Ind. Eng. Chem. Res. 2004, 43, 4233-4242
4233
Sustainability Assessment of Industrial Systems
Helen H. Lou,* Makarand A. Kulkarni, Aditi Singh, and Jack R. Hopper
Department of Chemical Engineering, Lamar University, Beaumont, Texas 77710
Sustainability is a vital issue for long-term development of industries and effective environmental
protection. The principal aim of sustainable development is the progress on all fronts, i.e.,
economy, environment, and society. In this paper, a set of new sustainability indices is introduced
to assess the environmental and economic performances as well as the sustainability of industrial
systems in a uniform structure. In these indices, the values of nonmoneyed and moneyed
resources, services, and commodities are quantified using a common unit: emergy. As compared
to the existing emergy-based sustainability indices that originated from the study on agricultural
or natural ecological systems, the newly defined indices improve the applicability and the
effectiveness of the existing indices by addressing the unique features of industrial systems
systematically. These new indices reveal strong interdependence among multiple objectives and
provide clear guidance to the industries on how to improve their performance on multiple fronts.
The utilities of the new indices are demonstrated by case studies.
Introduction
Sustainability is a vital issue for long-term development of industries and effective environmental protection. Sustainable development can be defined as the
development in which the needs of the present generation are met without compromising the ability of future
generations to meet their requirements. The principal
aim of sustainable development is the progress on all
fronts, i.e., economy, environment, and society.1
A number of environmental performance assessment
techniques are available, such as AIChE total cost
assessment,2 life cycle analysis,3 and integral biodiversity impact-assessment system.4 These techniques address the environmental impacts of a system from
different angles. However, in this challenging world, the
economic profitability of the industry needs to be well
addressed simultaneously.
A number of sustainability assessment methods have
also been developed and widely accepted. Ulgiati et al.5
introduced a set of general sustainability indices based
on the study of agricultural systems. This suggests considering more details in assessing industrial systems.6-8
The AIChE sustainability index metric (SIM) is an
excellent method of measuring the sustainability of an
industry. It is formed based on the ratios of different
process streams in plants.9 The streams bearing environmental impacts, such as resource consumption and/
or pollutant waste, are represented in the numerator,
while the outputs of the process in physical or financial
terms are represented in the denominator. If all quantities in the numerator and the denominator are normalized to per pound of product, then a number of metrics
can be formed to indicate economic, environmental, and
social sustainability of the process. To combine the
economic and environmental performances of an industrial process, the AIChE SIM uses six indicators, i.e.,
material intensity, energy intensity, water consumption,
toxic emissions, pollutant emissions, and greenhouse
gas emissions. However, different units of these indica* To whom correspondence should be addressed. Tel.: (409)
880-8207. Fax: (409) 880-2197. E-mail: louhh@hal.lamar.edu.
tors may cause difficulty in evaluating the overall
sustainability.
The growth and development of ecosystems is determined by the availability of energy and the ability of
organisms to convert it to useful work. As a given
amount of the available energy moves from lower to
higher organisms, its quality improves but its quantity
decreases. Thus, the ecological cost of nature’s products
and services may be estimated as the amount of energy
used directly or indirectly in its manufacture, which is
named emergy.10 In this paper, a new set of indices is
introduced to quantitatively assess the environmental
and economic performances as well as the sustainability
of industrial systems. In these indices, the values of
nonmoneyed and moneyed resources, services, and
commodities are effectively quantified using a common
unit, emergy, which provides a uniform framework for
the evaluation of the environmental performance and
the sustainability of industries. As compared to the
existing emergy-based sustainability indices that were
developed from the study of agricultural or natural
ecological systems, the new indices are devised by
addressing the unique features of industrial systems,
i.e., waste treatment, recovery, reuse, and recycle. By
considering all of the material/energy flows and investments in industrial systems, the applicability and
effectiveness of the emergy-based indices in analyzing
industrial systems can be improved significantly.
The concept of industrial ecology is a valuable aspect
of sustainable engineering. Industrial ecology can be
defined as the study of physical, chemical, and biological
interactions and interrelationships within and between
industrial and ecological systems.11 An industrial ecosystem is analogous to a natural ecosystem where
organisms survive by consuming other organism’s product/waste, so that no source of energy is wasted.
Similarly, in an industrial ecosystem, industries come
together in such a manner that the product/waste
generated by one industry is used as a resource by
another industry. The aim of an industrial ecosystem
is to change the industrial process from a linear one to
a cyclic one so that the waste from one industry is used
as an input for another. This system structure results
10.1021/ie049972w CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/15/2004
4234 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004
in strong interdependence among the member entities.
Therefore, the performance of each entity in an ecosystem will be partially determined by the activities of
other entities. These new indices can be a valuable tool
in revealing the interdependence among the member
entities in achieving the economic, environmental, and
sustainability goals.
