Combined Life Cycle Environmental and Exergetic Assessment of

Feb 13, 2014 - Four commonly used sewage sludge treatment techniques in China are compared, each with and without the combination of anaerobic digesti...
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Combined Life Cycle Environmental and Exergetic Assessment of Four Typical Sewage Sludge Treatment Techniques in China Jun Dong, Yong Chi,* Yuanjun Tang, Fei Wang, and Qunxing Huang Institute for Thermal Power Engineering, Zhejiang University, Hangzhou, China ABSTRACT: Four commonly used sewage sludge treatment techniques in China are compared, each with and without the combination of anaerobic digestion: composting, co-combustion in power plant, thermal drying-incineration, and cement manufacturing. Life cycle assessment (LCA) is used to quantify the environmental burden, while exergetic life cycle assessment (ELCA) is supplemented to measure the resources conversion efficiency. Afterward, abatement exergy is adopted to determine their degree of environmental sustainability, so that all environmental issues associated with resource use and environmental emissions can be solved simultaneously. Results show that anaerobic digestion is an effective pretreatment approach to reduce environmental burden. Thermal drying-incineration is preferable to co-combustion and cement production, since fossil fuel combustion is the dominant cause of emissions. Composting poses a positive effect to mitigate global warming, but it introduces high heavy metals to the soil. Results from ELCA reveal that thermal techniques present higher resources conversion efficiency than a biological system. Adopting anaerobic digestion obviously improves the performance of composting, but it has reduced the total energy recovered in thermal techniques. For process improvements, an efficient sludge predrying is important; and the use of a combined heat and power system can also provide more effective recovery of energy.

1. INTRODUCTION The vast growth of urbanization has resulted in a dramatic increase of sewage sludge generation. In China, approximately 31.1 and 57.8 million tons of sewage sludge was produced in 2001 and 2011,1 respectively, corresponding to an annual increase rate of 6.4%. Given that sludge contains various pollutants, it should be treated properly to avoid contamination to the environment. The threat of climate change and resource scarcity has become a critical bottleneck to global sustainable development. Sustainable management of sludge means to reduce environmental emissions and improve resource use efficiency. Landfill is not attractive, since it is land intensive and generates undesirable emissions to the environment. Meanwhile, a number of sludge treatment techniques have attracted increasing attention, such as composting, thermal dryingincineration, co-combustion, and cement manufacturing. They utilize the energy contained in the sludge and achieve cleaner and more efficient energy utilization. Composting decomposes the organics in the sludge, making it a soil conditioner, and returns nutrients back to the soil. Incineration is an approach for energy recovery. The main difference between cocombustion and thermal drying-incineration is the coal addition amount, since the drying process could reduce the amount of coal used to evaporate the sludge moisture. Sludge used in cement production also saves the fuel fed into the kiln, and the residual ash is a replacement of raw materials to produce cement. Besides, anaerobic digestion is a sludge pretreatment technique that utilizes the energy in the sludge. Anaerobic digestion could convert approximately 40−50% of the volatile solid into biogas;2 therefore, it not only permits the recovery of energy but also considerably reduces the sludge volume to facilitate its transportation. Since different techniques exhibit a wide range of environmental impacts and resource use, it is © 2014 American Chemical Society

valuable to search for a most superior one, especially for possible combination with anaerobic digestion. Recently, life cycle assessment (LCA) has occupied a prominent position in assessing the environmental burden and natural resource depletion associated with a process.3 Guided by ISO standards,4 LCA becomes an effective tool across all life cycle phases from raw materials acquisition to final disposal. For sludge treatment, a number of LCA studies have been carried out. In contrast, the analysis considering anaerobic digestion is rather limited. Cao et al.5 conducted a comparison of sludge pyrolysis with and without anaerobic digestion, but the impacts of other techniques are not included; Hong et al.6 contributed an array of sludge treatment processes, but only several impact categories are involved; Murray et al.7 evaluated different combinations of sludge treatment and end-use options, but a parallel comparison of techniques before and after anaerobic digestion is lacking. In this sense, it is essential to give a comprehensive LCA of different sludge treatment techniques. Each with and without anaerobic digestion should be investigated; and different environmental impacts should be involved to better obtain the most sustainable system. However, LCA is insufficient to characterize natural resource depletion, since it takes no account of the nonenergetic resources, or just uses reserve-to-use ratios to measure them. Exergy analysis can overcome this shortcoming. When extended to a life cycle perspective, exergetic life cycle assessment (ELCA) measures the total exergy of natural resources extracted out of the environment in all steps of a process. Using ELCA, both fuels and nonfuel resources can be quantified in the same unit (Joules), and the issue of natural Received: December 7, 2013 Revised: February 13, 2014 Published: February 13, 2014 2114

