Eco-Design of a Wastewater Treatment System Based on Process

Article pubs.acs.org/IECR

Eco-Design of a Wastewater Treatment System Based on Process Integration Donghee Park,† Dae Sung Lee,*,‡ and Seong-Rin Lim*,§ †

Department of Environmental Engineering, Yonsei University, Wonju 220-710, South Korea Department of Environmental Engineering, Kyungpook National University, Daegu, 702-701, South Korea § Department of Environmental Engineering, College of Engineering, Kangwon National University, Chuncheon, Gangwon, 200-701, South Korea ‡

S Supporting Information *

ABSTRACT: Because almost all industrial plants have wastewater treatment systems, eco-design of the systems is an effective way to reduce environmental impacts and economic costs of industry sectors. The eco-design using biobjective optimization has, however, a limitation due to the subjective weighting on the two objectives. The objective of this study is to eco-design existing wastewater treatment systems based on process integration by converting biobjective to single objective problems. For the mathematical optimization model, an objective function is formulated by monetizing environmental impacts to external costs and summing the external and economic costs. Mass balances and constraints are formulated to reflect the superstructure model and real situations. Two case studies are performed to verify the developed model. The eco-design outcomes are compared to their respective economic cost- and environmental impact-minimized designs. This comparison shows that the developed model optimizes the trade-offs between the biobjectives. This study can be applied to reduce environmental impacts and economic costs of other process systems. an increase in other ones in other life cycle stages.11 Therefore, eco-design should be employed to optimize the trade-offs among all impacts and costs in all life cycle stages. Although eco-design frequently employs biobjective optimization to reduce environmental impacts and economic costs,12,13 this approach has a limitation due to the decision makers’ subjective weighting on each objective in the objective function. Depending on the preference of decision makers, different optimum solutions can be obtained from a biobjective optimization problem. As an approach to overcome this limitation, it should be mentioned that, if environmental impacts and economic costs are in a unidimensional unit, this enables free trade-offs between the two objectives and thus generates a single optimum solution, irrespective of who decision makers are. Therefore, eco-design needs to seek a way to monetize environmental impacts to economic costs. The objective of this study is to eco-design existing wastewater treatment systems based on process integration by monetizing environmental impacts to economic costs in biobjective optimization problems. For the eco-designs, a mathematical optimization model is developed to integrate distributed and terminal wastewater treatment systems in an industrial plant and to simultaneously minimize environmental impacts and economic costs. Environmental impacts are monetized by using the EPS 2000 methodology.12 The model is applied for two case studies. In each case study, an eco-design outcome is compared to two other design alternatives (environmental impact-minimized and economic cost-minimized designs) to examine the characteristics

1. INTRODUCTION Because wastewater treatment systems consume considerable amounts of electricity and chemicals, the systems are one of the principal targets to be focused on to reduce economic costs in industrial plants. For economical design of the systems, many studies have been performed to integrate wastewater treatment systems distributed in industrial plants.1−5 This integration approach is to minimize the flow rate of the wastewater treated in distributed systems and to maximize the flow rate of the wastewater bypassing the distributed systems. This can reduce capital investment and operating costs for wastewater treatment because these costs are proportional to the flow rate of the wastewater treated in distributed systems.1,2,6 Therefore, the integration is a cost-effective strategy to comprehensively restructure existing wastewater treatment systems. In addition to the economic aspects of wastewater treatment systems, their environmental impacts need to be reduced to increase the sustainability of industrial plants. Since the systems directly and indirectly incur environmental impacts,6,7 the systems should be optimized and improved to proactively prevent environmental pollution. For instance, since electricity and chemicals used in the systems are produced by generating greenhouse gases and air pollutants in power plants, the systems need to be designed and operated to reduce the electricity and chemicals. Therefore, existing wastewater treatment systems should be restructured to enhance environmental and economic performance at the same time. Eco-design can be used to improve environmental and economic performance of existing wastewater treatment systems.8,9 In general, eco-design takes into account life cycle stages and supply chains associated with systems in reducing environmental impacts and economic costs.10 This is because a design can lead to a decrease in an impact/cost in a stage but simultaneously to © 2013 American Chemical Society

Received: Revised: Accepted: Published: 2379

July 17, 2012 October 30, 2012 January 11, 2013 January 11, 2013 dx.doi.org/10.1021/ie3018934 | Ind. Eng. Chem. Res. 2013, 52, 2379−2388

Industrial & Engineering Chemistry Research

Article

Figure 1. Superstructure model used to integrate distributed and terminal wastewater treatment systems (WW: wastewater, DS: distributed system, TS: terminal system).

