Hybrid Life-Cycle Environmental and Cost Inventory of Sewage

Mar 20, 2008 - This research uses life-cycle inventory and economic analysis to assess nine sewage sludge treatment technologies along with the enviro...
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Environ. Sci. Technol. 2008, 42, 3163–3169

Hybrid Life-Cycle Environmental and Cost Inventory of Sewage Sludge Treatment and End-Use Scenarios: A Case Study from China A S H L E Y M U R R A Y , * ,† A R P A D H O R V A T H , ‡ A N D KARA L. NELSON‡ Energy and Resources Group and Department of Civil and Environmental Engineering, University of California, Berkeley, California 94720

Received September 7, 2007. Revised manuscript received January 30, 2008. Accepted January 30, 2008.

Sewage sludge management poses environmental, economic, and political challenges for wastewater treatment plants and municipalities around the globe. To facilitate more informed and sustainable decision making, this study used life-cycle inventory (LCI) to expand upon previous process-based LCIs of sewage sludge treatment technologies. Additionally, the study evaluated an array of productive end-use options for treated sewage sludge, such as fertilizer and as an input into construction materials, to determine how the sustainability of traditional manufacturing processes changes with sludge as a replacement for other raw inputs. The inclusion of the life-cycle of necessary inputs (such as lime) used in sludge treatment significantly impacts the sustainability profiles of different treatment and enduse schemes. Overall, anaerobic digestion is generally the optimal treatment technology whereas incineration, particularly if coal-fired, is the most environmentally and economically costly. With respect to sludge end use, offsets are greatest for the use of sludge as fertilizer, but all of the productive uses of sludge can improve the sustainability of conventional manufacturing practices. The results are intended to help inform and guide decisions about sludge handling for existing wastewater treatment plants and those that are still in the planning phase in cities around the world. Although additional factors must be considered when selecting a sludge treatment andend-usescheme,thisstudyhighlightshowasystemsapproach to planning can contribute significantly to improving overall environmental sustainability.

Introduction Sewage sludge management is a growing challenge for mechanical wastewater treatment facilities around the world. Sludge is an unavoidable byproduct of primary, secondary, and advanced wastewater treatment processes, and is typically generated at a rate of 70–90 g/person equivalent, or 1 dry ton/10 000 person equivalents per day (1). Sludge management is neither economically nor institutionally trivial; thus, it cannot be overlooked in the planning phase of wastewater treatment infrastructure. For example, in a study of several treatment plants in Austria, it was found that * Corresponding author e-mail: [email protected]; phone: (510) 295-8986. † Energy and Resources Group. ‡ Department of Civil and Environmental Engineering. 10.1021/es702256w CCC: $40.75

Published on Web 03/20/2008

 2008 American Chemical Society

sludge management (stabilization, dewatering, treatment, and transport) accounted for between 49 and 53% of the total operating costs (2). Despite large capital and operation expenses, incineration is rapidly becoming a common means of sludge handling because the volume of the end product (ash) is only about 30% of the initial solids content of sewage sludge (1). One cost-effective approach to sludge management is choosing wastewater treatment technologies with minimal sludge production, such as waste stabilization ponds and constructed wetlands. When mechanical wastewater treatment is used, however, numerous sludge treatment and enduse options exist. These options exhibit a wide range of economic and environmental costs and benefits, which often depend on site-specific economic, environmental, social, and political constraints. Life-cycle assessment (LCA) has proven an apt tool for comparing different sludge handling schemes (3–5). Life-cycle inventory assessment (LCI) is used in the study herein to evaluate sludge handling options that combine treatment with productive end uses for sludge, thus treating sludge as a resource rather than waste. Scope. The goals of this research were 2-fold. The first objective was to expand upon the fragmented process-based LCAs that already exist for sewage sludge treatment, with emphasis on covering a broader selection of treatment technologies, enriching the environmental data by including the life-cycles of different inputs, and expanding the economic data. This work builds on previous research. Bridle and Skrypski-Mantle developed a framework for a holistic LCA of sludge treatment options but only applied a portion of it to five different treatment scenarios (4). Suh and Rousseaux’s LCA characterized the environmental profiles, based on nine impact categories, of the operation phase of various technologies; however, the transferability of their work is limited by the fact that they only report qualitative results (5). Hospido et al. compared anaerobic versus thermal sludge treatment processes, accounting for both the environmental and economic costs, but the boundary condition was limited to the operation phase (3). In ignoring the construction phase, they eliminate a comparison of capital costs, which is an important part of the decision-making process for many municipalities. Also, like Suh and Rousseaux, Hospido et al. consider land application as the final destination for anaerobically digested sludge but only consider landfills as the final destination for incineration ash. Considering that LCA is a very time- and data-intensive process, it is not surprising that previous studies on sludge treatment and handling processes have only addressed small components of a comprehensive LCA. The second objective of the research was to evaluate productive end-use options for treated sewage sludge, and determine how the environmental impacts of existing manufacturing processes change with sludge as a replacement for other raw inputs. There is a growing body of literature that looks at the costs and benefits of using sludge and sludge treatment byproducts as raw materials in different production processes (6–9). However, there do not appear to be any published studies that present a direct comparison of emissions and energy use between conventional and sludge-substituted manufacturing, nor do there appear to be studies that have looked at the comparative impact of the same quantity of sludge in different products. The results of this research will enable more informed decisions about how best to handle treated sludge, illuminate for manufacturers the potential benefits of accepting sewage sludge, and motivate additional research to improve the feasibility of VOL. 42, NO. 9, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Nine Sludge Handling Scenarios treatment scenario dewatering dewatering f lime stabilization