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during treatment are also omitted, since they are usually sent back to the wastewater treatment plant. Sludge treatment and background systems are defined to distinguish the direct and indirect emissions. Besides, allocation is necessary since useful commodities such as electricity, heat, compost, and cement are produced. Recovered energy is considered as substitution for fossil fuels, while recovered compost and cement avoid the conventional production of chemical fertilizer and cement, respectively. One ton of sludge on a dry basis (1 t-DS) is defined as the functional unit. Several typical sludge treatment facilities in China, representing anaerobic digestion, composting, co-combustion, thermal drying-incineration, and cement production, respectively, are selected to carry out the inventory. Data are mainly taken from the site-specific measurement, either by field survey or by operation report. Sludge characteristics are based on the average values of the sites investigated (Table 1). Background data related to raw material production are collected from Europe, since the relevant information in China is limited.

resource depletion can be solved by calculating the total exergy consumption. Therefore, ELCA can become part of LCA, representing one environmental impact as resource consumption. Furthermore, another advantage of exergy is its ability to measure environmental damage. Emissions can be quantified by abatement exergy, the exergy required for the abatement of emissions.8,9 Using abatement exergy, different impacts in LCA can be integrated into thesame unit. ELCA and LCA are combined together, and all environmental problems associated with resource use and environmental emissions can be evaluated simultaneously. So far, some ELCA researches have been applied to assess biofuel systems10,11 and green building design,12,13 or to determine the thermodynamic perfection degree of a process.14,15 However, only a few ELCA studies were conducted in the waste management field.16−18 Yet the sludge treatment has never been assessed, not to mention its combination with LCA. Therefore, a quantitative ELCA of different sludge treatment techniques is evaluated in the present study, not only to compare the resource use efficiency but also to measure the overall environmental damage. Accordingly, the overall goal of the present study is to conduct a combined life cycle environmental and exergetic assessment of different sewage sludge treatment techniques in China, each with and without the combination of anaerobic digestion. LCA is used to measure the environmental burden, while ELCA quantifies the relevant resource use. Afterward, the environmental sustainability degree of each system is determined by integrating the LCA and ELCA results. The study aims at identifying the most environmental friendly and energy efficient sludge treatment system, and the results can serve as the scientific basis for developing sludge management strategies in the future.

Table 1. Sludge Characteristics Employed in the Study parameter moisture content of mixed sludge (%) ultimate analysis (as % of DS)a C H O N S ash organic matter (% DS)b nitrogen (N) phosphor (P) low heating value (MJ/kg-DS)

value 99.30 30.05 3.65 16.57 4.25 0.92 44.56 47.20 4.25 1.59 10.45

a

The ultimate analysis follows the National Standard GB/T 476-2008. The organics are analyzed based on the National Standard CJ/T 3092009.

b

2. MATERIALS AND METHODS 2.1. Definition of the Systems. Figure 1 shows the system boundaries of the study. In general, the evaluation comprises sludge

2.1.1. Sludge Pretreatment. Sludge needs to be thickened and dewatered before treatment. The sludge moisture content after thickening is about 97%. It is then dewatered by belt filter. Polymer flocculation (polyacrylamide, PAM) is added to improve the dewatering ability, and the moisture content is further reduced to 78%. If anaerobic digestion is adopted, it is conducted prior to dewatering. The selected facility is mesophilic type, and the organic degradation attains ca. 38%. Energy recovery is accomplished, since a significant amount of biogas is generated, which accounts for 152.6 m3 per ton of dried sludge. The biogas, with energy content of 21.8 MJ/ m3, is further transferred into the gas engine to produce electricity. Some uncollected biogas is released as fugitive emission. Since the fugitive loss is difficult to measure, the average gas collection efficiency of 97% is estimated according to the literature.20 2.1.2. Composting. Composting is a biological technique that uses microorganisms to break down the organic matter aerobically. Straw, ash, and other structure materials are added to optimize the chemical properties (e.g., C/N ratio) of the sludge. The finished compost contains nutrients and, thus, can be agriculturally applied to replace mineral fertilizers. According to the operating data,21 around 57% organic matter degrades, and the sludge moisture after composting is approximately 40%. Total N and total P after composting is 2.01% and 1.27% of digested matter on a dry basis, respectively. Emissions from the degradation of sludge are mainly composed of biogenic-CO2, NH3, and N2O. Some CH4 and H2S may generate if anaerobic conditions occur within composting heaps. Due to the lack of data, the information of N2O is on the basis of the literature.22 Besides, indirect emissions are related to the provision of fuels for turning and managing of the sludge, as well as the transportation of feedstock and compost.