2.3. Mathematical Optimization Model. Based on the superstructure model, a mixed integer nonlinear programming (MINLP) model was developed by formulating environmental impacts and economic costs. This model includes (i) binary variables to formulate whether a stream exists or whether a wastewater treatment system is operated and (ii) bilinear variables to formulate the mass balances based on the superstructure model. 2.3.1. Monetization of Environmental Impacts. This monetization was performed by using the EPS 2000 methodology12 to evaluate external costs of principal contributors. The EPS methodology is based on the willingness-to-pay method to valuate environmental impacts in a monetary value (i.e., EUR).15,16 This is generally used in environmental economics. The monetization was performed by using the Gabi 4.0 software17 in order to evaluate the unit external cost for piping, electricity consumption, and pipe recycling. In this study, the principal contributors are (i) piping in the construction stage, (ii) electricity for pumping and for wastewater treatment, and maintenance and repair (M&R) in the operation and maintenance (O&M) stage, and (iii) pipe scrap recycling in the disposal stage. The external costs are not discounted to present values to take into account future generations.18 2.3.2. Economic Costs. The economic costs consist of (i) pipe material cost, labor cost, construction expense, overhead, and profit in the construction stage, (ii) electricity consumption for pumping and for wastewater treatment, labor, and M&R in the O&M stage, and (iii) decommissioning cost, construction expense, overhead, and profit in the disposal stage. These costs are discounted to present values by using interest and escalation rates. 2.3.3. Formulation. For the eco-design, the objective function of the mathematical model is formulated as the sum of the external costs and economic costs of the integrated wastewater treatment system. Mass balances are formulated based on the superstructure model: overall integrated system, wastewater source splitters; and mixers and splitters of distributed and terminal systems. Also, constraints are formulated to take into account real situations in industrial plants: flow rates and loads of distributed and terminal systems, flow rates of streams, and concentrations of the discharge quality in terminal systems.

of the eco-design. The developed model can be widely used in other industrial plants to obtain a single optimum solution for a generally acceptable eco-design.

2. METHODS The eco-designs were performed with existing distributed and terminal wastewater treatment systems in an industrial plant (for case study I, five distributed and one terminal systems; and for case study II, seven distributed and one terminal systems). The systems were integrated based on a mathematical optimization model that was developed by using a superstructure model and by monetizing environmental impacts to economic costs, so-called external costs.13−16 The solution to the developed mathematical model was schematically embodied into an eco-design outcome. This ecodesign was compared to the designs generated by minimizing environmental impacts (hereinafter, this design is called “envidesign”) and economic costs (hereinafter, “cost-design”), respectively. 2.1. Data. Data used for two case studies are presented in the Supporting Information (Tables S1−S6): characteristics of wastewater sources; design and operational conditions of distributed and terminal systems; and distance matrix between the wastewater sources and sinks. The raw wastewaters are primarily treated in the distributed systems and then secondarily in the terminal system. These systems consist of physical and chemical treatment processes (i.e., coagulation, flocculation, and sedimentation). The required discharge quality of the terminal system was set at 20 mg/L for CODcr, 5 mg/L for SS, and 8 mg/L for F−: it is assumed that the treated wastewater meeting these requirements has negligible environmental impact potential. 2.2. Superstructure Model. Figure 1 shows the superstructure model used to integrate the distributed and terminal systems. This model takes into account all possible interconnections:5,6 from wastewater sources to the inlets of distributed and terminal systems; and from the outlets of distributed systems to their inlets and to the inlets of terminal systems. The mixer nodes imply the combination of all possible streams into a stream, and the splitter nodes imply the division of a given stream into all possible streams. 2380

dx.doi.org/10.1021/ie3018934 | Ind. Eng. Chem. Res. 2013, 52, 2379−2388


Article

Figure 2. Configurations of the three designs for caste study I: (a) envi-design; (b) cost-design; and (c) eco-design (WW: wastewater, DS: distributed system, TS: terminal system).

Minimize ECO = ENVI + COST

2.3.4. Mathematical Optimization Models for Comparative Alternatives. For the envi-design and cost-design, the objective functions were formulated with the total external cost and economic cost of the integrated system, respectively. 2.4. MINLP Solver. GAMS19 is used to obtain the optimal solutions to the three models: CPLEX for linear programming, MINOS for nonlinear programming (NLP), and DICOPT for MINLP (note that these NLP and MINLP solvers do not always guarantee global optimum solutions). The optimum solutions are embodied to the schematic configurations of the three designs, respectively.

