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Chemical exergy based evaluation of water quality
e c o l o g i c a l m o d e l l i n g 2 0 0 ( 2 0 0 7 ) 259–268
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Short communication
Chemical exergy based evaluation of water quality G.Q. Chen a,b,∗ , Xi Ji a a
National Laboratory for Complex Systems and Turbulence, Department of Mechanics and Engineering Science, Peking University, Beijing 100871, China b Institute for Water Resources, School of Environment, Beijing Normal University, Beijing 100875, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
The thermodynamic concept of chemical exergy is introduced for water quality evaluation,
Received 22 September 2005
to develop unified objective indicators in contrast to conventional indicators characteristic of
Accepted 20 June 2006
subjectivity. While a quantity termed specific standard chemical exergy based on the global
Published on line 7 August 2006
reference substances is used to evaluate the standard water quality, an indicator as specific relative chemical exergy with reference to a spectrum of substances associated with some
Keywords:
specified water quality standard is developed for practical water quality evaluation, with
Chemical exergy
related concepts of carrying deficit and carrying capacity well embodied in exergy terms.
Water quality indicator
Based on the data collected in the GEMS/WATER project, water qualities of 72 rivers and 24
Water resources
lakes over the world are evaluated, as a detailed case study to illustrate the adaptability of the chemical exergy based indicators for water quality evaluation. © 2006 Elsevier B.V. All rights reserved.
1.
Introduction
The thermodynamic concept of exergy as a unified measure of the deviation of a system from its environment has gained wide acceptance in environmental and ecological fields (Jørgensen, 2001; Szargut, 2001; Chen, 2005, 2006). With a sound scientific foundation in physics, exergy has been adopted as an ecological indicator and goal function (Jørgensen, 1988, 1992a,b,c, 1994, 1995, 2000, 2001, p153; Jørgensen et al., 1995) for environmental evaluation and ecological modeling. In addition to the conventional application of evaluating the efficiency or efficacy of energy-utilization systems and detecting quantitatively the causes of the thermodynamic imperfection of thermal or chemical processes, exergy attracts escalating interests in environmental resource accounting, environmental impact assessment, ecological cost evaluation, and ecological modeling in recent years (e.g., Jørgensen et al.,
∗
Corresponding author. Tel.: +86 10 62767167; fax: +86 10 62750416. E-mail address: [email protected] (G.Q. Chen). 0304-3800/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ecolmodel.2006.06.015
1995; Rosen and Dincer, 1997b, 1999, 2001, 2003; Dincer, 2000, 2002; Gong and Wall, 2001; Wall, 2002; Szargut, 2003, 2004; Dincer and Rosen, 1998, 2005; Chen, 2005, 2006). The resource accounting in terms of exergy has been carried out based on nation or industrial sector scales (e.g., Reistad, 1975; Wall, 1987, 1990; Rosen, 1992; Wall et al., ˚ and Mielnik, 2000; 1994; Rosen and Dincer, 1997a; Ertesvag Hammond and Stapleton, 2001; Ayres et al., 2003; Dincer et al., 2003, 2004a,b,c,d,e; Ji and Chen, 2006; Chen and Chen, 2006, in press-a,b,c,d,e). Rosen and Dincer (Rosen and Dincer, 1997a,b, 1999, 2001, 2003; Dincer, 2000, 2002; Dincer and Rosen, 1998, 2005) paid much attention to the relationship between energy utilization and the environmental impacts, and highlighted the implication of the exergy analysis to the sustainable development. A number of researchers (e.g., Ulgiati et al., 1995; Creyts and Carey, 1997) have tried to devise unified objective measures for environmental impact assessment based upon exergy, via either estimating the chemical
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exergy associated with a waste stream or the total exergy consumption associated with corresponding human helped ¨ treatment courses of the waste stream. Hellstorm (1997, 2003) estimated and compared the exergy consumption of physical resources in some wastewater treatment plants and sew¨ erage systems. Finnveden and Ostlund (1997), Ayres et al. (1998), and Cornelissen et al. (1999) have developed an Exergy Based Life Cycle Analysis, introducing the concept of exergy into the methodology of environmental life cycle assessment and using it as a uniform indicator of total environmental impact. Sciubba and co-workers (Sciubba, 1999, 2001a,b, 2003a,b; Sciubba and Ulgiati, 2005) discussed a new paradigm for the calculation of the real environmental cost by performing the Extend Exergy Accounting, and provided a novel framework for integrated evaluation of concerned factors of capital, labor and environmental impact, etc. The concept of ecological cost has been defined as the cumulative consumption of non-renewable natural resources measured by exergy in the fabrication of particular products (Szargut et al., 2002). In a framework for systems evaluation based on exergy circuit language, ecological value for a waste stream is defined negative and equal in magnitude to corresponding embodied exergy in terms of the total exergy consumed in human helped treatment or natural degradation of the waste stream (Chen, 2006). Jørgensen and co-workers (Jørgensen et al., 1995; Bendoricchio and Jørgensen, 1997; Martins et al., 1997; Xu, 1997; Xu et al., 1999, 2001, 2002, 2004; Jørgensen, 2001; Jørgensen et al., 2002a,b,c; Marques et al., 2003; Zhang et al., 2003, 2004) have made much efforts in exergetic modeling for aquatic systems such as lakes and coastal areas, by demonstrating and illustrating the relationships between exergy and biomass, biodiversity, species composition, and other properties of ecosystems. A preliminary resource accounting for river water has been carried out by Zaleta-Aguilar et al. (1998) in terms of the river water availability quantified as the mechanical, thermal and chemical exergy flux with the river flow. A more general approach involving biological and sedimental exergy has been proposed for a unitary objective accounting of water resources, with application to the Yellow River basin (Chen, 2004). An exergy based evaluation of water quality remains to be initiated. The monitoring of water quality of rivers dates back to around 1890 when some European rivers, such as the Thames and the Seine, highly contaminated due to domestic sewage, were monitored in terms of a few simple parameters of dissolved oxygen, pH, etc. With the rapid industrialization and development of the energy sectors and high-input agriculture, there has been an exponential rise in the number of water quality indicators, corresponding to the increasing diversity of pollutants (Meybeck and Helmer, 1989). These indicators, including the earliest monitored simple indicators, major irons, organic, and inorganic matters, and toxic pollutants, etc., cover a broad range of water quality. In order to comprehensively evaluate the water quality, a variety of evaluation methods such as the single index, fuzzy mathematics, principal factor analysis, specialist evaluation, gray correlation, radial basis function, artificial neural network, and comprehensive index evaluation, etc. have been established (Cater, 1996, pp. 56–99, 122–143). Common to all those mod-
els is the unavoidable subjectivity, of the weighting factor for each involved indicator, mathematical models or corresponding parameters. Due to the subjective weighted factors out of so-called specialist inquiry, contradictory evaluations may be resulted for the same water quality data with different specialist groups. For reasonable and consistent water resource exploitation and management, it is essential to pursue a unified objective assessment of water quality. In the present paper, the chemical exergy based evaluation method is developed for a unified objective assessment of water quality. While a quantity termed specific standard chemical exergy based on the global reference substances might be adopted, an indicator as specific relative chemical exergy with reference to a spectrum of substances associated with the specified water quality standard is proposed for water quality evaluation with more practical implications, resulting in unified objective quantifiers for the carrying capacity and carrying deficit of water resources. With the data collected in the GEMS/WATER project, water qualities of 72 rivers and 24 lakes over the world are evaluated as a detailed case study to illustrate the adaptability of the chemical exergy based indicators for water quality evaluation.
2.