end-product

destination

sludge cake, 20% DS, no mass change sludge cake, 30% DS, mass increase 15%/DT

landfill agriculture, cement manufacturing mesophilic anaerobic digestion f dewatering sludge cake, 20% DS, mass decrease 29%/DT agriculture aerobic digestion f dewatering sludge cake, 20% DS, mass decrease 29%/DT agriculture dewatering f heat drying f composting finished compost, 57% DS, mass increase 32%/DT agriculture dewatering f heat drying sludge cake, 36% DS, no mass change cement manufacturing mesophilic anaerobic digestion f dewatering f sludge cake, 43% DS, mass decrease 29%/DT cement manufacturing heat drying aerobic digestion f dewatering f heat drying sludge cake, 43% DS, mass decrease 29%/DT cement manufacturing dewatering f fluidized bed combustion ash, mass reduction 70%/DT cement, clay brick (incineration) manufacturing

sludge substitution so that the environmental benefits can be realized. Decisions about sludge end use must also consider factors not explicitly accounted for in this research, such as the sludge quality and its suitability for land application, local demand for the products in question, the flexibility of local supply chains, and the willingness of manufacturers to accept sludge or ash.

Methodology This research compares the sustainability of sludge treatment and handling options by developing a hybrid LCA framework. The metrics of sustainability used in this research include economic costs and benefits, key air emissions, and energy consumption and production. The framework is a hybrid LCA with two independent components: sludge treatment and sludge end use. Sludge treatment options are evaluated using process-based LCA augmented with economic inputoutput analysis-based LCA (EIO-LCA) to include the lifecycles of inputs into the treatment process (10–12). EIO-LCA consists of an economy-wide matrix of economic inputs and outputs and an environmental matrix that relates economic output to resource use, emissions, and waste production. For a given production value, the model determines the impacts of any product or service in the economy by summing the costs incurred across all direct and indirect inputs into the supply chain. The environmental offsets associated with using sludge for different end uses are evaluated using EIOLCA. EIO-LCA tables based on the U.S. economy were used in this study because they are far more detailed than those available for the Chinese economy. However, for analyses that rely less on disaggregated data, the Chinese tables could have been employed (13). Boundary Conditions. For the sludge treatment component, the system boundaries for the direct economic costs include construction and operation of the treatment technologies, as well as transportation of the end product for 25 km (from the treatment plant to their point of end use). The boundaries of the environmental impacts exclude the construction phase, largely due to a dearth of available data; however, other studies have concluded that the environmental impacts of the operation phase of drinking water and sludge treatment processes are significantly greater than that of the construction phase (5, 12). Nonetheless, analysis of the construction phase impacts of sludge treatment options is intended as a follow-up to this study. Both direct and indirect impacts of sludge treatment were considered by including the life-cycles of different inputs into the operation of different technologies, such as polymer to aid the sludge dewatering process. In this research, sludge was defined as a byproduct of wastewater treatment; the sludge is assumed to be available for different productive end uses at no additional life-cycle cost because those costs are already accounted for in the sludge treatment component of the research. 3164