(1)

ENVI is the sum of external costs incurred in the stages of construction, O&M, and disposal. t

ENVI = EXCcon +

∑ EXCOt &M + EXCdis t=1

(2)

For the construction stage, external cost is derived from piping, which is estimated with a regression equation from the relationship between the cross-sectional area of the pipe and the unit external cost of piping. The cross-sectional area is determined by an optimum velocity, which varies depending on the flow rate.20

3. MATHEMATICAL OPTIMIZATION MODEL FOR THE ECO-DESIGN 3.1. Objective Function. The objective function for the ecodesign is to minimize ECO, which is the sum of the total external cost (hereinafter called “ENVI”) and total economic cost (hereinafter called “COST”) of the integrated wastewater treatment system.

EXCcon =

∑ ∑ EPi ,j i

j

EPi , j = (apeAi , j + bpeli , jBi , j )lijBij 2381

(3) (4)


Industrial & Engineering Chemistry Research Ai , j =

Article

Fi , j vi , j

(5)

vi , j = aopFi , j + bop

(6)

Table 1. Quantitative Characteristics of the Envi-Design, Cost-Design, and Eco-Design for Case Study I item pipe distributed systems

For the O&M stage, external costs are incurred from electricity consumption for pumping and for wastewater treatment, and M&R. t EXCOt & M = EEpt + EEwt + EMRC t

terminal systems power requirement

(7)

The external cost of electricity consumption for pumping is estimated by calculating pumping power based on the Darcy− Weisbach equation.21 EEpt = (∑ ∑ Pi , j)UEetop i

Pi , j =

ρFi , jg (HLi , j + Ha) 1 ηpηm 1000

HLi , j = f

life cycle

(9)

li , j ⎛ π ⎞ vi , j ⎜ ⎟ Bi , j g ⎜⎝ Ai , j ⎟⎠ 4 0.5

=

piping electricity for pumping during a year electricity for wastewater treatment during a year maintenance & repair (M&R) during a year subtotal during service time (15 years) disposal pipe recycling total during the life cycle

(10)

∑ PsysBsysUEetop

costdesign

ecodesign

m m3/h

1,650 2 146.0

750 2 85.2

1,410 2 144.4

m3/h

178.9

178.9

178.9

kW kW

19.6 2.7

24.0 2.7

19.8 2.7

envidesign

cost-design

ecodesign

1,848 43,872

953 53,605

1,638 44,221

13,403

13,403

13,403

55

29

49

859,950

1,005,540

865,094

2,693 864,491

1,389 1,007,882

2,387 869,119

(11)

sys

The external cost of M&R is assumed to be proportional to that of the piping in the construction stage. EMRC t = δEXCcon

equation from the relationship between the cross-sectional area of the pipe and the unit economic costs.

(12)

TDPpiping =

TDLpiping =

EDi , j = (adeAi , j + bde)li , jBi , j

t

∑ t=1

COt & M(1 + e)t (1 + i)t

j

DLi , j = (adlAi , j + bdl)li , jBi , j

(18)

(19) (20)

The construction expenses are assumed to be proportional to the sum of the material and labor costs.

(14)

For economic costs, COST is the sum of discounted economic costs in the stages of construction, O&M, and disposal. ⎛ COST = ⎜⎜Ccon + ⎝

(17)

∑ ∑ DLi ,j i

(13)

j

j

DPi , j = (adpAi , j + bdp)li , jBi , j

∑ ∑ EDi ,j i

∑ ∑ DPi ,j i

For the disposal stage, the external cost is estimated based on the recycling of pipelines, which is calculated by using a regression equation from the relationship between the crosssectional area of the pipeline and the unit external cost of the recycling. EXCdis =

item

construction operations & maintenance (O&M)

2

The external cost of electricity consumption for wastewater treatment is estimated based on the power requirement of agitators, flocculators, and sludge scrapers in the distributed and terminal systems. t EEwt

envidesign

Table 2. External Costs Due to Environmental Impacts from the Envi-Design, Cost-Design, and Eco-Design for Case Study I (Unit: EUR in Thousands)

(8)

j

length number (in use) flow rate of bypassing wastewater flow rate of treated wastewater electricity (pumping) electricity (wastewater treatment)

unit

EXPpiping = α(TDPpiping + TDLpiping )

(21)

The overheads are assumed to be proportional to the sum of the material and labor costs, and the construction expenses.