Chemical exergy and water quality
Standard chemical exergy of a substance is defined as the minimum amount of available energy or work necessary to produce the substance under consideration from environmental substances in the sense of Szargut’s global averaged model and associated with standard environment with temperature T0 of 298.15 K and pressure P0 of 1 atm, i.e., 101.325 kPa (Riekert, 1974; Morris and Szargut, 1986). To assess the chemical exergy of a substance, the properties of the chemical elements comprising the substance must be referred to the properties of some corresponding reasonably selected substances in the environment (Kotas, 1985, p. 44). The reference substances selected are the most abundant substances in the natural environment, and in equilibrium with the rest of the environment. Obviously, the most suitable reference substance for the chemical element in question is the one not only containing the chemical element but also with the lowest chemical potential among all the corresponding environmental substances. For instance, what qualifies to be a suitable reference substance in the global environment for the solid S is SO4 2+ in the hydrosphere rather than SO3 2+ , and what for C is CO2 in the atmosphere rather than CO rarely found in the environment as a whole. Na+ , K+ , Cl− , Ca+ , etc. in the hydrosphere, N2 , O2 , CO2 , etc. in the atmosphere, and Si O2 , Fe2 O3 , etc. in the external layer of the earth crust have been adopted as the reference substances (Morris and Szargut, 1986). For a general case the standard chemical exergy (SCE) comprises two parts as the standard standard mixing exergy (SME) and reaction exergy (SRE). The standard mixing exergy for the concentration effect of the reference substances is calculated as SMEi = RT0 ln
C0 , Ci00
(1)
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where SMEi with the unit J mol−1 , denotes the standard mixing exergy of the ith reference species with the conventional average molar concentration Ci00 , R the gas constant (8.314 J mol−1 K−1 ), T0 the standard absolute temperature of the environment in Kelvin (298.15 K) and C0 is the molar concentration of substances in the environment. Obviously, a reference substance of lower concentration in its dead state corresponds to a larger standard chemical exergy, and conversely, the more abundant the substance found in the environment, the less the chemical exergy it carries. When the substance under consideration is not a conventional reference substance, the standard chemical exergy of the substance can be obtained by the analysis of related chemical reaction between the substance and its reference substances, should the molar Gibbs function of formation and the activities of the substances involved be available. For simplify, consider the chemical combination of two mono-element substances. A general reversible reaction can be expressed as aAi + bBj ↔ cAm Bn ,
(2)
where Ai and Bj are the reactants, Am Bn the product, and a, b, and c are the stoichiometric coefficients. If the standard reaction exergy of Am Bn is referred to the substances Ai and Bj , with known standard mixing exergy values, we have
G0r =
[Ai ]a [Bj ]b
G0f −
,
(4)
G0f = c × G0fAm Bn − a × G0fA
i
reactants
products
− b × G0fB , j
SCEi × Ci ,
(6)
(3)
and [Am Bn ]c
SSCE =
i
c × SREAm Bn = a × SMEAi + b × SMEBj + Gr ,
Gr = G0r + RT ln
because of not only the diversity of such pollutants but also the technical difficulties associated with and intolerable expense in monitoring them. There have been two kind of monitoring, namely, basic monitoring covering electrical conductivity, pH, temperature, total suspended solids (TSS), and major ions and nutrients (e.g., Na+ , K+ , PO4 3− , NO3 − , NH4+ ), etc. and advanced monitoring covering organic and inorganic micropollutants. The costs of advanced monitoring per sample may range be two orders of magnitude higher than those of basic monitoring. Zaleta-Aguilar et al. (1998) supposed to use a typical organic molecule to represent the ‘mean organic substance’ to facilitate the estimation of the order of magnitude of exergy value for the organic components. Actually, oxygen equivalent indicators of BOD, COD, etc. have been adopted to indirectly reflect ¨ the concentration of organic matters in water. Hellstorm (1997, 2003) suggested alternatively the chemical exergy of organic contaminants in water can be calculated by using the data of COD with the converted coefficient of 13.6 kJ g−1 of COD. This provides a first approximation for practical estimate. The specific standard chemical exergy (SSCE, J l−1 ) of the water is the sum of the standard chemical exergy of the components under consideration in unit volume of water, namely,
(5)
where Gr is the change in Gibbs free energy (J mol−1 ) of the reaction, [Am Bn ], [Ai ], [Bj ] are the activities of Am Bn , Ai , Bj , respectively, G0r the change in the standard Gibbs free energy of the reaction, is equal to Gr when each product or reactant is present at the standard state with unit activity, and G0f is the free energy of formation, i.e., energy (exergy) needed to produce 1 mole of a substance from pure elements in the most stable form, and the values of G0f are available in Cox et al. (1989). Water contamination by organic pollutants has emerged as a ubiquitous and serious problem. The breakdown of the pollutants depletes the oxygen content of water coming either from the photosynthesis of the plants or from the atmosphere, thereby adversely affects the oxygen-dependent flora and fauna. To facilitate the calculations involved in the exergy methods, Morris and Szargut (1986) tabularized the values of the standard chemical exergy of various elements and some inorganic and organic compounds of these elements on the planet earth. Even standard chemical exergy values for some organic compounds are available, the calculation of the chemical exergy of organic substances remains a tough work,
where SCEi is the standard chemical exergy of the ith component (J mol−1 ), and Ci is the concentration of the ith component in water with the unit of mol l−1 . SSCE provides for water quality evaluation a unified normal indicator with a scale of global reference. In practical assessment of water quality with respect to unfriendly chemical spectrum, maximum concentration levels are usually specified for components in water, and the lower concentrations are preferred. It is then more practical to define a ‘specific relative chemical exergy (SRCE)’ referring to the specified water quality standard as SRCE =
i
SRCEi =
(SSCEi − SSCEi ref ),
(7)
i
where SRCEi is the difference between SSCEi and SSCEi ref , which are the specific standard chemical exergy of the ith component involved in the water body and the reference water quality standard, respectively. Obviously, the SRCE is a unified measure of the ‘chemical distance’ the concerned water is away from and the specified standard, in contrast to SSCE from the global reference. Evidently, SRCE is not positive definite, depending on the status of the assessed water and the chosen water quality standard. A positive SRCE value means that the amount of the pollutants in exergy in the water exceeds the amount the water body could carry as a limit specified by the standard: this represents a ‘carrying deficit’. On the contrary, a negative SRCE value means that the amount of the pollutants in exergy in the water is below the amount the water body could carry as a limit specified by the standard: there is a remaining ‘carrying capacity’ for the water. It is preferable to have greater carrying capacity or smaller carrying deficit in exergy unit.
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Table 1 – The conventional average molarities of the ions in the sea watera (the salinity of the sea water is 0.035) Ions
Na+
K+
Ca2+
Mg2+
Cl−
SO4 2−
PO4 3−b
NO3 −b
Molarities (mol kg−1 H2 O)
0.474
1.04E−2
9.6E−3
4.96E−2
0.5657
1.17E−2
–
–
SiO2 is one of the major irons considered, with its conventional standard mole fraction of 0.472 (Morris and Szargut, 1986) in the external layer of the earth crust. a b
3.
Refer to Dyrssen and Wedborg (1974). The molarities of PO4 3− and NO3 − are not offered in Dyrssen and Wedborg (1974), and their standard chemical exergy can be calculated based on the values of phosphates and nitrates offered by Morris and Szargut (1986) following Eqs. (2)–(5), respectively.
Case study based on GEMS/WATER data
3.1. The specific standard chemical exergy (SSCE) indicator Based on the data collected in the GEMS/WATER project (Fraser et al., 2002) through the voluntary efforts of the WHO member states, the specific standard chemical exergy (SSCE) analysis for 72 river and 24 lake waters over the world are performed. The water quality indicators adopted in the GEMS/WATER project cover a wide range and are classified into basic indicators (such as TSS, DO, pH, etc.), major ions (i.e., Si O2 , Ca2+ , Mg2+ , Na+ , K+ , Cl− , SO4 2− , etc.), nutrients and organic matters (i.e., PO4 3− , Ptot , NO3 − , NH4+ , Nk , TOC, BOD, COD, etc.), pollutants and miscellaneous elements (i.e., F, B, CN, Phenol, etc.), and highly toxic pollutants (i.e., As, Cd, Hg, Pb, DDT, PCB, etc.), though due to the high cost and difficulties associated with monitoring and collection, the data of the highly toxic chemicals have not been available. According to the results of the GEMS/WATER project, the BOD5 is with a low average value, denoting the trivial pollution due to organic pollutants (Fraser et al., 2002). The major ions, namely, Si O2 , Ca2+ , Mg2+ , Na+ , K+ , Cl− , SO4 2− , and phosphorus (PO4 3− ) and nitrogen (NO3 − ) are the major indicators monitored. The major ions in rivers and lakes are originated from both the natural courses, mainly including the action of the atmosphere, the erosion of weathered surface rocks and soils, and the degradation of terrestrial organic matters, and the anthropogenic activities, such as irrigation, mining, the domestic or industrial waste emission, etc. The components of phosphorus (PO4 3− ) and nitrogen (NO3 − ) are mainly due to the pesticide, fertilizer, dejecta and domestic wastewater. Most of those indicators are exactly corresponding to the reference substances in the environment, for which the standard chemical exergy can be calculated by Eq. (1),
with the mole fractions calculated based on the conventional molarities illuminated in Table 1. The calculated SSCE values for the river and lake waters are illustrated in Figs. 1–7. Most of the rivers in South America and Oceanic are shown with lower SSCE. Some river waters in North America, Asia and Europe are found with higher SSCE values. The Balsas River and Colorado River in North America, Yellow River, Arab River, Syr Darya River and Tapti River in Asia, Elbe River, Guadalquivir River, Rhine River, Seine River and Weser River in Europe are prominent in the figures, with much higher SSCE values, denoting that they are under relatively serious environmental impacts due to natural or anthropogenic actions. Compared with the river waters, the lake waters are estimated with much higher SSCE values as shown in Fig. 7. The Caspian Sea, Superior Lake, Kivu Lake, Albert Lake, Turkana Lake, Edward Lake, Dead Sea and Van Golu Lake have higher SSCE values. Due to its well-known high concentration of salts and other components, the Dead Sea is shown with the maximum SSCE value among all the lakes over the world, followed by the Van Golu Lake.
3.2. The specific relative chemical exergy (SRCE) indicator Now some sketchy information of the quality status of the river and lake waters has been presented with the SSCE indicator referring to the global reference, we resort to the specific standard chemical exergy (SRCE) indicator associated with some specified water quality standard for a more practical and particular evaluation. To facilitate evaluation and comparison, consider the standards for potable water. Among existing standards for potable water, the most widely applied or referred to over the world are those issued by the World Health Organization (WHO),
Fig. 1 – The specific standard chemical exergy (SSCE) of some rivers in North America.
e c o l o g i c a l m o d e l l i n g 2 0 0 ( 2 0 0 7 ) 259–268
Fig. 2 – The specific standard chemical exergy (SSCE) of some rivers in South America.
Fig. 3 – The specific standard chemical exergy (SSCE) of some rivers in Asia.