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Case Study. This study was conducted to evaluate the environmental and economic sustainability of sludge treatment and end use for four biological secondary wastewater treatment plants (employing activated sludge) in the urban core of Chengdu, the capital of Sichuan Province, China. The four treatment plants have an average flow of 707 000 m3/d and a sludge production of 84 dry tons (DT)/d. The research includes a comprehensive LCI of nine different sludge treatment schemes and three productive end-use options. The environmental costs and benefits of treatment and end use are separately calculated, as those costs and benefits are likely to be realized by different entities — the wastewater treatment plant and the recipient of treated sludge. The net environmental costs and benefits are also reported to facilitate comparison between different options. The analysis of different sludge treatment technologies accounts for the local conditions with respect to energy and transportation production and emissions (Supporting Information Table S1). Chengdu’s raw sludge lower heating value (LHV) is 10.5 MJ/kg dry solids (DS) and the digested sludge lower heating value is 7.5 MJ/kg DS. Although much of the data are specific to Chengdu, the trends and general results from the analysis can help inform decisions in other cities around the world. Sludge Treatment. Mechanical dewatering, as well as three sludge stabilization processes (lime stabilization, anaerobic digestion, or aerobic digestion) and three final treatment processes (composting, drying, or incineration) were considered. On the basis of a review of case studies in the literature (4, 5, 9, 14–17) and conversations with wastewater treatment plant engineers (18, 19), nine feasible combinations of these processes were developed for comparison (Table 1). The environmental and economic costs of mechanical dewatering using a belt filter press were included in the analysis of each of the treatment processes. The dewatering analysis is sensitive to the volume of sludge and includes the polymer added for dewatering. The fuel source for heat drying was assumed to be natural gas, and incineration was assessed using both natural gas and coal. The natural gas recovered from anaerobic digestion was used to offset fuel demand for heat drying, and the remainder was used to produce electricity; the steam recovered from incineration was used to produce electricity. In alignment with the Intergovernmental Panel on Climate Change (IPCC), CO2 released during the sludge treatment process is assumed to be carbon neutral. The combustion of methane recovered from anaerobic digestion is also assumed to be carbon neutral (i.e., burning the methane is not a net contribution to CO2 emissions); however, emissions of other air pollutants associated with burning biogas to make electricity are accounted for. Heat drying to 36 and 43% DS for undigested and digested sludge, respectively, were chosen for sludge used in cement manufacturing because they were the lowest

integer values that resulted in net energy recovery at the cement plant. Data related to the costs and performance of individual technologies were taken from case studies (4, 5, 9, 14–17) and from data provided by the East Bay Municipal Utilities District in Oakland, California, which utilizes anaerobic digestion, and the Central Contra Costa Sanitary District Wastewater Treatment Plant in Martinez, California, which employs incineration. The numbers used for direct fuel consumption during incineration are based on those reported by Mininni et al. in their scenario with no heat drying (15), but the energy recovery is based on the LHV of Chengdu’s raw sludge. Data related to the site-specific costs and sludge quality are from two sources; some were collected by the first author in Chengdu during the summer of 2006, and some are from a study conducted on sludge treatment options for the city of Beijing (20). EIO-LCA was used to account for the life-cycle impacts of any inputs to treatment, such as lime and polymer (10). See the Supporting Information for a complete breakdown of data inputs and their respective sources (Tables S1-S9). Sludge End Use. EIO-LCA is applied to the analysis of using sludge and its treatment end-products as substitutes for other raw materials in a variety of conventional production processes, or as a replacement for a product itself. The products considered in this study are phosphatic fertilizer (Sector #325312), nitrogenous fertilizer (Sector #325311), Portland cement (Sector #327310), and clay brick (Sector #327121). Environmental offsets for sludge-substituted manufacturing of each product are normalized to 1 DT of sludge substitution. In the cases of phosphatic and nitrogenous fertilizers, sludge is assumed to replace the production process entirely, whereas for Portland cement, sludge is assumed to replace components of the total manufacturing. Because the EIO-LCA model is formatted as a matrix of the entire supply chain associated with a given product, it is possible to single out the inputs along the supply chains that change as a result of sludge substitution. Phosphatic and Nitrogenous Fertilizer. The emissions and resource consumption due to conventional fertilizer production that would be eliminated by the use of sludge were based on the amount of commercial production that Chengdu’s sludge would replace. The ratio of nutrient concentrations of commercial fertilizer manufactured in Chengdu to average nutrient concentrations in sludge from wastewater treatment plants in Beijing and Chengdu was determined, and the equivalent percentage of nutrients that sludge accounted for was subtracted from the emissions and resource consumption of the entire fertilizer manufacturing process. For example, in 2006, 1 ton of nitrogenous fertilizer cost approximately $115 to produce, and in Chengdu the manufactured concentration is 170 kg N/ton, whereas the average concentration of nitrogen in sludge is 7 kg N/DT (21). On the basis of this ratio, 4% of the emissions, resource use, and waste from the production of one ton of commercial fertilizer is saved for every ton of sludge used as nitrogen fertilizer. The avoided transportation of conventional fertilizer to the fields and the additional fuel requirement of applying sludge instead of conventional fertilizer were also included in the total offsets (Supporting Information Tables S10-S18). The same rationale was used to determine the offsets in phosphatic fertilizer production, and the sum of these savings is reported in the results. Although the average nutrient concentrations of China-made fertilizers are lower than that of the United States, the overall impacts of producing one ton of each were assumed to be comparable because the technologies employed in China are likely to be less energy efficient and more polluting. Cement Manufacturing. The dollar value of cement production that corresponds to the substitution of 1 DT of