C t (1 + e)t ⎞ ⎟⎟CCF + dis (1 + i)t ⎠

OHpiping = β(TDPpiping + TDLpiping + EXPpiping )

(15)

(22)

The profits are assumed to be proportional to the sum of the labor cost, construction expenses, and overheads.

The construction cost consists of pipe material costs, labor costs, construction expenses, and the contractor’s overheads and profits.

PROpiping = γ(TDLpiping + EXPpiping + OHpiping )

Ccon = TDPpiping + TDLpiping + EXPpiping + OHpiping

(23)

For the O&M stage, the economic costs of electricity consumption and labor are calculated based on their unit economic costs. The M&R cost is assumed to be proportional to the construction cost for piping.

+ PROpiping

(16) The pipe material and labor costs are estimated by using their unit economic costs, which are calculated by using the regression

t COt & M = ECpt + ECwt + LC t + MRC t

2382

(24)



Article

Table 3. Economic Costs of the Envi-Design, Cost-Design, and Eco-Design for Case Study I (Unit: EUR in Thousands) life cycle stage construction

operations & maintenance (O&M)

disposal

item

envi-design

cost-design

eco-design

piping construction expenses overhead contractor’s profit subtotal electricity for pumping during a year electricity for wastewater treatment during a year labor during a year maintenance & repair (M&R) during a year subtotal during service time (15 years, discounted) pipe decommissioning construction expenses overhead contractor’s profit subtotal (discounted)

38,065 7,613 2,284 4,102 52,065 9,319 2,847 426,650 1,562 5,404,115 12,451 2,490 747 1,569 11,706 5,467,886

21,296 4,259 1,278 2,267 29,100 11,386 2,847 426,650 873 5,421,031 6,853 1,371 411 863 6,442 5,456,573

34,593 6,919 2,076 3,714 47,301 9,393 2,847 426,650 1,419 5,403,272 11,257 2,251 675 1,418 10,583 5,461,156

total

ECpt =

i t ECwt =

(25)

j

∑ PsysBsysUCetop (26)

sys

t

LC =

Table 4. Total Economic and External Costs of the Envi-Design, Cost-Design, and Eco-Design for Caste Study I (Unit: EUR in Thousands)

∑ ∑ Pi ,jUCetop

∑ UCl ,sysBsys (27)

sys

MRC t = δCcon

cost-design

eco-design

construction operations & maintenance (O&M) disposal total

53,913 6,264,065 14,399 6,332,378

30,053 6,426,570 7,831 6,464,455

48,939 6,268,366 12,970 6,330,275

For the contaminant mass balances of the distributed and terminal systems FC i c , i(1 − R c , sys) − FjCc , j = 0

(29)

(37)

For the constraints of flow rates and loads on the distributed and terminal systems

The pipe decommissioning cost is assumed to be proportional to the labor cost for piping. TDLdecom = εTDLpiping

envi-design

(28)

For the disposal stage, the economic cost consists of pipe decommissioning costs, expenses, overheads, and profits. t Cdis = TDLdecom + EXPdecom + OHdecom + PROdecom

life cycle stage

max 0 ≤ Fsys ≤ Fsys Bsys

(30)

FsysCc , sys −

The expenses, overheads, and profits in the disposal stage are calculated as in the construction stage.

max Lsys

≤0

(38) (39)

For the constraints of flow rates on all the streams

EXPdecom = αTDLdecom

(31)

Fi , j − Fimax , j Bi , j ≤ 0

(40)

OHdecom = β(TDLdecom + EXPdecom)

(32)

Fi , j − Fimin , j Bi , j ≥ 0

(41)

PROdecom = γ(TDLdecom + EXPdecom + OHdecom)

(33)

For the constraint of concentrations on the discharge quality of the terminal system required to meet the discharge limits of environmental regulations

The pipe decommissioning cost for the existing system is not taken into account in the formulation because all existing pipes are assumed to be at end-of-life: the cost is the same for the three designs and thus does not affect the total cost of a new integrated system. 3.2. Mass Balances. For the mass balances of the splitters Fspl =

Cc , tsout − Ccmax , tsout ≤ 0

The eco-design was performed by using eqs 1−42. For the comparison, the envi-design and cost-design were generated by using eqs 2−14 and 34−42 and eqs 5−6 and 15−42, respectively.

