Selecting sewage sludge treatment alternatives in modern wastewater treatment plants using environmental decision support systems

Accepted Manuscript Selecting sewage sludge treatment alternatives in modern wastewater treatment plants using environmental decision support systems M. Garrido-Baserba , M. Molinos-Senante , J.M. Abelleira-Pereira , L.A. Fdez-Güelfo , M. Poch , F. Hernández-Sancho PII:

S0959-6526(14)01201-3

DOI:

10.1016/j.jclepro.2014.11.021

Reference:

JCLP 4910

To appear in:

Journal of Cleaner Production

Received Date: 12 November 2013 Revised Date:

2 November 2014

Accepted Date: 4 November 2014

Please cite this article as: Garrido-Baserba M, Molinos-Senante M, Abelleira-Pereira JM, FdezGüelfo LA, Poch M, Hernández-Sancho F, Selecting sewage sludge treatment alternatives in modern wastewater treatment plants using environmental decision support systems, Journal of Cleaner Production (2014), doi: 10.1016/j.jclepro.2014.11.021. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT SELECTING SEWAGE SLUDGE TREATMENT ALTERNATIVES IN MODERN WASTEWATER TREATMENT PLANTS USING ENVIRONMENTAL DECISION SUPPORT SYSTEMS M. Garrido-Baserbaa,b, M. Molinos-Senantec, J.M. Abelleira-Pereirad, L. A. Fdez-Güelfoe, M.

a

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Pochb, F. Hernández-Sanchof

Department of Civil and Environmental Engineering, Urban Water Research Center,

b

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University of California, Irvine, California 92697, United States. E-mail: [email protected]

Laboratory of Chemical and Environmental Engineering (LEQUIA), Universitat de Girona,

c

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Facultat Ciències. Campus Montilivi, 17071 Girona, Spain. E-mail: [email protected] Department of Mathematics for Economics, Universitat de Valencia, Campus dels Tarongers,

46022, Valencia. Spain. E-mail: [email protected] d

Department of Chemical Engineering and Food Technology, Faculty of Sciences, Campus de

Excelencia Internacional Agroalimentario (CeiA3), University of Cádiz, 11510 Puerto Real

e

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(Cádiz), Spain. E-mail: [email protected]

Department of Environmental Technology, Faculty of Marine and Environmental Sciences,

University of Cádiz, 11510 Puerto Real (Cádiz), Spain. E-mail: [email protected] Department of Applied Economics II, Universitat de Valencia, Campus dels Tarongers, 46022,

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Valencia. Spain. E-mail: [email protected]

ABSTRACT

The importance of the sewage sludge treatment within the field of wastewater treatment plants (WWTPs) suggests new dimensions of analysis where the relevance of economic criteria combined with the associated environmental issues are increasing the sludge management complexity. For supporting the decision process and for comparative purposes, this study assesses five alternative configurations for sludge treatment, namely: mesophilic and termophilic anaerobic digestion plus composting, incineration, gasification, and supercritical

ACCEPTED MANUSCRIPT water oxidation (SCWO). The global warming potential (GWP) and the annual cash flow of each alternative are used to estimate a composite indicator for each alternative. Stakeholders’ preferences are integrated into the assessment through the development of five scenarios prioritizing economic or environmental aspects. A case study for a 1 million person equivalent

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WWTP proved that SCWO is the most adequate option if economic and environmental criteria are considered equally important. However, if the economic assessment is prioritized over the environmental one, thermophilic anaerobic digestion followed by composting turned out to be the most appropriate option. The proposed approach contributes to the implementation of more

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suitable sewage sludge treatment lines since it provides an indicator for each alternative

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embracing economic and GWP issues.

Keywords: Sewage sludge treatment technologies; Environmental Decision Support System; Knowledge-based methodology; Cost-Benefit Analysis (CBA); Composite indicator; Global

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Warming Potential (GWP).

ACCEPTED MANUSCRIPT 1. INTRODUCTION Sewage sludge is inevitably produced in urban wastewater treatment plants (WWTPs), being by far the largest constituent removed. As the number of WWTPs in operation increases, the quantity of sewage sludge generated is also expected to grow very substantially in the future.

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For instance, in 1992 the European Union produced around 5.5 million metric tons of sludge (dry matter) while in 2010 this figure increased to almost 10 million tons (European Commission, 2010). Moreover, the processing, reuse, and disposal present one of the most

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complex problems facing engineering in the field of wastewater treatment (Metcalf & Eddy, 2003). The complexity of the sludge treatment management includes offensive substances, mass

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balances, and variations of the solid characteristics. The selection of the most suitable process involves many possible options which are all linked. The accomplishment of a variety of objectives and multiple criteria increases the complexity of the selection of the most appropriate process to treat sewage sludge. Therefore, this task requires the inclusion of economic and or

processes flow diagrams.