In an industrial ecosystem, the member industries
can improve their economic and environmental performance by discovering the opportunities for internal
recycle and external mass/energy exchange and exploring market opportunities for waste. These not only help
to improve the economic status of the industry by
cutting down the requirement of fresh resources but also
reduce environmental pressure by reducing the amount
of waste disposed of and the consumption of fresh
nonrenewable resources.
On the basis of these principles, several industrial
ecoparks have been developed around the world. The
industrial complex in Kalundburg, Denmark, is an
example of such an industrial ecosystem. In that
example, five different industries together formed a
highly integrated industrial system that optimized the
use of its byproducts, thereby minimizing the amount
of net waste disposed of.11 Consequently, the sustainability of the industrial system was improved. Another
example is the Dalian Economic and Technological
Development Zone, China, which has around 1150
enterprises with over U.S.$10 billion investment. In that
park, air emission control, water management, solid
waste management greener technologies, and cleaner
production processes were implemented.12 Several such
industrial ecoparks and resource recovery parks are now
being developed in the U.S., for example, Cabazon
Resource Recovery Park, Mecca, CA, Urban Ore Resource Recovery Park, San Leandro, CA, and The
Brownsville Project, Brownsville, TX.13 Apart from such
ecoparks, several industrial complexes also exist around
the world implementing internal and external recycle
to minimize waste discharge. For example, the Mississippi River Corridor Industrial Complex comprises
around 150 chemical plants. Significant research activities have been directed toward minimizing waste disposal and maximizing material and energy reuse in this
complex.14 Governments and industries around the
world tried in many ways to promote research and
business opportunities in industrial sustainability and
external recycling.15 In the U.S., many different sources,
including federal, state, and local governments, industry, professional associations, universities, and nongovernmental organizations, have acknowledged the importance of sustainability and support the research and
business opportunities at various levels.16
In the case of an industrial ecosystem, the individual
performances as well as the performance of the entire
system need to be addressed clearly. When the fundamentals are revealed, the newly introduced indices
provide a clear guidance to the industries on how to
improve the economic, environmental, and sustainability performance of individual industries as well as the
entire industrial ecosystem.
Basics of Emergy Analysis
The theory of environmental accounting through
emergy analysis is based on the evaluation of the energy
used for making products or services. This “used” energy
is called emergy.10 In other words, emergy is the
Figure 1. Traditional emergy-flow diagram. Reprinted from ref
17, Copyright 1998, with permission from Elsevier.
available energy of one kind previously required, directly and indirectly, to make the products or services.
The unit of emergy is emjoule. To calculate the emergy
of a particular substance or service, transformity of that
substance or service will be needed. Transformity is
defined as the emergy of one kind of available energy
required to make 1 J of energy of another type.17 The
unit of transformity is solar emjoule per joule when
emergy is calculated in terms of solar energy. In case
the transformity is unknown, the emergy/money ratio
is helpful in calculating the emergy of the substances
or the services. Emdollars, i.e., the ratio of emergy to
money, is calculated by dividing the total emergy use
of a country by its gross economic product.10
Emergy analysis provides a common platform to
quantitatively express the economic values as well as
the environmental factors. It facilitates the comparison
of the economic and environmental status of different
industrial entities on a common ground. Hence, the
sustainability performance can be exploited conveniently.
Traditional Emergy-Flow Diagram and Emergy
Indices
In traditional emergy-flow analysis as shown in
Figure 1, the natural renewable resources (R; such as
water, air, and solar energy) and nonrenewable resources (N; such as fossil fuels) are consumed through
purchased resources (such as equipment) and services
(such as labor) (F) for economic use. As a result, yield
(Y) is generated along with waste (W), i.e., undesirable
byproduct. Yield (Y) will be sold in the market and waste
(W) will be disposed of to the environment.
On the basis of this emergy-flow diagram, several
indices have been developed to quantify the economic,
environmental, and sustainability performance of a
system.10,17,18 Those related to this work are reviewed
briefly below.
(a) Environmental Yield Ratio (EYR). It is defined
as the ratio of emergy of the yield (Y) to the emergy of
imported investment (F) required to convert the raw
materials to product, i.e., EYR ) Y/F. As the definition
suggests, a high value of EYR is always desired. Thus,
the yield should be high for a given investment, while
the investment is expected to be low for any given yield.
(b) Environmental Loading Ratio (ELR). It is
defined as the ratio of the sum of the imported emergy
(F) and emergy of nonrenewable resources (N) to the
emergy of renewable resources (R), i.e., ELR ) (F + N)/
R. Obviously, a low value of ELR, as an indicator of low
environmental pressure, is always desired. Therefore,
imported resource or service (F) and the consumption
Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4235
Figure 2. New emergy-flow diagram for an industrial system.
of nonrenewable resources used (N) should be kept low,
and the usage of renewable resources should be high.