Fig. 4 – The specific standard chemical exergy (SSCE) of some rivers in Europe.
Fig. 5 – The specific standard chemical exergy (SSCE) of some rivers in Africa.
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Fig. 6 – The specific standard chemical exergy (SSCE) of some rivers in Oceania.
Fig. 7 – (a) The specific standard chemical exergy (SSCE) of some lakes in the world. (b) The specific standard chemical exergy (SSCE) of the Dead Sea and the Van Golu Lake.
Fig. 8 – The specific relative chemical exergy (SRCE) of some rivers in North America.
Fig. 9 – The specific relative chemical exergy (SRCE) of some rivers in South America.
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Table 2 – The specified concentrations of the components in potable water standards Na+
Reference substance Concentration (mg l−1 ) (GL)a Concentration (mg l−1 ) (MAC)d a b
c d e f
20 150
K+
Ca2+
Mg2+
Cl−
SO4 2−
HCO3 −
SiO2
PO4 2−
NO3 −
10 12
100 100e
30 50
25 250f
25 250
30b 30b
1 10
2.5c 2.5c
25 50
The Guide Line in EU (80/778/EC), with lower permissible concentration, namely, denoting higher quality level. The specified concentration of HCO3 − , representing the alkalinity (opposite with others, it is the minimum allowed concentration, hence the standard chemical exergy of HCO3 − should be taken into account separately). The specified concentration of PO4 2− in the water quality standard of potability prescribed by Vietnam. The Maximum Allowed Concentration in EU (80/778/EC), with higher permissible concentration, namely, lower quality level. The specified concentration of Ca2+ in the water quality standard of potability prescribed by France and China. The specified concentration of Cl− in the water quality standard of potability prescribed by Malaysia and Thailand.
Fig. 10 – The specific relative chemical exergy (SRCE) of some rivers in Asia.
Fig. 12 – The specific relative chemical exergy (SRCE) of some rivers in Africa. Fig. 11 – The specific relative chemical exergy (SRCE) of some rivers in Europe.
the European Union (EU) and the Environmental Protection Agency in US (US EPA). Many developed countries adopt more stringent criteria, however, some developing countries have reconciled themselves to less stringent criteria duo to various practical difficulties. As most of the indicators applied in the GEMS/WATER project are involved in the potable water quality standard prescribed by EU (80/778/EC), it is then reasonable to devise a reference standard as shown in Table 2 with EU (80/778/EC) standard as a basis and some minor ones unavailable in the EU (80/778/EC) standard adopted from some other state standards (WHO, 1986). Based on the reference environment according to the Guide Line (GL) with lower permissible concentrations specified and the maximum allowed concentration (MAC) with higher permissible concentrations specified, respectively, the SREC val-
ues of the river and lake waters are calculated and shown in Figs. 8–14. River waters in South America, Africa and Oceania are shown with good quality status: all of them have remaining carrying capacity relative to the MAC standard, and most
Fig. 13 – The specific relative chemical exergy (SRCE) of some rivers in Oceania.
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water quality against a global earth averaged background. As a practical indicator for anthropogenic water exploitation, the specific relative chemical exergy (SRCE) with reference to a specified water quality standard is devised, with related concepts of carrying deficit and carrying capacity well embodied in exergy terms. With the data collected in the GEMS/WATER project, water qualities of 72 rivers and 24 lakes over the world are evaluated, as a detailed case study to illustrate the adaptability of the chemical exergy based indicators for water quality evaluation.
Acknowledgement Project supported by the National Key Basic Research Program (Grant No. 2005CB724204).
references
Fig. 14 – (a) The specific relative chemical exergy (SRCE) of some lakes in the world. (b) The specific relative chemical exergy (SRCE) of the Dead Sea and the Van Golu Lake.
of them (except for the rivers Burdekin and Murray in Oceania) also meet the stringent GL standard. In contrast, many river waters in North America, Asia and Europe and most lake waters over the world are shown associated with carrying deficit, especially when GL is applied. Those with carrying deficit with respect to GL are the Balsas River, Colorado River, Mississippi River and Nelson River in North America, Cauvery River, Yangtse River, Yellow River, Kistna River, Narmada River, Arab River, Syr Darya River and Tapti River in Asia, Danube River, Ebro River, Elbe River, Garonne River, Guadalquivir River, Loire River, Po River, Rhine River, Rhne River, Seine River, Taugs River, Volga River and Weser River in Europe, Burdekin River and Murray River in Oceania and the lakes of Caspian Sea, Superior, Huron, Victoria, Titicaca, Kivu, Albert, Turkana, Edward, the Dead Sea and the Van Golu. But with less stringent MAC standard, much fewer river and lake waters are shown under carrying deficit.
4.
Conclusions
The thermodynamic concept of chemical exergy, defined as the minimum work in need to bring the concerned system into chemical equilibrium with its environment and as an information measure of the chemical deviation of a concerned system from its environment, is introduced for water quality evaluation to develop unified objective indicators in contrast to conventional indicators characteristic of subjectivity. The concept of specific standard chemical exergy (SSCE) based on the global reference environment as modeled by Szargut is devised and elaborated as a normal indicator for
Ayres, R.U., Ayres, L.W., Martinas, K., 1998. Exergy, waste accounting and life-cycle analysis. Energy 23 (5), 353–363. Ayres, R.U., Ayres, L.W., Warr, B., 2003. Exergy, power and work in the US economy, 1900–1998. Energy 28, 219–273. Bendoricchio, G., Jørgensen, S.E., 1997. Exergy as goal function of ecosystems dynamic. Ecol. Model. 102, 5–15. Cater, L.W., 1996. Environmental Impact Assessment [M], second ed. McGraw-Hill Inc., Singapore, pp. 56–99, 122–143. Chen, B., 2004. A theory of ecological exergy and its application to the ecological modeling of the Yellow River basin. Ph.D Dissertation. School of Environment, Peking University. Chen, G.Q., 2005. Exergy consumption of the earth. Ecol. Model. 184, 363–380. Chen, G.Q., 2006. Scarcity of exergy and ecological evaluation based on embodied exergy. Communications in nonlinear science and numerical simulation 11, 541–552. Chen, G.Q., Chen, B., Resource analysis of the Chinese society 1980–2002 based on exergy-part 1: fossil fuels and energy minerals. Energy Policy, in press-a. Chen, G.Q., Chen, B., Resource analysis of the Chinese society 1980–2002 based on exergy-part 2: renewable energy sources and forest. Energy Policy, in press-b. Chen, B., Chen, G.Q., Resource analysis of the Chinese society 1980–2002 based on exergy-part 3: agricultural products. Energy Policy, in press-c. Chen, B., Chen, G.Q., Resource analysis of the Chinese society 1980–2002 based on exergy-part 4: fishery and rangeland. Energy Policy, in press-d. Chen, B., Chen, G.Q., Resource analysis of the Chinese society 1980–2002 based on exergy-part 5: resource structure and intensity. Energy Policy, in press-e. Chen, B., Chen, G.Q., 2006. Exergy analysis for resource conversion of the Chinese society 1993 under the material product system. Energy 31, 1115–1150. Cox, J.D., Wagman, D.D., Medvedev, V.A., 1989. CODATA Key Values for Thermodynamics, vol. 248. IAHS Publ, pp. 379–385. Cornelissen, R.L., Marquart, E.N., Hirs, G.G., 1999. The value of exergetic life cycle assessment besides LCA. In: Proceedings of ECOS’99, Tokyo. Creyts, J.C., Carey, V.P., 1997. Use of Exergy Analysis as a Tool for Assessment of the Environmental Impact of Industrial Processes. AES-37, ASME. Dincer, I., Rosen, M.A., 1998. A worldwide perspective on energy, environment and sustainable development. Int. J. Energy Res. 22, 1305–1321.