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environmental considerations during the selection or design of current solids-treatment

Some driving factors promoting changes in the design of wastewater treatment process flow diagrams (and subsequently increasing the complexity in the design) are: a) the rising energy

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costs and the need of more electricity and heat to operate the plants; b) sustainability and

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environmental concerns, such as global warming and greenhouse gases (GHG) emissions; c) regulation as factor stimulating the development of new technologies (Olsson, 2013). Along with technical factors, economic and environmental aspects must be considered in sludge treatment. The sludge management costs impact plays a central role in any type of WWTP analysis, since the solids handling and processing accounts for as much as 30% - 50% of wastewater treatment facility’s costs (Neyens et al., 2004; GWRC, 2008). However, it should be emphasized that sludge contains 10 times the energy required to treat it. Therefore, new emergent perspectives lead to consider this waste as a product to be used beneficially after

ACCEPTED MANUSCRIPT treatment (WEF, 1998). It has been proven to be technically feasible to recover energy from the sludge, which can be directly used in wastewater treatment or be sold to the network, reducing the facility’s dependency on conventional electricity and helping the stressed public budgets. In other words, from an economic point of view, energy recovery means incomes as direct benefit

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or avoided cost. Concerns about sustainability involve not just the consideration of technical and economic aspects during the decision-making process but also environmental issues. In this context,

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technologies for treating sludge are considered as solutions not exempt of impacts. The life cycle perspective and the carbon footprint analysis entails the consideration of direct impacts

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associated to the sludge treatment, combined with indirect impacts associated to the inputs (materials and energy use) and outputs (emissions and wastes generated). The most widely accepted and well-established procedure to quantify the environmental impacts regarding a product or process throughout its whole life cycle is the Life Cycle Assessment (LCA) (Cooney, 2009 ; ISO, 2006). Sludge treatment is not exempt from this trend and a wide number of works

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were carried out aimed to assess the environmental impacts of the sludge line in WWTPs (Hospido et al., 2005; Righi et al., 2013; Cao and Pawlowski, 2013). Although eutrophication, ozone depletion, photochemical ozone creation, depletion of abiotic resources and human

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toxicity are impact categories usually evaluated through LCA, it is well known that global warming potential (GWP) is not one of the most common impact categories in this

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methodology. Nevertheless, since WWTPs are big consumers of electric power, the GWP has been commonly applied in order to both quantify indirect emissions and include political and social concerns about this impact (Larsen, 2007; Rodriguez et al., 2011). As environmental and economic concerns increases, the interest shifts from just building technically suitable sludge-treatment options to also consider environmentally friendly and economically feasible ones (Bertanza et al., 2014). The growing number of treatment technologies which can be potentially implemented for the very same case provides water managers with a variety of alternatives. A high number of combinations of sludge-treatment

ACCEPTED MANUSCRIPT flow diagrams incorporating unit operations and processes can be proposed (Metcalf & Eddy, 2003). In this respect, the Environmental Decision Support Systems (EDSS) are assessment tools capable of supporting complex decision making processes. EDSS integrate coupled models, databases, numerical methods, environmental ontologies, etc. (He et al. 2006; Matthies

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et al. 2007, Shim et al. 2002; Huang 2010). EDSS assist decision makers in choosing between alternative solutions or actions by applying knowledge about the decision domain to reach recommendations for the various options (Fox and Das, 2000).

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Previous experiences successfully applied Decision Support System (DSS) tools for the selection of the best alternatives in the wastewater treatment domain. Alemany et al. (2005)

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used an EDSS to identify adequate small wastewater treatment technologies or low populated communities, although only technical aspects were considered. Molinos-Senante et al. (2012) also used an EDSS for the selection of SWWT, incorporating the economic vector in the selection analysis. Dinesh (2003) was assisted by a DSS for the evaluation and selection of treatment alternatives for reclamation and reuse applications. Joksimovic (2008) used a DSS for

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considering alternative options for reuse treatment and also network distribution aspects. Nevertheless, none of those approaches quantified or considered the potential environmental impacts of the selected alternatives. In this respect, the works from Hamouda (2011) and

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Garrido-Baserba (2013) integrate sustainability indicators (i.e. LCA) during the decisionmaking of the selection of the most appropriate treatment alternative. However, works that

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include the development and application of EDSS for supporting decisions regarding sewage sludge treatment technologies are much more limited, with only a few cases focusing in the selection of the best decision management option for composting or sludge application on agricultural soils (Horn et al., 2003; Passuello et al., 2008). In this work, the NovEDAR_EDSS software was used for the identification and assessment of the most appropriate sludge treatment technologies for the design of WWTPs. The NovEDAR _EDSS was conceived as an integrated software employing artificial intelligence techniques combined with different analytical tools: Multicriteria Decision Analysis (MCDA)

ACCEPTED MANUSCRIPT methodologies (Flores-Alsina et al., 2008), LCA (Lundin, 2000), Cost-Benefit Analysis (CBA) and

Environmental-Benefit

Analysis

(EBA)

(Molinos-Senante

et

al.,

2011).

The

NovEDAR_EDSS has previously been successfully used in feasible WWTP selections (Garrido-Baserba et al., 2011), including economic parameter evaluation (Molinos-Senante et

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al., 2012). The different databases were developed from a variety of sources, including information from the literature specific to our purposes, and interviews with experts within the NovEDAR Project. The proposed EDSS model was based on a hierarchical decision approach combined with a knowledge-based system, which uses the interaction of different main

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knowledge bases to provide a required number of optimum alternatives. Garrido-Baserba et al. (2010) reported additional development information regarding NovEDAR_EDSS. This software

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integrates not only water line information but also an exhaustive database about sewage sludge treatment technologies. It is to be highlighted that the NovEDAR_EDSS includes information about investment and operating costs, as well as direct and indirect GHG emissions of traditional and novel sludge treatment technologies. Hence, it is a useful tool for identifying

environmental criteria).