(c) Index of Sustainability (SI). This index combines both environmental and economic factors that
affect the performance of an industry. It is defined as
the ratio of EYR to ELR, i.e., SI ) EYR/ELR. This index
reflects the ability of a system in providing desired
products or services with a minimum environmental
stress and a maximum profit. Naturally, a high value
of SI is always preferred.
Industrial Emergy-Flow (IEF) Diagram and
New Emergy-Based Indices
The traditional emergy-flow diagram provides a general framework for depicting the relationships of different components in an ecosystem. However, for an
industrial system, a much more detailed portrait of its
unique features needs to be fully explored;7 especially,
the fate of waste and the cost associated with waste
treatment, recycle, and waste disposal should be fully
considered. Figure 2 is an IEF diagram that is to be
fully studied.
The IEF diagram encompasses all kinds of possible
mass/energy flows of an industrial system. The main
difference between the traditional and the new emergyflow diagram lies in the classification of the waste
generated by the process system. For an industrial
system, the waste generated during production may
either undergo waste treatment (e.g., desulferization
and detoxification) as per environmental regulations or
remain untreated if harmless. Sometimes, the waste can
be recycled internally as a renewable/nonrenewable
resource (like wastewater and residual heat). Under
some circumstance, it can be exported to other industries, as well as commercial, residential, or agricultural
units, to be used as a material/energy resource supplement (e.g., fly ash generated by burning coal in a power
plant can be sent to a cement factory as a raw material).
As shown in Figure 2, the fate of waste generated
during production (W) is taken into full consideration.
Some of the waste can be handled without treatment
(WU), while some needs to be discharged to the environ-
ment only after waste treatment (WT). Part of WU may
be directly discharged into the environment (WUW), part
of it may be recycled as a nonrenewable resource (WURN)
or renewable resource (WURR), and part can be sold in
the market (YUW). For WT, some of it may be discharged
to the environment after waste treatment (WTW), some
can be recycled as a nonrenewable resource (WTRN) or
renewable resource (WTRR), and some can be sold in the
market (YTW). WTRN and WURN together are the total
amount of waste recycled as nonrenewable resource
(WRN). Similarly, WTRR and WURR together are the total
amount of waste recycled as renewable resource (WRR).
The summation of WUW and WTW gives the total waste
discharge (Ww). YUW and YTW together are the total
waste that can be sold as a useful product (YW). The
investment of the industrial system can thus be classified into four parts: the investment on production (FP),
waste treatment (FW), waste recycle (FR), and waste
disposal (FD). FR consists of the cost for recycle of
nonrenewable resources from waste (FRN) and that for
recycle of renewable resources from waste (FRR) including the handling and transportation cost for each of
them. FD also consists of two parts: the cost of disposing
treated waste into the environment (FTD) and the cost
of disposing untreated waste (FUD).
To provide a more realistic analysis on the environmental, economic, and sustainability performances of
industry systems, a set of new emergy indices is
developed based upon the IEF diagram.
(a) Index of Economic Performance (IEcP). The
index of economic performance is defined as the ratio
of the total yield to the total investment needed in
obtaining the required quantity of the product of desired
quality, while satisfying the environmental regulations,
i.e.,
IEcP ) (YP + YW)/(FP + FW + FR + FD + I)
Note that the numerator of IEcP is the sum of the
emergy of all of the products produced by a plant,
including the products and byproducts (YP), and the
useful yield obtained from waste (YW). The denominator
of IEcP is the emergy of the investment for production
(FP) and the waste treatment (FW), recycle (FR), and
disposal (FD) under environmental constraints, and
resource intake (I ) N + R). As discussed before, a high
value of IEcP indicates a high product yield with less
investment, and this is desirable. If the IEcP value of a
plant is less than 1, its economic performance is unacceptable. In this case, the following approaches can be
considered.
(i) To improve the product yield (YP). Improvement
in product quality and quantity can be obtained through
better product and process design and improved operational strategy. On the contrary, any increase in the
production cost (FP) or resource investment (I) for better
yield may adversely affect IEcP. Hence, any attempt to
increase the yield should keep the investment as low
as possible.
(ii) To convert “waste” to “treasure” (YW) economically.
Identifying market opportunities for the waste generated will improve IEcP. If some waste can be exported
or recycled, it will cut down the waste treatment and
waste disposal cost and thus increase IEcP eventually.
