POLICY ANALYSIS pubs.acs.org/est
Should We Pretreat Solid Waste Prior to Anaerobic Digestion? An Assessment of Its Environmental Cost Marta Carballa,* Cecilia Duran, and Almudena Hospido* Department of Chemical Engineering, School of Engineering, University of Santiago de Compostela, Rua Lope Gomez de Marzoa s/n, E-15782 Santiago de Compostela, Spain
bS Supporting Information ABSTRACT: Many studies have shown the effectiveness of pretreatments prior to anaerobic digestion of solid wastes, but to our knowledge, none analyzes their environmental consequences/costs. In this work, seven different pretreatments applied to two types of waste (kitchen waste and sewage sludge) have been environmentally evaluated by using life cycle assessment (LCA) methodology. The results show that the environmental burdens associated to the application of pretreatments prior to anaerobic digestion cannot be excluded. Among the options tested, the pressurize-depressurize and chemical (acid or alkaline) pretreatments could be recommended on the basis of their beneficial net environmental performance, while thermal and ozonation alternatives require energy efficiency optimization to reduce their environmental burdens. Reconciling operational, economic and environmental aspects in a holistic approach for the selection of the most sustainable option, mechanical (e.g., pressurize-depressurize) and chemical methods appear to be the most appropriate alternatives at this stage.
’ INTRODUCTION Anaerobic digestion (AD) is a very promising option for the treatment of solid organic wastes due its ability to transform organic matter into biogas (with 60 70% CH4), with the concomitant reduction of the amount of final solids to be disposed.1 Moreover, solid byproduct (digestate) can be further used for agricultural purposes.2 However, AD of solid wastes is often limited by long retention times (20 30 days) and/or low overall degradation efficiencies (30 50%), probably associated with the hydrolysis stage.3 Therefore, significant effort has been dedicated in recent years to find alternatives to improve AD of solid wastes.4 Among the different options, the use of pretreatments is the most studied and its operational effectiveness has been demonstrated by many authors.1,5 8 All pretreatments entail the use of resources (chemicals and/or energy), thus deriving not only financial but also environmental costs. Preliminary economical analyses of several pretreatments have been recently published;8,9 however, to the best of our knowledge, their environmental evaluation has not been addressed yet. Life cycle assessment (LCA) is one of the most widely known and internationally accepted methodologies to compare environmental impacts of processes and systems.10 Several LCAs have focused on examining sewage sludge (SS) treatment options and/or end uses,11 16 most of them concluding that the combination of AD and agricultural application is the preferable alternative from an environmental point of view. This methodology has been also applied as a decision support tool in the selection of the most adequate municipal solid waste (MSW) management strategy for several countries or regions.17 21 In most cases, the r 2011 American Chemical Society
recycling of valuable materials together with the AD of the organic fraction of MSW, mostly consisting of food waste or kitchen waste (KW), turned to be the best scenario in environmental terms. Other studies were only focused on the conversion and management options of the organic fraction of MSW,22 24 concluding that AD was environmentally more favorable than incineration or aerobic composting. So, up to now, much LCA has validated AD and agricultural application as the best options for treatment and final disposal of organic solid wastes, respectively, but no information could be found on the environmental impact of the use of pretreatments prior to AD of solid wastes. In this study, we aim to fulfill this information gap by using LCA to evaluate the environmental consequences associated to the use of seven different pretreatments before AD of two organic solid wastes (kitchen waste and sewage sludge), and consequently, add the environmental vector to the technical and the economic evaluation toward a more sustainable decision making process.
’ EXPERIMENTAL SECTION Functional Unit Definition. The functional unit (FU) is usually defined in terms of the system output;25 however, when dealing with waste management systems, the FU might Received: June 1, 2011 Accepted: October 31, 2011 Revised: September 26, 2011 Published: October 31, 2011 10306
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POLICY ANALYSIS
be defined in terms of the system input, that is, the waste to be managed.26 Accordingly, the management (i.e., pretreatment, anaerobic digestion, and agriculture application) of 10 L of solid waste has been chosen in this work. For KW, the 10 L consisted of food waste provided by Trans Vanheede Environmental Group (Belgium) diluted with wastewater coming from a sewage treatment plant (Ossemeersen, Belgium) in order to achieve the required organic loading rate (OLR) in the digester (for further information, see Ma et al., 20118). In the case of SS, 10 L of a mixture of primary and secondary sludge (70:30, v/v) collected from the two thickeners existing in a sewage treatment plant (Galicia, NW Spain) was considered (for a more detailed description, see Carballa et al., 200727). The physic-chemical parameters, nutrients and heavy metals content in the KW and SS are reported in Table 1.
