Life-cycle environmental and economic assessment of sewage sludge treatment in China

Journal of Cleaner Production 67 (2014) 79e87

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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro

Life-cycle environmental and economic assessment of sewage sludge treatment in China Changqing Xu, Wei Chen, Jinglan Hong* Shandong Provincial Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Shanda South Road 27, Jinan 250100, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 June 2013 Received in revised form 2 December 2013 Accepted 3 December 2013 Available online 13 December 2013

A cost-combined life-cycle assessment was conducted to estimate the environmental and economic burdens of 13 sewage sludge-treatment scenarios in China. Results showed that anaerobic digestion was a suitable alternative to reduce both environmental and economic burdens because this approach reduced dry mass volume and applied energy recovery. Landfill and incineration technologies had the highest and lowest environmental burdens, respectively. Direct heavy metal emissions generated from landfill and incineration processes contributed significantly to human toxicity and marine ecotoxicity. However, energy recovery from the landfill and incineration stages was important to reduce both environmental and economic burdens. This study indicated that a sewage sludge-treatment scenario with anaerobic digestion, dewatering, and incineration technologies was the most environmentally and economically suitable method to treat sewage sludge because of energy recovery. All new sewage treatment plants should be constructed to operate according to this method, and existing plants should be retrofitted. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Life-cycle assessment Life-cycle costing Anaerobic digestion Incineration Energy recovery Sewage sludge

1. Introduction Significant amounts of sewage sludge are produced from sewage treatment plants worldwide. To date, the average annual outputs of sewage sludge in Germany, England, France, and America are 22, 12, 8.5, and 71 MT, respectively. In China, more than 20 MT of sewage sludge is generated annually (MOUHUR and NDARC, 2011). Aggravated environmental problems and increasing cost of sludge processing pose great challenges for wastewater treatment plants and policy makers. Sludge contains large amounts of pathogenic organisms and heavy metals, which are harmful to human health and the environment (US EPA, 2007). Therefore, useful and effective methods are needed to remove pollutants, such as organic micro-pollutants and heavy metals. Approximately 71.5%, 40.6%, 10.8%, 38%, and 3.5% sewage sludge are treated in China by using thickening, dewatering, drying, anaerobic digestion, and composting, respectively (Ministry of Environmental Protection, 2010), with end-of-life treatment processes including landfill (31.03%), incineration (3.45%), and agriculture use (44.83%) (Wang et al., 2006).

* Corresponding author. E-mail address: [email protected] (J. Hong). 0959-6526/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jclepro.2013.12.002

Anaerobic digestion can regenerate electricity and heat by using the methane produced by sewage sludge. However, during the 11th Five-Year Plan in China, only 50 sewage treatment plants used anaerobic digestion. This number is less than 5% of all sewage treatment plants in China (Zhou, 2010). To provide useful information for policy makers on the rectification of sewage sludgetreatment plants, a comprehensive method for evaluating both environmental and economic burdens is highly needed. Life-cycle assessment (LCA) is used to evaluate the environmental burdens associated with the whole life-cycle treatment of a product, process, or activity (ISO 14040, 2006). LCA has been widely used for eco-labeling programs, strategic planning, and marketing. LCA applications also include product design, process improvement, and consumer education. LCA for sewage sludge-treatment has been widely studied worldwide. Yang et al. (1999) analyzed sludge treatment and disposal in China. However, they did not quantify the life-cycle inventory, and their results were ambiguous. Suh and Rousseaux (2002) evaluated five scenarios of sewage sludge treatment in France. These scenarios included incineration and landfill, lime stabilization and landfill, lime stabilization and land application, composting and land application, and anaerobic digestion and land application. Without considering energy recovery from landfill and incineration, they found that the anaerobic digestion and land application scenario was the most environmentally friendly