(iii) To reduce fresh resource consumption (I). Adopting highly efficient processing techniques will help to
improve the yield while using fewer resources. Efficient
mass and energy networks for recycling/reuse can also
4236 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004
considerably lower the demand on fresh raw material
and utility in plants.19 Through recycle, the net amount
of waste discharge will be reduced, so the waste treatment (FW) and waste disposal cost (FD) will be decreased
correspondingly. All of these will help to improve IEcP.
However, the investment on recycle (FR) should be kept
as low as possible to maintain a good economic performance.
(iv) To reduce waste treatment cost (FW) and waste
disposal cost (FD). Waste reduction or elimination in
production can dramatically reduce the need for waste
treatment and disposal.18 The invention and adoption
of economically and technically effective waste treatment and disposal technologies will also help to reduce
FW and FD.
As a rule of thumb, any process that can generate a
maximum yield with minimum processing cost is economically attractive. The improvement in the product
quality and yield may have a positive effect on the
production cost (FP), resource requirement (I), and waste
generation. Hence, there should be an attempt to
minimize the overall investment while improving the
product yield or its quality. Another approach is to
reduce the cost on waste treatment, waste disposal, and
recycle or to convert waste to useful merchandise. These
can also help to reduce environmental pressure and thus
improve sustainability.
(b) Index of Environmental Performance (IEvP).
IEvP quantifies the environmental load from a plant.
Because the majority of industrial activities consume
renewable resources and discharge waste into the
environment, a high environmental load is frequent.
IEvP is defined as the ratio of the sum of the emergy of
nonrenewable (N) resources and the waste released into
the environment with or without waste treatment (WUW
+ WTW) to the total emergy of renewable resources used
in the plant and the recycle streams for both renewable
as well as nonrenewable resources (WRR + WRN) in the
plant. That is
IEvP ) (N + WUW + WTW)/(R + WRR + WRN)
To make the plant environmentally sustainable, IEvP
should be kept as low as possible. Any plant relying
heavily on nonrenewable resources is not environmentally friendly. The environmental performance of a plant
is further degraded if it releases a large amount of waste
into the environment. The following strategies are
applicable for a plant to reduce IEvP.
(i) To reduce the consumption of nonrenewable resources (N) and to replace them by appropriate renewable resources (R). This can lessen the stress exerted
by the plant on the environment. Nevertheless, the
increased consumption of renewable resources in the
form of raw material and energy may require some
process or product modification.
(ii) To minimize the total amount of waste discharge
to the environment (WUW + WTW) and to improve the
recycle and reuse of waste generated by the plant (WRR
+ WRN). The waste discharge from the plant can be
reduced through improvement of plant design, operation, and control. An efficient recycling and reuse
mechanism within the plant and industrial ecosystem
can also effectively reduce waste discharge. If the
recycled resource is nonrenewable, it can further reduce
the consumption of fresh nonrenewable resources. This
again helps to reduce the environmental pressure.
Clearly, the increased utilization of renewable resources
(R) and improved recycling or reuse of waste (YW) can
reduce IEvP.
(c) Index of Sustainable Performance (ISP). ISP
is defined as the ratio of IEcP and IEvP, i.e.,
ISP ) IEcP/IEvP ) [(YP + YW)/(FP + FW + FR +
FD + I)]/[(N + WUW + WTW)/(R + WRR + WRN)]
Thus, a higher value of ISP indicates a higher sustainability of the plant. To improve the sustainability, all
factors involved in IEcP and IEvP must be analyzed
carefully. For example, the net waste discharge can be
reduced by generating market opportunities for the
waste, adopting efficient recycle networks, and implementing effective and affordable waste treatment technologies. These will reduce the disposal cost (FD). On
the other hand, recycle of waste (WRR + WNR) can reduce
the environmental stress, i.e., IEvP. However, if the
waste treatment cost (FW) or recycle cost (FR) is too high,
it may decrease the value of IEcP. Thus, the influence
of these factors on sustainability may be complicated
because of the combined effects on IEcP and IEvP. ISP
emphasizes implementing the optimal strategy to improve the sustainability of the industry and eventually
the entire industrial ecosystem. It seeks the balance
between economic and environmental aspects of the
plant by providing proper considerations to the production, waste, and cost factors. Conclusively, the sustainability of a plant can be enhanced in the following ways.
(i) To improve the product yield (YP) with no considerable increment in production cost (FP).20
(ii) To improve the yield from the waste (YW) with
minimum waste treatment cost (FW).
(iii) To replace the nonrenewable resources (N) with
renewable resources (R), while keeping the production
cost (FP) at the minimum level and maintaining product
quality.
(iv) To reduce waste generation in production (W),
hence reducing the waste treatment cost (FW) and
eventually the waste disposal cost (FD).
(v) To reduce waste release into the environment
(WUW + WTW) so that the waste disposal cost (FD) can
be minimized.