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strategies for sludge treatment based on the stakeholders’ perspective (economic, technical or

The aim of this study is to assess the selection between five alternatives (see Table 3 in section

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2.1) for sludge treatment, embracing economic and GWP issues. In doing so, five scenarios regarding the stakeholders’ preferences (see Table 5 in section 3.3) are evaluated using a

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WWTP with a one million person equivalent (PE) capacity as a case study. The most relevant factors contributing to the overall plant feasibility and GWP of the evaluated alternatives will be identified and discussed. 2. METHODOLOGY Data from each scenario or defining a case study constitutes the input information which the user introduces to the NovEDAR_EDSS. After introducing all the required input data, the software will compute it and proceed with the calculations of the complete process flow

ACCEPTED MANUSCRIPT diagram (PFD) for the sludge line. Using the simulated output values the software can rank the most suitable sludge lines according to the parameters introduced and the user’s specifications. In this way, the NovEDAR_EDSS is able to rank the most feasible treatment units depending on the scenario characteristics or user objective’s prioritization. It corresponds to the user to select

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one technology (normally the one that achieved the highest score) providing the user with a complete set of estimative output data, which can be used for carrying out comparisons between different feasible designs, in case the user needs to consider various options.

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2.1 Case study

This work focused on one case study according to population size, corresponding to a large

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plant serving 1,000,000 person equivalents (Peq) (See Table 1). The study of large WWTPs enables exploration of a wide variety of alternatives in the design of sludge treatment. *** INSERT TABLE 1 ***

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The estimated parameter outputs using the scenario input data is shown in Table 2. *** INSERT TABLE 2 ***

The output information of the EDSS shows technically feasible sludge process flow diagrams.

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In this study, five representatives and different configuration alternatives among the set of technically suitable options were selected according to the three main criteria implemented in

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the EDSS: economy, environmental and technical as it shown in Table 3. The five alternatives proposed to be evaluated involve six technological processes (Table 4). For each alternative the NovEDAR_EDSS provides a complete set of estimated output data based on cost and environmental prioritization which can be used for carrying out comparisons between the different alternatives. As it is shown in Figure 1, the NovEDAR_EDSS suggests a WWTP process flow diagram based on the case study data (top of screen in Figure 1). In addition, it provides a complete set of estimated output data from the corresponding groups of technologies within sludge line-related sections (overlapped screens in Figure 1).

ACCEPTED MANUSCRIPT *** INSERT TABLE 3 *** *** INSERT TABLE 4 *** ***INSERT FIGURE 1***

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The first two options are integrated by two processes, namely anaerobic digestion (AD) and composting, and they represent two of the most widely used processes to treat the sludge in WWTPs (Fdez-Güelfo et al., 2011a, b). The mesophilic AD consists in a series of biological

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reactions in which microorganisms break down biodegradable material in the absence of oxygen. It is used for industrial or domestic purposes for the stabilization of organic slurries

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and/or to release energy. Mesophilic AD is widely used as a renewable energy source since, besides carbon dioxide, the process produces hydrogen and/or methane that could be suitable for energy production, helping to replace fossil fuels. The nutrient-rich digestate which is also produced can be used as fertilizer after some specific treatments (i.e. composting) are applied. Nowadays many large WWTPs use mesophilic AD for sludge treatment, mainly due to its

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relatively easy implementation and operation. However, thermophilic AD has been upgraded so that the process is easier to control (Ahring, 2003; Starberg et al., 2005; Willis and Schafer, 2006). The main advantages of thermophilic over mesophilic processes are potentially better gas

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production, reduction in sludge volume. Furthermore, depending on the treatment, thermophilic treatment does not require further sterilization for agricultural use of the digestate (Fdez-Güelfo

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et al, 2010; Rulkens, 2008; Appels et al., 2008). All these potential advantages would significantly lower the total cost for sludge handling (Arnold, 2011). However, AD also produces H2S that is highly corrosive and must be removed from the biogas before it is suitable for an efficient energy production. Many techniques are being studied and applied to remove H2S and upgrade the biogas quality. Unfortunately, this involves reduced economic benefits from the generation of biogas. It should be noted that the digestate from the AD must be dewatered before conducting subsequent composting. Dewatering processes

ACCEPTED MANUSCRIPT requires many resources in terms of energy and/or chemicals, especially in the case of thermophilic AD than in mesophilic AD. The third alternative is incineration, which is able to recover the energy contained in sewage sludge by means of its conversion into heat and electric power. Sewage sludge incineration can

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be a convenient solution in many specific situations as it is regarded as cost-efficient in large urban areas where the distance to agricultural land or landfill site makes transportation expensive and when restrictions on landfilling are imposed (Arnold, 2010). However, costs

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become an important driving factor: incineration operates at extremely high temperatures (> 850°C), producing harmful gaseous emissions (containing among others, dioxins and furans),

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whose treatment can be very expensive and sometimes cost prohibitive.