Scenarios Description. The system included the different pretreatments applied, the anaerobic digestion process with energy recovery and the disposal of the digestate in agricultural land. Based on the different pretreatments and waste used (KW and SS), 16 scenarios (8 per type of waste) were considered for comparison (Table 2), including the reference scenario. The latter refers to the anaerobic treatment of 10 L of waste without being previously pretreated. A detailed description of the pretreatments applied to KW and SS can be found in Ma et al. (2011)8 and Carballa et al. (2006,5 20076), respectively. It is important to make clear that not all scenarios were experimentally tested; instead, estimations were conducted on the basis of the experimentally tested pretreatments. A detailed description of the assumptions required for those estimated alternatives can be found in the Supporting Information (SI). AD was carried out in a lab-scale continuously stirred tank reactor at thermophilic conditions (55 ( 2 °C) and with a sludge retention time (SRT) of 10 and 20 days for SS and KW, respectively. A detailed description of the equipment and their performance can be found elsewhere.8,27 However, due to the lack of information, the infrastructure related to lab-scale operation was excluded from the analysis. Final disposal on agricultural land was modeled on the basis of literature data (see the Inventory Analysis section). The generation of SS and KW was left beyond the boundaries of the system, and the common elements within the system boundaries, such as digested solids conditioning, transportation and the spreading procedure, were not included in the analysis for comparative reasons. Inventory Analysis. In this stage, the raw materials consumed, the energy used, the products and coproducts obtained, and the emissions to air, water and soil, were identified and quantified for each scenario. Lab-scale experimental data of the KW and SS characteristics for each scenario and AD performance were provided by Duong (2009),28 Ma et al. (2011),8 and Carballa et al. (2006;5 2007;6 200929), except for the levels of nutrients and heavy metals in KW that were taken from literature.30 Since
Table 1. Main Characteristics of Sewage Sludge (n = 20) and Kitchen Waste (n = 10, except Nutrients and Heavy Metals Which Come from Literature30) Used in the Experiments parameter
kitchen waste
sewage sludge
pH
3.8 ( 0.2
5.6 ( 0.2
CODt (g/kg KW or L SS)
268 ( 20
50 ( 18
CODs (g/kg KW or L SS) TS (g/kg KW or L SS)
75 ( 7 166 ( 14
4(2 53 ( 19
VS (g/kg KW or L SS)
155 ( 13
34 ( 13
N (g/kg TS)
31.6
21.5
P (g/kg TS)
5.2
30.5
Zn (mg/kg TS)
76
868
Cu (mg/kg TS)
31
293
Cd (mg/kg TS)
1
2
Cr (mg/kg TS) Pb (mg/kg TS)
2 4
167 93
Ni (mg/kg TS)
2
79
Table 2. Description of the Pre-Treatments Applied in the Different Scenarios of KW and SS OLR scenariosa (kg COD m‑3d 1) KW1b
3.0
SS1
4.3
pretreatment
description Reference (nonpre-treated)
KW2b
4.0
alkaline
Addition of lime until pH 12, checking this value after 24 h, and neutralization with HCl (10 N)
SS2 KW3b
3.6 4.0
acid
before feeding the digester. Doses: 0.125 g CaO g 1 VSS and 0.058 g HCl g 1 VSS. Addition of HCl (10 N) until pH 2, checking this value after 24 h, and neutralization with NaOH (10 N)
SS3
4.0
KW4b
3.0
thermal
Heating and maintenance at 120 °C during 30 min. Cooling to room temperature before
thermo-acid
Acidification with HCl (10 N) until pH 2 followed by thermal treatment at 120 °C during 30 min. Freezing at
SS4
8.7
KW5b
3.0
before feeding the digester. Doses: 0.026 g HCl g
1
VSS and 0.040 g “extra” NaOH g
3.0
KW6b
5.0
freeze thaw
SS6 KW7b
5.0 5.0
feeding the digester. pressurize depressurize Pressurization to 10 bar with airc and quick depressurization to atmospheric pressure.