e c o l o g i c a l m o d e l l i n g 2 0 0 ( 2 0 0 7 ) 259–268
Dincer, I., 2000. Thermodynamics, exergy and environmental impact. Energy Sources 22, 723–732. Dincer, I., 2002. Technical, environmental and exergetic aspects of hydrogen energy systems. Int. J. Hydrogen Energy 27, 265–285. Dincer, I., Hussain, M.M., Al-Zaharnah, I., 2003. Energy and exergy use in the industrial sector of Saudi Arabia. Proceedings of the I MECH E Part A. J. Power Energy 217 (12), 481–492. Dincer, I., Hussain, M.M., Al-Zaharnah, I., 2004a. Energy and exergy utilization in transportation sector of Saudi Arabia. Appl. Thermal Eng. 24, 525–538. Dincer, I., Hussain, M.M., Al-Zaharnah, I., 2004b. Energy and exergy use in public and private sector of Saudi Arabia. Energy Policy 32, 1615–1624. Dincer, I., Hussain, M.M., Al-Zaharnah, I., 2004c. Energy and exergy use in the utility sector of Saudi Arabia. Desalination 169, 245–255. Dincer, I., Hussain, M.M., Al-Zaharnah, I., 2004d. Energy and exergy use in residential sector of Saudi Arabia. Energy Sources 26, 1239–1252. Dincer, I., Hussain, M.M., Al-Zaharnah, I., 2004e. Analysis of sectoral energy and exergy use of Saudi Arabia. Int. J. Energy Res. 28, 205–243. Dincer, I., Rosen, M.A., 2005. Thermodynamic aspects of renewables and sustainable development. Renewable Sustainable Energy Rev. 9, 169–189. Dyrssen, D., Wedborg, M., 1974. The Sea (5) (Goldberg, E.D., ed.), Wiley Interscience, N.Y.; In: Morris, D.R., Szargut, J., 1986. Standard chemical exergy of some elements and compounds on the planet earth. Energy 11 (8), 733–755. ˚ I.S., Mielnik, M., 2000. Exergy analysis of the Norwegian Ertesvag, society. Energy 25, 957–973. ¨ Finnveden, G., Ostlund, P., 1997. Exergies of natural resources in life-cycle assessment and other applications. Energy 22 (9), 923–931. Fraser, A.S., Meybeck, M., Ongley, E.D., 2002. Water quality of the rivers over the world. In: Zheng, S.R. (Ed.), Comprehensively Evaluation of the Freshwater Resource over the World. Hubei Science and Technology Press, Wuhan. Gong, M., Wall, G., 2001. On exergy and sustainable development–Part 2: indicators and methods. Exergy—Int. J. 1 (4), 217–233. Hammond, G.P., Stapleton, A.J., 2001. Exergy analysis of the United Kingdom energy system. IMech J. Power Energy 215, 141–162. ¨ Hellstorm, D., 1997. An exergy analysis for a wastewater treatment plant—an estimation of the consumption of physical resources. Water Environ. Res. 69, 44–51. ¨ Hellstorm, D., 2003. Exergy analysis of nutrient recovery processes. Water Sci. Technol. 48 (1), 27–36. Ji, X., Chen, G.Q., 2006. Exergy analysis of energy utilization in the transportation sector in China. Energy Policy 34, 1709–1719. Jørgensen, S.E., 1988. Use of models as experimental tool to show that structural changes are accompanied by increased exergy. Ecol. Model. 41, 117–126. Jørgensen, S.E., 1992a. Intergration of Ecosystem Theories: A Pattern. Kluwer, Dordrecht. Jørgensen, S.E., 1992b. Parameters, ecological constrains and exergy. Ecol. Model. 62, 163–170. Jørgensen, S.E., 1992c. Development of models able to account for changes in species composition. Ecol. Model. 62, 195–208. Jørgensen, S.E., 1994. Review and comparison of goal function in system ecology. Vie Milieu 44 (1), 11–20. Jørgensen, S.E., 1995. Exergy and ecological buffer capacities as measures of ecosystem health. Ecosyst. Health 1 (3), 150–160. Jørgensen, S.E., Nielsen, S.N., Mejer, H., 1995. Energy, environ, exergy and ecological modeling. Ecol. Model. 77, 99–109. Jørgensen, S.E., 2000. Application of exergy and specific exergy as ecological indicators of coastal areas. Aquat. Ecosyst. Health Manag. 3, 419–430.
267
Jørgensen, S.E., 2001. The thermodynamic concept: exergy. In: Jørgensen, S.E. (Ed.), Thermodynamics and Ecological Modeling. CRC Press, Boca Raton, Florida, USA. Jørgensen, S.E., Marques, J., Nielsen, S.N., 2002a. Structural changes in an estuary, described by models and using exergy as orientor. Ecol. Model. 158, 140–233. Jørgensen, S.E., Verdonschot, P., Lek, S., 2002b. Explanation of the observed structure of functional feeding groups of aquatic macro-invertebrates by an ecological model and the maximum exergy principle. Ecol. Model. 158, 223–231. Jørgensen, S.E., Ray, S., Berec, L., Straskraba, M., 2002c. Improved calibration of a eutrophication model by use of the size variation due to succession. Ecol. Model. 153, 269–277. Kotas, T.J., 1985. The Exergy Method of Thermal Plant Analysis. Anchor Brendon Ltd., Essex, p. 44. Morris, D.R., Szargut, J., 1986. Standard chemical exergy of some elements and compounds on the planet earth. Energy 11 (8), 733–755. Meybeck, M., Helmer, R., 1989. The quality of rivers: from pristine stage to global pollution. Global Planet Change 1, 283–309. Martins, I., Marques, J.C., Jørgensen, S.E., Nielsen, S.N., 1997. Modeling the effects of green macroalgae blooms on the population dynamics of Cyathura carinata (Crustacea: Isopoda) in an eutrophied estuary. Ecol. Model. 102, 33–53. Marques, J.C., Nielsen, S.N., Pardal, M.A., Jørgensen, S.E., 2003. Impact of eutrophication and river management within a framework of ecosystem theories. Ecol. Model. 166, 147–168. Reistad, G.M., 1975. Available energy conversion and utilization in the United States. J. Eng. Power 97, 429–434. Riekert, L., 1974. Chem. Eng. Sci. 29, 1613. Rosen, M.A., 1992. Evaluation of energy utilization efficiency in Canada using energy and exergy analyses. Energy—Int. J. 17, 339–350. Rosen, M.A., Dincer, I., 1997a. Sectoral energy and exergy modeling of Turkey. ASME—J. Energy Resources Technol. 119 (3), 200–204. Rosen, M.A., Dincer, I., 1997b. On exergy and environmental impact. Int. J. Energy Res. 21, 643–654. Rosen, M.A., Dincer, I., 1999. Exergy analysis of waste emissions. Int. J. Energy Res. 23, 1153–1163. Rosen, M.A., Dincer, I., 2001. Exergy as the confluence of energy, environment and sustainable development. Exergy—Int. J. 1 (1), 3–13. Rosen, M.A., Dincer, I., 2003. Exergy-cost-energy-mass analysis of thermal systems and processes. Energy Conversion Manag. 44, 1633–1651. Sciubba, E., 1999. Exergy as a direct measure of environmental impact. ASME Adv. Energy Syst. Div. Publaes 39, 573–581. Sciubba, E., 2001a. Using exergy to evaluate environmental externalities. In: Invited Lecture, IV NTVA Seminar on Industrial Ecology, Trondheim. Sciubba, E., 2001b. Beyond thermoeconomics? The concept of extended exergy accounting and its application to the analysis and design of thermal systems. Exergy—Int. J. 1 (2), 68–84. Sciubba, E., 2003a. Cost analysis of energy conversion systems via a novel resource-based quantifier. Energy 28, 457–477. Sciubba, E., 2003b. Extended exergy accounting applied to energy recovery from waste: the concept of total recycling. Energy 28, 1315–1334. Sciubba, E., Ulgiati, S., 2005. Emergy and exergy analyses: complementary methods or irreducible ideological options? Energy 30, 1953–1988. Szargut, J., 2001. Sequence method of determination of partial exergy losses in thermal systems. Exergy 1, 85–90. Szargut, J., Andrzej, Z., Stanek, W., 2002. Depletion of the nonrenewable natural exergy resources as a measure of
268
e c o l o g i c a l m o d e l l i n g 2 0 0 ( 2 0 0 7 ) 259–268
the ecological cost. Energy Conversion Manag. 43, 1149–1163. Szargut, J.T., 2003. Anthropogenic and natural exergy losses. Energy 28, 1047–1054. Szargut, J.T., 2004. Optimization of the design parameters aiming at the minimization of the depletion of non-renewable resources. Energy 29, 2161–2169. Ulgiati, S., Brown, M.T., Bastianoni, N., Marchettini, N., 1995. Emergy based indices and ratios to evaluate sustainable use of resources. Ecol. Eng. 5, 23–28. Wall, G., 1987. Exergy conversion in the Swedish society. Resources Energy 9, 55–73. Wall, G., 1990. Exergy conversion in the Japanese society. Energy—Int. J. 15, 435–444. Wall, G., Sciubba, E., Naso, V., 1994. Exergy use in the Italian society. Energy—Int. J. 19, 1267–1274. Wall, G., 2002. Conditions and tools in the design of energy conversion and management systems of a sustainable society. Energy Conversion Manag. 43, 1235–1248. WHO, 1986. The Guide Line of the Potable Water Quality. People Hygienic Press, Beijing. Xu, F.L., 1997. Exergy and structural exergy as ecological indicators for the development state of the Lake Chaohu ecosystem. Ecol. Model. 99, 41–49.
Xu, F.L., Jørgensen, S.E., Tao, S., 1999. Ecological indicators for assessing freshwater ecosystem health. Ecol. Model. 116, 77–106. Xu, F.L., Tao, S., Dawson, R.W., Li, P.G., Cao, J., 2001. Lake ecosystem health assessment: indicators and methods. Water Res. 35, 3157–3167. Xu, F.L., Dawson, R.W., Tao, S., Li, B.G., Cao, J., 2002. System-level responses of lake ecosystems to chemical stresses using exergy and structural exergy as ecological indicators. Chemosphere 46, 173–185. Xu, F.L., Lam, K.C., Zhao, Z.Y., Zhan, W., Chen, Y.D., Tao, S., 2004. Marine coastal ecosystem health assessment: a case study of the Tolo Harbour, Hong Kong. Chin. Ecol. Model. 173, 355–370. Zaleta-Aguilar, A., Ranz, L., Valero, A., 1998. Towards a unified measure of renewable resources availability: the exergy method applied to the water of a river. Energy Conversion Manag. 39, 1911–1917. Zhang, J.J., Jørgensen, S.E., Tan, C.O., Beklioglu, M., 2003. A structurally dynamic modelling—Lake Mogan. Turkey as a case study. Ecol. Model. 158, 103–120. Zhang, J.J., Jørgensen, S.E., Mahler, H., 2004. Examination of structurally dynamic eutrophication model. Ecol. Model. 173, 313–333.