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method to treat sludge. Lundin et al. (2004) assessed the environmental and economic consequences of four options for sewage sludge in Sweden. These consequences include agricultural application, co-incineration with waste, incineration with phosphorus recovery, and fractionation. The conclusion was that agricultural use is a cost-effective solution that is appropriate for local conditions. However, they did not consider the electricity recovery of incineration. Houilon and Jolliet (2005) compared six scenarios of sludge treatment in Switzerland. These scenarios included agricultural spreading, incineration, wet oxidation, landfill, pyrolysis, and cement production. The results showed that specific incineration in fluidized beds and agricultural spreading are the most attractive processes. Notably, this study only focused on energy conservation and on the effect of emissions on global warming without considering other environmental burdens. Mario et al. (2007) established an environmental LCA of sludge treatment and found electricity generation from incineration process proved to be an environmentally friendly option in Italy. Nevertheless, they only analyzed three scenarios: anaerobic digestion of sludge plus incineration, incineration of undigested sludge, and anaerobic digestion of sludge with composting. They also did not present quantified inventory data. Murray et al. (2008) evaluated the environmental and economic effects of sewage sludge treatment in Chengdu, China and reported that the superior sludge handling option is anaerobic digestion followed by the use of sludge as fertilizer. However, they only listed an inventory of economic costs. Benefits, key air emissions, energy consumption, and other inventory data (e.g., material consumption and heavy metal emissions) were not considered. Hong et al. (2009) conducted an integrated study of sewage sludge-treatment options in Japan by considering both environmental and economic effects and drew the conclusion that the environmentally optimal and economically affordable method of sludge treatment in Japan was thickening, digestion, dewatering, and melting. Similar with aforementioned research (Houilon and Jolliet, 2005), they considered very few categories: global warming, acidification, human toxicity, and land use. Almudena et al. (2010) studied the reuse of anaerobic digested sludge in agriculture in Spain and found that land application is an acceptable option to handle digested sludge. However, they only analyzed the environmental effects with four categories: eutrophication, global warming, human toxicity, and terrestrial toxicity. Nakakubo et al. (2012) compared two sewage sludge disposal technologies, namely, sludge and food treatment and sludge-food waste treatment. The results showed that food waste digested with sludge was superior to the conventional separate processing of sewage and food waste. Nevertheless, only greenhouse gas emissions and phosphorus recovery were studied. Wang et al. (2013) evaluated the assessment of environmental effects of sludge-treatment processes in Taiwan. The treatment processes included carbonization, direct landfill, co-incineration with municipal solid waste, and mono-incineration. They drew the conclusion that carbonization, followed by co-incineration and landfill was the most preferable sludge-handling option overall. However, the environmental effect generated from heavy metal emissions in landfill and incineration was excluded. To our knowledge, most previous studies consider only limited aspects of the environmental effects of sludge treatment or assess only certain sludge-treatment processes. Few studies have concentrated on both the environmental and economic assessment of sludge disposal in China. Thus, research needs to address certain issues to present a more credible assessment. First, a Chinese database of sewage sludge treatment should be introduced, and effective decisions for waste management should be encouraged. Second, the environmental and economic performances of all sludge-treatment scenarios with and without anaerobic digestion

in China have to be compared with those employed in other countries. Third, the efficiency of raw material, energy, and processes in sludge treatment in China must be improved. Finally, managers and policy makers should be provided with useful information to help them make decisions regarding the problem amendment in sewage treatment plants. In this study, LCA and life-cycle costing (LCC) are integrated to address the aforementioned needs, evaluate the environmental and economic effects of sewage sludge-treatment scenarios, and identify the optimal handling scenario. After investigating nearly all sewage treatment plants in China (Wang et al., 2006), 13 main scenarios of sewage sludge treatment are compared in this study. 2. Scope definition 2.1. Functional unit The functional unit is the base for the treatment comparison in the life-cycle inventory. The management of one tone of dry sludge (DS) is selected. All materials, emissions, cost, energy consumption, and recovery levels are referred to this functional unit. 2.2. System boundary Thirteen scenarios for sewage sludge treatment are considered in this study. Except for the scenario of gravity thickening with landfill (GL), six scenarios are included: (a) gravity thickening, anaerobic digestion, dewatering, and landfill (GADwL); (b) gravity thickening, anaerobic digestion, dewatering, and incineration (GADwI); (c) gravity thickening, anaerobic digestion, dewatering, and agricultural use (GADwA); (d) gravity thickening, anaerobic digestion, and landfill (GAL); (e) gravity thickening, anaerobic digestion, and agricultural use (GAA); and (f) gravity thickening, anaerobic digestion, drying, and agricultural use (GADrA). The remaining six scenarios are similar to the six aforementioned scenarios but without anaerobic digestion. Fig. 1 shows the system boundaries of the GL scenario and the six scenarios with digestion process. The processes of raw materials and energy production, road transport, direct emissions, wastewater treatment, energy recovery from anaerobic digestion, incineration, and landfill stages are also included. The infrastructure is excluded because of the lack of detailed information on sewage sludge-treatment plants and their raw material production sites. Moreover, the infrastructure exhibited very low contribution to the overall potential environmental effect (Hong et al., 2009). 2.3. Methodology The life-cycle environmental effect results are calculated at midpoint by using the ReCiPe (Goedkoop et al., 2009; De Schryver et al., 2009) method. This method is the most recent indicator that is available for LCA analysis. The ReCiPe method can also define 18 midpoint categories: climate change, ozone depletion, human toxicity, photochemical oxidant formation, particulate matter formation, ionizing radiation, terrestrial acidification, freshwater eutrophication, marine eutrophication, terrestrial ecotoxicity, freshwater ecotoxicity, marine ecotoxicity, agricultural land occupation, urban land occupation, natural land transformation, water depletion, metal depletion, and fossil depletion. The IMPACT 2002þ (Jolliet et al., 2003) method is used as a comparison to supplement and verify the applicability of the results attained from the ReCiPe method. The conventional costs of all scenarios are assessed by using the LCC method, which is based on LCA but considers the costs rather than the environmental effects (Hong et al., 2009, 2012; Castella

