretreatment exceeds a threshold value above which start-up of the methanogenesis begins to be affected. About the A. awamori reactor, it is the only one that presents an accumulated methane production clearly lower than to the control reactor, with a value of 21.8 l. This fact may be associated with a fraction of the solubilized organic matter during the pretreatment may be consumed by the fungus for its own metabolism and/or the generation of new biomass. If that occurs, there is less available substrate to be transformed to gas and the methane production is lower. Finally, with regard to the successful effect of the Precomposting on the biomethanization, it may be associated with the mature compost used in this work comes from a composting pile constituted by a proportion 85:15 (dry base) of OFMSW and digested sewage sludge respectively. As consequence of the high proportion of OFMSW during the composting stage, the consortia of microorganisms resulting on the compost employed as biological agent may contain specific enzymes that are capable to hydrolyze more easily the refractory fractions of organic matter. 3.2. Discussion based on new parameter tOPT As in the previous section, according to Fig. 2, a classification of the pretreatments on the basis of the tOPT values (from lowest to highest) may be reported: Mature compost (12 days); fungus A. awamori (14 days); Thermochemical NaOH–Air (16 days); Sludge from WWTP (18 days); Thermochemical NaOH–N2 (22 days, practically the same value of the control reactor). Again, the best pretreatment is the Precomposting since it presents the lower tOPT, 12 days. In this case, it very important to highlight that all reactors with pretreated OFMSW have presented a tOPT value lower than the Control reactor (23 days). This fact shows that the application of pretreatments gets accelerate the hydrolysis stage due to the pre-hydrolysis of the waste.
About the VFA removal, all reactors except Thermochemical (NaOH–N2), present a final VFA concentration lower than 60 mgAcH/L. This is an important aspect since the digestate obtained by means of the anaerobic digestion of pretreated OFMSW could be employed as agricultural soils improver. Hence, the final product is a stabilized digestate: low content of dissolved organic matter easily assimilate by the crops and aseptic from the microbiological point of view since in this study the biodegradation of the waste has been developed at thermophilic regime of temperature (55 °C). 4. Conclusions According with the above discussion, the following conclusions may be established: 1. Classical parameters of biomethanization performance such as specific methane production, accumulated methane production or kinetic parameters are not enough to determine the best pretreatment to enhance the anaerobic digestion of solid wastes. 2. The definition of new parameters (tMAX and tOPT) allows the easily interpretation of the data and, therefore, kinetic adjustment or complex mathematical treatments are not necessary. Taking into account the tMAX and tOPT values, the biological pretreatment with mature compost (Precomposting) has been the best among the tested in this study since the biomethanization of the pretreated OFMSW by means of dry-thermophillic anaerobic digestion at batch regime of operation is significantly improved (increment of 37.36%). From the above conclusion, a two-stage sequential treatment may be designed for treating this type of wastes: Precomposting and dry-thermophilic anaerobic digestion. The final product is a stabilized digestate that may be employed as agricultural soils improver. 3. The application of the pretreatments tested in this study, except the Thermochemical (NaOH–N2), allows reducing the values of tMAX. This exception states that an excessive generation of VFA during the pretreatment may partially inhibit the methanogenesis of the waste. 4. The application of all pretreatments tested in this study allows reducing the values of tOPT due to that the waste is pre-hydrolyzed and, hence, the hydrolytic stage of the anaerobic digestion process is faster.
Acknowledgements This work was supported by the ‘‘Ministerio de Ciencia e Inovación’’ of Spain (Project CTM2010-17654), the ‘‘Consejería de Inno-
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vación, Ciencia y Empresa’’ of the ‘‘Junta de Andalucía, Spain’’ (Project P07-TEP-02472), the European Regional Development Fund (ERDF) and the ‘‘Ministerio de Educación y Ciencia’’ of Spain (Project NovEDAR_Consolider CSD2007-00055).