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Short communication
Chemical exergy based evaluation of water quality G.Q. Chen a,b,∗ , Xi Ji a a
National Laboratory for Complex Systems and Turbulence, Department of Mechanics and Engineering Science, Peking University, Beijing 100871, China b Institute for Water Resources, School of Environment, Beijing Normal University, Beijing 100875, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
The thermodynamic concept of chemical exergy is introduced for water quality evaluation,
Received 22 September 2005
to develop unified objective indicators in contrast to conventional indicators characteristic of
Accepted 20 June 2006
subjectivity. While a quantity termed specific standard chemical exergy based on the global
Published on line 7 August 2006
reference substances is used to evaluate the standard water quality, an indicator as specific relative chemical exergy with reference to a spectrum of substances associated with some
Keywords:
specified water quality standard is developed for practical water quality evaluation, with
Chemical exergy
related concepts of carrying deficit and carrying capacity well embodied in exergy terms.
Water quality indicator
Based on the data collected in the GEMS/WATER project, water qualities of 72 rivers and 24
Water resources
lakes over the world are evaluated, as a detailed case study to illustrate the adaptability of the chemical exergy based indicators for water quality evaluation. © 2006 Elsevier B.V. All rights reserved.
1.
Introduction
The thermodynamic concept of exergy as a unified measure of the deviation of a system from its environment has gained wide acceptance in environmental and ecological fields (Jørgensen, 2001; Szargut, 2001; Chen, 2005, 2006). With a sound scientific foundation in physics, exergy has been adopted as an ecological indicator and goal function (Jørgensen, 1988, 1992a,b,c, 1994, 1995, 2000, 2001, p153; Jørgensen et al., 1995) for environmental evaluation and ecological modeling. In addition to the conventional application of evaluating the efficiency or efficacy of energy-utilization systems and detecting quantitatively the causes of the thermodynamic imperfection of thermal or chemical processes, exergy attracts escalating interests in environmental resource accounting, environmental impact assessment, ecological cost evaluation, and ecological modeling in recent years (e.g., Jørgensen et al.,
∗
Corresponding author. Tel.: +86 10 62767167; fax: +86 10 62750416. E-mail address: [email protected] (G.Q. Chen). 0304-3800/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ecolmodel.2006.06.015
1995; Rosen and Dincer, 1997b, 1999, 2001, 2003; Dincer, 2000, 2002; Gong and Wall, 2001; Wall, 2002; Szargut, 2003, 2004; Dincer and Rosen, 1998, 2005; Chen, 2005, 2006). The resource accounting in terms of exergy has been carried out based on nation or industrial sector scales (e.g., Reistad, 1975; Wall, 1987, 1990; Rosen, 1992; Wall et al., ˚ and Mielnik, 2000; 1994; Rosen and Dincer, 1997a; Ertesvag Hammond and Stapleton, 2001; Ayres et al., 2003; Dincer et al., 2003, 2004a,b,c,d,e; Ji and Chen, 2006; Chen and Chen, 2006, in press-a,b,c,d,e). Rosen and Dincer (Rosen and Dincer, 1997a,b, 1999, 2001, 2003; Dincer, 2000, 2002; Dincer and Rosen, 1998, 2005) paid much attention to the relationship between energy utilization and the environmental impacts, and highlighted the implication of the exergy analysis to the sustainable development. A number of researchers (e.g., Ulgiati et al., 1995; Creyts and Carey, 1997) have tried to devise unified objective measures for environmental impact assessment based upon exergy, via either estimating the chemical
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exergy associated with a waste stream or the total exergy consumption associated with corresponding human helped ¨ treatment courses of the waste stream. Hellstorm (1997, 2003) estimated and compared the exergy consumption of physical resources in some wastewater treatment plants and sew¨ erage systems. Finnveden and Ostlund (1997), Ayres et al. (1998), and Cornelissen et al. (1999) have developed an Exergy Based Life Cycle Analysis, introducing the concept of exergy into the methodology of environmental life cycle assessment and using it as a uniform indicator of total environmental impact. Sciubba and co-workers (Sciubba, 1999, 2001a,b, 2003a,b; Sciubba and Ulgiati, 2005) discussed a new paradigm for the calculation of the real environmental cost by performing the Extend Exergy Accounting, and provided a novel framework for integrated evaluation of concerned factors of capital, labor and environmental impact, etc. The concept of ecological cost has been defined as the cumulative consumption of non-renewable natural resources measured by exergy in the fabrication of particular products (Szargut et al., 2002). In a framework for systems evaluation based on exergy circuit language, ecological value for a waste stream is defined negative and equal in magnitude to corresponding embodied exergy in terms of the total exergy consumed in human helped treatment or natural degradation of the waste stream (Chen, 2006). Jørgensen and co-workers (Jørgensen et al., 1995; Bendoricchio and Jørgensen, 1997; Martins et al., 1997; Xu, 1997; Xu et al., 1999, 2001, 2002, 2004; Jørgensen, 2001; Jørgensen et al., 2002a,b,c; Marques et al., 2003; Zhang et al., 2003, 2004) have made much efforts in exergetic modeling for aquatic systems such as lakes and coastal areas, by demonstrating and illustrating the relationships between exergy and biomass, biodiversity, species composition, and other properties of ecosystems. A preliminary resource accounting for river water has been carried out by Zaleta-Aguilar et al. (1998) in terms of the river water availability quantified as the mechanical, thermal and chemical exergy flux with the river flow. A more general approach involving biological and sedimental exergy has been proposed for a unitary objective accounting of water resources, with application to the Yellow River basin (Chen, 2004). An exergy based evaluation of water quality remains to be initiated. The monitoring of water quality of rivers dates back to around 1890 when some European rivers, such as the Thames and the Seine, highly contaminated due to domestic sewage, were monitored in terms of a few simple parameters of dissolved oxygen, pH, etc. With the rapid industrialization and development of the energy sectors and high-input agriculture, there has been an exponential rise in the number of water quality indicators, corresponding to the increasing diversity of pollutants (Meybeck and Helmer, 1989). These indicators, including the earliest monitored simple indicators, major irons, organic, and inorganic matters, and toxic pollutants, etc., cover a broad range of water quality. In order to comprehensively evaluate the water quality, a variety of evaluation methods such as the single index, fuzzy mathematics, principal factor analysis, specialist evaluation, gray correlation, radial basis function, artificial neural network, and comprehensive index evaluation, etc. have been established (Cater, 1996, pp. 56–99, 122–143). Common to all those mod-
els is the unavoidable subjectivity, of the weighting factor for each involved indicator, mathematical models or corresponding parameters. Due to the subjective weighted factors out of so-called specialist inquiry, contradictory evaluations may be resulted for the same water quality data with different specialist groups. For reasonable and consistent water resource exploitation and management, it is essential to pursue a unified objective assessment of water quality. In the present paper, the chemical exergy based evaluation method is developed for a unified objective assessment of water quality. While a quantity termed specific standard chemical exergy based on the global reference substances might be adopted, an indicator as specific relative chemical exergy with reference to a spectrum of substances associated with the specified water quality standard is proposed for water quality evaluation with more practical implications, resulting in unified objective quantifiers for the carrying capacity and carrying deficit of water resources. With the data collected in the GEMS/WATER project, water qualities of 72 rivers and 24 lakes over the world are evaluated as a detailed case study to illustrate the adaptability of the chemical exergy based indicators for water quality evaluation.
2.