(vi) To employ efficient recycle networks (WRR + WRN)
for reducing the consumption of fresh resources with
minimum recycle cost (FR).
The relationships among the variables in the definition equations for IEcP, IEvP, and ISP indicate possible
approaches to improve sustainability of industrial systems. Some of the relationships are already well accepted but have not been validated using sustainability
theory yet. The effectiveness of the three indices is
justified by those well-accepted practices.
Case Studies
An industrial system of two plants is selected for
evaluating the newly introduced emergy-based indices.
Three operational modes between the two plants are
considered. In case 1, plants 1 and 2 are completely
isolated from each other. Neither do they recycle waste
internally nor do they sell it to the external market.
Case 2 introduces internal waste recycle in each of the
two plants. Case 3 demonstrates the concept of an
industrial ecosystem where plants 1 and 2 have internal
Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4237
Figure 3. Case 1: isolated plants without internal recycle and waste reuse.
Table 1. Resource and Product Price for Plants 1 and 2
plant 1 ($/ton)
R
N
YP
FTD
FUD
YTW
YUW
30
45
200
10
15
60 (to plant 2, YTW1)
40 (to market, YUW1)
plant 2 ($/ton)
Table 3. Coefficients for Waste Streams in Plant 2
waste stream
50
80
400
30
40
40 (to plant 1, YUW2A)
50 (to market, YUW2B)
WU2
WT2
WTW2
WTRN2
WTRR2
WUW2
YUW2
YUW2A (to plant 1)
YUW2B (to market)
coefficient
b11
b12
b21
b22
b23
b31
b32
b41
b42
case 1
case 2
case 3
0.3
0.7
1
0
0
1
0
0
0
0.3
0.7
0.2
0.4
0.4
1
0
0
0
0.5
0.5
0.2
0.4
0.4
0.2
0.8
0.5
0.5
Table 2. Coefficients for Waste Streams in Plant 1
waste stream
coefficient
case 1
case 2
case 3
WU1
WT1
WTW1
WTRN1
YTW1 (to plant 2)
WUW1
WURR1
YUW1 (to market)
a11
a12
a21
a22
a23
a31
a32
a33
0.2
0.8
1
0
0
1
0
0
0.2
0.8
0.5
0.5
0
0.7
0.3
0
0.2
0.8
0.2
0.5
0.3
0.1
0.5
0.4
recycles by themselves as well as material/energy
exchanges between them.
For all of these cases, a mathematical model for each
plant is listed in the appendix. The models contain
various coefficients to signify material flow ratios when
process and waste streams are split. Figures 3-5 depict
three cases, where the coefficients (aij or bij) are listed.
The price of the resources and products for the two
plants are given in Table 1. Note that the cost for
handing and transporting the marketable waste (YUW
or YTW) is included in their price. Tables 2 and 3 list
the coefficient values for each case of plants 1 and 2,
respectively.
Case 1: Isolated Plants without Internal Recycle and Waste Reuse. As shown in Figure 3, case 1
represents a scenario of two isolated plants with neither
internal recycle nor external utilization of the waste.
The untreated waste streams (WUW1 and WUW2) and the
treated waste streams (WTW1 and WTW2) are discharged
into the environment. In this case, the coefficients (a21,
Table 4. Performance Indices of Plant 1 in Three
Different Cases
performance index
profit ($/year)
IEcP
IEvP
ISP
case 1
9.591 ×
1.499
2.833
0.529
106
case 2
1.496 ×
2.082
0.828
2.513
107
case 3
2.099 × 107
2.715
0.359
7.567
a31, b21, and b31) of the streams, WTW1, WUW1, WTW2, and
WUW2, are all equal to 1. This indicates that all of the
waste generated during production is discharged into
the environment.
For plant 1, 80% of its total waste needs to be treated
before entering the environment. For plant 2, this
amounts to 70% of the total waste. It is clear that the
waste treatment cost affects the profit of both plants
and the waste discharge exerts a high pressure on the
environment. In addition to waste treatment cost, the
plants have to pay for waste disposal, the cost of which
is proportional to the amount of waste being disposed
of. The disposal cost causes a considerabe reduction of
the overall profit and hence the IEcP value for both
plants.
Another factor that affects the profit and economic
performance of both plants is the amount of fresh
resources being consumed. The absence of any mechanism for recycling useful material from the waste
streams results in excessive consumption of fresh
resources that are either from market or the environment. As shown in Tables 4 and 5, both plants have
4238 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004
Figure 4. Case 2: isolated plants with internal recycle.