SS7
5.0 5.0
SS8
6.6
VSS.
feeding the digester.
SS5
KW8b
1
Cooling to room temperature and neutralization with NaOH (10 N) before feeding the digester.
ozone
20 °C for 6 h and thawing at 55 °C for 30 min. Cooling to room temperature before
Ozonation in a 10 L bubble column operated in batch for 2 h at room temperature. The ozone dose was set approximately at 20 mg O3 g
1
TSS.
a
Names in italics indicate the scenarios that have not been experimentally tested but inventoried based on estimations. b NaOH (10 N) was used in all KW scenarios in order to neutralize the influent prior to feed the digester. c Air was considered instead of CO2 because no chemical effect (e.g., increase in pH) was observed from the use of CO28. 10307
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Table 3. Summary of the Inventory Data for the 16 Scenarios Compared (see Table 2 for the Description of the Scenarios)a KW1 KW2 KW3 KW4 KW5
KW6
KW7
KW8
24
24
24
SS1
SS2
SS3
SS4
SS5
SS6
SS7
SS8
chemicals (g FU 1) NaOH
24
40
HCl
23
CaO
50
24
10
36 8
13
4
382
O2
355
22
energy used (kJ FU 1)
CH4 (%)
30
7
28
Na2S2O3 biogas production (L FU 1)
34
160
176
280
959
948
1,866
68
1,476
220
240
280
400
185
20
202 58.3
222 62.3
354 65.0
4,375
4,584
5,536
68
1,476
350
303
354
505
234
65.0
65.0
65.0
65.0
65.0
65.0
65.0
65.0
59.4
65.0
65.0
65.0
65.2
N
9.5
15.1
15.1
10.2
11.6
19.6
20.2
19.9
8.9
10.4
9.6
12.2
9.6
9.6
9.6
6.7
P
1.6
2.5
2.5
1.7
1.9
3.2
3.3
3.3
12.7
14.8
13.6
17.3
13.6
13.6
13.6
9.5
nutrients content (g FU 1)
a
Names in italics indicate that those scenarios have not been experimentally tested but inventoried based on estimations.
nutrients and heavy metals content in sludge varied during the experimental period, the average values reported in Table 1 were considered for all scenarios. One operational advantage of the use of pretreatments is that it allows operating the anaerobic digesters at higher OLR. Consequently, the OLR varied among the different scenarios, each corresponding to “optimal operational conditions”, that is, the highest OLR enabling steady state performance and conversion of at least 50% of the COD input into biogas (Table 2). Energy was not only used in some pretreatments but also during the AD process for stirring and heating. In this study, it was assumed that the biogas produced in the reference scenarios (KW-1 and SS-1) was sufficient to cover the heating and stirring requirements of the digester. The “extra” biogas produced as a consequence of the pretreatment application was considered to be burned in a cogeneration plant (gas engine) in order to produce both electricity and heat31 (power generation rate of 40% and a heat generation rate of 50%), and the associated air emissions of CO, CO2, CH4, and NMVOC (nonmethane volatile organic compounds) from the biogas combustion were included by using emission factors reported in literature.32 Regarding digestate application in agriculture, P and N were regarded as organic fertilizers that reduce the need of synthetic fertilizers by 70% and 50%, respectively.33 Nutrientrelated emissions (N to air as N2O and NH3 and P to water as PO43‑) were estimated by means of emission factors from literature.32 Background data related to the production of chemicals, energy, and fertilizers came from the Ecoinvent Database v2.34 36 Table 3 shows a summary of the input and output data collected or calculated for the 16 scenarios. A more detailed compilation of data can be found in the SI (Tables S1 and S2). Life Cycle Impact Assessment. This stage characterizes the environmental pressures related to the inventory by means of impact assessment models, and makes use of category indicators to condense and explain the inventory results. In this study, a well-established midpoint methodology was applied, the CML 2 baseline 2000 v2.05 implemented in the SimaPro 7.3 software (http://www.pre.nl/content/simapro-lca-software). Among the impact categories described by this method,37 the following were selected: abiotic resource depletion potential (ADP), eutrophication potential (EP), global warming potential (GWP), human toxicity potential (HTP) and terrestrial ecotoxicity potential (TTP).