C. Xu et al. / Journal of Cleaner Production 67 (2014) 79e87

81

Sewage sludge (1t-DS) 3.65 m3 wastewater

4 kg PAM

Gravity thickening

39.5 kwh

(1t-DS)

153.85 kwh

1.25× 107 kJ 3.65 m3 wastewater

(1t-DS)

4.31 kg PAM 1.5× 106kJ

Dewatering

61.54 kwh

(0.6t-DS)

39.0 kwh

Main treatment

Energy and raw materials production

615.38 kwh

Anaerobic digestion

Drying

(0.6t-DS)

3.65 m3 wastewater

Road transport (Assume 40 km) Incineration 1024.5 kWh

20.25kg NaOH

45L Diesel

200kg Coal

275 kWh

220 kg Lime

0.0377 kg Pesticide 7.54× 10-5 m3 LDPE 0.11m3Water

0.41 m3 Sand

0.38L Diesel 0.42 kWh 1.835 kg heavy metals 111.92 kWh

Agricultural use

Post treatment

Fly ash

Landfilling

Fig. 1. System boundary.

et al., 2009). The price of raw materials, energy, and wastewater treatment based on the current Chinese market is used to conduct LCC. The costs of labor in raw materials, energy production, and transport stages are also included. Moreover, additional equipment and maintenance costs are integrated in the product manufacturing stage. 2.4. Data sources The evaluation of the environmental effects of products, processes, and activities is where LCA studies originated and is still most widely applied. Life-cycle inventory (LCI) usually serves a fundamental function in LCA analysis. The geographically representative and time-related LCI for products is critical for LCA studies. Accordingly, data from China were used when available. In case such data were unavailable, data and relevant background data (i.e., chemicals and sewage treatment) from Europe were used (Ecoinvent Centre, 2007). Based on the data from China, the inventories on gravity thickening and agricultural use processes (i.e., electricity consumption, flocculants, wastewater disposal, and heavy metal emissions) are obtained from the best available techniques directive of sewage sludge treatment in China (Ministry of Environmental Protection, 2010). These data include those directly collected from 140 wastewater treatment plants in China. The annual average monitoring data of anaerobic digestion and dewatering processes in Maidao wastewater treatment plant of Qingdao in Shandong Province of China, which is the most typical wastewater treatment plant that uses anaerobic digestion technology to treat sewage sludge in the country, were used in this study. LCI data on drying and incineration processes are obtained from the “Urban Sludge Treatment Technology Guide” (MOHURD and NDARC, 2011), which represents the average level of sludge drying and incineration technology in China. Data on the landfill stage (i.e., electricity consumption, raw materials consumption, and direct gas emissions) are collected from references (Liao et al., 2009; Hong et al., 2010a,b). The standard for pollution control in

the municipal solid waste landfill site in China (GB/T23485, 2009) is used to determine the leachate emissions because of the limited information on leachate. Data on electricity generation and road transport in China are obtained from references (Wei et al., 2009; Cui et al., 2012). The prices of raw materials, energy, labor, and wastewater disposal are based on the current Chinese market (currency exchange rate at 6.13 CNY/USD, http://chem.chem99.com/; http:// www.stats.gov.cn/tjsj/; http://tongji.cnki.net/kns55/index.aspx). The equipment, transportation, and maintenance costs of the aforementioned scenarios are collected from relevant references and field research on sewage treatment plants in China (Deng et al., 2000; He et al., 1997; Qi and Cao, 2009; Zhang et al., 2006). 2.5. Life-cycle inventory The main characteristics of the sewage treatment plant considered in this study are described in Table 1. The CO2 emissions from anaerobic digestion, landfill, incineration, and agricultural use processes are omitted from the inventory because sewage sludge is considered a biogenic source. Table 2 presents the main inventory results for the sludge-treatment processes.