Besides incineration, the most common thermal sludge treatment technology is gasification. Gasification was chosen as the fourth alternative (see Table 3). Gasification has recently been upgraded to modern standards for reliability, emissions, automation and safety. Gasification

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consists of the partial oxidation of organics (biomass) and conversion to carbon monoxide, hydrogen (syngas) in the presence of limited air (BCWWA, 2010). Syngas produced from sewage sludge has comparable energy content to syngas from biomass and the generated sub-

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product (ashes) has the potential to be beneficially re-used (cement kiln, asphalt, ceramics). Supercritical water oxidation (SCWO) is the fifth alternative. SCWO is considered an

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environmentally friendly, innovative process, efficient in the treatment of the sewage sludge. In the SCWO sludge treatment process aqueous organic waste is combined with an oxidant (usually, air or pure oxygen) at conditions over the critical point of water (373.95 ºC, 220.64 bar), commonly 400 – 600 ◦C and 250 bar. SCWO can achieve an organic matter removal >99% operating at reaction times of less than a minute. Main products are water, CO2 and inorganic salts. Although the SCWO’s gas emissions mainly and inherently contain CO2, plus possibly low concentration of nitrous oxide (N2O), no more harmful emissions (NOx, SOx, furans, dioxins) do occur (Marrone, 2013).

ACCEPTED MANUSCRIPT 2.2 Economic assessment The economic assessment of the five alternatives (A, B, C, D, E - Table 3) studied for treating the sewage sludge was made using conventional cost-benefit analysis since it is fully valid as an instrument for decision makers (Molinos-Senante et al. 2010; Hardisty et al. 2013). In order to

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be able to compare alternatives with different life span, the economic assessment was based on the estimation of the annual cash flow (ACF) (Eq. 1) instead of the traditional net present value. The ACF allows comparing the economic viability associated with the implementation of

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different proposals. A project is only economically feasible if the NPV is positive.

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 =  − 
 (1) where: ACF is the annual cash flow (€); INC is the annual income (€/year); TAEC is the total annual equivalent cost (€/year).

The total annual equivalent cost (TAEC) (Eq. 2) is calculated by adding the annualised


 =

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investment cost (IC) to the annual operations and maintenance cost (OMC) as: (1 + )  +  (2) (1 + ) − 1

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where TAEC is the total annual equivalent cost (€/year); IC are the investment costs (€); OMC are the annual operational and maintenance costs (€/year); t is the useful life of the technology

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(alternative); and r is the discount rate. IC includes the capital and financial costs of implementing each alternative while OMC includes reagents, energy, staff, maintenance and overhaul costs. Regarding the incomes, processes considered in the alternatives A and B (mesophilic and thermophilic AD + windrow composting) involved the generation of two products with economic value, namely biogas and compost. Biogas produced from AD is used mainly for heating the digester but also for producing electric energy. Due to institutional, policy and technical conditions in most of the cases, the electricity produced is not used in the own WWTP

ACCEPTED MANUSCRIPT but it is sold in exchange for energy credits. The compost generated is also a valuable product since its application provides organic matter and nutrients to the soil, i.e., it can be used as organic amendment. After a quality control aimed to ensure that the concentration of heavy metals is below the established by the legislation, the sale of the compost entails certain

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incomes. The annual income gained from anaerobic digestion was estimated using Eq. (3) and the incomes related to compost production though Eq. (4).

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 =
 ∗  ∗   (3)

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where INCtad is the annual income from the anaerobic digestion of the sludge (€/year); ABTt is the annual biogas production (Kg/year); EPBt is the electricity produced from biogas (kWh/Kg biogas), and SPEt is the present selling price of the electricity (€/kWh).  =
 ∗  (4)

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where INCtc is the annual income from composting the digested sludge (€/year); ACPt is the annual compost production (Kg/year) and; SPCt is the present selling price of the compost (€/kg).

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It should be noted that our economic assessment is a financial analysis, i.e., only takes into account costs and benefits with market value. In the assessment of the economic feasibility of

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the WWTPs’ framework, other studies also included the economic value of the externalities associated to each alternative. For example, Molinos-Senante et al. (2012) quantified the environmental benefits of removing pollutants from wastewater to avoid their discharge into different water bodies. Murray et al. (2008) incorporated the external costs of six different air pollutants emitted during sewage sludge treatment and disposal. Our study integrates the GHG emissions of each alternative through the assessment of its global warning potential (GWP). The addition of the economic value of GHG in the quantification of the ACF would involve a double

ACCEPTED MANUSCRIPT counting of the environmental aspects. Hence, in order to avoid overlapping, the economic feasibility of the alternatives is based on tangible costs and benefits. 2.3 Global warming potential (GWP) assessment

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The United Nations included the reduction of CO2 and other GHG as one of their targets (UN, 2000). In order to implement mitigation policies it is necessary to properly account for the GHG emissions of any economic activity. Thus, to deal with the different nature of the generated GHG emissions (CO2, CO, NOx, CH4 and N2O) they are converted into units of CO2 equivalents

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(CO2eq). WWTPs generate CO2 due to several reactions that occur both in the water and the sludge line (Rittman and McCarty, 2001). The quantification of CO2 emissions have been found

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to be relevant since Griffith et al. (2009) stated that up to 20% of the carbon present in wastewaters can be of fossil origin (mainly related to detergents). Therefore all CO2 sources in WWTPs should be taken into account. In addition, WWTPs are big consumers of electric power so the energy used during the wastewater treatment implied indirect emissions that must be

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quantified. WWTPs are also a source of another carbonaceous and nitrous GHG such as carbon oxide (CO), methane (CH4), nitrous oxide (N2O) and nitrous oxides (NOx), which have an emission factor of 3, 25, 310 and 8 kg CO2eq per GHG respectively (IPCC, 2006; Forster et al.,

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2007).