sludge was entered into the EIO-LCA model. For example, at the sludge substitution rate of 10% used in this study, 10 tons of cement must be produced per 1 DT of sludge; therefore, the dollar value entered into the model was $760, assuming $76/ton, (22). The range of sludge substitution in the literature is 2–15% (7, 17, 23). Within the production chain, noncombustible components of sludge provide the chemical components such as CaO, SiO2, Al2O3, and Fe2O3, which are traditionally supplied by lime, clay, and iron ore (17); therefore, emissions and resource consumption equivalent to the fraction of raw materials that the sludge ultimately replaces at a given substitution rate are subtracted from the “stone mining and quarrying” and “transportation” sectors of the cement manufacturing supply chain. On the basis of other studies of sewage sludge incineration, the quantity of material that gets incorporated into cement was assumed to be 0.3 tons ash/DT sludge (1). When incineration ash is used, this is the only offset; when sludge cake is substituted, it not only provides chemical components but also energy. In modeling the sludge cake scenarios, the recoverable energy content of 1 wet ton of sludge with a lower heating value of 10.5 MJ/kg DS at 36% DS, and a kiln efficiency of 75% was added to the overall fuel offset for the scenario with heat drying as the sludge treatment. For the scenarios with digestion and heat drying as the sludge treatment, the recoverable energy content of 1 wet ton of sludge with a lower heating value of 7.5 MJ/kg DSat 43% dry solids was added to the overall fuel offset. Fuel and associated emissions offsets are assumed to replace coal consumption at the cement plant. This study assumes that emissions controls at the cement plant prevent any appreciable changes in stack emissions and cement kiln dust with the combustion of municipal sewage sludge. See the Supporting Information for a detailed explanation of model and data inputs (Tables S19-S25). Clay Brick Manufacturing. Whereas in cement manufacturing dewatered sludge and ash from sludge incineration can both be substituted into full-scale production processes, ash is the more common substitute in clay brick manufacturing, to maintain the structural integrity of the brick (24, 25). The dollar value of production that corresponded with the substitution of 1 ton of ash was entered into the EIO-LCA model. One ton of clay brick was taken to cost $119 to produce (26), and an ash substitution rate of 10% was assumed. Therefore, the emissions and energy offsets for every dry ton of sludge are equivalent to those avoided for every 0.3 tons of ash used at a 10% substitution rate (Supporting Information Tables S26-S28). Similar to cement, within the clay brick production chain, sewage sludge ash provides chemical components such as SiO2, Al2O3, and Fe2O3, which are traditionally supplied by clay (17). Therefore, emissions and resource consumption equivalent to the substitution rate are subtracted from the “stone and gravel mining” and “transportation” sectors of the cement manufacturing supply chain.

Results and Discussion Sludge Treatment. Environmental Assessment. The results from the LCI of the nine treatment schemes show that anaerobic digestion, particularly if lime is not required for pH stabilization, is overall the most preferable sludge treatment option (Table 2). In comparison to dewatering with no further treatment, anaerobic digestion results in offsets of SO2, CO, global warming effect (GWE) (27), and electricity by harnessing the embodied energy in the sludge. Similarly, if the sludge requires heat drying as preparation for an input to cement manufacturing, the combination of heat drying with anaerobic digestion is preferable to heat drying alone because the biogas from digestion can be used to meet the fuel demand of heat drying. Despite requiring VOL. 42, NO. 9, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Air Emissions and Energy Consumption Associated with Sludge Treatment Schemesa anaerobic anaerobic heat lime digestion digestion aerobic drying/ dewatering stabilization (no lime) (lime) digestion compost SO2 (kg) CO (kg) NOx (kg) VOC (kg) PM10 (kg) GWE (kg CO2) electricityb (kWh) fuelc (MJ)