∑ Fspl ,j

4. RESULTS AND DISCUSSION 4.1. Case Study I. Figure 2 shows the configurations of the envi-design, cost-design, and eco-design. The three designs have similar configurations. For instance, in all the designs, the three distributed systems (i.e., DSs 2, 4, and 5) were not used because the other distributed and terminal systems (i.e., DSs 1 and 3, and TS) can treat all the wastewaters generated in the plant by fully utilizing their design capacities. This is because all the

(34)

j

For the mass balances of the mixers Fmix =

∑ Fi ,mix i

FmixCc , mix =

∑ Fi ,mixCc ,i i

(42)

(35)

(36) 2383



Article

Figure 3. Configurations of the three designs for caste study II: (a) envi-design; (b) cost-design; and (c) eco-design (WW: wastewater, DS: distributed system, TS: terminal system).

later. It is noted that the eco-design seems to be generated by combining the characteristics of the envi-design and cost-design (see Table 1).

objective functions for the three designs have common targets (for instance, piping and electricity consumption) in reducing environmental impacts and/or economic costs, as discussed 2384



Article

The eco-design exhibits a little higher total external cost than the envi-design but significantly lower than the cost-design (see Table 2). Since the envi-design was generated by minimizing environmental impacts, the envi-design leads to the lowest total external cost. It is interesting that the values of each external cost for the eco-design and the envi-design are in similar ranges, even though the eco-design was intended to minimize environmental impacts and economic costs at the same time. For the contributors to external costs, the eco-design exhibits lower external costs from electricity consumption for pumping than the costdesign but higher external costs from piping, pipe recycling, and M&R. It is noted that the cost difference for the pumping outweighed that for the piping, pipe recycling, and M&R; thus, the eco-design has lower external cost than the cost-design. It is noted that, because the electricity consumption for pumping is

the most significant contributor to the total external costs, the envi-design and eco-design seem to be focused on reducing pumping power with the first priority. This can be supported by the similar configurations of the eco-design and envi-design. For economic cost as well, the eco-design lies between the envi-design and cost-design. Table 3 shows the economic costs of the three designs. Like the results for external costs, economic costs of the eco-design and envi-design are in similar ranges. The eco-design optimization was performed by reducing external costs rather than economic costs. The cost-design has the lowest total economic cost. The cost-design exhibits higher O&M cost than the eco-design and envi-design but lower construction and disposal costs. The cost difference for the construction and disposal is greater than for the O&M. It should be noted that, although the principal contributor to economic cost is the costs for labor and electricity consumption in the O&M stage, the highest contributors affecting the extent order of the total economic costs are pipe-related costs in the construction and disposal stages. This is because in this study three designs use the same distributed and terminal systems, respectively. The eco-design exhibits the lowest total external and economic cost due to the optimization of the trade-off between external and economic costs in the life cycle stages. Table 4 shows the total external and economic costs of the three designs. In the construction and disposal stages, the eco-design exhibits lower economic and external costs than the envi-design but higher than the cost-design. In contrast, in the O&M stage the eco-design has higher total economic and external cost than the envi-design but lower than the cost-design. Due to this trade-off, the eco-design is the most environmentally and economically friendly. These results demonstrated that the developed mathematical model

Table 5. Quantitative Characteristics of the Envi-Design, Cost-Design, and Eco-Design for Case Study II item

unit

envidesign

costdesign

ecodesign

length number (in use) flow rate of bypassing wastewater flow rate of treated wastewater electricity (pumping) electricity (wastewater treatment)

m m3/h

89,570 7 148.6

59,310 5 144.2

66,790 6 147.7

m3/h

464.5

464.5

464.5

kW

1093.5

1331.1

1177.7

kW

17.6

16.4

17.0

pipe distributed systems

terminal systems power requirement

Table 6. External Costs Due to Environmental Impacts from the Envi-Design, Cost-Design, and Eco-Design for Case Study II (Unit: EUR in Thousands) life cycle

item

envi-design

cost-design

eco-design

construction operations & maintenance (O&M)

piping electricity for pumping during a year electricity for wastewater treatment during a year maintenance & repair (M&R) during a year subtotal during service time (15 years) pipe recycling