Chemical exergy and water quality
Standard chemical exergy of a substance is defined as the minimum amount of available energy or work necessary to produce the substance under consideration from environmental substances in the sense of Szargut’s global averaged model and associated with standard environment with temperature T0 of 298.15 K and pressure P0 of 1 atm, i.e., 101.325 kPa (Riekert, 1974; Morris and Szargut, 1986). To assess the chemical exergy of a substance, the properties of the chemical elements comprising the substance must be referred to the properties of some corresponding reasonably selected substances in the environment (Kotas, 1985, p. 44). The reference substances selected are the most abundant substances in the natural environment, and in equilibrium with the rest of the environment. Obviously, the most suitable reference substance for the chemical element in question is the one not only containing the chemical element but also with the lowest chemical potential among all the corresponding environmental substances. For instance, what qualifies to be a suitable reference substance in the global environment for the solid S is SO4 2+ in the hydrosphere rather than SO3 2+ , and what for C is CO2 in the atmosphere rather than CO rarely found in the environment as a whole. Na+ , K+ , Cl− , Ca+ , etc. in the hydrosphere, N2 , O2 , CO2 , etc. in the atmosphere, and Si O2 , Fe2 O3 , etc. in the external layer of the earth crust have been adopted as the reference substances (Morris and Szargut, 1986). For a general case the standard chemical exergy (SCE) comprises two parts as the standard standard mixing exergy (SME) and reaction exergy (SRE). The standard mixing exergy for the concentration effect of the reference substances is calculated as SMEi = RT0 ln
C0 , Ci00
(1)
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where SMEi with the unit J mol−1 , denotes the standard mixing exergy of the ith reference species with the conventional average molar concentration Ci00 , R the gas constant (8.314 J mol−1 K−1 ), T0 the standard absolute temperature of the environment in Kelvin (298.15 K) and C0 is the molar concentration of substances in the environment. Obviously, a reference substance of lower concentration in its dead state corresponds to a larger standard chemical exergy, and conversely, the more abundant the substance found in the environment, the less the chemical exergy it carries. When the substance under consideration is not a conventional reference substance, the standard chemical exergy of the substance can be obtained by the analysis of related chemical reaction between the substance and its reference substances, should the molar Gibbs function of formation and the activities of the substances involved be available. For simplify, consider the chemical combination of two mono-element substances. A general reversible reaction can be expressed as aAi + bBj ↔ cAm Bn ,
(2)
where Ai and Bj are the reactants, Am Bn the product, and a, b, and c are the stoichiometric coefficients. If the standard reaction exergy of Am Bn is referred to the substances Ai and Bj , with known standard mixing exergy values, we have
G0r =
[Ai ]a [Bj ]b
G0f −
,
(4)
G0f = c × G0fAm Bn − a × G0fA
i
reactants
products
− b × G0fB , j
SCEi × Ci ,
(6)
(3)
and [Am Bn ]c
SSCE =
i
c × SREAm Bn = a × SMEAi + b × SMEBj + Gr ,
Gr = G0r + RT ln
because of not only the diversity of such pollutants but also the technical difficulties associated with and intolerable expense in monitoring them. There have been two kind of monitoring, namely, basic monitoring covering electrical conductivity, pH, temperature, total suspended solids (TSS), and major ions and nutrients (e.g., Na+ , K+ , PO4 3− , NO3 − , NH4+ ), etc. and advanced monitoring covering organic and inorganic micropollutants. The costs of advanced monitoring per sample may range be two orders of magnitude higher than those of basic monitoring. Zaleta-Aguilar et al. (1998) supposed to use a typical organic molecule to represent the ‘mean organic substance’ to facilitate the estimation of the order of magnitude of exergy value for the organic components. Actually, oxygen equivalent indicators of BOD, COD, etc. have been adopted to indirectly reflect ¨ the concentration of organic matters in water. Hellstorm (1997, 2003) suggested alternatively the chemical exergy of organic contaminants in water can be calculated by using the data of COD with the converted coefficient of 13.6 kJ g−1 of COD. This provides a first approximation for practical estimate. The specific standard chemical exergy (SSCE, J l−1 ) of the water is the sum of the standard chemical exergy of the components under consideration in unit volume of water, namely,
(5)
where Gr is the change in Gibbs free energy (J mol−1 ) of the reaction, [Am Bn ], [Ai ], [Bj ] are the activities of Am Bn , Ai , Bj , respectively, G0r the change in the standard Gibbs free energy of the reaction, is equal to Gr when each product or reactant is present at the standard state with unit activity, and G0f is the free energy of formation, i.e., energy (exergy) needed to produce 1 mole of a substance from pure elements in the most stable form, and the values of G0f are available in Cox et al. (1989). Water contamination by organic pollutants has emerged as a ubiquitous and serious problem. The breakdown of the pollutants depletes the oxygen content of water coming either from the photosynthesis of the plants or from the atmosphere, thereby adversely affects the oxygen-dependent flora and fauna. To facilitate the calculations involved in the exergy methods, Morris and Szargut (1986) tabularized the values of the standard chemical exergy of various elements and some inorganic and organic compounds of these elements on the planet earth. Even standard chemical exergy values for some organic compounds are available, the calculation of the chemical exergy of organic substances remains a tough work,
where SCEi is the standard chemical exergy of the ith component (J mol−1 ), and Ci is the concentration of the ith component in water with the unit of mol l−1 . SSCE provides for water quality evaluation a unified normal indicator with a scale of global reference. In practical assessment of water quality with respect to unfriendly chemical spectrum, maximum concentration levels are usually specified for components in water, and the lower concentrations are preferred. It is then more practical to define a ‘specific relative chemical exergy (SRCE)’ referring to the specified water quality standard as SRCE =
i
SRCEi =
(SSCEi − SSCEi ref ),
(7)
i
where SRCEi is the difference between SSCEi and SSCEi ref , which are the specific standard chemical exergy of the ith component involved in the water body and the reference water quality standard, respectively. Obviously, the SRCE is a unified measure of the ‘chemical distance’ the concerned water is away from and the specified standard, in contrast to SSCE from the global reference. Evidently, SRCE is not positive definite, depending on the status of the assessed water and the chosen water quality standard. A positive SRCE value means that the amount of the pollutants in exergy in the water exceeds the amount the water body could carry as a limit specified by the standard: this represents a ‘carrying deficit’. On the contrary, a negative SRCE value means that the amount of the pollutants in exergy in the water is below the amount the water body could carry as a limit specified by the standard: there is a remaining ‘carrying capacity’ for the water. It is preferable to have greater carrying capacity or smaller carrying deficit in exergy unit.
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Table 1 – The conventional average molarities of the ions in the sea watera (the salinity of the sea water is 0.035) Ions
Na+
K+
Ca2+
Mg2+
Cl−
SO4 2−
PO4 3−b
NO3 −b
Molarities (mol kg−1 H2 O)
0.474
1.04E−2
9.6E−3
4.96E−2
0.5657
1.17E−2
–
–
SiO2 is one of the major irons considered, with its conventional standard mole fraction of 0.472 (Morris and Szargut, 1986) in the external layer of the earth crust. a b
3.
Refer to Dyrssen and Wedborg (1974). The molarities of PO4 3− and NO3 − are not offered in Dyrssen and Wedborg (1974), and their standard chemical exergy can be calculated based on the values of phosphates and nitrates offered by Morris and Szargut (1986) following Eqs. (2)–(5), respectively.
Case study based on GEMS/WATER data
3.1. The specific standard chemical exergy (SSCE) indicator Based on the data collected in the GEMS/WATER project (Fraser et al., 2002) through the voluntary efforts of the WHO member states, the specific standard chemical exergy (SSCE) analysis for 72 river and 24 lake waters over the world are performed. The water quality indicators adopted in the GEMS/WATER project cover a wide range and are classified into basic indicators (such as TSS, DO, pH, etc.), major ions (i.e., Si O2 , Ca2+ , Mg2+ , Na+ , K+ , Cl− , SO4 2− , etc.), nutrients and organic matters (i.e., PO4 3− , Ptot , NO3 − , NH4+ , Nk , TOC, BOD, COD, etc.), pollutants and miscellaneous elements (i.e., F, B, CN, Phenol, etc.), and highly toxic pollutants (i.e., As, Cd, Hg, Pb, DDT, PCB, etc.), though due to the high cost and difficulties associated with monitoring and collection, the data of the highly toxic chemicals have not been available. According to the results of the GEMS/WATER project, the BOD5 is with a low average value, denoting the trivial pollution due to organic pollutants (Fraser et al., 2002). The major ions, namely, Si O2 , Ca2+ , Mg2+ , Na+ , K+ , Cl− , SO4 2− , and phosphorus (PO4 3− ) and nitrogen (NO3 − ) are the major indicators monitored. The major ions in rivers and lakes are originated from both the natural courses, mainly including the action of the atmosphere, the erosion of weathered surface rocks and soils, and the degradation of terrestrial organic matters, and the anthropogenic activities, such as irrigation, mining, the domestic or industrial waste emission, etc. The components of phosphorus (PO4 3− ) and nitrogen (NO3 − ) are mainly due to the pesticide, fertilizer, dejecta and domestic wastewater. Most of those indicators are exactly corresponding to the reference substances in the environment, for which the standard chemical exergy can be calculated by Eq. (1),
with the mole fractions calculated based on the conventional molarities illuminated in Table 1. The calculated SSCE values for the river and lake waters are illustrated in Figs. 1–7. Most of the rivers in South America and Oceanic are shown with lower SSCE. Some river waters in North America, Asia and Europe are found with higher SSCE values. The Balsas River and Colorado River in North America, Yellow River, Arab River, Syr Darya River and Tapti River in Asia, Elbe River, Guadalquivir River, Rhine River, Seine River and Weser River in Europe are prominent in the figures, with much higher SSCE values, denoting that they are under relatively serious environmental impacts due to natural or anthropogenic actions. Compared with the river waters, the lake waters are estimated with much higher SSCE values as shown in Fig. 7. The Caspian Sea, Superior Lake, Kivu Lake, Albert Lake, Turkana Lake, Edward Lake, Dead Sea and Van Golu Lake have higher SSCE values. Due to its well-known high concentration of salts and other components, the Dead Sea is shown with the maximum SSCE value among all the lakes over the world, followed by the Van Golu Lake.
3.2. The specific relative chemical exergy (SRCE) indicator Now some sketchy information of the quality status of the river and lake waters has been presented with the SSCE indicator referring to the global reference, we resort to the specific standard chemical exergy (SRCE) indicator associated with some specified water quality standard for a more practical and particular evaluation. To facilitate evaluation and comparison, consider the standards for potable water. Among existing standards for potable water, the most widely applied or referred to over the world are those issued by the World Health Organization (WHO),
Fig. 1 – The specific standard chemical exergy (SSCE) of some rivers in North America.
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Fig. 2 – The specific standard chemical exergy (SSCE) of some rivers in South America.
Fig. 3 – The specific standard chemical exergy (SSCE) of some rivers in Asia.
Fig. 4 – The specific standard chemical exergy (SSCE) of some rivers in Europe.
Fig. 5 – The specific standard chemical exergy (SSCE) of some rivers in Africa.