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
New parameters to determine the optimum pretreatment for improving the biomethanization performance L.A. Fdez.-Güelfo a,⇑, C. Álvarez-Gallego a, D. Sales Márquez b, L.I. Romero García a a b
Department of Chemical Engineering and Food Technology, Faculty of Science, University of Cádiz, 11510 Puerto Real, Cádiz, Spain Department of Environmental Technologies, Faculty of Sea and Environmental Sciences, University of Cádiz, 11510 Puerto Real, Cádiz, Spain
h i g h l i g h t s " The biological pretreatment with mature compost improves the OFMSW anaerobic digestion. " The total methane production is increased up to 37% over the control. " The hydrolytic stage of the anaerobic digestion process is faster by means of application of pretreatments. " New parameters (tMAX and tOPT) have been defined to select the optimum pretreatment. " New parameters as new user-friendly tools have been used to characterize the biomethanization performance.
a r t i c l e
i n f o
Article history: Received 11 April 2012 Received in revised form 21 May 2012 Accepted 22 May 2012 Available online 29 May 2012 Keywords: Anaerobic digestion Biomethanization OFMSW Pretreatments Parameters Performance
a b s t r a c t Thermochemical and biological pretreatments have been applied to the organic fraction of the municipal solid wastes (OFMSW) coming from an industrial 30 mm-trommel placed in a full-scale mechanical–biological treatment (MBT) plant in order to study their effects on the organic matter solubilization yield and the biomethanization process. To compare the effect of theses pre-treatments on anaerobic biodegradability of pretreated OFMSW, a series of batch experiments were carried out. The accumulated methane production and the temporary evolutions of the volatile fatty acids (VFA) have been used to define two new parameters (tMAX and tOPT) in order to compare the effect of the pre-treatments on anaerobic digestion of pretreated OFMSW. From the point of view of the biomethanization, the results indicate that the optimum pretreatment is the ‘‘Precomposting’’, since it presents the lower tOPT (12 days) and in only 15 days of operation (tMAX = 15) the reactor achieves the maximum accumulated methane production, 37.5 l vs. 7 l of the control digester for the same operation time. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The organic fraction of municipal solid waste (OFMSW) is a substrate that may contain many lignocellulosic and fatty materials. These difficulty biodegradable organic matters may involve limitations process on anaerobic digestion (AD) yield. The AD processes are usually developed through four main stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis. Generally, the hydrolysis is considered the rate-limiting step of anaerobic digestion of solid wastes [1]. Hence, the hydrolysis stage is decisive for the OFMSW biodegradation and it may determine the overall rate of the process.
⇑ Corresponding author. Tel.: +34 956016379. E-mail address: [email protected] (L.A. Fdez.-Güelfo). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.05.077
To avoid the above aspects, in the literature have been reported many studies in which thermochemical and biological pre-treatments are applied in order to pre-hydrolyze the waste and improve the organic matter solubilization to the liquid phase during the hydrolysis stage of the AD [2–13]. However, the literature about the applications of pretreatments on industrial solid wastes with high particle size, as the organic fraction of municipal solid wastes (OFMSW) coming from a fullscale mechanical–biological treatment (MBT) plant used in this study, is very limited. In fact, only three papers have been found about this topic [14–16]. The above information indicates the relevance of studying different pretreatments in order to increase the biodegradation scope by enhancing the hydrolytic stage of complex solid wastes such as the industrial OFMSW used in this work. For all the stated previously, the present paper has two main goals:
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Fig. 1. Batch stirred tank reactor (Patent no. WO2006/111598A1-World Intellectual Property Organization).
About the second goal, in many cases classical operational and control parameters to check the performance of biomethanization process as the specific methane production, accumulated methane production or kinetic parameters not provide unambiguous information to determine the optimum pretreatment. For these reasons, two new parameters (as new user-friendly tools) to evaluate the performance/efficiency of pretreatments on the biomethanization, without resorting to the kinetic characterization of the process, are defined in this work. Below, these new parameters are presented to the scientific community:
if tMAX is low, it represents that the substrate biodegradation and, hence, the biomethanization process occurs rapidly. In this case the effect of the pretreatment will be better compared to the opposite cases (high tMAX). 2. Kinetics of batch anaerobic processes is well established. In general, in any microbiological process, the microorganisms require a lag phase to adapt to substrate and subsequently, both the acceleration phase and the exponential growth phase take place. However, in anaerobic processes the complexity of microbiota leads to a concatenation of the phases for the different microorganism groups and hence, in most of the cases, a spontaneous separation of stages can be observed. Taking into account the above mentioned, VFA evolution in the system shows an initial increase as consequence of the hydrolysis and acidogenesis phases, followed of a stationary stage in which the microorganisms are adapting to these new conditions. Later, acetoclastic population degrades the VFA to methane causing a continuous decreasing in VFA concentration. Simultaneously, the methane production is related initially to the hydrogenotrophic microorganisms in the hydrolysis and acidogenesis phases and, later, to the VFA degradation in the methanogenic phase by means of acetoclastic microorganisms. In this sense, VFA degradation is considered the main route for methane production. As a consequence, the maximum removal rate of VFA and the maximum rate for methane production are located in the methanogenic phase of the process developed by acetoclastic microorganisms. The operation time necessary for both maximum rates (maximum values of the slopes) match, it is a specific parameter that can be denominated ‘‘tOPT’’. Obviously, lower tOPT values imply that the system reaches the methanogenic phase faster and, hence, the pre-treatment is more effective.