Table 5. Performance Indices of Plant 2 in Three
Different Cases
performance index
case 1
case 2
case 3
profit ($/year)
IEcP
IEvP
ISP
2.342 × 107
1.641
2.347
0.699
3.718 × 107
2.630
0.874
3.009
4.303 × 107
2.972
0.603
4.925
maintained good profit ($9.591 × 106/year for plant 1
and $2.342 × 106/year for plant 2) and reasonable values
of IEcP (1.499 for plant 1 and 1.641 for plant 2). Their
environmental performance based on IEvP is 2.833 for
plant 1 and 2.347 for plant 2. This is very poor, which
eventually affects the overall performance, as depicted
in the their ISP values (0.529 for plant 1 and 0.699 for
plant 2).
In this case, the plants are able to maintain their
economic performance, which is a prerequisite for their
survival, but they have ignored the environmental
pressure and thus the sustainability issue. High values
of IEvP of these two plants show their inability to
recognize the opportunities of waste recovery and
recycle. On the other hand, the attempts in the direction of reducing IEvP through reducing waste discharge or recycling may eventually increase IEcP of the
plant. However, the extent to which the sustainability
performance can be improved also depends on the costs
related to waste treatment, recycle, and reuse. Further
investigation is needed in order to evaluate the impacts
of each of these factors on the sustainability performance.
Case 2: Isolated Plants with Internal Recycle.
As shown in Figure 4, the treated and untreated waste
streams are now split into recyclable and disposable
waste streams. There are two types of recycling
streams: WRR as the recycle of renewable resources and
WRN as the recycle of nonrenewable resources. In plant
1, part of the treated waste can be recycled as a
nonrenewable resource (WTRN1) and part of the untreated waste can be recycled as a renewable resource
(WURR1). In plant 2, only treated waste is recycled. Part
of it belongs to a renewable resource (WTRR2), while part
of it belongs to a nonrenewable resource (WTRN2).
In plant 1, 50% of the treated waste is recycled
(a22 ) 0.5), which saves an equal amount of fresh
nonrenewable resources. Thus, the disposal of treated
waste in plant 1 is now reduced to 50% of its value in
case 1 (a21 is reduced from 1 in case 1 to 0.5 in case 2).
Besides, plant 1 recycles 30% of its total untreated
waste in the form of renewable resources (a32 ) 0.3),
which also helps to save fresh resources and release
environmental pressure. In plant 2, a total of 40% of
its treated waste stream is recycled as nonrenewable material (b22 ) 0.4), another 40% are recycled
as renewable material (b23 ) 0.4), and the remaining 20% are disposed of into the environment (b21 ) 0.2).
Tables 2 and 3 list the coefficients of waste streams
for plants 1 and 2 in case 2, respectively (see column 4
of each table). With recycle, the economic and environmental performances of plants 1 and 2 improve significantly, as reflected in the values of the
performance indices listed in column 3 of Tables 4
and 5.
The change in the IEcP value is mainly due to the
following reasons: a reduction in the investment for
fresh resources and a reduction in the investment for
waste disposal/treatment. However, now there is an
additional cost factor, i.e., recycle cost, that adds to the
total investment of the plant. This is the reason the
improvement in IEcP is not significant as compared to
that in IEvP.
The environmental pressure of both plants is reduced
considerably as compared to case 1 because of the
following reasons: (i) a reduction of the consumption
of fresh nonrenewable resources, (ii) a reduction of the
net amount of waste disposal, and (iii) a recycle of
nonrenewable and renewable resources.
Finally, a higher IEcP and a lower IEvP contribute
to the higher sustainability performance of both plants.
As shown in Table 4, ISP of plant 1 is increased by 375%
(from 0.529 in case 1 to 2.513 in case 2). Table 5 shows
that ISP of plant 2 is increased by 330% (from 0.699 in
case 1 to 3.009 in case 2).
Case 3: Plants with Internal Recycle and External Exchange. In this case, plants 1 and 2 explore
Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4239
Figure 5. Case 3: plants with internal recycle and external exchange.
the option of finding market opportunities for their
waste. A plant may not be able to fully recycle and reuse
all of the waste it generates because of material and
energy specification, quality purpose, capacity limit,
and/or cost issues. However, there is a possibility that
other systems may utilize their waste. Under such
circumstances, it will be reasonable for these plants to
hunt for potential customers who will buy their waste
material and waste energy. This can improve the
sustainability through improved economic performance
and reduce the environmental pressure by reducing the
total waste discharged into the environment.
Figure 5 depicts the case of an industrial ecosystem
where plants 1 and 2 explore the option of selling their
residual waste either to each other or to the market.
This helps reduce the consumption of fresh resources.
The treated and untreated waste streams from plant 1
are split into three streams: disposable, recyclable, and
marketable streams. The marketable streams of the
treated or untreated waste for plant 1 are denoted as
YTW1 and YUW1, respectively, and their corresponding
coefficients are a23 and a33.