-6.0 -1.6 0.2 0.0 0.1 -5.7 × 101

heat drying

anaerobic digestion aerobic FBC FBC (no lime)/heat digestion/ incineration incineration drying heat drying (NG) (coal)

0.1 0.1 0.1 0.0 0.0 1.2 × 101

0.3 1.2 0.2 0.1 0.0 5.5 × 102

-6.1 -2.0 0.1 0.0 0.1 -2.8 × 102

0.1 0.3 1.5 0.0 0.0 3.4 × 102

-2.7 -0.4 1.8 0.0 0.1 -1.2 × 102

4.9 2.5 1.9 0.0 0.0 2.1 × 102

-6.9 -3.0 -0.9 0.0 0.0 2.1 × 103

45 29 5 0.0 0.0 3.0 × 103

1.9 × 101

8.1 × 101

-9.2 × 102 -8.9 × 102 7.3 × 102 7.0 × 101 1.9 × 101

-4.0 × 102

7.6 × 102

-1.0 × 103

-1.0 × 103

9.2 × 101

3.5 × 103

6.5 × 101

3.3 × 101

4.8 × 103

2.3 × 104

2.3 × 104

1.5 × 103

4.9 2.5 0.7 0.0 0.0 2.1 × 102

0.5 0.3 1.2 0.0 0.0 2.6 × 102

6.5 × 101 4.3 × 103 5.9 × 103

a

Values are for the treatment of 1 DT of sludge and include mechanical dewatering to 20% DS with a belt filter press. Values account for potential resource recovery from anaerobic digestion first used to cover heat drying fuel demand and remainder as electricity, detailed in Supporting Information Table S31. c Values for incineration account for energy recovery used to offset electricity, detailed in Supporting Information Table S31. b

TABLE 3. Lifetime Breakdown of Economic Costsa to Treat 84 Dry Tons of Sludge per Day

dewatering lime stabilization anaerobic digestion (no lime) anaerobic digestion (lime) aerobic digestion heat drying/compost heat drying anaerobic /heat drying heat drying/aerobic FBC incineration (natural gas) FBC incineration (coal) b

capital cost ($)

operation PV ($)

transportation PVb ($)

total direct cost ($)

total cost including external cost ($)

10,000,000 14,000,000 47,000,000 47,000,000 47,000,000 32,000,000 21,000,000 56,000,000 56,000,000 94,000,000 94,000,000

13,000,000 18,000,000 -11,000,000 -9,600,000 27,000,000 59,000,000 27,000,000 1,500,000 43,000,000 87,000,000 6,000,000

1,500,000 450,000 410,000 420,000 410,000 310,000 320,000 270,000 270,000 35,000 35,000

25,000,000 32,000,000 36,000,000 39,000,000 75,000,000 91,000,000 48,000,000 58,000,000 100,000,000 180,000,000 100,000,000

25,000,000 35,000,000 31,000,000 35,000,000 80,000,000 93,000,000 50,000,000 56,000,000 105,000,000 190,000,000 150,000,000

a Costs are reported in present value, adjusted to 2006 prices, assuming a 6% discount rate and 20 year time horizon. Transportation is assumed to be 25 km from the treatment plant to the point of end use.

a higher dry solids content if the sludge is digested (due to a reduction in the LHV), the volume of sludge diverted to heat drying in each scenario works out to be almost equivalent because the digested sludge has less total volume (Supporting Information Tables S6-S7). This analysis assumes that all methane is captured and used to offset fuel or electricity demand; if methane leaks directly into the atmosphere, the GWE will increase. For example, if 10% of the methane leaks, GWE increases from -283 to 64 kg/DT for anaerobic digestion without lime. Importantly, the results of this study indicate that incineration has the highest GWE and fuel consumption of the nine schemes evaluated (Table 2). The air emissions associated with natural gas-fueled FBC incineration are the lowest of the treatment options: clean burning fuel is used to harness the energy embodied in the sludge, which is then used to offset electricity demand. In contrast, the air emissions associated with coal-fired incineration are at least 1 order of magnitude greater than all other treatment options, despite offsets in electricity demand (Table 2). Adding heat drying to aerobic digestion has little impact on emissions or GWE because of the low carbon intensity of natural gas (used for heat drying) compared to electricity and transportation; however, total fuel consumption increases significantly (Table 2). The results of the LCI are highly correlated with the energy demand of the treatment technology; the overall ranking of the different options closely matches the technology’s rank in operation energy (Table 2, Supporting Information Table S29). Interestingly, lime stabilization has the lowest operation energy after anaerobic digestion, yet its overall profile is comparable to the more energy intensive processes (Table 2, Supporting Information Table S29). 3166