’ RESULTS Table 4 displays the outcomes of the classification and characterization steps of the impact assessment stage. The selected category indicators are separately presented since each one is expressed in its corresponding unit of reference. For a quick comparison of the different alternatives examined, the relative performance of the individual scenarios within each category indicator (stated as the percentage ratio between the value of the scenario and the maximum value of that indicator within the same waste group) is also shown. Pressurize-depressurize (KW7, SS7) and chemical pretreatments (KW2, KW3, SS2, SS3) shared the top positions with minimum or even positive net impact on the environment, except for eutrophication, where the references (KW1, SS1) were the best scenarios. The latter is probably related to the OLR applied in each scenario, because taking into account that nutrients are hardly removed during AD (some precipitation can occur), the higher the OLR applied, the higher the amount of solids entering the digester, and thus, higher nutrients levels being discharged per FU. Among KW scenarios, KW-8 (ozone) was located at the end of the ranking in all categories, which can be explained by the high use of chemicals and energy. In the case of SS, the greatest impacts were observed in the most energydemanding scenarios, that is, SS4 (thermal), SS5 (thermo-acid) and SS6 (freeze thaw). Abiotic Resource Depletion Potential (ADP). ADP covers all potential impacts of the extraction of mineral and fossil fuels.37 Figure 1 shows the contribution of the different elements to this impact category (see Table S3 in the SI for detailed information). Taking into account that the Spanish energy profile is heavily reliant on fossil energy (80%), it was not surprising that the energy use was the main contributor to this category indicator. Therefore, the most damaging alternatives were those scenarios consuming large amounts of energy, that is, thermal (KW4, KW5, SS4, SS5) and freeze thaw (KW6, SS6) pretreatments. This fact was particularly remarkable in the SS scenarios because the volume of SS pretreated was higher (KW was pretreated without dilution). On the contrary, the consumption of chemicals had a minor impact, except for ozonation, with contributions of 62% and 57% in KW8 and SS8, respectively. This is mainly due to the higher amounts of oxygen consumed (Table 3) rather than the emission factor associated to its manufacture process (0.00301 g Sb-eq g 1 O2), which is 10308
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Table 4. Absolute and Relative (Within Each Waste Group) Results from the Impact Assessment Stage (See Table 2 for the Description of the Scenarios). Positive Values Mean a Negative Impact on the Environment, While Negative Values Mean a Positive Impact (Avoided Impact) on the Environmenta category indicator scenario
3‑
ADP (g Sb-eq/FU)
EP (g PO4 -eq/FU)
KW1
0.20
7.4%
1.27
35.8%
99.8
20.2%
38.1
KW2
0.16
5.7%
1.85
52.1%
106.7
21.6%
22.1
KW3
1.21
43.7%
2.06
58.0%
23.1
KW4
0.50
18.2%
1.65
46.4%
170.7
KW5 KW6
0.46 0.58
16.4% 20.9%
1.92 3.01
54.1% 84.7%
176.8 237.1
KW7
3.25
117%
2.48
KW8
2.77
100%
3.55
69.8% 100%
GWP (g CO2-eq/FU)
232.5 494.5
4.68%
HTP (g 1,4-PDB-eq/FU) 14.7% 8.53%
TTP (g 1,4-PDB-eq/FU) 1.8
22.1%
1.7
21.0%
60.8
23.2%
3.1
38.4%
34.5%
113.1
43.6%
4.6
56.8%
35.8% 48.0%
139.1 180.8
53.6% 69.6%
5.4 7.9
66.4% 97.9%
3.4%
2.7
47.0% 100%
8.7 259.6
100%
8.1
33.2% 100%
SS1
0.50
12.1%
1.71
46.1%
11.3
1.6%
548.1
49.4%
19.3
49.5%
SS2
0.66
15.8%
2.04
55.2%
21.8
3.1%
652.0
58.7%
22.7
58.4%
57.0%
22.2
SS3
2.03
49.1%
2.07
SS4
2.66
64.2%
3.70
SS5 SS6
4.15 3.97
100% 95.9%
3.53 3.54
SS7
4.67
113%
SS8
2.20
53.0%
56.1% 100% 95.5% 95.6%
128.4
18.4%
521.2
74.7%
1,110
633.1
698.0 697.5
100% 99.9%
1,029 1,047
100% 92.7% 94.3%
38.9 35.3 36.6
57.0% 100% 90.6% 94.0%
1.78
48.1%
420.7
60.3%
552.3
49.8%
20.7
53.1%
2.31
62.3%
388.6
55.7%
626.8
56.5%
19.8
50.9%
a
Names in italics indicate that those scenarios have not been experimentally tested but inventoried based on estimations. ADP: Abiotic Resource Depletion Potential; EP: Eutrophication Potential; GWP: Global Warming Potential; HTP: Human Toxicity Potential; TTP: Terrestrial Ecotoxicity Potential.