A comprehensive life cycle category is proposed to calculate the environmental impact of GHG

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emissions for the selected sewage sludge treatment alternatives. The GWP category was chosen for its capacity to quantify both the direct and indirect impacts from the selected lines (Table 4), considering direct emissions from the sludge processing alternatives and the indirect emissions related to the net power (the difference between energy usage and production). 2.4 Integration of criteria The economic and GWP assessment are two different criteria for decision-making. However, many times the policy-making process requires the integration of the multi-dimensional evaluation into a single index (Blancas et al., 2012). In this context, and mainly in the

ACCEPTED MANUSCRIPT framework of sustainability, there is an increasing interest in the development of synthetic indicators embracing different criteria (Mondejar-Jimenez et al. 2010; Lozano-Oyola, 2012; Perez et al., 2013). To integrate the economic and GWP criteria a composite indicator (CI), based on the distance to

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a reference point approach, was defined. m initial indicators (NPV and GWP) to evaluate n alternatives (5 sludge process flow diagrams) were considered. According to Blancas et al. (2011), the first step to obtain the composite indicator was to homogenize the direction of

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improvement of our two initial indicators, since the highest NPV and the lowest GWP are optimal. In doing so, the sign of the values of the NPV were changed. Then, the value of the ith

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alternative in the jth indicator was normalized. This is a common procedure to ensure that the measuring units of each indicator do not affect to the final results (Floridi et al., 2011). Since in our case, the lowest value of the indicator means the best, the procedure to normalize involves dividing the distance to the ideal point by the difference between the maximum and the minimum value (Eq. 5):

  −  (5)   − !"

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 =

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where  is the normalised value of the ith alternative in the jth indicator;  is the value of the ith alternative in the jth indicator; 



is the maximum value of the jth indicator and; !" is the

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minimum value of the jth indicator.

These quotients allow expressing the indicator values using a dimensionless scale with values that vary between 0 and 1 (Blancas et al. 2011). Once the initial indicators have been normalized, different weights can be assigned to them in order to incorporate the preferences of the decision-makers (Eq. 6). Hence a composite indicator  is obtained for each alternative i considering the weights for the indicator j ($ ). It should be noted that if the weights are defined as $ ∈ [0 − 1] then  ∈ [0 − 1]. For each scenario (different weights for each initial indicator) the best alternative will be the one with the highest value of .

ACCEPTED MANUSCRIPT  = ∑+ ,- $ ∗  (6) This methodology is a useful tool for selecting the most suitable alternative according to the criteria and preferences defined by the decision-makers.

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3. RESULTS AND DISCUSSION 3.1 Economic assessment

Based on the information gathered in Table 4, the TAEC of the five studied alternatives was

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estimated. It should be noted that lifespan of the alternatives D and E (gasification and SCWO) is 10 years whereas the life span of the remaining alternatives is 20 years. However, this fact did

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not affect the interpretation of results since economic assessment was based on annual estimations (ACF). To annualize IC the choice of the discount rate is of particular relevance because it can lead to different results (Almansa and Martínez-Paz, 2011). Although Monte Carlo techniques, declining discount rates, dual rates or Delphi panel, are useful methodologies for defining the discount rate, a more traditional approach was conducted in this work, taking

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into account that the economic feasibility assessment developed here is a financial analysis,. In particular, a constant discount rate of 4% was chosen (Berbel et al. 2011).

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Figure 2 shows significant differences between the five alternatives evaluated, since the highest TAEC (corresponding to incineration) is almost three times higher than the lowest TAEC,

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corresponding to SCWO. From the costs´ perspective, alternatives A, B and C (mesophilic AD + composting, thermophilic AD + composting, and incineration) are quite similar while a second group integrated by alternatives D (gasification) and E (SCWO) have lower costs. Focusing on IC, anaerobic digestion is the most expensive technology while SCWO is the cheapest one. A different pattern is observed regarding OMC, since maximum costs do not correspond to anaerobic digestion but to incineration. Gasification and SCWO present similar OMC (the lowest). Both,mesophilic and thermophilic anaerobic digestion have quite similar OMC, with a value in between the OMC values of gasification and incineration . It should be noted that incineration has a special cost distribution since it has medium-low IC but high OMC

ACCEPTED MANUSCRIPT and therefore, TAEC is also high. This fact illustrates the importance of integrate both types of costs (investment and operational) during the decision making process to select the best alternative from the TAEC point of view, which is the one with the lowest total costs during the

***INSERT FIGURE 2***

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life-span of the project.

Once TAEC was estimated, in order to calculate the ACF, incomes were quantified. As it was described in Section 2.2, only the alternatives A and B entail incomes. Regarding the generation

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of biogas, it has been considered that 65% of the total production was used for heating the digester and electrical energy was produced with the 35% remained biogas (Metcalf and Eddy,

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2003). Based on the Spanish Royal Decree 661/2007 and on the Order ITC/3353/2010 it was assumed that the selling price of the electricity fed to network is 0.037 €/kWh. The second product with market value obtained in the alternatives A and B is the compost. Although the price paid by this product is quite variable depending on its quality (MMA, 2005), a value of 6.5

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€/T as the market price of compost was assumed (Sevilla et al., 2005). It should be emphasized that incomes from the sale of the energy are around 98% of the total ones illustrating the importance of optimizing the generation of biogas in order to increase the profits associated

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with sludge treatment.