Figure 1. System boundaries of the study. pretreatment, main treatment, final disposal, and transportation. Four systems are analyzed: composting (S1), co-combustion in power plant (S2), thermal drying-incineration, (S3) and cement manufacturing (S4). They are representative of commonly used sludge treatment techniques in China. In order to investigate the effect of anaerobic digestion, each system is further divided into two subunits: one is the baseline case (situation a), while the other is in combination with anaerobic digestion (situation b). Anaerobic digestion is conducted prior to dewatering, and a drying step is included in both S3 and S4. Plant construction is not included because its emissions are negligible compared to the total impacts.19 Waste liquids generated 2115

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2.1.3. Co-combustion in a Coal-Fired Power Plant. The dewatered sludge, with a moisture content of 78%, is mixed with coal at a ratio of 24% for co-combustion.23 The chosen power plant is of fluidized bed type. Sludge and coal are combusted at 850 °C, and exhaust gases from the combustion chamber are used for electricity generation. Twenty percent of the produced electricity is required to meet the input energy demand; with the remaining delivered to the city’s power grid. The inorganic fraction from both sludge and coal, which accounts for about 200 kg per ton of the feedstock, is left as incineration ash. The bottom ash and the fly ash are transported to a special waste treatment center, where the former is further reused as road materials and the latter is deposited in a hazardous landfill site. 2.1.4. Thermal Drying-Incineration. The sludge is semidried to 40% before incineration. An important amount of coal can be saved for evaporating moisture contained in the sludge. A combined heat and power (CHP) system is used, with an overall thermal efficiency of 83%.24 The exhaust steam from the backpressure turbine is used to offset the energy demand for drying, with the surplus sent for district heating. The drying machine is a disc type, and the thermal utilization efficiency of the drying process is 85%.24 The semidried sludge can realize monoincineration; however, an extra 9% coal is added per ton of the dried sludge to maintain stable combustion and ensure adequate electricity generation. Meanwhile, incineration residues are also transported to the special waste treatment center. 2.1.5. Cement Manufacturing. Sludge is first dried to 15% using the waste heat from the rotary kiln. Afterward, it is combusted at 1400 °C with other feedstock (1.6 tons of coal, 15.8 tons of limestone, 1.6 tons of iron ore, and 1.4 tons of clay per dried ton of sludge) to produce cement. Compared to conventional production, the dried sludge can be regarded as a biofuel and reduces the amount of coal fed into the kiln. Meanwhile, material recovery is also attained. The sludge ash is rich in minerals and substitutes the use of limestone and clay. According to the operating data, about 12.7 tons of cement is produced per ton of the dried sludge considering the combustion loss in the kiln. 2.1.6. Final Use and Disposal. Compost produced in S1 is transported to farmland as soil conditioner. The N and P nutrients avoid the production of an equivalent amount of N-based and P-based chemical fertilizer. Diesel and electricity are consumed during compost spreading; unfortunately, heavy metals are also transferred to the soil. A government-qualified treatment center is responsible for the treatment of incineration residues. Bottom ash is reused, and the subsequent process is ignored. Fly ash is stabilized followed by landfill. Stabilization data from Europe25 are used because of the limited statistical information in China. This assumption leads to some deviations but is still reasonable due to the small amount of fly ash generated. No air emissions are assumed to be released after landfill,26 but the leaching of heavy metals is considered based on the study of Fruergaard et al.25 For S4, cement is the only product and is sold as a building material. 2.1.7. Transportation. The same coal is assumed to be used in S2, S3, and S4 and is transported via train. It is mined from Shanxi Province, with its composition presented as follows: Mar 7.5%, Aar 22.3%, Car 60.1%, Har 3.2%, Oar 5.2%, Nar 0.9%, and Sar 0.8%. Incineration ash, compost, and cement are all transported by diesel trucks. Diesel consumption is 0.4 L/km, and the combustion technology is in compliance with Euro 3 standards. For a clear and rational assessment of all systems, the coal delivery distance is set at 1500 km, with that distance for ash, compost, and cement estimated at 50 km. They are representative of typical situations in China. 2.2. Methods. 2.2.1. Life Cycle Assessment. LCA is conducted to assess the environmental impacts. According to the ISO 14040 standards,4 the LCA framework consists of four phases: (1) goal and scope definition; (2) life cycle inventory (LCI); (3) life cycle impact assessment (LCIA); and (4) interpretation. Once the system is defined, all resources and emissions that cross the boundary are collected to compile the LCI. LCIA is aimed at aggregating the LCI result into some concerned impacts. Based on the findings,