93,418 2,442,770 39,315 2,803 37,273,319 136,138 37,502,875

25,685 2,973,452 36,634 771 45,162,855 37,437 45,225,977

29,727 2,630,719 37,974 892 40,043,775 43,327 40,116,829

disposal total during the life cycle

Table 7. Economic Costs of the Envi-Design, Cost-Design, and Eco-Design for Case Study II (Unit: EUR in Thousands) life cycle stage construction

operations & maintenance (O&M)

disposal

item

envi-design

cost-design

eco-design

piping construction expenses overhead contractor’s profit subtotal electricity for pumping during a year electricity for wastewater treatment during a year labor during a year maintenance & repair (M&R) during a year subtotal during service time (15 years, discounted) pipe decommissioning construction expenses overhead contractor’s profit subtotal (discounted)

1,967,343 393,469 118,041 211,292 2,690,145 518,887 8,351 826,663 80,704 17,604,853 640,565 127,113 38,434 80,711 602,228 20,897,226

595,289 119,058 35,717 63,031 813,095 631,613 7,782 666,664 24,393 16,326,722 190,212 38,042 11,413 23,967 178,828 17,318,645

676,297 135,259 40,578 71,799 923,933 558,810 8,066 746,664 27,718 16,459,328 216,862 43,372 13,012 27,325 300,571 17,683,832

total 2385



■

optimized the trade-offs between economic and external costs to simultaneously reduce the environmental impacts and economic costs of the integrated system. In addition, it is noted that the envi-design is more environmentally and economically friendly than the cost-design. 4.2. Case Study II. Although the configurations of the three designs for case study II look different due primarily to the number of distributed systems (see Figure 3), overall, case study II showed almost the same consequences as case study I. For pipe length and pump power as principal contributors to economic and external costs, respectively, the eco-design was between the envi-design and cost-design (see Table 5). Thus, the eco-design had the lowest economic and external costs (detailed results are presented in Tables 6, 7, and 8). Consequently, the developed

life cycle stage

envi-design

cost-design

eco-design

2,783,563 54,878,172

838,780 61,489,577

953,660 56,503,103

738,366 58,400,101

216,265 62,544,622

343,898 57,800,661

ASSOCIATED CONTENT

S Supporting Information *

Additional calculations and tables as noted in the text. This material is available free of charge via the Internet at http://pubs. acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-33-250-6358. Fax: +82-33-254-6357. E-mail: srlim@ kangwon.ac.kr (S.-R.L.). Tel: +82-53-950-7286. Fax: +82-53950-6579. E-mail: [email protected] (D.S.L.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (2011-0008373) and by 2011 Research Grant from Kangwon National University. This work was also supported by the Priority Research Centers Program through the NRF grant funded by MEST (2010-0028301).

Table 8. Total Economic and External Costs of the Envi-Design, Cost-Design, and Eco-Design for Case Study II (Unit: EUR in Thousands) construction operations & maintenance (O&M) disposal total

Article

■

NOMENCLATURE

Sets

mathematical model was verified again. In addition, it is noted that the cost-design utilizes the five distributed systems excluding DSs 4 and 5 to minimize labor cost significantly affecting the total of economic cost, while the eco-design utilizes the six distributed systems excluding DS 2, and the envi-design utilizes all the systems. 4.3. Implications from the Two Case Studies. Eco-design for integrated wastewater treatment systems should be focused on optimizing the flow rates of streams by pump in order to minimize the total of economic and external costs. Lower flow rates lead to a decrease in external cost from pumping because of a decrease in head loss (eqs 5, 6, 9, and 10) but to an increase in pipe-related cost because of an increase in the number (i.e., length) of pipelines (eqs 5 and 6); and vice versa. In the two case studies, the eco-designs were generated by optimizing the flow rates of streams, whereas the cost-designs and envi-designs were generated by increasing and decreasing flow rates, respectively. Next to flow rates, labor cost significantly affects the eco-design of integrated wastewater treatment systems due to its high contribution to economic cost.

C = {c|c is a contaminant in wastewater}, c = 1, 2, ..., Nc WW = {ww|ww is a wastewater source}, ww = 1, 2, ..., Nm SYS = {sys|sys is a distributed or terminal system}, sys = 1, 2, ..., Nn and 1, 2, ..., Nk SPL = {spl|spl is a splitter}, spl = 1, 2, ..., Ns MIX = {mix|mix is a mixer}, mix = 1, 2, ..., Nt

5. CONCLUSIONS Existing wastewater treatment systems were eco-designed based on process integration. For these eco-designs, a mathematical optimization model was developed by monetizing environmental impacts to external costs and thus converting biobjective to single objective problems. The developed model optimized the trade-offs between environmental impacts and economic costs of the integrated wastewater treatment systems. Thus, this ecodesign strategy can be used to eliminate the limitation from subjective weighting on environmental impacts and economic costs in biobjective optimization and to generate a single design outcome, regardless of who decision makers are. Since industrial plants have been trying to reduce freshwater and wastewater by applying water reuse and recycling, the developed model can be widely used to eco-design existing wastewater treatment systems in various industrial plants.