Figure 1. Contribution of different elements to abiotic resource depletion (ADP).
the second lowest among all chemicals used in this study. Actually, oxygen was responsible for 66% and 85% of the impact associated to chemicals consumption in KW8 and SS8, respectively (SI Table S3). In most scenarios, the impact was compensated by the enhancement of the AD performance, which is reflected in the avoided products (energy and fertilizers). In this category, the avoided energy derived a greater benefit than the avoided fertilizers in most cases. Yet, it was not enough to balance the impact associated to the energy requirements of the pretreatments, except for the pressurize-depressurize method. In fact, the best results were achieved in these scenarios (KW7, SS7), which were net energy and fertilizers producers (Table 4).
Eutrophication Potential (EP). EP covers all potential impacts of excessively high environmental levels of macronutrients (mainly N and P), which may cause an undesirable shift in species composition and elevated biomass production.37 In all scenarios, nutrient-related direct emissions from the application of digestates on agricultural soil dominated the impact in this category (Figure 2 and SI Table S4). The poorest performance was observed in those scenarios where higher OLRs were applied, that is, higher solids entering the digester, and therefore, higher amounts of nutrients per FU. This impact is correlated with the benefit derived from the provision of a biosolids product that displaces the use of N- and P-based fertilizers, as both elements directly 10309
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Figure 2. Contribution of different elements to eutrophication potential (EP).
Figure 3. Contribution of different elements to global warming potential (GWP).
depend on the nutrient content (Tables 3, SI S1 and S2). The use of energy and chemicals shared the second position, each being more relevant in the energy-using (KW4, KW5, KW6, SS4, SS5, SS6) and chemical (KW2, KW3, KW8, SS2, SS3, SS8) pretreatments, respectively. In this category, the avoided products did not compensate the environmental impact, and consequently, the reference scenarios were the most preferable options for both types of waste (Table 4). Global Warming Potential (GWP). GWP is defined as the impact of human emissions on the radiative forcing of the atmosphere.37 In this work, the emissions of biogenic CO and CO2 have been disregarded (characterization factors equal to 0) according to Docka (2010).32 As expected, Figure 3 shows a relationship between energy use and the GWP, being the worst scenarios KW6, KW8, SS4, SS5, and SS6 (Table 4), all of them characterized by a high energy demand (>1.4 MJ FU 1). Behind the background emissions associated with energy production, this category is dominated by the direct emissions from biogas burning (CH4 and NMVOC) and digestate application in the soil (N2O) (SI Table S5). Only in KW8 and SS8 (ozonation), the emissions derived from the use of chemicals accounted for more
than 40% of total impact (Figure 3), once again due to the use of oxygen, whose production is highly energy intensive. In this category, the environmental benefit (negative values in figures) was always higher in the pretreatment scenarios than in the reference, and the rate of this benefit was mainly related to the increased biogas produced as a consequence of the application of the pretreatment (Figure 3 and Table 3). For both types of waste, pressurize-depressurize scenarios (KW7, SS7) performed the best, followed by the acid scenarios (KW3, SS3), all of them having net positive impact (i.e., negative category indicators) on global warming potential (Table 4). Human and Terrestrial Toxicity Potential (HTP and TTP). HTP covers the impacts of toxic substances present in the environment on human health, while TTP refers to impacts of toxic substances on terrestrial ecosystems.37 In these impact categories, there is a significant difference between both wastes (Figure 4), caused by the extremely high contribution of the direct emissions of heavy metals to soil in the SS scenarios (>63% in HTP, SI Table S6, and >58% in TTP, SI Table S7). This pattern can be explained by the nature of heavy metal pollution, mostly diffuse with industrial origin, and thus making very difficult 10310
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Figure 4. Contribution of different elements to human toxicity potential (A) and terrestrial toxicity potential (B).