Table 1 Characteristics of the sewage treatment plant and sewage sludge considered in the study. Values are presented per functional unit. Characteristics Wastewater quantity Water content of influent sludge Water content of gravity thickened sludge Organics content Digestion rate Water content of dewatered sludge Water content of dried sludge Construction lifetime Equipment lifetime

Value 3  10 99 97 80 50 75 40 30 10

Unit 4

m3/day % % % % % % Year Year

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C. Xu et al. / Journal of Cleaner Production 67 (2014) 79e87

Table 2 Life cycle inventories of sewage sludge treatment processes. Values are presented per functional unit. Gravity thickeninga

Anaerobic digestionb

Material consumption

Material consumption

Direct gas emissions

Dewateringb

Drying

c

Agriculture usea

Landfill

Material consumption

Material consumption Direct soil emissions

Material consumptiond

Direct gas emissionse

Direct water emissionsf

Incinerationc

Material consumption

Direct gas emissions

a b c d e f

Electricity consumption

39.50 kWh

PAM Wastewater Electricity consumption

4.00 kg 3.56 m3 153.85 kWh

Electricity regeneration Energy regeneration Wastewater CH4

615.38 kWh 1.25  107 kJ 3.56 m3 250.00 kg

N2 H2 H2S Electricity consumption

10.08 kg 0.71 kg 0.43 kg 61.54 kWh

PAM Wastewater Electricity consumption

4.31 kg 3.00 m3 39.00 kWh

Energy consumption T-Cd

1.53  106 kJ 5.00  103 kg

T-Hg T-Zn T-Cr T-Ni T-As T-Pb T-Cu Electricity consumption

5.00 5.00 0.60 0.10 7.50 0.30 0.25 0.42

Electricity regeneration Diesel Sand Pesticide Water LDPE Lime CH4

111.92 kWh 0.38 L 0.41 m3 3.77  102 kg 0.11 m3 7.54  105 m3 220.00 kg 13.40 kg

NH3 SO2 NOx H2S COD

10.08 kg 0.22 kg 7.80  102 kg 4.54  103 kg 8.25 kg

T-Cd T-Hg T-Zn T-Cr T-Ni T-As T-Pb T-Cu Electricity consumption

0.02 kg 2.50  102 kg 4.00 kg 1.00 kg 0.20 kg 7.50  102 kg 1.00 kg 1.50 kg 275.00 kWh

Electricity regeneration Diesel Coal NaOH NO2

1024.50 kWh 45.00 L 200.00 kg 20.25 kg 2.81 kg

SO2 Dust Cd Hg Dioxin

1.12 0.34 0.56 0.56 2.25

Ministry of Environmental Protection (2010). Field research at Maidao sewage treatment plant. MOHURD and NDARC, 2011. Hong et al., 2010a. Liao et al., 2009. GB/T23485-2009.

 103 kg  103 kg kg kg  102 kg kg kg kWh

kg kg g g ug

3. Results 3.1. Midpoint The life-cycle impact assessment (LCIA) results of all scenarios are compared using the ReCiPe method. For most impact categories except for climate change, scenarios without anaerobic digestion had a higher impact than those with anaerobic digestion (data not shown). This result was attributed to the direct greenhouse gas emissions (GHG) generated from anaerobic digestion. Table 3 presents the ReCiPe LCIA results for six scenarios with anaerobic digestion and GL. For human toxicity, GL, GADwL, and GAL showed the highest impact. For photochemical oxidant formation, particulate matter formation, terrestrial acidification, freshwater ecotoxicity, marine ecotoxicity, and fossil depletion, GADwI had the lowest value. For terrestrial ecotoxicity, GADwA, GAA, and GADrA had the highest value. The most significant contribution of the processes in each scenario is shown in Fig. 2. Anaerobic digestion contributed the most to climate change in most scenarios except for GL. This result is consistent with that in Table 3. For GL, GADwL, and GAL, landfill had a dominant contribution to human toxicity, whereas anaerobic digestion had a high contribution to the other categories. Gravity thickening and dewatering served insignificant functions in the overall environmental burden. For GADwA, GAA, and GADrA, agricultural use served an important function in terrestrial ecotoxicity, whereas gravity thickening, dewatering, and drying had insignificant contributions. For GADwI, incineration had a dominant contribution in ozone depletion, photochemical oxidant formation, and terrestrial ecotoxicity. Anaerobic digestion contributed the most to the other categories, whereas gravity thickening and dewatering had insignificant contributions to the overall environmental burden. Fig. 3 shows the normalized midpoint values of each scenario with and without digestion. For all scenarios, the impact observed from human toxicity and marine ecotoxicity served an important function. The impacts observed from freshwater eutrophication, terrestrial ecotoxicity, freshwater ecotoxicity, and climate change had small contributions, whereas those observed from the other categories could be neglected. For human toxicity, GL, GADwL, and GAL had higher impacts compared with the other scenarios. For marine ecotoxicity, GADwI had the highest impact compared with the other scenarios. To understand and describe better the dominant pollutants in human toxicity and marine ecotoxicity, the contributions of the most significant substances to these midpoints are shown in Fig. 4. Direct mercury and lead emissions generated from the landfill process significantly contributed to human toxicity. Direct mercury and arsenic emissions generated from incineration also served important functions in human toxicity. For marine ecotoxicity, the direct mercury emissions generated from the landfill stage contributed the most, whereas direct vanadium and lead emissions served an insignificant function. Direct vanadium emissions generated from incineration significantly contributed to marine ecotoxicity, whereas direct nickel and arsenic emissions had insignificant contributions. 3.2. LCC analysis Fig. 5 presents the LCC results of each scenario in this study. The total costs of GL, GADwL, GADwI, GADwA, GAL, GAA, and GADrA with anaerobic digestion were 47.5, 24.6, 6.3, 39.5, 11.2, 31.9, and 53.8 USD/T-DS, respectively; scenarios without anaerobic digestion had total costs of 47.5, 69.9, 19.3, 94.7, 47.6, 82.0, and 118.5 USD/T-DS, respectively. Notably, scenario GADwI had a negative cost because