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Fig. 6 – The specific standard chemical exergy (SSCE) of some rivers in Oceania.
Fig. 7 – (a) The specific standard chemical exergy (SSCE) of some lakes in the world. (b) The specific standard chemical exergy (SSCE) of the Dead Sea and the Van Golu Lake.
Fig. 8 – The specific relative chemical exergy (SRCE) of some rivers in North America.
Fig. 9 – The specific relative chemical exergy (SRCE) of some rivers in South America.
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Table 2 – The specified concentrations of the components in potable water standards Na+
Reference substance Concentration (mg l−1 ) (GL)a Concentration (mg l−1 ) (MAC)d a b
c d e f
20 150
K+
Ca2+
Mg2+
Cl−
SO4 2−
HCO3 −
SiO2
PO4 2−
NO3 −
10 12
100 100e
30 50
25 250f
25 250
30b 30b
1 10
2.5c 2.5c
25 50
The Guide Line in EU (80/778/EC), with lower permissible concentration, namely, denoting higher quality level. The specified concentration of HCO3 − , representing the alkalinity (opposite with others, it is the minimum allowed concentration, hence the standard chemical exergy of HCO3 − should be taken into account separately). The specified concentration of PO4 2− in the water quality standard of potability prescribed by Vietnam. The Maximum Allowed Concentration in EU (80/778/EC), with higher permissible concentration, namely, lower quality level. The specified concentration of Ca2+ in the water quality standard of potability prescribed by France and China. The specified concentration of Cl− in the water quality standard of potability prescribed by Malaysia and Thailand.
Fig. 10 – The specific relative chemical exergy (SRCE) of some rivers in Asia.
Fig. 12 – The specific relative chemical exergy (SRCE) of some rivers in Africa. Fig. 11 – The specific relative chemical exergy (SRCE) of some rivers in Europe.
the European Union (EU) and the Environmental Protection Agency in US (US EPA). Many developed countries adopt more stringent criteria, however, some developing countries have reconciled themselves to less stringent criteria duo to various practical difficulties. As most of the indicators applied in the GEMS/WATER project are involved in the potable water quality standard prescribed by EU (80/778/EC), it is then reasonable to devise a reference standard as shown in Table 2 with EU (80/778/EC) standard as a basis and some minor ones unavailable in the EU (80/778/EC) standard adopted from some other state standards (WHO, 1986). Based on the reference environment according to the Guide Line (GL) with lower permissible concentrations specified and the maximum allowed concentration (MAC) with higher permissible concentrations specified, respectively, the SREC val-
ues of the river and lake waters are calculated and shown in Figs. 8–14. River waters in South America, Africa and Oceania are shown with good quality status: all of them have remaining carrying capacity relative to the MAC standard, and most
Fig. 13 – The specific relative chemical exergy (SRCE) of some rivers in Oceania.
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water quality against a global earth averaged background. As a practical indicator for anthropogenic water exploitation, the specific relative chemical exergy (SRCE) with reference to a specified water quality standard is devised, with related concepts of carrying deficit and carrying capacity well embodied in exergy terms. With the data collected in the GEMS/WATER project, water qualities of 72 rivers and 24 lakes over the world are evaluated, as a detailed case study to illustrate the adaptability of the chemical exergy based indicators for water quality evaluation.
Acknowledgement Project supported by the National Key Basic Research Program (Grant No. 2005CB724204).
references
Fig. 14 – (a) The specific relative chemical exergy (SRCE) of some lakes in the world. (b) The specific relative chemical exergy (SRCE) of the Dead Sea and the Van Golu Lake.
of them (except for the rivers Burdekin and Murray in Oceania) also meet the stringent GL standard. In contrast, many river waters in North America, Asia and Europe and most lake waters over the world are shown associated with carrying deficit, especially when GL is applied. Those with carrying deficit with respect to GL are the Balsas River, Colorado River, Mississippi River and Nelson River in North America, Cauvery River, Yangtse River, Yellow River, Kistna River, Narmada River, Arab River, Syr Darya River and Tapti River in Asia, Danube River, Ebro River, Elbe River, Garonne River, Guadalquivir River, Loire River, Po River, Rhine River, Rhne River, Seine River, Taugs River, Volga River and Weser River in Europe, Burdekin River and Murray River in Oceania and the lakes of Caspian Sea, Superior, Huron, Victoria, Titicaca, Kivu, Albert, Turkana, Edward, the Dead Sea and the Van Golu. But with less stringent MAC standard, much fewer river and lake waters are shown under carrying deficit.
4.
Conclusions
The thermodynamic concept of chemical exergy, defined as the minimum work in need to bring the concerned system into chemical equilibrium with its environment and as an information measure of the chemical deviation of a concerned system from its environment, is introduced for water quality evaluation to develop unified objective indicators in contrast to conventional indicators characteristic of subjectivity. The concept of specific standard chemical exergy (SSCE) based on the global reference environment as modeled by Szargut is devised and elaborated as a normal indicator for
Ayres, R.U., Ayres, L.W., Martinas, K., 1998. Exergy, waste accounting and life-cycle analysis. Energy 23 (5), 353–363. Ayres, R.U., Ayres, L.W., Warr, B., 2003. Exergy, power and work in the US economy, 1900–1998. Energy 28, 219–273. Bendoricchio, G., Jørgensen, S.E., 1997. Exergy as goal function of ecosystems dynamic. Ecol. Model. 102, 5–15. Cater, L.W., 1996. Environmental Impact Assessment [M], second ed. McGraw-Hill Inc., Singapore, pp. 56–99, 122–143. Chen, B., 2004. A theory of ecological exergy and its application to the ecological modeling of the Yellow River basin. Ph.D Dissertation. School of Environment, Peking University. Chen, G.Q., 2005. Exergy consumption of the earth. Ecol. Model. 184, 363–380. Chen, G.Q., 2006. Scarcity of exergy and ecological evaluation based on embodied exergy. Communications in nonlinear science and numerical simulation 11, 541–552. Chen, G.Q., Chen, B., Resource analysis of the Chinese society 1980–2002 based on exergy-part 1: fossil fuels and energy minerals. Energy Policy, in press-a. Chen, G.Q., Chen, B., Resource analysis of the Chinese society 1980–2002 based on exergy-part 2: renewable energy sources and forest. Energy Policy, in press-b. Chen, B., Chen, G.Q., Resource analysis of the Chinese society 1980–2002 based on exergy-part 3: agricultural products. Energy Policy, in press-c. Chen, B., Chen, G.Q., Resource analysis of the Chinese society 1980–2002 based on exergy-part 4: fishery and rangeland. Energy Policy, in press-d. Chen, B., Chen, G.Q., Resource analysis of the Chinese society 1980–2002 based on exergy-part 5: resource structure and intensity. Energy Policy, in press-e. Chen, B., Chen, G.Q., 2006. Exergy analysis for resource conversion of the Chinese society 1993 under the material product system. Energy 31, 1115–1150. Cox, J.D., Wagman, D.D., Medvedev, V.A., 1989. CODATA Key Values for Thermodynamics, vol. 248. IAHS Publ, pp. 379–385. Cornelissen, R.L., Marquart, E.N., Hirs, G.G., 1999. The value of exergetic life cycle assessment besides LCA. In: Proceedings of ECOS’99, Tokyo. Creyts, J.C., Carey, V.P., 1997. Use of Exergy Analysis as a Tool for Assessment of the Environmental Impact of Industrial Processes. AES-37, ASME. Dincer, I., Rosen, M.A., 1998. A worldwide perspective on energy, environment and sustainable development. Int. J. Energy Res. 22, 1305–1321.