1. ‘‘tMAX’’ is defined as the operation time necessary to reach the accumulated methane production value from which the accumulated methane production is increased by less than 5%. Thus,
The value of both parameters (tMAX and tOPT) must be determined graphically from the normalized curves of cumulative methane production and temporal evolution of VFA.
Table 1 Configuration and operational conditions in batch pretreatment tests. Reactor
Pretreatment
Conditions
1 2
– 2.5% (v/v)
5
Control Biological – mature compost Biological – sludge (WWTP) Biological – Aspergillus awamori Thermochemical
6
Thermochemical
3 4
2.5% (v/v) 2.5% (v/v) 180 °C - 5 bar - 3 g/L of NaOH (inert atmosphere with N2) 180 °C - 5 bar - 3 g/L of NaOH (oxidizing atmosphere with synthetic air)
To examine different pretreatments applied to OFMSW to enhance the hydrolysis stage, in a previous stage to the drythermophilic AD process, in order to increase the methane production and the organic matter (expressed in terms of volatile fatty acids) removal rate. To define new parameters in order to determine the optimum pretreatment among the tested in this work.
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(a) The cumulative methane production curve must be normalized (NAMPi) by dividing each daily value of accumulated methane production (AMPi) by the accumulated methane production value at the end of the assay (maximum accumulated methane production, AMPMAX).
NAMPi ¼
AMPi AMPMAX
(a) On the other hand, the VFA curve must be normalized (NVAFi) by dividing each daily value of VFA (VFAi) by the maximum VFA concentration determined along the assay (VFAMAX). This maximum VFA concentration is associated, generally, at the end of hydrolytic stage and its value is usually higher than the VFA value at the beginning of the assay.
NVFAi ¼
VFAi VFAMAX
2. Materials and methods A series of experiments were carried out to compare the effect of different pretreatments on the anaerobic degradation of OFMSW and determine their influence on the scope of the biomethanization process. The methodology of the anaerobic digestion tests of the pretreated OFMSW were developed according to Fdez.-Güelfo et al. [15]. As experimental equip, a battery of batch 2-L stirred tank reactors with a temperature control system (Fig. 1, Patent no. WO/2006/ 111598 – World Intellectual Property Organization) was used. The experimental design was defined according to these authors and it is detailed in Table 1. The initial and final physicochemical characterization for each reactor is summarized in Table 2. To check the solubilization yield of the pretreatments and the scope of the biomethanization process, the following analytical determinations were used: total solids (TS), volatile solids (VS), alkalinity, pH, dissolved organic carbon (DOC), ammonium, volatile fatty acids (VFA), biogas composition and volume. All the parameters were analyzed once a day and the determinations were performed according to Fdez.-Güelfo et al. [15]. 3. Results and discussion To develop this section, two new parameters, related to the rate of the AD process, have been defined in the ‘‘Introduction’’ section
in order to facilitate the discussion of the results. This additional information may help to determine, from the point of view of the improving of the waste biomethanization, the best pretreatment in cases in which classical performance parameters are not sufficiently concise. On the basis of the previous definitions, the following interpretation may be developed:
3.1. Discussion based on new parameter tMAX In Fig. 2, as can be seen from the shape of the normalized-accumulated methane production curves, when the waste is pretreated no step-wise gas productions were detected. However, when the OFMSW is non-pretreated (control digester), step-wise gas generation is clearly observed [17]. According to Fdez.-Güelfo et al. [15], the methane production rate is conditioned by the oxidation state of the anaerobically-biodegraded organic matter. Thus, equals amount of CH4 and CO2 are generated from the carbohydrates biodegradation and higher CH4 proportion are achieved from the lipids digestion. Therefore, if the shape of the curves of Fig. 2 is taken into account, it may be stated that the application of pretreatments may partially pre-hydrolyze the different organic matter fractions of the OFMSW and, consequently, these prehydrolyzed fractions may be converted to methane faster and more efficiently. As can be seen in Table 3, when classical parameters (specific methane production expressed as LCH4/gVSr, maximum accumulated methane production or kinetic parameters) of the biomethanization performance are compared, it is very difficult to determine the best pretreatment. In addition, this situation may be aggravated if the specific methane production is expressed in terms of other forms of organic matter (VFA, DOC or COD). To avoid this situation of uncertainty, a classification of the pretreatments on the basis of the tMAX values (from lowest to highest) may be reported: Mature compost (15 days); fungus Aspergillus awamori (20 days); Sludge from WWTP (23 days); Thermochemical NaOH– Air (28 days); Thermochemical NaOH–N2 (higher than 31 days, characteristic of the control reactor). As can be seen in the above classification, the best pretreatment is the Precomposting since it presents the lower tMAX (15 days vs. 31 days for the control reactor) and the higher accumulated methane production. Furthermore, the Precomposting reactor is the only one that presents an accumulated methane production clearly superior to the Control reactor, 37.5 l vs. 27.3 l respectively. Thus, the total methane generation has been increased to 37.36%. This
Table 2 Initial and final physicochemical characterization for each reactor. Control
Sludge from WWTP
Aspergillus awamori
Thermochemical (NaOH–N2)
Thermochemical (NaOH–Air)
Analytical parameters
Initial
Final
Initial
Final
Initial
Final
Initial
Final
Initial
Final
Initial
Final
Total solids (g/g) Volatile solids (g/g) Fixed solids (g/g) Dissolved organic carbon (mg/g) Dissolved inorganic carbon (mg/g) Alkalinity (gCaCO3/g) Ammonium (gNH3-N/g)
0.195 0.116 0.079 12.4
0.092 0.045 0.046 7.69
0.218 0.111 0.106 12.6
0.014 0.070 0.072 5.68
0.200 0.112 0.088 13.2
0.078 0.038 0.039 7.69
0.209 0.115 0.093 13.1
0.1183 0.062 0.056 5.43
0.250 0.133 0.116 13.8
0.0944 0.052 0.041 11.0
0.210 0.112 0.102 14.6
0.046 0.016 0.029 5.66
0.35
4.29
0.24
1.05
0.22
4.59
0.23
1.04
0.24
3.34
0.24
2.36
0.13 0.17101
0.28 0.14101
0.13 0.17101
0.26 0.12101
0.13 0.17101
0.27 0.13101
0.12 0.17101
0.28 0.13101
0.12 0.16101
0.32 0.13101
0.12 0.16101
0.26 0.11101
20 <10 <10 <10 <10
315 24 78 26 533
40 <10 <10 <10 <10
237 26 87 29 566
34 <10 <10 <10 <10
180 20 78 34 416
26 <10 <10 <10 <10
587 43 148 55 1005
51 341 <10 <10 636
430 83 104 39 790
42 <10 <10 <10 <10
Main volatile fatty acids -VFA- (mg/L)a Acetic 342 Propionic 27 Butyric 92 Caproic 30 Total VFA (mgAcH/ L) 595 a
Mature compost
Isobutyric, Isovaleric, Valeric, Isocaproic and Heptanoic acids were not detected during the experiment.