C. Xu et al. / Journal of Cleaner Production 67 (2014) 79e87

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Table 3 ReCiPe mid-point results for all scenarios with digestion. Values are presented per functional unit. Catogries

Unit

GL

GADwL

GADwI

GADwA

GAL

GAA

GADrA

Climate change Ozone depletion Human toxicity Photochemical oxidant formation Particulate matter formation Ionising radiation Terrestrial acidification Freshwater eutrophication Marine eutrophication Terrestrial ecotoxicity Freshwater ecotoxicity Marine ecotoxicity Agricultural land occupation Urban land occupation Natural land transformation Water depletion Metal depletion Fossil depletion

kg CO2 eq kg CFC-11 eq kg 1,4-DB eq kg NMVOC kg PM10 eq kg U235 eq kg SO2 eq kg P eq kg N eq kg 1,4-DB eq kg 1,4-DB eq kg 1,4-DB eq m2a m2a m2 m3 kg Fe eq kg oil eq

5.6  102 7.0  106 8.3  102 0.1 3.2 2.8 24.8 6.5  103 1.3 1.7  102 2.3 1.7 0.1 0.1 3.8  103 8.7  102 1.2 22.8

4.7  103 5.0  108 3.6  102 17.5 2.5 7.2 1.9 0.1 3.0 4.5  103 1.7 1.9 0.7 1.5 2.8  104 12.4 1.3 4.7  102

4.0  103 1.1  104 46.8 1831.5 4.8 5.8 13.8 0.2 2.1 2.1  102 6.1 5.6 0.3 2.5 2.8  103 11.9 0.9 5.0  102

4.4  103 3.5  106 85.4 17.1 4.3 7.0 12.7 8.9  102 2.4 5.8 2.1 2.3 0.6 1.3 8.5  106 12.4 1.2 4.7  102

4.6  103 5.7  107 3.5  102 17.8 2.6 8.2 1.7 0.1 2.9 5.7  103 2.0 2.2 0.7 1.6 7.3  104 12.4 1.8 4.8  102

4.3  103 4.0  106 94.6 17.4 4.4 7.9 12.9 0.1 2.3 5.8 2.4 2.6 0.7 1.4 1.3  103 12.4 1.7 4.8  102

4.5  103 3.7  106 85.9 16.2 4.1 7.2 12.1 0.1 2.4 5.8 2.2 2.4 0.6 1.3 9.7  104 11.5 1.5 4.5  102

of the relatively higher energy recovery during anaerobic digestion and incineration processes. By assessing the entire life cycle of all scenarios with and without anaerobic digestion, the costs of raw materials and electricity were found to serve a significant function. Electricity recovery significantly contributed to the scenarios with anaerobic digestion. 3.3. Carbon balance To comprehend the dependability of the midpoint results of LCIA better, Table 4 presents the carbon mass balance of direct emissions for all scenarios. In all sewage sludge-treatment processes, only anaerobic digestion, landfill, incineration, and agricultural use generated gases. The primary carbon mass for each scenario was 380 kg C/T-DS. After treatment, the total mass of carbon in GL, GADwL, GADwI, GADwA, GAL, GAA, and GADrA was 346, 360.5, 361, 349.4, 360.5, 349.4, and 349.4 kg C/T-DS, respectively. The lost mass of carbon in each scenario might be attributed to the limitations related to the current data and tools available. Measurement error problems might have also influenced the results. 3.4. Energy recovery Fig. 6 presents the sensitivity analysis to electricity recovery capacity. The potential effect on human toxicity and total cost for landfill and incineration decreased with increasing electricity recovery capacity. The electricity recovery capacity in landfill ranged from approximately 46 kWh/T-DS to 400 kWh/T-DS, whereas that in incineration ranged from approximately 287 kWh/T-DS to 600 kWh/T-DS (Hong et al., 2010a,b). A linear correlation among electricity recovery capacity, midpoint score for human toxicity, and landfill and incineration costs was observed. This linear relation showed that a 100 kWh/T-DS increase in electricity recovery decreased the total cost and human toxicity potential score by approximately USD 8.2 and 23.5 kg 1,4-DB eq in both the landfill and incineration stages, respectively. 3.5. Sensitivity analysis 3.5.1. Sensitivity to the impact assessment method To verify these results, the Impact 2002 þ method was used for comparisons with the ReCiPe method. Table 5 presents the Impact 2002 þ midpoint LCIA results for six scenarios with anaerobic digestion and GL. The results obtained by Impact 2002 þ for all