the control of heavy metals presence in sewage sludge.38 On the contrary, in order to protect human health, food products are very controlled and, consequently, the presence of heavy metals is not expected. This dominance of heavy metals has been found by other LCA studies,11,12 and consequently, the application of digestates in sectors where no risk of contaminating the animal or human food chain exists (e.g., floriculture) has been suggested. 13 In addition, the environmental burdens associated to HTP (Figure 4A) were more than 25-fold those of TTP (Figure 4B), which positions the human being in a more protective perspective than the ecosystem. Overall, the relative position of the different scenarios within these two categories was quite similar, being the pressure-depressure pretreatment (SS7, KW7) one of the best options, whereas thermal and freeze thaw methods showed the poorest performance due to the indirect emissions of toxic substances of the background processes associated (electricity production).
’ DISCUSSION Sixteen scenarios combining pretreatment, anaerobic digestion and agricultural disposal were environmentally modeled using life cycle assessment. The results show that the environmental burdens associated to the application of any pretreatment
prior to anaerobic digestion cannot be excluded. If for illustrative purposes, we value all the selected indicators equally, pressurizedepressurize and chemical (acid or alkaline) methods could be recommended on the basis of their net environmental performance. On the contrary, thermal, freeze thaw and ozonation alternatives would entail an environmental damage as the improvement of the AD process does not compensate the environmental burdens associated to the pretreatment. Energy vs Chemicals. Any pretreatment makes use of some form of energy (pressure, translational, rotational, thermal, or electrical) and/or chemicals and both resources can have a very diverse effect on the environment and humans. In this study, the scenarios using energy in their pretreatments possess a higher impact than those using chemicals. Therefore, though thermal pretreatments appear to be the most suitable for the improvement of waste stabilization, efforts must be done to enhance the energy balance by using the waste residual heat to maintain the temperature of the digester, by applying more efficient methods for waste disintegration such as microwave heating9 and for biogas utilization39 or by making use of a more sustainable energy production profile, that is, less dependent on fossil fuels. Among energy-using pretreatments, mechanical disintegration (i.e., pressurize-depressurize) is preferred over thermal methods due to the lower energy demand without compromising the increase in 10311
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Environmental Science & Technology biogas production. Among chemical processes, category indicators invert the option preferences between acid and alkaline methods. The former performs better in terms of ADP and GWP, while the latter does in EP and toxicity potentials. Further research on alternative chemicals as well as on the optimization of the required dose are likely to entail a reduction of the environmental burdens of these scenarios. Biogas Production vs Digestate Quality. The two beneficial impacts derived from AD of solid waste are the biogas production (avoided energy) and a biosolids product (digestate) suitable for agricultural application (avoided fertilizers). When analyzing the operational effectiveness of the use of pretreatments, most researchers focus on the increase in biogas production and scarce information is provided on digestate quality. In terms of environmental performance, this study shows that the benefit derived from pretreatments application regarding biogas production is greater than in terms of improving the fertilizing capacity of digestates when both avoided products were measurable. This is probably related to the intrinsic purpose of pretreatments, that is, making organic matter available to boost biogas production. However, the more stringent limitations for biosolids application in agriculture would make practitioners to work hard to control digestate quality. Substrate Characteristics. Overall, no significant differences were observed between the analysis of kitchen waste and sewage sludge scenarios. Yet, there are some aspects that need to be addressed. Organic solids content in the waste is relevant not only because it determines the amount of nutrients and pollutants contained in the waste, affecting particularly eutrophication and toxicity potentials, but also because it establishes the loading rate at which the digesters can be run, and consequently, the biogas production rate. Moreover, substrate composition (i.e., biodegradability) will determine the effect of the pretreatments, since those wastes containing poorly digestable materials (e.g., cellulose