1,0
0,9
0,9
Total VFA Accumulated
0,8
tMAX=15 days tOPT=12 days
0,7
0,7
0,6
0,6
0,5
0,5
0,4
0,4
0,3
0,3
0,2
0,2
0,1
0,1
0,0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
0,0
0,9 0,8 0,7 0,6
tMAX=23days tOPT=18 days
0,9 0,8 0,7
0,6
0,6
0,5
0,5
0,4
0,4
0,3
0,3
0,2
0,2
0,1
0,1
0,0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
0,0
Normalized-total VFA (mgAcH/ L)
Total VFA Accumulated
1,0
Normalized-accumulated of CH4 (L)
Normalized-total VFA (mgAcH/ L)
0,9 0,7
0,8
0,7
0,7
tMAX=31days
0,6
0,6
tOPT=23 days
0,5
0,4
0,4
0,3
0,3
0,2
0,2
0,1
0,1
0,0
0,0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
Time (days)
Normalized-total VFA (mgAcH/ L)
0,9
Normalized-accumulated of CH4 (L)
Normalized-total VFA (mgAcH/ L)
Total VFA Accumulated
1,0
0,8
0,5
0,7 0,6 0,5 0,4
0,3
0,3
0,2
0,2
0,1
0,1
0,0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
0,0
1,0 0,9
Thermochemical (NaOH-Air) tMAX=28days Total VFA Accumulated
0,8 0,7
1,0 0,9 0,8 0,7
0,6
tOPT=16 days
0,5
0,6 0,5
0,4
0,4
0,3
0,3
0,2
0,2
0,1
0,1
0,0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
0,0
Time (days)
Control
0,9
0,8
tMAX=20 days tOPT=14 days
0,4
Time (days)
1,0
0,9
Time (days)
Sludge from WWTP
0,8
1,0
Total VFA Accumulated
0,5
Time (days)
1,0
Aspergillus awamori
Normalized-accumulated of CH4 (L)
0,8
1,0
Normalized-accumulated of CH4 (L)
Mature compost
1,0
Thermochemical (NaOH-N2) Total VFA Accumulated
0,9 0,8
0,9 0,8
0,7 0,6
1,0
0,7
tOPT=22 days
0,6
0,5
0,5
0,4
0,4
0,3
0,3
0,2
0,2
0,1
0,1
0,0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
Normalized-accumulated of CH 4
1,0
Normalized-total VFA (mgAcH/ L)
L.A. Fdez.-Güelfo et al. / Chemical Engineering Journal 198–199 (2012) 81–86
Normalized-accumulated of CH4 (L)
Normalized-total VFA (mgAcH/ L)
84
0,0
Time (days)
Fig. 2. Normalized-temporary evolutions of the accumulated methane production and total VFA expressed as liters of CH4 and mgAcH/L respectively.
fact may be checked in Table 3 if the kinetic parameters reported by Fdez.-Güelfo et al. [15] are considered. As can be seen, the maximum specific growth rates of the microorganisms are obtained when mature compost is used as biological pretreatment, 0.540 and 0.309 days1 for the ‘‘Product-Generation’’ and ‘‘SubstrateConsumption’’ kinetic adjustment respectively. As it is reported by Fdez.-Güelfo et al. (2011c), most of the research works are focused on pretreatments and anaerobic biodegradability of pretreated sewage sludge (liquid waste with small
particle size) with increments of methane production between 11% and 100%. The higher increment of methane production achieved in this study by means of Precomposting has been 37.36%. According to these authors, this value could be highlighted since the OFMSW is a semi-solid waste (20% of TS concentration) with a high particle size (30 mm) and, hence, the increments of methane generation may be limited by mass transfer phenomena on the solid organic fractions. On the other hand, it must be noted that a high fraction of the organic matter contained on this OFMSW
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L.A. Fdez.-Güelfo et al. / Chemical Engineering Journal 198–199 (2012) 81–86 Table 3 New parameters and classical parameters values to characterize the biomethanization performance.
New parameters tOPT (days) tMAX (days) Maximum accumulated methane production (L) Average specific methane production (LCH4/gVSr) b lmax (day–1) Product – generation fitting Substrate – consumption fitting a b
Control
Mature compost
Sludge from WWTP
Aspergillus awamori
Thermochemical (NaOH–N2)
Thermochemical (NaOH– Air)
23 31 27.35
12 15 37.50
18 23 27.85
14 20 21.85
22 – 28.05a
16 28 29.88
0.225
0.308
0.215
0.404
0.210
0.130
0.178 0.118
0.540 0.309
0.401 0.227
0.413 0.289
0.117 0.116
0.201 0.161
This value is the accumulated methane production after 34 days but, in this case, the maximum value has not been reached during the experimentation time. These data are reported by Fdez.-Güelfo et al. [15].