Figure 3 shows that ACF of all alternatives assessed is negative i.e., none of the proposed are

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economically feasible, since total costs are higher than incomes. In this context, the cost benefit analysis allows identifying the best alternative which is the one whose ACF is closer to zero. It has been verified that, from an economic point of view, the best alternative is B, i.e., the thermophilic anaerobic digestion followed, after the subsequent and required dewatering, by the composting process. On the other hand, the worst alternative is the incineration since its ACF is 9.5 times lower than thermophilic AD + composting (alternative B). The present economic assessment illustrated the importance of taking into account not just total costs, but also the incomes associated to each alternative, in the decision making process since,

ACCEPTED MANUSCRIPT although gasification and SCWO were the cheapest technologies they are not the best alternatives if the ACF is used as decision criteria. The energy production in the anaerobic digestion is a key point for the economic feasibility of this process. Although under current conditions it is an economically unfeasible technology, if the 40% of the biogas were available

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for producing energy (thanks to a hypothetic technology upgrade), then thermophilic anaerobic digestion + composting (alternative B) would be a feasible technology from an economic point of view.

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3.2 Global warming potential (GWP) assessment

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***INSERT FIGURE 3***

Alternatives A and B produce energy so their indirect carbon dioxide emission per population equivalent are low, since the anaerobic digestion of sewage sludge produces biogas that enables the process to be partially self-sufficient concerning its electricity demand (Daelman et al., 2013). In this case it is assumed that the biogas is fed directly into a gas-fired combustion

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turbine converting the CH4 into CO2 and generating electricity and heat (used to heat the anaerobic digester). A value of 0.94 kg CO2 per kWh is assumed for any external energy production (based on the efficiency of a coal-burning power plant (Bridle et al., 2008). The CO2

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generated during anaerobic digestion and the CO2 produced in the combustion process are assumed to be released to the atmosphere. Figure 4 shows that both alternatives (A & B) have

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similar indirect CO2 emissions (close to 50 Kg CO2 eq./Ton DM), corresponding the slight differences between them to the higher energy consumption of the thermophilic digestion and to the fact that the biogas increase do not compensate the internal energy demands. Nevertheless, it should be noted that for both alternatives the direct emissions (close to 70-75%) are produced by the composting process. ***INSERT FIGURE 4*** It can be stated that thermal technologies such as incineration (C) and gasification (D) have the higher CO2 emissions in comparison to the rest of alternatives. Both are considered energy

ACCEPTED MANUSCRIPT recovery strategies. However, the higher energy consumption to reach the high temperatures required to destroy the organic matter cannot be fully compensated or recovered, making both alternatives not self-sufficient. These results are according to Rand, et al., 2000 and Sevilla et al., 2005. It can be noted that for the alternative C (incineration) the energy used from the grid

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is relatively higher than D (gasification), as can be checked looking at the lower indirect emissions in the gasification-related option. However, concerning the direct emissions to the environment from both alternativesit can be seen that the emission values are closer in

impact parameter for comparison purposes.

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magnitude stating that, in this case, the direct emission could not be used as an environmental

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SCWO (alternative E) showed the lowest GWP among the evaluated alternatives. As stated, among others, by Gidner et. al., (2001), Patterson et al. (2001), Cocero et al. (2002), Svanström et al. (2004) and Abelleira et al. (2013) the energy recovery from this innovative technology is expected to satisfy all its operating energy requirements, making this option self-sufficient working off the grid, which implies no indirect emissions. Another characteristic advantage of

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SCWO is the low emission of GHG (Griffith and Raymond, 2002; Svanström et al., 2005; Sevilla et al., 2005). The combination of both factors points out the most promising option regarding the GWP parameter. It is to be highlighted that, specifically, SCWO is widely

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proposed as an environmentally friendly alternative to incineration (Veriansyah and Kim, 2007; Boachen et al., 2009; Marulanda et al., 2010), which is a fact that has been shown and strongly

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confirmed in this work through the NovEDAR_EDSS software. With a sludge residence time of a few seconds, SCWO is able to remove more than 99% of the organic matter releasing the following major products: water, CO2 and inorganic solids [which could also be further exploited to recover phosphorous (Stark et al., 2006) and even precious metals (Matthey, 2003); however, although this valuable production has not been taken into account in this work, since it is novel and far from being implemented as a common procedure related to SCWO]. 3.3 Integrated results of global warming potential and annual cost

ACCEPTED MANUSCRIPT As mentioned, the CI developed for decision-making integrates economic and GWP assessments. Summarized in Table 5, five scenarios were defined to prioritize one of the both criteria. Scenario 1 and 2 consider only one of the two criteria since a weight of one was assigned to the economic or GWP assessment, respectively. Scenarios 3, 4 and 5 integrate both

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criteria while considering different weights. In particular, scenario 3 reflects that economic and GWP assessments have the same importance because the weight of both criteria is the same, i.e, 0.5. Scenario 4 considers that economics is more important than GWP (a weight of 0.75 for economic versus a weight of 0.25 for GWP). The scenario 5 is the opposite of the scenario 4,

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i.e., a weight of 0.75 for GWP criteria versus a weight of 0.25 for economic assessment.

