LCA as a Decision Support Tool for the Environmental Improvement of

Environ. Sci. Technol. 2009, 43, 3300–3307

LCA as a Decision Support Tool for the Environmental Improvement of the Operation of a Municipal Wastewater Treatment Plant JORGELINA C. PASQUALINO,† MONTSE MENESES,† MONTSERRAT ABELLA,‡ AND F R A N C E S C C A S T E L L S * ,† AGA, Chemical Engineering Department, Rovira i Virgili University, Avinguda dels Paı¨sos Catalans 26, 43007 Tarragona, Spain, EMATSA, AGBAR group, 43005 Tarragona, Spain

Received July 28, 2008. Revised manuscript received February 6, 2009. Accepted March 12, 2009.

Life cycle assessment (LCA) methodology is used to evaluate the environmental profile of a product or process from its origin to its final destination. In this paper we used LCA to evaluate the current situation of a wastewater treatment plant and identify improvement alternatives. Currently, the highest environmental impacts are caused by the stages of the plant with the highest energy consumption, the use of biogas from anaerobic digestion (95% burned in torch) and the final destination of the sludge (98.6% for agricultural use and 1.4% for compost). We propose four alternatives for biogas applications and five alternatives for sludge applications and compare them to the current situation. The alternatives were incorporated in a decision support system to identify and prioritize the most positive environmental option. Using biogas to produce electricity or a combination of electricity and heat providedthebestenvironmentaloptionssincetheenergyproduced would be enough to supply all the stages of the plant, thus reducing their environmental impact. The best environmental option for the final destination of the sludge is to combine the current situation (fertilizer replacement) with use of the sludge in a cement plant (as a replacement for fuel and raw material).

Introduction Wastewater treatment plants (WWTP) have been designed to minimize the environmental impact of discharging untreated water into natural water systems. Different WWTP options have different performance characteristics and different direct impacts on the environment. If one of the main functions of wastewater treatment systems is to minimize the impact on the environment, they should be designed accordingly (1). Given the long-term needs for ecological sustainability, the goals for wastewater treatment systems need to move beyond the protection of human health and surface waters to also minimizing the loss of resources, reducing the use of energy and water, reducing waste generation, and enabling the recycling of nutrients (2). * Corresponding author phone: [email protected]. † Rovira i Virgili University. ‡ EMATSA, AGBAR group. 3300

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Environmental issues concerning wastewater treatment are numerous. Legislation is continuously refining the requirements for the level of removal of various pollutants in treated water (3) (4). This often leads to a higher production of sludge, though ways to eliminate this sludge are becoming increasingly restricted. Most sludge has traditionally been incinerated (with energy recovery), disposed of in landfill sites or dumped into the sea. Today, however, ocean dumping has been banned in many areas, while landfilling will be banned in Europe by 2015 (5). Sludge incineration is heavily regulated and facing strong social opposition, as it leads to small atmospheric emissions of substances such as mercury and dioxins and generates ashes that need to be landfilled or handled as hazardous waste (6). Recycling sludge as fertilizer on agricultural soils, which accounts for about 40% of European sludge, is held by many governments to be the best environmental option, adding nutrients and organic content to the soil. However, the content of heavy metals, pathogens, and persistent and toxic organic compounds in sewage sludge make stringent regulations necessary. If the sludge is clean enough to be used in agriculture, this is a simple and often the least expensive way to recycle materials (5) (7) (8) (9). Life cycle assessment (LCA) is used to evaluate the environmental performance of goods, processes and services (collectively termed products). International standards (10) (11) define LCA as a compilation and evaluation of the inputs, outputs and the potential environmental impacts of a system throughout its life cycle: from the production of raw materials to the disposal of the waste generated. Several LCA works are focused in the treatment of residual water and the technology applied, comparing different treatment stages and techniques, or specific case studies (1) (2) (8) (12) (13) (14) (15) (16) (17). Considering that LCA application has not always been performed in the same way, the main contributors to the environmental profile have been reported as the energy consumption (15), the water discharge to rivers (13) (17), or the sludge application to land (13). When sludge is recycled on agricultural soils, mineral fertilizers are often substituted in the inventory to take into account the sludge’s agronomical benefits. The N and P sludge content allows the equivalent N and P mineral fertilizer substitution. An important distinction between LCA studies is whether sewage sludge is regarded as a resource or a waste product. The terms “land application” and “recycling” are often used and include not only agricultural use but also other “beneficial” uses such as land reclamation and land restoration (5). Since the nutrients in the sludge may be used as fertilizers, thereby reducing the need for mineral fertilizers, the production of these fertilizers should be included within the system boundaries (2) (6) (7) (9) (18) (19). Other authors, however, do not consider recycling the sludge as a fertilizer among the system boundaries (8) (20). The anaerobic digestion of sludge, within the sludge line of a WWTP, produces biogas as a byproduct. Most WWTP use this biogas to provide heat to the digester, where sludge must be kept at 35 °C. The rest of the biogas is usually burned in torch. However, the biogas produced in wastewater treatment can be considered a renewable energy source since it can replace the use of fossil fuels for the production of energy and electricity (cogeneration). The main component of biogas is methane, which can be used to produce electricity, or a combination of electricity and heat by means of a cogeneration system. Using biogas 10.1021/es802056r CCC: $40.75