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A key finding of this analysis is the magnitude of impact that accounting for lime production has on the environmental performance of a given treatment scheme. The Supporting Information shows the contribution of lime production to treatment processes that utilize lime (Supporting Information Table S30). Depending on the characteristics of the incoming wastewater, lime is sometimes an input in anaerobic digestion to raise the alkalinity of the sludge and to buffer against decreases in pH, to which methanogens (microbes that produce methane) are particularly sensitive (28). On the basis of the lime application rate used in this study (6% of the dry ton sludge flow (29)), lime production significantly impacts the environmental cost of anaerobic sludge digestion; its contribution to SO2, GWE, and electricity is greater than the net total, and it accounts for more than 50% of the NOx emissions and 93% of the fuel consumption (Supporting Information Table S30). Economic Cost Assessment. This analysis accounted for the capital, operational and transportation costs of the individual treatment schemes. Operational costs included the inputs, such as lime and polymer, and the power supply. The value of recoverable energy was subtracted from the operational costs, and transportation costs are sensitive to the volume of the end-product. The total present value of each treatment scheme was calculated assuming an annual discount rate of 6% and a 20-year time horizon. Costs calculated in Table 3 are for the capacity to treat all 84 DT of Chengdu’s sludge per day. To account for the total costs or benefits to society due to the emissions and energy consumption, the lifetime cost of the nine different sludge treatment schemes was also calculated with the inclusion of

TABLE 4. Avoided Emissions and Energy Consumption through Productive Sludge Usea

SO2 (kg) CO (kg) NOX (kg) VOC (kg) PM10 (kg) GWE (kg CO2) electricityb (kWh) fuel (MJ)

offset w/sludge applied as fertilizer

offset w/heat dried sludge used in portland cement

offset w/digested and heat dried sludge used in portland cement

offset w/ash used in portland cement

offset w/ash used in clay brick

-2.0 × 103 -8.4 × 103 -1.3 × 103 -1.5 × 103 -4.5 × 102 -2.4 × 106 -5.9 × 108 -4.8 × 107

-6.8 × 103 -4.2 × 103 -9.7 × 102 -7.1 × 101 -1.0 × 101 -4.4 × 105 -2.2 × 103 -3.4 × 106c

-2.4 × 103 -2.0 × 103 -3.9 × 102 -7.1 × 101 -1.0 × 101 -2.3 × 105 -2.2 × 103 -1.2 × 106d

-4.6 × 101 -8.3 × 102 -8.5 × 101 -7.1 × 101 -1.0 × 101 -1.2 × 105 -2.2 × 103 -9.4 × 104

-1.4 × 100 -1.8 × 102 -1.4 × 101 -1.4 × 101 -2.8 × 10-1 -2.7 × 103 0 -1.9 × 104

a Values assume 1 y sludge production in Chengdu (30 660 DT raw sewage sludge, 21 769 DT digested sludge, or 9198 t incineration ash) substituted for raw materials in conventional manufacturing processes. b Conversion from MJ to kWh assumes 38% efficiency. c Added fuel offset assumes a lower heating value of 10.5 MJ/kg dry sludge at 36% DS, a starting sludge temperature of 16 °C, and a kiln efficiency of 75%. d Added fuel offset assumes a lower heating value of 7.5 MJ/kg dry sludge at 43% DS.