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***INSERT TABLE 5***

Figure 5 shows the composite indicator for the five scenarios and for the five sludge treatment alternatives analyzed. Because scenario 1 and 2 are focused just on economic and GWP assessment respectively, the ranking of alternatives for these scenarios based on the CI,

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obviously is the same than those shown in Figure 3 and 4, respectively. Under scenario 3, where economic and GWP criteria have the same importance, the best alternative for treating the sludge, according to the CI value, is SCWO followed by the anaerobic digestion processes + composting, specifically thermophilic AD. The same ranking is obtained under scenario 5.

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However, if the weight of the economic criterion increases to 0.75 (scenario 4), then

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thermophilic AD + composting (alternative B) becomes favored over SCWO. It is pretty clear that depending of the priorities of stakeholders and policy makers different alternatives are identified as the most suitable. ***INSERT FIGURE 5***

It should be noted that because incineration is the worst option from both an economic and GHG emissions point of view, the value of the CI of this alternative is zero for all scenarios evaluated. In other words, based on the economic and GWP criteria, incineration never should

ACCEPTED MANUSCRIPT be implemented for treating sewage sludge. A similar discussion can be applied to gasification, since this alternative has lower CI values than the other three alternatives in all scenarios. In brief, the integration of the economic and GWP criteria into a single indicator illustrates that SCWO is the most adequate option for treating the sludge if both criteria are considered equally

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important. Nevertheless, if economics are prioritized (0.75 versus 0.25); then thermophilic anaerobic digestion + composting turn out to be the most appropriate option.

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4. CONCLUSIONS

The recommendations stated in this study are based on the results of the NovEDAR_EDSS

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software for different scenarios in sewage sludge treatment and management. The results can be used by policy makers and engineers as they embrace a variety of different criteria, offering several technological alternatives adapted for each specific situation.

From the economic point of view, the integration of investment and operational costs is decisive during the decision making process to select the best alternative, which is the one with the

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lowest total annual equivalent costs (TAEC). Its value together with the expected incomes of each alternative is the basis for the annual cash flow (ACF) calculation. Although none of the five alternatives are economically feasible, the economic assessment allows identification of the

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best alternative. In this work, thermophilic anaerobic digestion followed by a composting process proved to be the best of the five alternatives since its ACF value is the closest to zero

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(i.e. least negative). The worst alternative is incineration, whose ACF is 9.5 times lower than the ACF value of thermophilic anaerobic digestion. The Global Warming Potential (GWP) showed that thermal technologies as incineration and gasification presented the higher CO2 emissions, since they are not self-sufficient alternatives from the energetic point of view. Supercritical water oxidation showed the lowest GWP. It is expected that the energy recovery from this innovative technology may satisfy all its operating energy requirements making this option self-sufficient and, therefore, indirect emissions free. While analyzing mesophilic and thermophilic anaerobic digestion, it was assumed that the

ACCEPTED MANUSCRIPT biogas generated in these processes is fed directly into a gas-fired combustion turbine to convert the CH4 into CO2 while generating electricity and heat, so that these alternatives may have GWPs halfway between thermal and SCWO technologies. The integration of economic (ACF) and environmental (GWP) criteria into a single composite

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indicator (CI) allows determination of the best alternative depending on the weight that it is assigned to each criterion. Thus, if both criteria are considered equally important, SCWO is the most suitable option followed by the anaerobic digestion processes (especially the thermophilic

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one). The same ranking is obtained if the weight of the GWP criteria is higher than the economic criteria. However, if economic assessments are prioritized over environmental,

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thermophilic anaerobic digestion followed by a composting process appears to be the most appropriate option. About thermal technologies [incineration and gasification (alternatives C and D)], they are the worst options since the value of the composite indicator is the lowest (zero in the case of incineration) for all scenarios evaluated.

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The proposed approach efficiently explores different possibilities providing useful information for supporting sewage sludge treatment decisions. Its results contribute to the development of more feasible and environmentally friendly sewage sludge treatment lines. Since stakeholders’ preferences are integrated into the assessment, its results are very useful not only for water

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authorities but also from a policy perspective.

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ACKNOWLEDGMENTS

The authors would like to thank to all NOVEDAR project members. We also want to thank Doug Hendry at Duke University for proofreading this paper. This study has been financed by the Spanish Ministry of Education and Science (Consolider Project-NovEDAR) (CSD200700055). María Molinos also would thank to Generalitat Valenciana Generalitat Valenciana (APOSTD/2013/110). REFERENCES

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Table 2. Process estimations according to the case study

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Table 1. Case study parameters required for the sludge treatment selection Case study Input Data 1,000,000 Peq Scenario characteristics Flow rate (m3/d) 200,000 450 Total SS Influent characteristics BOD 400 (mg/L) 800 COD 10 Biosolids SRT (days) (Peq: person equivalent; COD: Chemical Oxygen Demand; BOD: Biological Oxygen Demand; SRT: Solid Retention Time)

Process Data

Case study

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Technology composition Mesophilic anaerobic digestion + Windrow Composting Thermophilic anaerobic digestion + Windrow Composting Incineration Gasification Supercritical Water Oxidation (SCWO)

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Alternative A B C D E

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Table 3. Assessed alternatives

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SRT (days) 10 Sludge production (Kg/day)* 128,800 Sludge production (Dry Matter Tone/ day)** 12 Volatile Solids removal (Kg/day)* 47,200 Biogas production Biogas production (m3/day)* 37,500 characteristics Energy production (KWh/day) 188,500 *Estimations calculated according to Metcalf & Eddy (1991 and 2004) and Hamoda (1988). Biosolid line characteristics

ACCEPTED MANUSCRIPT Table 4. Summary of the technologies embracing alternatives evaluated regarding energy demands, air emissions, associated costs [investment costs (IC) and operation and maintenance costs (OMC)] and expected lifetime.