 2009 American Chemical Society

Published on Web 04/06/2009

TABLE 1. Inventory Fluxes (Data from 2006) treatment stage

energy consumption (% of total WWTP)

inputs (annual amount)

outputs (annual amount)

Water Line bar screen

0.98

10 220 000 m3 residual water

solid residues disposed to landfill

sand chamber/degreaser

7.79

air

sand disposed to landfill grease disposed to stabilization

primary settler

1.67

anaerobic reactor aerobic reactor

primary sludge sent to sludge line

3.86 46.41

secondary settler

6.48

primary sludge sieve

1.29

gravity thickener

2.03

air secondary sludge sent to sludge line treated water disposed to sea Sludge Line

flotation thickener

12.01

mixing chamber

3.08

anaerobic digester

7.00

tampon storage

0.39

centrifuge

6.62

final storage

0.40

primary sludge

solid residues disposed to landfill

secondary sludge air FeCl3 (40%) biogas, used partially to provide energy for this equipment, and the rest burned in torch polyelectrolyte (ZETAG 8848FS)

final disposal

as an energy provider can produce two environmental benefits: lower emissions (compared to the emissions of the biogas burned in torch), and lower electricity consumption (21) (22). In this work, we have applied the LCA methodology to the operation stage of a WWTP in order to identify the main environmental contributors and propose alternatives for impact reduction. The alternatives chosen have been selected according to the common practice in Catalonia, where the WWTP evaluated is located. Some of the alternatives selected have not been previously evaluated or compared within LCA works.

Objective This study assesses and identifies the environmental impact of a WWTP in order to determine the environmental loads associated with the plant’s operation and compare the total environmental impact among the various stages in both water and sludge treatment lines. This will enable us to identify the stages at which effort must be focused to in order to find environmental improvements. Thus, the main objective of this study is to assess alternatives into the wastewater treatment for their environmental improvement. The alternatives will focus on the stages identified in the first part of the study as those with the greatest environmental impact.

Materials and Methods CASE STUDY - Tarragona WWTP. Today there are 327 WWTP in Catalonia (Spain) with a useful treatment capacity close to 2 850 000 m3/day (25). The Tarragona WWTP, located at the basin of the river Francolı´, treats an average of 28 000

98.6% sludge sent to agricultural application (replacement of 39.6 kg of CAN and 6.23 kg of TSP per ton of sludge)(9) 1.4% sludge sent to composting plant (replacement of 79.2 kg of CAN and 12.5 kg of TSP per ton of sludge)(9)

m3/day of residual water from urban sewer and rainwater, serving a population of 144 000 inhabitants equivalent, in Tarragona and the surrounding area. The amount of water entering the plant is 10 220 000 m3/year, which is provided by urban collectors and rainwater. The Tarragona WWTP is divided into three main parts, including the water line, the sludge line (see Supporting Information Figure S1), and the services. The services include maintenance (diesel fuel, lubricating oil, and lubricating tallow) and packaging (cardboard, plastic, wood and iron disposed of in landfill). The inventory fluxes taken into account in the water and sludge lines are shown in Table 1. LCA Methodology. When applying LCA, we followed the methodology indicated by international standards (10) (11). As the functional unit we took 1 m3 of residual water entering the plant. All data thus refer to this unit. When performing LCA to WWTP, we considered all the energy and mass input and output flows. We therefore considered all the additives used at the different treatment stages, their packaging, their transport to the WWTP, the wastes generated and their treatments or final disposal, the transport of wastes to their respective treatment plants, the energy consumption at every treatment stage, the service consumptions, and the maintenance material. Because of the comparative nature of the study and the long lifespan of WWTPs (more than 50 years), we did not consider the infrastructure or dismantling of buildings or equipment, and the water distribution system. We have considered that recommended hierarchy on waste management establishes the following order: reduce, reuse, recycle, treat, and finally dispose. Thus, the main wastes VOL. 43, NO. 9, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Total Impact for Every Category water line impact category AP (kg SO2-Eq) GWP (kg CO2-Eq) EP (kg PO4-Eq) PHO (kg formed ozone) DAR (kg antimony-Eq) ODP (kg CFC-11-Eq) ETP (kg 1,4-DCB-Eq)

value

sludge line %

-3

1.58 × 10 1.76 × 10-1 8.92 × 10-5 8.88 × 10-6 1.28 × 10-3 9.60 × 10-9 2.06 × 10-1