the external costs of six different air pollutants: CO ($0.52/ kg), CO2 ($0.014/kg), NOx ($1/kg), particulate matter (PM) ($2.80/kg), SO2 ($1.8/kg), and volatile organic carbons (VOCs) ($1.4/kg) (30). The inclusion of these environmental costs decreases the total cost of sludge treatment by up to 14% in the case of anaerobic digestion (no lime), and adds up to 50% to the total cost in the case of coal-fired FBC incineration. With the inclusion of the environmental costs, anaerobic digestion (no lime) becomes slightly less expensive than lime stabilization; otherwise, the rank from least to most expensive is unchanged. The limited change in ranking by cost is in part due to the fact that capital costs account for a significant portion of the total cost of the different treatment schemes. Resource Recovery. Energy can be recovered at wastewater treatment plants from anaerobic digestion and incineration of sewage sludge. Off-site resource recovery is addressed below in the Sludge End Use section. Potential energy recovery through incineration is very sensitive to the water content and LHV of the sludge, which can vary dramatically between treatment plants and municipalities (31). LHVs are sensitive to the wastewater source and treatment processes; for example, the value of approximately 10.5 MJ/kg DS (undigested sludge) in Chengdu is much lower than the 21 MJ/kg DS reported as the average for European cities (1), illustrating the importance of site-specific considerations when evaluating sludge handling options. Sludge End Use. Using data for the annual sludge production for the core of the city of Chengdu, Table 4 shows the annual avoided environmental costs if all of the currently generated sludge was used as a replacement for raw materials in a given manufacturing process. For perspective, the offset of 3.4 × 106 MJ if heat dried sludge is substituted into Portland cement is equivalent to the annual output of a 25 MWe power plant, or enough electricity for about 24 000 households. Overall, the offset in air emissions, GWE, electricity, and fuel consumption per DT of sludge is greatest when sludge is used as a replacement for conventional fertilizers (Table 4). Land application of sewage sludge is only feasible if the sludge quality meets the local standards with respect to pathogens and heavy metal content and if there is local fertilizer demand. Additional treatment may be necessary to further reduce pathogens (beyond the options in Table 1). In contrast, heavy metal concentrations can only be reduced through improved source control. Using sludge directly in cement manufacturing also has substantial environmental offsets, and allows recovery of the embodied energy in the sludge at the cement plant. Using either sludge or ash improves the sustainability of the cement manufacturing process. The greater the dry solids content and the higher the LHV of the sludge, the more attractive it becomes to cement plants because of its increased fuel offset value. It

should also be noted that heavy metals are immobilized in cement, even when exposed to acidic environments (32, 33). The impact of incineration ash as a substitute is greater when used to replace materials in Portland cement than in clay brick for all parameters except PM10 (Table 4). Net Environmental and Economic Costs and Benefits. The net environmental and economic costs of potential handling schemes are discussed for two scenarios, differing by whether land application is or is not allowed. The most favorable option may vary significantly depending on the weight given to each objective and on the feasibility of including land application of sewage sludge in the portfolio of options. No Limit on Land Application. Anaerobic digestion followed by using the sludge as fertilizer is the superior sludge handling option even if lime addition is required (Table 5). Economically and environmentally, the costs of heat drying and compost are greater than anaerobic digestion largely due to the significant electricity savings (captured by the wastewater treatment plant) associated with the latter (Table 2). However, composting has potential environmental benefits over mesophilic anaerobic digestion that were not captured in this study. Whereas mesophilic anaerobic digestion only meets the pathogen standards for what the US EPA has defined as Class B biosolids, compost meets the standards for Class A biosolids because of its higher temperature (34). In practice, the Class A biosolids designation means that compost meets the highest level of quality control and can be used with fewer restrictions than Class B biosolids. Furthermore, the composted product is dryer and is less odorous than anaerobically digested sludge, making it easier to store and potentially more socially acceptable (34). These factors could prove to elevate the cost-effectiveness of heat drying and compost if the end product attracts a higher demand from local farmers or from fertilizer processing plants. No Land Application. If land application is not feasible, the optimal sludge handling scheme is anaerobic digestion and heat drying followed by end use in cement manufacturing (Table 5). This combination results in a net elimination of SO2, GWE, and electricity, and the fuel consumption is the lowest after anaerobic digestion (no lime) combined with land application (Table 5). It should be noted that the optimal degree of heat drying (at the treatment plant) followed by end use in cement manufacturing for minimizing environmental costs results in trade-offs between fuel use at the wastewater treatment plant, for transportation, and at the cement plant. The subsequent allocation of economic costs may require redistribution via institutional negotiations or policy-driven incentives. Sludge incineration with coal or natural gas, compared to anaerobic digestion with heat drying VOL. 42, NO. 9, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 5. Net Environmental and Economic Analysis of Potential Sludge Handling Schemes environmental assessmenta treatment dewatering lime stabilization anaerobic digestion (no lime) anaerobic digestion (lime) heat drying + compost heat drying anaerobic digestion (no lime) + heat drying anaerobic digestion (lime) + heat drying FBC incineration (natural gas) FBC incineration (coal)

end use

total economic costb ($)

SO2 (kg)

GWE (kg CO2) electricity (kWh)

fuel (MJ)

landfill land application land application land application land application cement cement

26,000,000 35,000,000 31,000,000 35,000,000 80,000,000 93,000,000 50,000,000

4.0 × 3.8 × 8.7 × 103 1.5 × 107 5 -1.9 × 10 -1.1 × 107 -1.9 × 105 -4.2 × 106 1.2 × 104 5.6 × 106 -2.6 × 103 1.0 × 107 -8.4 × 104 -4.1 × 106