conclusions are obtained as interpretation, in order to propose practical improvements. The EDIP 97 method27 is used to establish the impact assessment. Six impacts are quantified: global warming (GW), acidification (AC), nutrient enrichment (NE), human toxicity via air and soil (HTa, HTs), and ecotoxicity via soil (ETs). Toxic impacts via water are not considered, since they are mainly caused by the heavy metals in the wastewater, but the relevant pollutants are not within the scope of the study. According to EDIP 97, equivalence factors are used for the characterization of each category, for example, CO2-equivalents for GW. Table 2 summarizes the selected impact categories together with the characterization unit.

Table 2. Selected Impact Categories and Characterization Unit Based on EDIP 97 impact category

characterization unit

global warming (GW) acidification (AC) nutrient enrichment (NE) human toxicity via air (HTa) human toxicity via soil (HTs) ecotoxicity via soil (ETs)

kg CO2-equivalent kg SO2-equivalent kg NO3−-equivalent m3 air m3 soil m3 soil

2.2.2. Exergetic Life Cycle Assessment and Abatement Exergy. Exergy is based upon the second law of thermodynamics that all macroscopic processes are irreversible; that is, the quality of energy decreases. By definition,28 exergy is the maximum amount of work obtained from a system in equilibrium with the reference environment. Hence, unlike energy, exergy reflects the quality of energy. Due to this property, exergy analysis is widely applied to quantify the magnitudes and the location of exergy losses within industrial processes.29 But, in recent years, exergy has gained considerable attention as an environmental tool to assess the quantity and quality of a resource, since it represents the maximum portion of the resource that can be converted into work. Particularly, the life cycle-based exergy quantifies the cumulated exergy consumption of a system from cradle to grave. Its definition is similar to that of LCA; thus, ELCA can be supplemented as part of LCA,17 representing one impact category to assess resource consumption. Cumulative exergy consumption (CExC) is used to measure ELCA, with the reference environment taken from Szargut et al.30 CExC represents the total exergy of natural resources delivered to the system in all links of the production chain that starts with resource exploitation and finally leads to the product.14,30 CExC of different resources are additive; thus, the resources conversion efficiency (ηrce) can be obtained by calculating the ratio of all CExC of output products and input resources:

ηrce =

(∑i Oi )CExC (∑j Ij)CExC

=

(Ouseful products )CExC (Ienergy + I materials)CExC

(1)

where I and O represent input and output flows; i and j are the number of output and input flows, respectively; input flows are divided into energy and nonfuel materials; and output flows stand for different useful products. When comparing different sludge treatment techniques, the useful products are specified as compost, electricity, heat, and cement: ηrce = =

(∑i Oi )CExC (∑j Ij)CExC (Ocompost + Oelectricity + Oheat + Ocement )CExC (Ienergy + I materials)CExC

(2)

In addition, ELCA is also applied to assess the trade-off between the environment and the natural resources. Environmental sustainability should consider not only the exergy required as resource consumption but also the abatement exergy for the treatment of emissions.8 For 2116

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example, Cornelissen9 has calculated the abatement exergy of CO2 at 5.86 MJ, which means the exergy used to store 1 kg of CO2 in a depleted oil well at 8 MPa. Therefore, the abatement exergy is added to the input flows, and the indicator of environmental sustainability degree (ηesd) is proposed:

Table 4. CExC of Resources Required for Each System

(∑i Oi )CExC

ηesd = =

based on mass balance, but the recovered energy is determined according to energy conservation. Based on Table 3, a CExC library for the required resources is given in Table 4. CExC of sludge itself is

resource

(∑k Ik)CExC

energy

(Ocompost + Oelectricity + Oheat + Ocement )CExC (I resource + Ienergy )CExC + AbatEx