93.60 67.72 6.47 47.03 71.43 19.91 34.08

value

Results for the Current Situation WWTP Tarragona. Table 2 shows the results for the environmental impact categories for the Tarragona WWTP. Negative values mean environmental benefit effects. With regard to the influence of each stage in the total impact factors, the services are negligible compared to the water and sludge lines. The water line has the highest environmental impacts in acidification, climate change, photochemical oxidation and depletion of abiotic resources, due to the high energy consumption in this line (67.2% of the total energy consumption in the WWTP). Figure 1 shows the contribution to environmental impact of every stage in the water line. The highest environmental impact of all categories (on average, 70% of the total impact for the water line) is assigned to the aerobic reactor, where energy consumption by the equipment represents 70.85% of the total. Improvement efforts must therefore concentrate on this operation stage, focusing especially on a more efficient utilization of energy to reduce energy consumption and an increase in the use of renewable energies whenever possible. Use of additives, their transport to the plant, the wastes 3302

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% -7

-1.03 × 10 -7.04 × 10-2 1.28 × 10-3 7.33 × 10-6 4.12 × 10-4 3.77 × 10-8 3.82 × 10-1

generated (biogas and sludge) are considered as resources that can be recycled into valuable products, and their final destination has been included within the system boundaries. LCA inventory was performed by adapting the data from ecoinvent V2.01 database (23) to the Spanish energy mix and the European model for transport and water. The infrastructure of the processes and products was not taken into account. We have used two main kinds of data: (a) site specific operating data collected from internal reports and personal interviews from the Tarragona WWTP, and (b) data from databases (23). These data consist of annual material and energy inputs and outputs, detailed for every treatment stage, provided for the operation of the plant in 2006. Data quality is assured by the accuracy of the operating data of the plant and the reported deviation of values from database. Environmental impact was assessed with SiSOSTAQUA, an environmental management tool adapted from LCAmanager tool (24). In this study, we used the CML2000 method, which obtains a single score for each impact category. The impact categories considered are: AP (acidification potential, global, kg SO2 eq.), GWP100a (global warming potential, kg CO2 eq.), EP (eutrophication potential, global, kg PO4 eq.), PHO (photochemical oxidation, kg formed ozone), DAR (depletion of abiotic resources, kg antimony eq.), ODP (ozone depletion potential, kg CFC-11 eq.), and ETP (ecotoxicity potential, kg 1,4-DCB eq.) (see Supporting Information Table S1). The ETP category is the result of adding freshwater aquatic and sediment ecotoxicity, marine aquatic and sediment ecotoxicity, human toxicity and terrestrial ecotoxicity.


services

0.01 27.09 92.91 38.82 22.99 78.17 63.20

value -4

1.08 × 10 1.35 × 10-2 8.50 × 10-6 2.67 × 10-6 1.00 × 10-4 9.26 × 10-10 1.64 × 10-2

%

total

6.40 5.19 0.62 14.14 5.58 1.92 2.71

1.69 × 10-3 1.19 × 10-1 1.37 × 10-3 1.89 × 10-5 1.79 × 10-3 4.82 × 10-8 6.05 × 10-1

generated (except for sludge) and their final disposal have a lower contribution to the total environmental impact of the water line. The sludge line has the highest environmental impacts in eutrophication, ecotoxicity and depletion of stratospheric ozone. The high impacts in eutrophication and ecotoxicity categories are due to excess nutrients and heavy metals, respectively, caused by the final disposal of sludge (application in soil). The depletion of stratospheric ozone is caused by the use of FeCl3 as a desulphurant additive in the mix chamber. Figure 2 shows the contribution to environmental impact of every stage in the sludge line. Taking into account the effects of sludge application to soil and avoidance of the use of chemical fertilizers, we find that sludge disposal is the major contributor to total environmental impact and it has a considerable environ-

FIGURE 1. Comparison of impact factors for the operation stages of the water line.

FIGURE 2. Comparison of impact factors for the operation stages of the sludge line.