scenarios were almost similar to those obtained by the ReCiPe method for ozone layer depletion, acidification, and agricultural land occupation. For particulate matter formation, the LCIA results obtained from the Impact 2002 þ method differed from those obtained from the ReCiPe method. This difference was caused by the different equivalent values of the dominant substances of these two methods. If PM10 was considered the label substance in the Impact 2002 þ method, the potential impact of particulate matter formation would be close to the ReCiPe method. For the energy category, the ReCiPe results were consistent with the Impact 2002 þ results by conversion (i.e., change rate at 42.62 MJ/kg oil eq). For climate change, the LCIA results of all scenarios obtained from the Impact 2002 þ method were lower than those obtained from the ReCiPe method. This finding was mainly caused by the difference in the time horizon of the Impact 2002þ and ReCiPe methods (500- and 100-year time horizons, respectively). If a 100-year time horizon is considered in the Impact 2002 þ method, the potential impact of climate change will be similar to that of the ReCiPe LCIA results. Other LCIA results were difficult to compare because other categories and label substances differed widely. These comparisons indicated that the ReCiPe method was reliable for this study. 3.5.2. Sensitivity to main contributors To recognize the crucial influence on the LCIA results obtained from this study, the results of the sensitivity analysis of dominant processes for the main impact categories are presented in Table 6. Incineration efficiency had the highest environmental benefit in all categories, except for human toxicity, because of the increase in electricity recovery capacity in the incineration stage. Similarly, landfill efficiency had a lower environmental benefit than incineration in all categories, except for human toxicity, because of the direct emissions of lead and mercury to water. Anaerobic digestion efficiency had the highest environmental benefit in terms of climate change, which could be attributed to the decrease in direct methane emissions. Agricultural use efficiency had a relatively higher environmental benefit than landfill because of the decrease in direct heavy metal emissions to soil. Therefore, incineration and landfill had the highest and lowest environmental benefits, respectively. 4. Discussion As mentioned earlier, scenarios with anaerobic digestion had a higher global warming impact than those without anaerobic digestion. This finding was mainly attributed to the GHG emissions

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Fig. 3. Normalized midpoint scores for the full life cycle a) with anaerobic digestion b) without anaerobic digestion.

approach to reduce the global warming potential impact. Accordingly, increasing the efficiency of energy recovery and carbon fixation is a key to reduce the GHG emissions from anaerobic process. GL, GADwL, and GAL contributed the most to human toxicity because of heavy metal emissions (i.e., mercury and lead to water) in the landfill stage (Fig. 2 and Table 3). GADwI had the lowest impact in most categories because of the energy recovery during the incineration stage. GADwA, GAA, and GADrA had the highest contributions to terrestrial ecotoxicity because of the relatively high heavy metal emissions in the agricultural use stage. Table 5 shows that incineration and landfill have the highest and lowest environmental benefits, respectively. This result was consistent with that of a previous study on waste management strategies in Austria (Francesco et al., 2009), in which the landfill system was found to be the worst waste management option because significant environmental savings were gained through energy recycling. This result was consistent with the Best Available Techniques Directive for Treatment and Disposal of Sludge from Wastewater Treatment Plant (Ministry of Environmental Protection, 2010). This directive indicates that landfills cause significant environmental problems in all sewage sludge-treatment processes. By contrast, incineration was deemed advantageous because it can reduce the total volume of sludge significantly, and the recovered energy could reduce environmental burden.

Fig. 2. Contributions of sludge treatment processes to mid-point value of each scenario.

during anaerobic digestion process. Researchers are focused on identifying effective technologies to reduce GHG emissions. Erik et al. (2013) demonstrated that the two-stage anaerobic digestion configuration coupled with algae production can reduce GHG emissions by approximately 60% compared with a traditional anaerobic lagoon. Hong et al., 2010a,b reported that increasing electricity recovery efficiency from methane gases is an efficient

Fig. 4. Contributions of substances to the mid-point score a) human toxicity b) marine ecotoxicity.

a)

85

900

20 y = -0.24x + 848.10 R² =1

Cost ($/t-DS)

10

800 0

- 10 700

y = -0.08x + 8.30 R² = 1

- 20 Fig. 5. LCC analysis.