In this case, plant 1 sells 30% of its total treated waste
to plant 2 (a23 ) 0.3). Consequently, the net disposal of
treated waste from plant 1 is reduced to only 20% of
that in case 1 (a21 ) 0.2). In addition, plant 1 sells 40%
of its untreated waste as a renewable resource to the
market (a33 ) 0.4). Thus, the net disposal of untreated
waste is reduced to 10% of that in case 1 (a31 ) 0.1).
On the other hand, plant 2 sells its untreated waste
in the form of nonrenewable resources to plant 1 as well
as other entities in the market. The new marketable
untreated waste stream is denoted as YUW2 and has the
coefficient of b32. This stream is further divided into two
substreams: one to plant 1 and the other to the market.
The coefficients for these subdivided streams are b41 and
b42, respectively.
Because of the new demand for its untreated waste,
plant 2 can increase the production of untreated waste
(b11 is increased from 0.3 in both cases 1 and 2 to 0.5 in
case 3) and directly sell the majority of it to the market
without treatment (b32 ) 0.8). In this case, the waste
disposal for plant 2 is reduced to 20% of the total waste
(b31 ) 0.2 and b21 ) 0.2). For the marketable waste from
plant 2, plant 1 buys 50% (b41 ) 0.5) and other plants
buy the remaining 50% (b42 ) 0.5).
Tables 4 and 5 give the index values for plants 1 and
2, respectively (see the last column of each table). The
three indices, especially IEcP and ISP, are improved in
case 3 as compared to the first two cases. The improvement in the economic (IEcP) and environmental (IEvP)
performances of the plant is the result of (i) a realization
of the market opportunities for the waste, (ii) a reduced
investment in fresh resources, and (iii) a reduced waste
disposal cost. The improvement in the environmental
performance mainly is attributed to the following reasons: (i) a replacement of fresh nonrenewable/renewable
resources by the waste exported from other plants and
(ii) a further reduction in the net discharge of the waste
into the environment.
Discussion. The case studies illustrated how to use
these three indices to quantitatively analyze the sustainability of industrial systems. Under the conditions
given in the case studies, the external recycle option
(case 3) gives the best sustainability performance,
followed by the one with internal recycle only (case 2),
while the case without external or internal recycle (case
1) gives the worst performance. However, under a
different scenario, for example, the recycle cost for waste
is higher than the waste treatment cost, the comparison
4240 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004
result may be different. Hence, the optimal strategy for
maximizing sustainability depends on the various cost
factors involved, and a general solution applicable to all
cases cannot be arrived at.
Money Value of the Streams or Investments
CFP1 ) 25 + 8.68X10.7
CFW1 ) 2.5 + MWTW10.6
Concluding Remarks
CFRR1 ) 25MWRR10.5
Through systematic consideration of the unique features of industrial systems, the newly developed indices,
i.e., IEcP, IEvP, and ISP, can improve significantly the
applicability of existing emergy-based sustainability
indices. The new indices can be employed to effectively
evaluate the economic, environmental, and eventually
sustainability status of an industry or industrial ecosystem. This advancement brings deep insight into the
sustainability issue of industrial systems and provides
directions for the improvement of sustainability. Although the case study problem is relatively simple, the
indices are, in principle, applicable to an industrial
system of any complexity.
CFRN1 ) 25MWRN10.5
CFUD1 ) 15MWUW1
CFTD1 ) 10MWTW1
CR1 ) 30MR1
CN1 ) 45MN1
CNP1 ) 40MYUW2A
Acknowledgment
This work is in part supported by NSF under Grant
DMI-0225844 and Gulf Coast Hazardous Substance
Research Center.
Appendix: Mathematical Models for Plants
Profit
P ) 200MYP1 + 60MYTW1 + 40MYUW1 - 25 8.68 - X10.7 - 2.5 - MWTW10.6 - 30MR1 45MN1 - 40MYUW2A - 25MWRR10.5 - 25MWRN10.5 10MWTW1 - 15MWUW1
Mathematical Models for Plant 1.