TABLE 3. Environmental Alternatives Alternatives Using Biogas from Anaerobic Digestion

present situation combustion in torch electricity generation electricity and heat generation use as natural gas

a minor fraction of the biogas is used to provide thermal energy to the digester (energy needed to keep sludge at 35 °C) and the rest of the biogas is burned in a torch all the biogas is burned in torch produce electricity form the biogas and use it to serve equipment in the WWTP use part of the biogas to provide heat to the digester and the rest of the biogas to produce electricity for other equipment in the plant use the biogas as a substitute for natural gas Alternatives for the Final Disposal of Sludge

present situation agriculture compost cement plant

incineration landfill

99.86% of the sludge is allocated for agricultural uses and 0.14% is allocated to a compost plant, thus getting a benefit for saving fertilizer in both cases all the sludge is allocated for agricultural uses, thus getting a benefit for saving fertilizer ( all the sludge is allocated to a compost plant and then applied in agriculture, thus getting a benefit for saving fertilizer all the sludge is allocated to a cement plant, where it is burned as fuel, and the ashes are used as raw material substitutes, thus getting a benefit for saving fossil fuel and raw material all the sludge is allocated to an incineration plant, where it is used to produce electricity, thus obtaining a benefit, and the ashes, generated as waste, are sent to landfill all the sludge is disposed to landfill

mental benefit (negative percentage). This shows the importance of a sustainable sludge management. There is an environmental benefit from substituting the use of chemical fertilizers by the agricultural use of sludge and composting. For example, the global warming impact is -7.04 ×10-2 kg CO2 eq, resource recovery due to its use in agriculture avoids the CO2 emissions caused by the manufacture of chemical fertilizers. However, the impact values for eutrophication and ecotoxicity categories are positive. The concentrations of excess nutrients (which affect the eutrophication potential) and heavy metals (which affect the ecotoxicity potential) are weaknesses that can limit the amount of sludge that can be used in agricultural applications and encourage the search for sludge management alternatives. It should be taken into account the amount of sludge applied to the soil is based on bibliography (9). As the biogas is burned in torch, the impact of the anaerobic digester is important and strongly affects global warming potential. These results are useful for making decisions on improvements to current technologies and show the importance of adequate biogas use and sludge disposal management.

Improvement Alternatives Proposed Anaerobic Digestion. The anaerobic digestion of sludge produces biogas that can be used for the cogeneration of electricity and heat. Currently in the Tarragona WWTP, only a minor fraction of biogas is used to provide thermal energy to the digester (energy that is needed to keep the sludge at 35 °C). The rest of the biogas is burned in torch. However, the amount of biogas produced (0.11 m3 biogas/ m3 treated water) could be converted to 0.68 kWh/m3 of electricity, which would be enough to cover all the plant’s electricity needs. Possible alternatives to this situation are given in Table 3. Sludge Final Destination. Appropriate use of WWTP sludge could represent an environmental benefit thanks to

the use of waste. In Catalonia, about 89.3% of sludge produced during 2007 was used in agriculture, both directly and as compost (27), while the rest was used in cement plants (1.6%), landfilled (1.9%), or used for other purposes (0.2%). Sludge final destination in Spain has followed a similar trend with 66, 7.8, 16, and 10.2% used in agriculture, incinerated, landfilled, or applied to other uses, respectively (26). According to that, we have compared the current situation with five alternative sludge destinations (see table 3), all of them being of common application in Spain and other European countries (26) (28).

Results for Improvement Alternatives Figure 3 compares the environmental impact of the total Tarragona WWTP (current situation) with the alternatives for the use of biogas. We observe that the energy production from biogas (electricity or electricity plus heat) represents an environmental improvement to the current situation in all the impact categories studied. This benefit is particularly significant in acidification, climate change, and depletion of abiotic resources categories, due to energy saving credit. If this energy were used to provide electricity to the water line equipment, especially the aerobic reactor, the environmental profiles of this equipment and the water line would be significantly reduced. If biogas were used as a replacement for natural gas, results would be similar to the current situation, though there would be an improvement in the depletion of abiotic resources category. Biogas burning in torch is the worst scenario for all impact categories studied. Figure 4 compares the current situation of the sludge line and the five alternative sludge destinations. We find that the results for the agricultural application and the current situation are similar. This is because of the low amount of sludge that is currently used as compost (0.14%). Allocating the sludge for agricultural uses is the best environmental option for all impact categories (except eutrophication and ecotoxicity). This is due to the nutrients (eutrophication) and heavy metals concentraVOL. 43, NO. 9, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Comparison of the environmental impact of the total WWTP for the current situation and the biogas application alternatives.

tion (ecotoxicity) in the sludge that may limit the amount of sludge that can be used in agricultural soil, which justifies the search for alternatives. 3304