(3) materialsa

where AbatEx means the abatement exergy obtained in LCA. 2.3. Inventory Analysis. Based on the data obtained, the LCI for both LCA and ELCA is compiled. Since sludge is a biogenic source, its direct CO2 emission is omitted.31 Data on electricity production is taken from the average national electricity supply mix (75.9% coal, 3% oil, 2% natural gas, 17.6% hydropower, and 1.5% nuclear power);32 and the produced heat displaces the same amount of heat generated by boilers fed with coal. Table 3 shows the main inputs and outputs per functional unit. The data need to be recalculated if anaerobic digestion is adopted, since the sludge weight after anaerobic digestion is reduced. The calculation is

thickening dewatering anaerobic digestion main treatment

compost spreading ash treatment transportation

anaerobic digestion main treatment

S1

S2

S3

S4

Inputs electricity (kWh) 13.2 electricity (kWh) 48.8 PAM (kg) 5 electricity (kWh) 89.9

13.2 48.8 5 89.9

13.2 48.8 5 89.9

13.2 48.8 5 89.9

7.5 7479 2.0 18.9

7.5 196.4 0.3 0.9

7.5 733.5 0.2 1.6

55.5

6.1

49.7

5.4

0.8

0.07

16.8 1.2 31440

2.3 0.2 1418

1710

93.3

12.8

633.1

349.5

349.5

349.5

37396

982 11800

4.7

0.6

diesel (kg) electricity (kWh) diesel (kg) coal (t) straw (kg) hydrogen chloride (kg) sodium hydroxide (kg) limestone (t) iron ore (t) electricity (kWh)

7.5 76.5 11.3

electricity (kWh) heat (MJ) compost (t) cement (t) incineration ash (t)

reference

MJ/kg MJ/kg MJ/kWh MJ/MJ MJ/kg MJ/kg MJ/kg MJ/kg MJ/kg MJ/kg MJ/kg MJ/tkm MJ/tkm

14 14 32, 33 14 34 35 17 14 14 17 14 17 17

not considered, since sludge is a waste and the starting point of the process chain; therefore, only its exergy content (11.62 MJ/kg) is taken into account.

3. RESULTS 3.1. Results of LCA. Figure 2 provides the environmental impacts of GW, AC, and NE, while the toxic impacts (HTa, HTs, and ETs) are illustrated in Figure 3. Emissions and their contribution to each impact are distributed over six processes: pretreatment, main treatment, materials recovery, energy recovery, transportation, and final disposal. In general, cocombustion (S2) exhibits the highest impact on GW, NE, HTa, HTs, and ETs, while composting (S1) mostly contributes to AC. Oppositely, thermal drying-incineration (S3) is the most preferable to NE, HTs, and ETs. Compared to baseline cases, anaerobic digestion will pose a positive effect to the majority of impacts. With reference to each stage, the impacts are mainly affected by treatment or compensated by energy and materials recovery. For composting, land spreading is the dominant contributor causing toxic impacts to the solid. Results from GW show that composting leads to the least impact. With the implementation of anaerobic digestion, 72%, 19%, 33%, and 27% decrease in GW has been achieved for S1, S2, S3, and S4, respectively. Energy recovery from biogas utilization is the main contributor, compensating a remarkable part of GW caused by fossil-fuel based electricity generation. Although CH4 is emitted due to incomplete gas collection, its amount is small and provides an insignificant effect to the total GW. With respect to different techniques, GW is mainly attributed to the combustion of fossil fuels, since carbon from sludge is biogenic derived, especially for S2, which occupies the highest moisture content and consumes the largest quantity of coal. GW from S3 is low, since the predrying of sludge has saved a considerable amount of coal used. N2O emission is the dominant cause of GW in S1, which accounts for 320 times the GW potential than that of CO2. The principal contributors for AC are SO2, NOx, HCl, H2S, and NH3. Both negative values appear in S2 and S3, indicating that environmental savings can be achieved by sludge incineration with energy recovery. It needs to be mentioned

455a

15.8 1.6

59.6

diesel (kg) 0.7 electricity (kWh) diesel (kg) train transport (tkm) truck transport 34 (tkm) outputs electricity (kWh) 349.5

unit

61.23 26.81 12.37 1.98 17.20 4.50 14.50 9.96 2.69 4.36 6.18 0.19 3.13

CExC of PAM is not included due to the lack of data. This assumption is assumed to be acceptable because of the small amount used.