- 30

Table 4 Carbon balance (kg C/t-DS). Parameter

GL

GADwL

GADwI

GADwA

GAL

GAA

GADrA

Input Anaerobic digestion Landfill Incineration Agricultural use Total Lost

380

380 187.5

380 187.5

380 187.5

380 187.5

380 187.5

380 187.5

346 e e 346 34

173 e e 360.5 19.5

e 173.5 e 361 19

e e 161.9 349.4 30.6

173 e e 360.5 19.5

e e 161.9 349.4 30.6

e e 161.9 349.4 30.6

100

200

300

400

600 500

b)

350

100 y = -0.24x + 380.00 R² = 1

90

300 80 70

250 y = -0.08x + 107.90 R² = 1

60 50 100

200

300

400

500

600

Human toxicity (kg 1,4-DB eq)

Electricity recovery (kWh/t-DS)

Cost ($/t-DS)

Eriksson et al. (2005) reported that the global warming potentials of landfill and incineration are 510 and 350 kg CO2/t-DS, respectively. Murray et al. (2008) reported that the global warming potentials of drying, landfill, and incineration are 340, 1500, and 800 kg CO2/T-DS, respectively. Wang et al. (2013) reported that the global warming potentials of drying, landfill, and incineration are 209, 300, and 230 kg CO2/T-DS, respectively. In this study, the global warming potentials of drying, landfill, and incineration were 225, 509, and 626 kg CO2/T-DS, respectively. The ranges of the global warming potential impact in this study have been reported by the aforementioned studies. From an environmental perspective, GADwI and GAA had larger environmental benefit than other scenarios. However, the heavy metal emissions in the agricultural use stage should meet the environmental capacity requirement. After calculating the dynamic and static environmental capacities (Zhao et al., 2008), the reasonable applied amount for sewage sludge was 0.33 T/hm2 for agriculture use. The LCC results for scenarios with anaerobic digestion were significantly lower than those of scenarios without anaerobic digestion (Fig. 4). This finding was mainly attributed to the capability of anaerobic digestion to reduce the volume of organics in sewage sludge at a rate of 50%. Thus, the costs for subsequent treatment processes were reduced. Deng et al. (2000) analyzed the cost of two scenarios of sewage sludge treatment (i.e., incineration and landfill) in Shanghai City, China. The costs of these two scenarios are 123.9 and 142.8 USD/TDS. Lundin et al. (2004) performed an environmental and economic analysis of four sludge-treatment scenarios (i.e., incineration, incineration plus phosphorus recovery, fractionation, and agricultural application) in Sweden. The cost of incineration was calculated to be 402 USD/T-DS. Murray et al. (2008) analyzed on the environmental and economic burdens of Chengdu in China. The cost of incineration was calculated to be 2.5  106 USD/T-DS. Hong et al. (2009) reported on cost-combined life-cycle environmental and economic analyses of six sewage sludge-treatment scenarios (i.e., dewatering, composting, drying, incineration, incineration

0

Human toxicity (kg 1,4-DB eq)

C. Xu et al. / Journal of Cleaner Production 67 (2014) 79e87

200 700

Electricity recovery (kWh/t-DS) Fig. 6. Relationship among the electricity recovery capacity, human toxicity midpoint score and cost a) landfill b) incineration.

plus melting, and melting) with and without anaerobic digestion in Japan. The cost of incineration with anaerobic digestion was calculated to be 223 USD/T-DS. Notably, the cost of incineration reported by Murray et al. (2008) is approximately four orders of magnitude higher than the cost of incineration in Sweden (Lundin et al., 2004) and Japan (Hong et al., 2009). In this study, the costs of the incineration and landfill scenarios with digestion were 24.6 and 6.3 USD/T-DS, respectively. This result was considerably lower than that in Shanghai and Sweden because the benefit of energy recovery was considered in this study. This result was also lower than that in Japan because carbon tax was not considered in this study. In addition, the price of raw materials, energy, labor, and maintenance used in the sewage treatment plant in China was lower than that in Japan and Sweden. From an economic perspective, GADwI and GAL with anaerobic digestion had larger economic benefits than the other scenarios. Combined with the environmental results, the sewage sludge treatment in GADwI with anaerobic digestion could be considered as the most optimal method. However, in China, approximately 38% and 3.45% sewage sludge are treated by using anaerobic digestion and incineration, respectively (Ministry of Environmental Protection, 2010; Wang et al., 2006), which is significantly different from the approaches of sewage sludge treatment in developed countries (e.g., America, EU, and Japan). In particular, approximately 19%, 73.9%, and 67% of sewage sludge are treated by using incineration technology in America, EU, and Japan, respectively (US EPA, 2007; Ministry of land of Japan, 2002; http://www. eea.europa.eu/). Therefore, all projects in China, including large,