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All the alternatives produce better results for the eutrophication impact category since they all prevent or reduce the accumulation of nutrients in the soil. However, only the

FIGURE 4. Comparison of the environmental impact of the sludge line for the current situation and the sludge application alternatives. compost and cement applications are able to reduce ecotoxicity impact. The composting process of the sludge considerably reduces its total volume, which facilitates its application in soil and its transportation to agricultural land. However, the composting

process itself presents a high environmental impact in the acidification and photochemical oxidation categories. As well as reducing the impact in the eutrophication and ecotoxicity categories, the allocation of sludge to a cement furnace also improves photochemical oxidation VOL. 43, NO. 9, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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and the depletion of abiotic resources categories (thanks to savings in fossil fuels and raw materials in the furnace). For the acidification and climate change categories, like all the alternatives, allocating sludge to a cement furnace has positive impact values that are higher than in the current situation. Allocating sludge to the incineration plant or to landfill does not generally improve the environmental impact. We should remember, however, that incineration alternative presents an important environmental benefit due to energy production. Landfill destination represents the worst scenario for disposing of sludge, though it is the most common means of doing so.

General Discussion The highest environmental impacts of the water line are due to the energy consuming equipment, especially the aerobic reactor, which accounts for 70% of the electricity consumed by the water line. The recommendations here are to reduce energy consumption, use energy efficiently, and use more renewable forms of energy. With regard to the sludge line, the highest environmental impacts are found in the anaerobic digester (due to biogas burning in torch), in the stages with the highest energy consumption or that use chemical additives, and in the sludge final destination. Considering the alternatives for the use of biogas (table 3), we can therefore conclude that if the WWTP operates a cogeneration unit, the environmental profile of the anaerobic digester will be reduced, thus creating an environmental benefit for the whole plant, especially the water line, which uses almost 70% of the total energy of the plant. In this case the plant should be self-sufficient in terms of energy consumption. We should also note that this option is economically feasible. Of the alternative final destinations of sludge we studied (table 3), the best environmental option is to combine the current situation (use sludge mainly for agriculture) with using the sludge in a cement plant because this emphasizes their respective advantages and diminishes their respective disadvantages. Regulations limit the composition of the sludge and thus the sludge amount that can be applied to agricultural soil. Regulations and sludge composition are related, as the concentration of specific components in the sludge may exceed the limits allowed for agricultural applications, and these limits may vary from one country to another. In the case the sludge composition does not accomplish with local regulations, the direct application of the sludge for agricultural uses will not be possible. In such cases, the use of sludge in a cement plant clearly appears as the best option. If concentration limits are not exceeded, the final disposal of the sludge may combine several options. The application in agriculture is determined by the type of crop, and thus its cultivation requirements in terms of nutrients needs, application period, etc. However, sludge is being continuously generated at WWTPs, thus accumulating until its application period. Due to the high volume of sludge, the economic and environmental cost of storage is not of minor importance, does inducing the decisions to more immediate applications as cement plant furnaces. The transport of the sludge to agricultural applications is not a fixed parameter, as it depends on specific needs, though the sludge is usually applied in soil relatively close to the plant location. The cement plants, however, are not usually close to the WWTPs, thus incrementing the cost and environmental impact of the transport of the sludge. Final decisions should then consider several factors such as the time period, cost and impact of the sludge storage versus the distance, cost and impact of 3306

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the transport to the cement plant. Use of sludge in a cement plant is therefore the best alternative when the amount and quality of the sludge prevent or limit its application in agricultural soil, also it can be a process in continuo. Therefore, final decisions have to be taken considering the following aspects: sludge composition (especially nutrients and heavy metals content), regulations, distance to the cement plant, agricultural application period and economical feasibility. According to the explained before, may be there is not unique solution for final destinations alternative of sludge. Use of sludge in a cement plant is therefore the best alternative when the amount and quality of the sludge prevent or limit its application in agricultural soil. The final sludge destination may be planed considering its use in agriculture during the application periods and the use in cement plants during the rest of the year because this emphasizes their respective advantages and diminishes their respective disadvantages.

Acknowledgments This work is part of the SOSTAQUA Project, led by Aguas de Barcelona (AGBAR) and founded by CDTI in the framework of the Ingenio 2010 Program under the CENIT call. We gratefully acknowledge the data acquisition from the wastewater treatment team from EMATSA (Tarragona WWTP).

Supporting Information Available Table 1 and an additional figure. This material is available free of charge via the Internet at http://pubs.acs.org.

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LCA as a Decision Support Tool for the Environmental Improvement of

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