Production and Waste Streams
MYP1 ) 0.8X1
MW1 ) MWU1 + MWT1
MWT1 ) MWTW1 + MWTRN1 + MYTW1
MWU1 ) MWUW1 + MWURR1 + MYUW1
MW1 ) 1.4X1
MWU1 ) a11MW1
MWT1 ) a12MW1
MWTW1 ) a21MWT1
MWTRN1 ) a22MWT1
MYTW1 ) a23MWT1
MWUW1 ) a31MWU1
MWURR1 ) a32MWU1
MYUW1 ) a33MWU1
MWRR1 ) MWURR1
MWRN1 ) MWTRN1
Constraints for the Coefficients of Waste Streams
Index of Economic Performance
IEcP ) (200 × MYP1 + 60 × MYTW1 + 40 × MYUW1)/
0.6
+ 30 × MR1 + 45 ×
(25 + 8.68 × X0.7 + 2.5 + MW
TW1
0.5
0.5
MN1 + 40 × MYUW2A + 25 × MW
+ 25 × MW
+
RR1
RN1
10 × MWTW1 + 15 × MWUW1)
Index of Environmental Performance
IEvP ) (45MN1 + 10MWTW1 + 15MWUW1)/(30MR1 +
40MYUW2A + 45MWRN1 + 30MWRR1)
Index of Sustainability Performance
ISP ) IEcP/IEvP
Mathematical Model for Plant 2.
Production and Waste Streams
MYP2 ) 0.75X2
MW2 ) MWU2 + MWT2
MYT2 ) MWTW2 + MWTRN2 + MWTRR2
MWU2 ) MYUW2 + MWUW2
MYUW2 ) MYUW2A + MYUW2B
a11 + a12 ) 1
MWRR2 ) MWTRR2
a21 + a22 + a23 ) 1
MWRN2 ) MWTRN2
a31 + a32 + a33 ) 1
MW2 ) 1.35X2
Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4241
MWU2 ) b11MW2
MWT2 ) b12MW2
MWTW2 ) b21MWT2
MWTRN2 ) b22MWT2
MWTRN2 ) b23MWT2
MWUW2 ) b31MWU2
MYUW2 ) b32MWU2
MYUW2A ) b41MYUW2
MYUW2B ) b42MYUW2
Constraints for the Coefficients of Waste Streams
Index of Environmental Performance
IEvP ) (80MN2 + 40MWUW2 + 30MWTW2)/(50MR2 +
60MYTW1 + 80MWRN2 + 50MWRR2)
Index of Sustainability Performance
ISP ) IEcP/IEvP
In the models, X1 is the amount of resources being
processed in plant 1 (tons/year), X2 is the amount of
resources being processed in plant 2 (tons/year), M is
the amount of the stream (tons/year), and C is the
money value of the streams or investment ($/year).
In the calculation of the index values, the emergy
value of each term is calculated based on its money
value divided by the money-to-emergy ratio, ξ ) 1.75 ×
1012 SeJ/$.10 Because ξ appears in both the numerator
and denominator, eventually ξ is eliminated from the
indices.
b11 + b12 ) 1
b21 + b22 + b23 ) 1
b31 + b32 ) 1
b41 + b42 ) 1
Investment
CFP2 ) 30 + 1.68X20.7
CFW2 ) 3 + MWT20.6
CFRR2 ) 10MWRR20.5
CFRN2 ) 12MWRN20.5
CFUD2 ) 40MWUW2
CFTD2 ) 30MWTW2
CR2 ) 50MR2
CN2 ) 80MN2
CNP1 ) 60MYTW1
Profit
P ) 400MYP2 + 50MYUW2A + 50MYUW2B 30 - 1.68X20.7 - 3 - MWTW20.6 - 50MR2 80MN2 - 60MYTW1 - 10MWRR20.5 - 12MWRN20.5 40MWUW2 - 30MWTW2
Index of Economic Performance
IEcP ) (400MYP2 + 50MYUW2A + 50MYUW2B)/
(30 + 1.68Y0.7 + 3 + MWTW20.6 + 50MR2 +
80MN2 + 60MYTW1 + 10MWRR20.5 + 12MWRN20.5 +
40MWUW2 + 30MWTW2)
Nomenclature
FD ) total disposal cost
FP ) production cost
FR ) total cost for recycle
FRN ) cost for recycle of nonrenewable resources from
waste
FRR ) cost for recycle of renewable resources from waste
FTD ) cost of disposing treated waste into the environment
FUD ) cost of disposing untreated waste into the environment
FW ) waste treatment cost
I ) total resources consumed
N ) nonrenewable resources
R ) renewable resources
W ) total waste generated during production
WR ) total recycled waste
WRN ) waste recycled as nonrenewable resources
WRR ) waste recycled as renewable resources
WT ) waste that should be treated
WTRN ) nonrenewable resource recycle from treated waste
WTRR ) renewable resource recycle from treated waste
WTW ) net discharge of treated waste into the environment
WU ) waste that can be disposed without treatment
WURN ) nonrenewable resource recycle from untreated
waste
WURR ) renewable resource recycle from untreated waste
WUW ) net discharge of untreated waste into the environment
Ww ) total waste discharge
YP ) product yield
YTW ) yield from treated waste
YUW ) yield from untreated waste
YW ) total marketable waste
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Received for review January 6, 2004
Revised manuscript received March 24, 2004
Accepted May 3, 2004
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