or lignin), such as agroindustrial residues, would experience a more significant improvement in biogas production at the same cost (same energy and/or chemical consumption), thus yielding a more positive environmental impact. On the other hand, literature data were required to fill the gap of metals content in KW and differences are likely to exist between food wastes due to, for example, food legislation or habits of consumption. This demonstrates that general recommendations should be avoided and individual analyses should be conducted. Consideration of Offsets. One of the uncertainties of this study is the extent of potential offsets. The biogas produced in the reference scenarios was assumed to be sufficient to cover the energy needs of heating and stirring of the digester; however, Hospido et al. (2010)16 only compensated the heating demand and reported values for stirring of 1.68 and 0.84 kWh FU 1 for SRT of 20 and 10 days, respectively. Soda et al. (2010)40 showed that AD coupled to power generation processes resulted in excess energy production if high sludge-loading rates were applied. The inclusion of energy for stirring in this study would increase significantly the environmental burdens of all scenarios, especially those of KW due to the 2-fold SRT applied. Yet, since this study is focused on a comparative analysis, the ranking of the scenarios would not be affected. Methodology Aspects. In this study, biogenic CO2 and CO emissions have been excluded from the impact assessment. However, according to Griffith et al. (2009),41 around 20% of the total organic carbon found in wastewater has a fossil origin and therefore the figures obtained on GWP for SS scenarios would have
POLICY ANALYSIS
been underestimated. A sensitivity analysis was performed on the SS scenarios assuming the distribution reported,41 that is, 20% of fossil origin and 80% of biogenic C source, and the results revealed that, although the direct emissions associated to biogas combustion increased 3-fold, the general conclusions and the option preferences would not be reversed. Another aspect to be mentioned is the high significance that, in general, impact assessment methodologies give to heavy metals in comparison to other pollutants,42 44 probably as a combined result of the well-established toxicity models existing for these compounds and their bioaccumulative character. The FU has been here defined in terms of the system input as the function of our system was to guarantee the appropriate management of a waste stream rather than obtaining a particular product. In fact, our system had a combined objective: increase biogas production and provide a final substance with fertilizing value. Therefore, the definition of the FU in an output basis as recommended by other authors25 was not appropriate. Finally, we have used a “well to tank” approach when defining the boundaries related to the final use of energy (both heat and electricity) instead of a “well to wheel” system as suggested by other authors25 due to the comparative characteristics of this study. Sustainable Pretreatments Application. Reconciling operational, economic and environmental aspects for sustainable pretreatment application is not straightforward. Yet, this study in combination with financial and operational data from literature5,6,8,9,45,46 suggests that mechanical (e.g., pressurizedepressurize) and chemical pretreatments appear to be the most suitable at this stage. Further work may examine full-scale experience and a more integrated and energy-efficient scheme of waste management with the inclusion of subsequent digested solids treatment processes (dewatering, transportation, spreading) and biogas utilization pathways.
’ ASSOCIATED CONTENT
bS
Supporting Information. Assumptions made during the inventory analysis of nonexperimentally tested scenarios, additional inventory data for KW and SS scenarios and contribution of different elements to abiotic resource depletion potential, eutrophication potential, global warming potential, human toxicity potential and terrestrial toxicity potential. This material is available free of charge via Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +34-881-816020; fax: +34-881-816015; e-mail: marta.
[email protected] (M.C.),
[email protected] (A.H.). Author Contributions
Both authors equally contributed to the work.
’ ACKNOWLEDGMENT This research was funded by postdoctoral contracts from the Xunta de Galicia for Dr. Marta Carballa (IPP-08-37) and Dr. Almudena Hospido (IPP-06-57). We also thank the Spanish Ministry of Education and Science (Project CTM2010-17196) and the Xunta de Galicia (Projects 09MDS010262PR and GRC2010/37) for its financial assistance. Dr. Jingxing Ma and 10312
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Environmental Science & Technology Thu Hang Duong are acknowledged for their well-done experimental work.
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