may be considered as refractory (low biodegradability), since the VS/TS ratio is 0.5 and, hence, lower than to the limit of 0.7 established by other authors for the organic matter could be considered easily-biodegradable [18]. The Thermochemical (NaOH–N2) reactor is the only one that it has presented a worse performance than the control digester. In fact, this reactor is not able to reach its maximum accumulated methane production in 34 days of operation time. It may be due to the high VFA concentration (1005 mgAcH/L) at the beginning of the assay which may cause partial inhibition of the methanogenesis. In Fig. 2, it can be seen that there is a latency time, for the start-up of the methanogenesis, until the sixth day that it does not occur in the other reactors. Therefore, it can be stated that the biodegradability of the waste is slowed when the release of VFA during pretreatment exceeds a threshold value above which start-up of the methanogenesis begins to be affected. About the A. awamori reactor, it is the only one that presents an accumulated methane production clearly lower than to the control reactor, with a value of 21.8 l. This fact may be associated with a fraction of the solubilized organic matter during the pretreatment may be consumed by the fungus for its own metabolism and/or the generation of new biomass. If that occurs, there is less available substrate to be transformed to gas and the methane production is lower. Finally, with regard to the successful effect of the Precomposting on the biomethanization, it may be associated with the mature compost used in this work comes from a composting pile constituted by a proportion 85:15 (dry base) of OFMSW and digested sewage sludge respectively. As consequence of the high proportion of OFMSW during the composting stage, the consortia of microorganisms resulting on the compost employed as biological agent may contain specific enzymes that are capable to hydrolyze more easily the refractory fractions of organic matter. 3.2. Discussion based on new parameter tOPT As in the previous section, according to Fig. 2, a classification of the pretreatments on the basis of the tOPT values (from lowest to highest) may be reported: Mature compost (12 days); fungus A. awamori (14 days); Thermochemical NaOH–Air (16 days); Sludge from WWTP (18 days); Thermochemical NaOH–N2 (22 days, practically the same value of the control reactor). Again, the best pretreatment is the Precomposting since it presents the lower tOPT, 12 days. In this case, it very important to highlight that all reactors with pretreated OFMSW have presented a tOPT value lower than the Control reactor (23 days). This fact shows that the application of pretreatments gets accelerate the hydrolysis stage due to the pre-hydrolysis of the waste.
About the VFA removal, all reactors except Thermochemical (NaOH–N2), present a final VFA concentration lower than 60 mgAcH/L. This is an important aspect since the digestate obtained by means of the anaerobic digestion of pretreated OFMSW could be employed as agricultural soils improver. Hence, the final product is a stabilized digestate: low content of dissolved organic matter easily assimilate by the crops and aseptic from the microbiological point of view since in this study the biodegradation of the waste has been developed at thermophilic regime of temperature (55 °C). 4. Conclusions According with the above discussion, the following conclusions may be established: 1. Classical parameters of biomethanization performance such as specific methane production, accumulated methane production or kinetic parameters are not enough to determine the best pretreatment to enhance the anaerobic digestion of solid wastes. 2. The definition of new parameters (tMAX and tOPT) allows the easily interpretation of the data and, therefore, kinetic adjustment or complex mathematical treatments are not necessary. Taking into account the tMAX and tOPT values, the biological pretreatment with mature compost (Precomposting) has been the best among the tested in this study since the biomethanization of the pretreated OFMSW by means of dry-thermophillic anaerobic digestion at batch regime of operation is significantly improved (increment of 37.36%). From the above conclusion, a two-stage sequential treatment may be designed for treating this type of wastes: Precomposting and dry-thermophilic anaerobic digestion. The final product is a stabilized digestate that may be employed as agricultural soils improver. 3. The application of the pretreatments tested in this study, except the Thermochemical (NaOH–N2), allows reducing the values of tMAX. This exception states that an excessive generation of VFA during the pretreatment may partially inhibit the methanogenesis of the waste. 4. The application of all pretreatments tested in this study allows reducing the values of tOPT due to that the waste is pre-hydrolyzed and, hence, the hydrolytic stage of the anaerobic digestion process is faster.
Acknowledgements This work was supported by the ‘‘Ministerio de Ciencia e Inovación’’ of Spain (Project CTM2010-17654), the ‘‘Consejería de Inno-
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vación, Ciencia y Empresa’’ of the ‘‘Junta de Andalucía, Spain’’ (Project P07-TEP-02472), the European Regional Development Fund (ERDF) and the ‘‘Ministerio de Educación y Ciencia’’ of Spain (Project NovEDAR_Consolider CSD2007-00055).
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