scenario parameter

CExC

a

Table 3. Main Input and Output Flows per Functional Unit (1 t-DS sludge) unit

parameter diesel coal electricity heat straw hydrogen chloride sodium hydroxide limestone iron ore compost cement train transport truck transport

1.7 12.7

a

Ash and structure materials for composting are omitted. They are byproducts from the coal industry, and no environmental burdens are associated with their production. 2117

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Figure 2. Standard environmental impacts (GW, AC, and NE) of different systems: S1, composting; S2, co-combustion; S3, thermal dryingincineration; S4, cement manufacturing.

highest contribution. NE of S2 is the highest of all. NOx emission is the main contributor, which is mainly attributed to the combustion of coal. Compared to S2, thermal dryingincineration poses a great positive effect on pollution reduction. NE from S1 is also high. NH3 emission is the main attributor, which occupies a high NE equivalent factor. Although part of emissions can be avoided by the substitution of chemical fertilizer production, the effect is not comparable to the overall environmental loading. Heavy metals are elements that decisively contribute to HTa, HTs, and ETs. S2 has the most contribution to all these three impacts, not only because of the high airborne heavy metal emissions, but also owing to their extremely high equivalent factors to these toxic impacts. Oppositely, S3 performs the best to HTs and ETs. HTa of S1 is the least, since no airborne heavy metals are generated during composting. All impacts from different systems have a significant decrease after anaerobic digestion except for HTs and ETs of S1. Generally, S1 exhibits high impacts to the soil. These impacts contribute greatly to the disposal phase, since heavy metals contained in the sludge will transfer to the soil after land spreading. HTs and ETs of S1 after anaerobic digestion are higher than the baseline situation. Based on mass balance, adopting anaerobic digestion before

that the acidity of ash itself is not considered. After anaerobic digestion, AC from S1 and S4 is effectively reduced by 26% and 31%, respectively. The benefit is brought by biogas utilization, which avoids the same quantity of energy produced by fossil fuels. This environmental saving is especially obvious for S4. Cement manufacturing uses the sludge ash as raw production material, and conducting anaerobic digestion prior to this process has attained extra energy recovery besides its material recovery. However, AC is increased by 16% and 7% in S2 and S3, respectively, since adopting anaerobic digestion is at the sacrifice of energy fed into the furnace. While taking a closer look at different stages, the loadings are mostly generated by S2 and S4 during treatment. These findings reveal that fossil fuel combustion is the dominant contributor producing SO2, NOx, and HCl. Using dried sludge for incineration is an effective way to reduce the additional amount of auxiliary fuel. Meanwhile, S1 exhibits the highest AC. A remarkable amount of NH3 is generated during sludge aerobic degradation; and the avoided emissions are relatively small compared to those of other techniques. NE for all techniques decreases after anaerobic digestion, with S3 presenting the least impact. Similar to GW and AC, emissions from the main treatment of S2 and S4 give the 2118

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Figure 3. Toxic environmental impacts (HTa, HTs, and ETs) of different systems: S1, composting; S2, co-combustion; S3, thermal dryingincineration; S4, cement manufacturing.

Figure 4. Resources conversion efficiency (ηrce) of different systems: S1, composting; S2, co-combustion; S3, thermal drying-incineration; S4, cement manufacturing.

Based on the LCA results, a parallel comparison of each technique is conducted. In general, the majority of impacts will have a remarkable decrease after anaerobic digestion. Compared to thermal drying-incineration, co-combustion is not an ideal technique, owing to its higher impact on most categories. The largest quantity of coal is consumed to

composting will reduce the amount of compost generated; therefore decreasing the avoided emissions from the substitution of chemical fertilizer production. Meanwhile, the toxic impacts of S4 are also relatively high, because the equivalent factors of airborne heavy metals, especially Hg and Cd, are more crucial than those of soil emissions. 2119

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Table 5. Abatement Exergy and Environmental Sustainability Degree of Different Systemsa baseline b

abatement exergy (MJ/t-DS sludge) environmental sustainability degree (%)c

with anaerobic digestion

S1-a

S2-a

S3-a

S4-a

S1-b

S2-b

S3-b

S4-b

3390.6 16.9

110004.6 62.7

4071.7 79.8

15201.8 32.1

1224.4 34.6

89441.0 63.0

2613.0 78.3

10978.2 33.9

a

S1, composting; S2, co-combustion; S3, thermal drying-incineration; S4, cement manufacturing. bAbatement exergy: the exergy for the abatement of emissions before releasing. cEnvironmental sustainability degree: cumulative exergy consumption output/(cumulative exergy consumption input + abatement exergy).