ECCcon ECCtO&M ECCtdis ECtp ECtwt EXPdecom EXPpiping EDi,j

Variables

Ai,j Bi,j Bsys Cc,i Cc,j Cc,mix Cc,sys Cc,tsout DLi,j DPi,j

EEtp EEtwt EXCcon EXCtO&M EXCdis EMRCt 2386

cross-sectional area of a pipe from i to j binary variable for the existence of a pipe from i to j binary variable for the existence of a distributed or terminal system concentration of a stream i concentration of a stream j concentration at the outlet of a mixer concentration at the inlet of a distributed or terminal system concentration at the outlet of a terminal system labor cost for piping from i to j (i.e., WW to DS; WW to TS; DS to DS; and DS to TS) pipe material cost for piping from i to j (i.e., WW to DS; WW to TS; DS to DS; and DS to TS) economic cost in the construction stage economic cost in the O&M stage economic cost in the disposal stage electricity cost for pumping electricity cost for wastewater treatment construction expenses for pipe decommissioning construction expenses for piping external cost of pipe recycling from i to j (i.e., WW to DS; WW to TS; DS to DS; and DS to TS) external cost of electricity consumption for pumping external cost of electricity consumption for wastewater treatment external cost in the construction stage external cost in the O&M stage external cost in the disposal stage external cost of M&R dx.doi.org/10.1021/ie3018934 | Ind. Eng. Chem. Res. 2013, 52, 2379−2388

Industrial & Engineering Chemistry Research EPi,j Fi,j Fi,mix Fmix Fspl Fspl,j Fsys HLi,j COST LCt MRCt OHdecom OHpiping Pi,j Psys PROdecom PROpiping TDPpiping TDLdecom TDLpiping TEC vi,j

Article

li,j

external cost of piping from i to j (i.e., WW to DS; WW to TS; DS to DS; and DS to TS) flow rate from i to j (i.e., WW to DS; WW to TS; DS to DS; and DS to TS) flow rate from i to a mixer flow rate from a mixer flow rate to a splitter flow rate from a splitter to j flow rate at the inlet of a distributed or terminal system head loss through a pipe from i to j (i.e., WW to DS; WW to TS; and DS to DS) life cycle cost labor cost M&R cost contractor’s overhead for pipe decommissioning contractor’s overhead for piping power requirement for pumping wastewater from i to j (i.e., WW to DS; WW to TS; and DS to DS) power requirement for wastewater treatment in a distributed or terminal system contractor’s profits for pipe decommissioning contractor’s profits for piping total direct pipe material cost total direct labor cost for pipe decommissioning total direct labor cost for piping total external cost during the life cycle optimum velocity in a pipe from i to j (i.e., WW to DS; WW to TS; and DS to DS)

Lmax sys ρ Rc,sys t top UCe UCl,sys UE

■

pipe length from i to j (i.e., WW to DS; WW to TS; DS to DS; and DS to TS) maximum contaminant load of a distributed or terminal system density of wastewater removal efficiency of a distributed or terminal system service lifetime (15 year) operating hours per annum (8,760 h) unit economic cost of electricity (0.065 USD per kWh) unit economic cost of the labor to operate a distributed or terminal system (96,000 USD and 320,000 USD per year, respectively) unit external cost of electricity consumption (0.255 EUR per kWh)