5.7 × -5.9 × 108 -6.2 × 108 -6.2 × 108 -5.9 × 108 5.7 × 105 -1.2 × 107

2.8 × 106 5.8 × 107 -4.6 × 107 -2.9 × 106 8.3 × 107 1.8 × 108 -2.0 × 105

cement

59,000,000

-8.1 × 104

2.9 × 106

-1.2 × 107

4.5 × 107

190,000,000 150,000,000

-2.1 × 105 1.4 × 106

6.5 × 107 9.2 × 107

-3.2 × 107 -3.2 × 107

7.1 × 108 7.1 × 108

clay brick/cementd clay brick/cement

c

103

105

105

a Data are reported on annual basis. b Data are reported for a 20 year time horizon with 6% discount rate and include environmental externalities. c Cost includes landfill tipping fee. d When combined with incineration, there is no significant difference between the net combination of incineration with end use in cement or brick on this scale.

(no lime), is almost 3-4 times as expensive, and the overall handling schemes (including cement manufacturing offsets) have substantially higher GWE and fuel consumption. If fueled by coal, incineration contributes significantly to SO2 (Tables 2 and 5). Incineration schemes do, however, provide the largest electricity offsets (Tables 2 and 5). Using heatdried sludge directly in cement manufacturing takes advantage of existing infrastructure to perform sludge incineration, with the additional advantage that the ash gets incorporated into the cement product. Also, more than half of the fuel demand for incineration is for treating the flue gas in the afterburning chamber; this process already occurs at the cement plant so is not an added cost of sludge handling (15). This study demonstrates that, even when land application is not an option, there are more cost-effective and environmentally sustainable options for sludge handling than incineration. Nonetheless, existing incineration plants that are currently diverting ash to landfills could improve the overall environmental profile of their handling scheme by providing their ash to cement manufacturers. The most significant differences among the optimal handling scenarios are not in the overall environmental profiles, but in the economic cost of minimizing the environmental impacts of sludge treatment. As seen from the outlined scenarios, it is increasingly more expensive to minimize across all of the objectives as land application is eliminated as an end use for sludge; with no limit on land application the net present value of the lowest-cost option over 20 years is approximately $31 million versus approximately $50 million with no land application (Table 5). Economic efficiency is certainly an important part of the sludge management decision-making process. Therefore, measures that protect the suitability of sewage sludge for land application have the potential to greatly decrease the cost of sludge handling. Data Quality Assessment. This study was highly data intensive and thus the results are largely predicated on the quality of those data; a qualitative and quantitative quality assessment of each data input is provided in the Supporting Information (Tables S32-S35).

Outlook and Recommendations Increasingly, governments are setting legal limits on the amount of sludge that can be brought to landfills: in China, sludge tipping cannot exceed 5% of the daily capacity of a landfill, in the EU the goal is to reduce the year 2000 landfilling rate by 20% in 2010 and by 50% by 2050 (1, 35). Thus, additional regulatory policies and guidelines at local and 3168

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international levels can play a large role in determining the sustainability of the future in sewage sludge management. Policies could include enforcing source control to pretreat industrial wastewater before discharge to municipal sewers and thus keeping sludge safe for land application or tax incentives to entities that accept sludge and use it productively. On an international scale, carbon–off-set markets, such as the Clean Development Mechanism, could help to make the capital costs of some of the sludge handling schemes evaluated in this research more affordable in developing urban areas. This study demonstrates a variety of ways for utilizing the resources embodied in sludge during both the treatment and end-use phases of sludge handling. Anaerobic digestion followed by land application takes advantage of both the embodied energy, to make electricity, and nutrients, to enhance agriculture. Other schemes take advantage of some but not all of the resources: heat drying with compost takes advantage of embodied nutrients, whereas digestion with end use in cement takes advantage of embodied energy. The aim of this study was to use life-cycle assessment tools to independently evaluate the environmental and economic costs and benefits of a comprehensive set of sludge treatment and end-use options and to determine which combinations yield the most sustainable profiles. Although the study was motivated by a need to determine the optimal sludge handling options for the city of Chengdu, the results are intended to help guide decisions about sludge handling for both existing wastewater treatment plants and those that are still in the planning phase in cities around the world.

Acknowledgments The authors wish to thank the Chang Lin Tien Scholarship for supporting this research. They would also like to thank the people in Chengdu who have assisted with the collection of local data including Frederic Vallet, Chen Xia, Tang Ya, the Southwest Planning Bureau, and the Chengdu Engineering Consulting Firm.

Supporting Information Available A comprehensive description of the analytical methods, data inputs and data sources for this research is included in the Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org.

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