the trade-off between resource use and environmental emissions, some findings can be revealed. Co-combustion is inferior to thermal drying-incineration. The burning of coal significantly deteriorates the environment, and the moisture contained in the sludge will lower the energy to be utilized. Cement production leads to a low ηesd, not only due to its high environmental burden, but also the relatively low resources conversion efficiency. Meanwhile, ηesd of composting is also low, indicating that material recovery as soil conditioner is not an efficient approach for the conversion of energy contained in the sludge. When in combination with anaerobic digestion, ηesd of composting has considerably increased by 18% due to energy recovery from biogas utilization. However, ηesd of thermal techniques before and after anaerobic digestion is of most equal size. Adopting anaerobic digestion is at the sacrifice of energy fed into the furnace, reducing the net energy to be recovered. In this sense, conducting anaerobic digestion before sludge thermal treating needs to be further judged and discussed.

evaporate the moisture contained in the wet sludge. The burning of such a high amount of coal has resulted in an increasing amount of emissions released into the atmosphere. Realizing that fact, sludge predrying is an effective improvement. Besides, cement production is superior to co-combustion in all impacts except AC, but it is inferior to S3 due to the lack of energy recovery. Composting poses a positive effect to reduce GW and airborne heavy metals, but great efforts should be paid to control heavy metals released to the soil. 3.2. Results of ELCA. Figure 4 presents the resources conversion efficiency of different situations. For the baseline cases, ηrce of each technique in descending order is as follows: thermal drying-incineration, co-combustion, cement manufacturing, and composting. If anaerobic digestion is adopted, composting will surpass cement production and rank the third. ηrce of composting has increased by 88% after anaerobic digestion, proving that the presence of biogas utilization can achieve more efficient conversion of energy. Oppositely, its effect on S2, S3, and S4 is not obvious. Even in S3, ηrce is decreased. This can be explained from the perspective of energy balance. For anaerobic digestion, the energy is recovered through a gas turbine with 39% efficiency, which is lower than the overall thermal efficiency, 83%, brought by the CHP system for drying-incineration. With respect to different techniques, thermal techniques are more energy efficient than the biological system. S3 leads to the highest ηrce; S2 is inferior to S3, since more coal is consumed and the CExC value of coal is high. ηrce of S4 is lower than S2 and S3, attributed to the low CExC value of the output cement. Meanwhile, ηrce of S1 in the baseline case is the lowest, indicating that material recovery as soil conditioner is not a good way for the efficient utilization of sludge. Regarding that ηrce of S1 after anaerobic digestion is significantly increased, it can be concluded that direct heat recovery is superior to materials recovery for the use of sludge. 3.3. Results of Combined LCA and ELCA. Table 5 presents the abatement exergy and environmental sustainability degree of different systems. Abatement exergy for CO2, SO2, and NOx is 5.86, 57, and 16 MJ/kg, as provided by Cornelissen9 based on available technologies. Since the abatement exergies for other emissions have not been assessed, they are estimated proportional to the respective environmental impacts.12 Abatement exergy for heavy metals is omitted, because the relevant research is lacking and the values are unavailable to obtain. However, it can be easily updated if the data become available in the future. Results show that 64%, 19%, 36%, and 28% of abatement exergy has been saved for S1, S2, S3, and S4 after anaerobic digestion, respectively. Similar to LCA, S2 and S4 present high abatement exergy, which is primarily attributed to the combustion of coal. The environmental sustainability degree declines compared to the resources conversion efficiency. Especially in S2, ηesd is ca. 11% lower than ηrce. Since the indicator of ηesd represents