e c o l o g i c a l m o d e l l i n g 2 0 0 ( 2 0 0 7 ) 259–268
Dincer, I., 2000. Thermodynamics, exergy and environmental impact. Energy Sources 22, 723–732. Dincer, I., 2002. Technical, environmental and exergetic aspects of hydrogen energy systems. Int. J. Hydrogen Energy 27, 265–285. Dincer, I., Hussain, M.M., Al-Zaharnah, I., 2003. Energy and exergy use in the industrial sector of Saudi Arabia. Proceedings of the I MECH E Part A. J. Power Energy 217 (12), 481–492. Dincer, I., Hussain, M.M., Al-Zaharnah, I., 2004a. Energy and exergy utilization in transportation sector of Saudi Arabia. Appl. Thermal Eng. 24, 525–538. Dincer, I., Hussain, M.M., Al-Zaharnah, I., 2004b. Energy and exergy use in public and private sector of Saudi Arabia. Energy Policy 32, 1615–1624. Dincer, I., Hussain, M.M., Al-Zaharnah, I., 2004c. Energy and exergy use in the utility sector of Saudi Arabia. Desalination 169, 245–255. Dincer, I., Hussain, M.M., Al-Zaharnah, I., 2004d. Energy and exergy use in residential sector of Saudi Arabia. Energy Sources 26, 1239–1252. Dincer, I., Hussain, M.M., Al-Zaharnah, I., 2004e. Analysis of sectoral energy and exergy use of Saudi Arabia. Int. J. Energy Res. 28, 205–243. Dincer, I., Rosen, M.A., 2005. Thermodynamic aspects of renewables and sustainable development. Renewable Sustainable Energy Rev. 9, 169–189. Dyrssen, D., Wedborg, M., 1974. The Sea (5) (Goldberg, E.D., ed.), Wiley Interscience, N.Y.; In: Morris, D.R., Szargut, J., 1986. Standard chemical exergy of some elements and compounds on the planet earth. Energy 11 (8), 733–755. ˚ I.S., Mielnik, M., 2000. Exergy analysis of the Norwegian Ertesvag, society. Energy 25, 957–973. ¨ Finnveden, G., Ostlund, P., 1997. Exergies of natural resources in life-cycle assessment and other applications. Energy 22 (9), 923–931. Fraser, A.S., Meybeck, M., Ongley, E.D., 2002. Water quality of the rivers over the world. In: Zheng, S.R. (Ed.), Comprehensively Evaluation of the Freshwater Resource over the World. Hubei Science and Technology Press, Wuhan. Gong, M., Wall, G., 2001. On exergy and sustainable development–Part 2: indicators and methods. Exergy—Int. J. 1 (4), 217–233. Hammond, G.P., Stapleton, A.J., 2001. Exergy analysis of the United Kingdom energy system. IMech J. Power Energy 215, 141–162. ¨ Hellstorm, D., 1997. An exergy analysis for a wastewater treatment plant—an estimation of the consumption of physical resources. Water Environ. Res. 69, 44–51. ¨ Hellstorm, D., 2003. Exergy analysis of nutrient recovery processes. Water Sci. Technol. 48 (1), 27–36. Ji, X., Chen, G.Q., 2006. Exergy analysis of energy utilization in the transportation sector in China. Energy Policy 34, 1709–1719. Jørgensen, S.E., 1988. Use of models as experimental tool to show that structural changes are accompanied by increased exergy. Ecol. Model. 41, 117–126. Jørgensen, S.E., 1992a. Intergration of Ecosystem Theories: A Pattern. Kluwer, Dordrecht. Jørgensen, S.E., 1992b. Parameters, ecological constrains and exergy. Ecol. Model. 62, 163–170. Jørgensen, S.E., 1992c. Development of models able to account for changes in species composition. Ecol. Model. 62, 195–208. Jørgensen, S.E., 1994. Review and comparison of goal function in system ecology. Vie Milieu 44 (1), 11–20. Jørgensen, S.E., 1995. Exergy and ecological buffer capacities as measures of ecosystem health. Ecosyst. Health 1 (3), 150–160. Jørgensen, S.E., Nielsen, S.N., Mejer, H., 1995. Energy, environ, exergy and ecological modeling. Ecol. Model. 77, 99–109. Jørgensen, S.E., 2000. Application of exergy and specific exergy as ecological indicators of coastal areas. Aquat. Ecosyst. Health Manag. 3, 419–430.
267
Jørgensen, S.E., 2001. The thermodynamic concept: exergy. In: Jørgensen, S.E. (Ed.), Thermodynamics and Ecological Modeling. CRC Press, Boca Raton, Florida, USA. Jørgensen, S.E., Marques, J., Nielsen, S.N., 2002a. Structural changes in an estuary, described by models and using exergy as orientor. Ecol. Model. 158, 140–233. Jørgensen, S.E., Verdonschot, P., Lek, S., 2002b. Explanation of the observed structure of functional feeding groups of aquatic macro-invertebrates by an ecological model and the maximum exergy principle. Ecol. Model. 158, 223–231. Jørgensen, S.E., Ray, S., Berec, L., Straskraba, M., 2002c. Improved calibration of a eutrophication model by use of the size variation due to succession. Ecol. Model. 153, 269–277. Kotas, T.J., 1985. The Exergy Method of Thermal Plant Analysis. Anchor Brendon Ltd., Essex, p. 44. Morris, D.R., Szargut, J., 1986. Standard chemical exergy of some elements and compounds on the planet earth. Energy 11 (8), 733–755. Meybeck, M., Helmer, R., 1989. The quality of rivers: from pristine stage to global pollution. Global Planet Change 1, 283–309. Martins, I., Marques, J.C., Jørgensen, S.E., Nielsen, S.N., 1997. Modeling the effects of green macroalgae blooms on the population dynamics of Cyathura carinata (Crustacea: Isopoda) in an eutrophied estuary. Ecol. Model. 102, 33–53. Marques, J.C., Nielsen, S.N., Pardal, M.A., Jørgensen, S.E., 2003. Impact of eutrophication and river management within a framework of ecosystem theories. Ecol. Model. 166, 147–168. Reistad, G.M., 1975. Available energy conversion and utilization in the United States. J. Eng. Power 97, 429–434. Riekert, L., 1974. Chem. Eng. Sci. 29, 1613. Rosen, M.A., 1992. Evaluation of energy utilization efficiency in Canada using energy and exergy analyses. Energy—Int. J. 17, 339–350. Rosen, M.A., Dincer, I., 1997a. Sectoral energy and exergy modeling of Turkey. ASME—J. Energy Resources Technol. 119 (3), 200–204. Rosen, M.A., Dincer, I., 1997b. On exergy and environmental impact. Int. J. Energy Res. 21, 643–654. Rosen, M.A., Dincer, I., 1999. Exergy analysis of waste emissions. Int. J. Energy Res. 23, 1153–1163. Rosen, M.A., Dincer, I., 2001. Exergy as the confluence of energy, environment and sustainable development. Exergy—Int. J. 1 (1), 3–13. Rosen, M.A., Dincer, I., 2003. Exergy-cost-energy-mass analysis of thermal systems and processes. Energy Conversion Manag. 44, 1633–1651. Sciubba, E., 1999. Exergy as a direct measure of environmental impact. ASME Adv. Energy Syst. Div. Publaes 39, 573–581. Sciubba, E., 2001a. Using exergy to evaluate environmental externalities. In: Invited Lecture, IV NTVA Seminar on Industrial Ecology, Trondheim. Sciubba, E., 2001b. Beyond thermoeconomics? The concept of extended exergy accounting and its application to the analysis and design of thermal systems. Exergy—Int. J. 1 (2), 68–84. Sciubba, E., 2003a. Cost analysis of energy conversion systems via a novel resource-based quantifier. Energy 28, 457–477. Sciubba, E., 2003b. Extended exergy accounting applied to energy recovery from waste: the concept of total recycling. Energy 28, 1315–1334. Sciubba, E., Ulgiati, S., 2005. Emergy and exergy analyses: complementary methods or irreducible ideological options? Energy 30, 1953–1988. Szargut, J., 2001. Sequence method of determination of partial exergy losses in thermal systems. Exergy 1, 85–90. Szargut, J., Andrzej, Z., Stanek, W., 2002. Depletion of the nonrenewable natural exergy resources as a measure of
268
e c o l o g i c a l m o d e l l i n g 2 0 0 ( 2 0 0 7 ) 259–268
the ecological cost. Energy Conversion Manag. 43, 1149–1163. Szargut, J.T., 2003. Anthropogenic and natural exergy losses. Energy 28, 1047–1054. Szargut, J.T., 2004. Optimization of the design parameters aiming at the minimization of the depletion of non-renewable resources. Energy 29, 2161–2169. Ulgiati, S., Brown, M.T., Bastianoni, N., Marchettini, N., 1995. Emergy based indices and ratios to evaluate sustainable use of resources. Ecol. Eng. 5, 23–28. Wall, G., 1987. Exergy conversion in the Swedish society. Resources Energy 9, 55–73. Wall, G., 1990. Exergy conversion in the Japanese society. Energy—Int. J. 15, 435–444. Wall, G., Sciubba, E., Naso, V., 1994. Exergy use in the Italian society. Energy—Int. J. 19, 1267–1274. Wall, G., 2002. Conditions and tools in the design of energy conversion and management systems of a sustainable society. Energy Conversion Manag. 43, 1235–1248. WHO, 1986. The Guide Line of the Potable Water Quality. People Hygienic Press, Beijing. Xu, F.L., 1997. Exergy and structural exergy as ecological indicators for the development state of the Lake Chaohu ecosystem. Ecol. Model. 99, 41–49.
Xu, F.L., Jørgensen, S.E., Tao, S., 1999. Ecological indicators for assessing freshwater ecosystem health. Ecol. Model. 116, 77–106. Xu, F.L., Tao, S., Dawson, R.W., Li, P.G., Cao, J., 2001. Lake ecosystem health assessment: indicators and methods. Water Res. 35, 3157–3167. Xu, F.L., Dawson, R.W., Tao, S., Li, B.G., Cao, J., 2002. System-level responses of lake ecosystems to chemical stresses using exergy and structural exergy as ecological indicators. Chemosphere 46, 173–185. Xu, F.L., Lam, K.C., Zhao, Z.Y., Zhan, W., Chen, Y.D., Tao, S., 2004. Marine coastal ecosystem health assessment: a case study of the Tolo Harbour, Hong Kong. Chin. Ecol. Model. 173, 355–370. Zaleta-Aguilar, A., Ranz, L., Valero, A., 1998. Towards a unified measure of renewable resources availability: the exergy method applied to the water of a river. Energy Conversion Manag. 39, 1911–1917. Zhang, J.J., Jørgensen, S.E., Tan, C.O., Beklioglu, M., 2003. A structurally dynamic modelling—Lake Mogan. Turkey as a case study. Ecol. Model. 158, 103–120. Zhang, J.J., Jørgensen, S.E., Mahler, H., 2004. Examination of structurally dynamic eutrophication model. Ecol. Model. 173, 313–333.