Windrow Composting

Incineration

Gasification

30

CO2: 150 N2O: 0.2 CH4: 2.9

410

CO2: 450 CO: 1.5·10-4 NOx: 1·10-3 N2O: 0.64

y = 494x-0.20 3

y ($/m /day) x (m3/day)

220-500 €/T DM

1500 €/T DM/year

CO2: 580 CO: 0.48 NOx: 0.21 N2O: 3·10-3

244

650 €/T DM

Source

Hospido et al. (2005); Pradel et al. (2012); Haandel and Van der Lubbe (2007); Sato et al. (2007); Portela (2010)

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y ($/m3/day) x (m3/day)

95 – 105 (€/T DM)

20

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93.7

CO2: 3.87 CO: 2.52 NOx: 2.5 N2O: 0.06 CH4: 0.18

y = 494x-0.20

Lifetime (year)

OMC

100-115 (€/T DM)

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Thermophilic Anaerobic Digestion

88.6

IC

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Mesophilic Anaerobic Digestion

CO2: 1.29 CO: 0.84 NOx: 0.85 N2O: 0.02 CH4: 0.18

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Process Description

Direct emissions of Energy GHG consumption (kWh/t DM) (Kg CO2 eq./t DM)

20

60-140 (€/T DM)

20

250-300 (€/T DM)

20

30-110 (€/T DM)

10

Sato et al. (2007); Pradel et al. (2012); Portela (2010) U.S. EPA (2010); Houillon et al. (2006); Suh et al. (2001); Escaler et al. (2011); Sevilla et al. (2005); Lenart (2010) Suh et al., (2010); Houillon et al. (2006); Hospido et al. (2005); IPCC (2006); Rand et al. (2000); Sogama and Murakami (2009) Judex et al. (2012); Hospido et al. (2005); Sevilla et al. (2005)

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Stasta et al. (2006); Svanstrom et al. (2004, CO2* 60-140 410 2005); 6.5 CO: 0.1 10 €/T DM Griffith and Raymond ($/T DM) N2O: 0.17 (2002); Patterson et al. (2001) *Svanstrom et al. 2004: All gases are considered to be released into the environment but are assumed to have no adverse impacts.

Supercritical Water Oxidation (SCWO)

ACCEPTED MANUSCRIPT Table 5. Weights assigned to economic and GWP assessment to define scenarios of prioritization

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1 2 3 4 5

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Scenario

Weight Weight Economic GWP assessment assessment 1.00 0.00 0.00 1.00 0.50 0.50 0.75 0.25 0.25 0.75

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Figure 1. NovEDAR_EDSS snapshot.

Figure 2. Investment costs (IC), operational and maintenance costs (OMC) and total annual

TE D

equivalent cost (TAEC) of the alternatives studied expressed in thousands of euro per year. 1,800 1,600

1,000

EP

1,200

AC C

Cost (103 €/year)

1,400

800 600 400 200 0

A

B IC

C OMC

D TAEC

E

Sludge treatment alternatives

ACCEPTED MANUSCRIPT Figure 3. Total annual equivalent costs (TAEC), incomes (INC) and annual cash flow (ACF) of the alternatives studied expressed in thousands of euro per year. 2,000 1,500

B A

500 D

-500 -1,000 -1,500 -2,000 INC

ACF

Sludge treatment alternatives

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TAEC

E

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C

0

SC

103 €/year

1,000

Figure 4. Indirect, direct and total emissions of the alternatives studied expressed in Kg CO2 eq./Ton DM 900 800

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600 500 400 300 200 100 0

EP

Kg CO2 eq./t DM

700

AC C

A

B

Indirect emissions

C

Direct emissions

D

Total emissions

E

ACCEPTED MANUSCRIPT Figure 5. Composite indicator embracing economic and GWP assessment for 5 scenarios (1-5) and for 5 sludge treatment alternatives (A-E). 1.0 0.8

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CI

0.6 0.4 0.2 0.0 2 B

C

4

D

AC C

EP

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A

3

5

SC

1

E

Scenarios

ACCEPTED MANUSCRIPT Selecting sewage sludge treatment alternatives in modern wastewater treatment plants using an environmental decision support system

Highlights

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NovEDAR_EDSS software was applied to explore sewage sludge treatment alternatives in different scenarios. The most suitable and common alternatives in sewage sludge treatment were identified and evaluated using economic and environmental criteria. Integration of economic and environmental criteria into a single composite indicator enhanced the obtained results. This paper provides useful information for supporting sewage sludge treatment decisions by engineers and stakeholders.

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