4. DISCUSSION The issues of pathogens, odor gas emissions, and leachate leakage will be tough without proper handling of the sludge. Hence, it is essential to investigate the situation if sludge is not treated. The LCA study from Hong et al.6 reveals that direct disposal of sludge exhibits the highest total environmental impact compared to composting, drying, incineration, and melting. Research from Pasqualino et al.36 also shows that landfill represents the worst scenario among agricultural use, composting, cement production, and incineration. With respect to ELCA, both the resources conversion efficiency and environmental sustainability degree of landfill are zero, since no useful commodity is produced. In this sense, sludge treatment techniques considered in this study are all useful measures to achieve cleaner and more efficient energy utilization. Meanwhile, some changes in operating conditions are conducted with the aim of process improvements. In the cases examined, the relatively good performance of thermal drying-incineration can be partly attributed to its efficient recovery of energy by the CHP system. Realizing that the total energy efficiency will increase if the production of electricity can be combined with heat recovery, the CHP system is assumed to be adopted in an anaerobic digestion process. Its effect on LCA and ELCA for each technique is illustrated in Figure 5, where GW is selected to represent LCA, since the changed tendency is the same. Results show that the GW loading from all systems is mitigated due to the increasing offset emissions from energy recovery. Meanwhile, both ηrce and ηesd of different systems are higher than the case when only electricity is recovered, indicating that the implementation of the CHP system is a good choice for the more efficient recovery of the energy. 2120

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natural resources extraction is much higher than the added loading.

5. CONCLUSIONS Four different sludge treatment techniques in China are compared, each with and without the combination of anaerobic digestion: composting, co-combustion in power plant, thermal drying-incineration, and cement manufacturing. LCA is used to quantify the environmental burden, while the resources conversion efficiency is examined by ELCA. Afterward, the environmental sustainability degree is measured, so that all environmental issues associated with resource use and environmental emissions can be solved simultaneously. Results from LCA show that the environmental burdens will have a remarkable decrease after anaerobic digestion. Thermal drying-incineration leads to the least impacts. Co-combustion is not an ideal technique, owing to its highest impact on most categories. Cement production also provides a high emission level due to fossil fuel combustion. Composting poses a positive effect to reduce GW and HTa, but great efforts should be paid to control its amount of heavy metals released to the soil. ELCA results further reveal that thermal techniques exhibit higher resources conversion efficiency than the biological system. Anaerobic digestion obviously increases the efficiency of composting but provides an insignificant effect to thermal techniques. Considering the combined effects of LCA and ELCA, the environmental sustainability degree shows that thermal drying-incineration has the best performance. For process improvements, sludge predrying is a prerequisite to thermal techniques; and the use of CHP system could provide a great positive effect to both anaerobic digestion and cocombustion. In conclusion, results from the present study can provide experience and a scientific basis for decision makers regarding developing sludge management strategies in the future. For further research, sensitivity analysis should be conducted to obverse the variations of the results. The effect of anaerobic digestion on sludge dewatering and the application of dried sludge monoincineration without coal should also be analyzed, to better find a more environmental friendly and energy efficient system.

Figure 5. LCA and ELCA performance of different systems after anaerobic digestion, if the combined heat and power system is equipped in the anaerobic digestion process: S1, composting; S2, cocombustion; S3, thermal drying-incineration; S4, cement manufacturing.

Co-combustion exhibits relatively high environmental burdens, and this process also needs to be improved. Previous analysis has revealed that the predrying of sludge is the key point. Meanwhile, the use of the CHP system can also provide additional advantages (Table 6). If the CHP system is adopted, all environmental impacts are reduced compared to the baseline situation in Figures 2 and 3. Especially, 31% and 58% decrease in GW and AC has been achieved, respectively. ELCA results further show that co-combustion will surpass thermal dryingincineration and occupies the highest ηrce and ηesd. Both these two efficiencies exceed 100%, since the avoided exergy from

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Corresponding Author

*Telephone: +86-0571-87952687. Fax: +86-0571-87952438. Email: [email protected]. Notes

The authors declare no competing financial interest.

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Table 6. LCA and ELCA Performance of S2-a (Cocombustion without Anaerobic Digestion), if the Combined Heat and Power System Is Equipped impact category

value

GW (kg CO2-equiv) AC (kg SO2-equiv) NE (kg NO3−-equiv) HTa (m3 air) HTs (m3 soil) ETs (m3 soil) resources conversion efficiency (%) environmental sustainability degree (%)

13510.0 −108.6 96.7 9.9 × 1009 858.7 54.5 130.1 116.6

AUTHOR INFORMATION

ACKNOWLEDGMENTS This project is supported by the National Basic Research Program of China (No. 2011CB201506), the National Natural Science Foundation of China (No. 51276168), and the Program of Introducing Talents of Discipline to University (B08026).

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REFERENCES

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