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Table 5 Impact 2002þ mid-point results for all scenarios with digestion. Values are presented per functional unit. Categories

Unit

GL

GADwL

GADwI

GADwA

GAL

GAA

GADrA

Carcinogens Non-carcinogens Respiratory inorganics Ionizing radiation Ozone layer depletion Respiratory organics Aquatic ecotoxicity Terrestrial ecotoxicity Terrestrial acid/nutri Land occupation Aquatic acidification Aquatic eutrophication Global warming Non-renewable energy Mineral extraction

kg C2H3Cl eq kg C2H3Cl eq kg PM2.5 eq Bq C-14 eq kg CFC-11 eq kg C2H4 eq kg TEG water kg TEG soil kg SO2 eq m2 org.arable kg SO2 eq kg PO4 P-lim kg CO2 eq MJ primary MJ surplus

0.9 5.6  1.9 2.9  7.0  0.1 6.7  3.6  1.5  0.1 19.0 0.2 3.2  1.1  0.4

3.7 2.9  102 1.5 7.6  102 4.4  108 1.2 4.0  106 7.0  103 5.1 0.7 3.0 0.1 2.1  102 2.0  104 0.2

8 87.1 2.8 6.3  102 1.1  104 1.2 6.1  104 9.5  103 1.0  102 0.9 15.5 2.2  102 3.2  102 2.1  104 0.1

4.3  102 4.8  103 2.5 7.4  102 3.5  106 1.2 3.8  106 4.2  106 93.3 0.6 14.1 2.1  102 43.4 2.0  104 0.2

4.3 2.9  102 1.5 8.7  102 5.6  107 1.2 4.0  106 5.4  103 6.9 0.8 3.2 0.1 1.7  102 2.1  104 0.4

4.3  102 4.8  103 2.6 8.5  102 4.0  106 1.1 3.8  106 4.2  106 95.1 0.7 14.4 1.4  102 6.8 2.0  104 0.4

4.3  102 4.8  103 2.4 7.7  102 3.7  106 1.2 3.8  106 4.2  106 88.5 0.7 13.3 1.5  102 1.3  102 1.9  104 0.3

102 102 106 106 103 102

102 103

Table 6 Sensitivity analysis of dominant contributors. Categories

Unit

Landfill

Incineration

Agricultural use

Anaerobic digestion

Variation Climate change Human toxicity Photochemical oxidant formation Particulate matter formation Terrestrial acidification Freshwater eutrophication Terrestrial ecotoxicity Freshwater ecotoxicity Marine ecotoxicity Fossil depletion

% kg kg kg kg kg kg kg kg kg kg

5% 6 40 4.9  102 1.3  102 3.6  102 1.4  103 7  103 1.0  101 6  102 1.4

5% 48 4 4.6  101 1.1  101 3.2  101 1.3  102 2.6  103 3.8  101 3.5  101 12.3

5% 12 2.7 1.3  101 6.2  102 5.4  102 1.2  103 9  102 2.3  101 8  102 10.5

5% 280 7 3.0  101 7  102 2.0  101 8.0  103 4.0  104 2.2  101 2.1  101 7

CO2 eq 1,4-DB eq NMVOC PM10 eq SO2 eq P eq 1,4-DB eq 1,4-DB eq 1,4-DB eq oil eq

medium, small, newly built, reconstruction, expansion, and technology transformation, are suggested to operate according to the results obtained by this study.

5. Conclusion This study identifies the contributions of sewage sludgetreatment processes and their potential for improvement. The most suitable environmental and economic method to treat sewage sludge in China was GADwI (i.e., gravity thickening, anaerobic digestion, dewatering, and incineration) mainly because of the energy recovery during anaerobic digestion and incineration processes. The LCIs, potential impacts, and LCC found in this study could help policy makers make decisions on sewage treatment plants in China. The additional main conclusions drawn are as follows:  In all scenarios, the impacts generated from human toxicity and marine ecotoxicity had significant contributions.  Direct mercury and lead emissions generated from landfill process significantly contributed to human toxicity, whereas direct vanadium emissions generated from incineration process served an important function in marine ecotoxicity.  The costs of raw materials and electricity served an important function in all scenarios and the entire life cycle.  Anaerobic digestion resulted in the lowest environmental and economic burden because of the significantly reduced volume.  The sequence of environmental impact was as follows: GADwI < GAA < GADrA < GADwA < GAL < GADwL < GL.  The sequence of the LCC results was as follows: GADwI < GAL < GADwL < GAA < GADwA < GL < GADrA.

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