REFERENCES

(1) Wang, Y.-P.; Smith, R. Design of distributed effluent treatment systems. Chem. Eng. Sci. 1994, 49 (18), 3127. (2) Galan, B.; Grossmann, I. E. Optimal design of distributed wastewater treatment networks. Ind. Eng. Chem. Res. 1998, 37 (10), 4036. (3) Hernandez-Suarez, R.; Castellanos-Fernandez, J.; Zamora, J. M. Superstructure decomposition and parametric optimization approach for the synthesis of distributed wastewater treatment networks. Ind. Eng. Chem. Res. 2004, 43 (9), 2175−2191. (4) Dzhygyrey, I.; Jezowski, J.; Kvitka, O.; Statyukha, G. Distributed wastewater treatment network design with detailed models of processes. Comput.-Aided Chem. Eng. 2009, 26, 853−858. (5) Lim, S. R.; Lee, H.; Park, J. M. Life cycle cost minimization of a total wastewater treatment network system. Ind. Eng. Chem. Res. 2009, 48 (6), 2965−2971. (6) Lim, S.-R.; Park, D.; Park, J. M. Environmental and economic feasibility study of a total wastewater treatment network system. J. Environ. Manage. 2008, 88 (3), 564−575. (7) Lim, S. R.; Park, J. M. Environmental impact minimization of a total wastewater treatment network system from a life cycle perspective. J. Environ. Manage. 2009, 90 (3), 1454−1462. (8) Erol, P.; Thoming, J. ECO-design of reuse and recycling networks by multi-objective optimization. J. Cleaner Prod. 2005, 13 (15), 1492− 1503. (9) Donnelly, K.; Beckett-Furnell, Z.; Traeger, S.; Okrasinski, T.; Holman, S. Eco-design implemented through a product-based environmental management system. J. Cleaner Prod. 2006, 14 (15−16), 1357− 1367. (10) Spitzley, D. V.; Grande, D. E.; Keoleian, G. A.; Kim, H. C. Life cycle optimization of ownership costs and emissions reduction in US vehicle retirement decisions. Transp. Res. Part D-Transp. Environ. 2005, 10 (2), 161−175. (11) Swarr, T. E. Life cycle management and life cycle thinking: Putting a price on sustainability. Int. J. Life Cycle Assess. 2006, 11 (4), 217−218. (12) Steen, B. A Systematic Approach to Environmental Priority Strategies in Product Development (EPS). Version 2000 - Models and Data of the Default Method; Center for Environmental Assessment of Products and Material Systems (CPM): Stockholm, 1999. (13) Matthews, H. S.; Lave, L. B. Applications of environmental valuation for determining externality costs. Environ. Sci. Technol. 2000, 34 (8), 1390−1395. (14) Gibbons, E.; O’Mahony, M. External cost internalisation of urban transport: A case study of Dublin. J. Environ. Manage. 2002, 64 (4), 401− 410. (15) Rey-Martinez, F. J.; Velasco-Gomez, E.; Martin-Gil, J.; Gracia, L. M. N.; Navarro, S. H. Life cycle analysis of a thermal solar installation at a rural house in Valladolid (Spain). Environ. Eng. Sci. 2008, 25 (5), 713− 723. (16) Swanstrom, L.; Reiss, H.; Troitsky, O. Y. Environmental balances of thermal superinsulations. Int. J. Thermophys. 2007, 28 (5), 1653− 1667.

Parameters

ade, bde regression parameter for an external cost of pipe recycling (ade = 96.955, bde = 1.3418) adl, bdl regression parameter for a labor cost for piping (adl = 2106.3, bdl = 16.326) adp, bdp regression parameter for a pipe material cost (adp = 714.15, bdp = 2.9053) aop, bop regression parameter for an optimum velocity (aop = 0.0297, bop = 0.6173) ape, bpe regression parameter for an external cost of piping (ape = 66.457, bpe = 0.921) α coefficient for construction expenses (0.2) β coefficient for contractor’s overhead (0.05) CCF currency conversion factor (0.8 for USD-to-EUR) Cmax maximum concentration at the outlet of a terminal c,tsout system Cc,ww concentration of a wastewater source γ coefficient for a contractor’s profits (0.1) δ coefficient for the external and economic costs of M&R (0.3) e escalation rate (0.03) ε coefficient for the total direct labor cost of pipe decommissioning (0.4) f friction factor (0.02) Fmax maximum flow rate from i to j (i.e., WW to DS; WW to i,j TS; DS to DS; and DS to TS) Fmax maximum flow rate at the inlet of a distributed or sys terminal system Fmin minimum flow rate from i to j (i.e., WW to DS; WW to i,j TS; DS to DS; and DS to TS) g acceleration of gravity (9.8 m/sec2) Ha additional head for the elevation of systems (10 m H2O) i interest rate (0.057) ηm motor efficiency (0.85) ηp pump efficiency (0.6) 2387



Article

(17) IKP PE Europe GMBH. Gabi 4.0 Software; 2004. (18) Lim, S.-R.; Park, J. M. Environmental indicators for communication of life cycle impact assessment results and their applications. J. Environ. Manage. 2009, 90 (11), 3305−3312. (19) GAMS Development Corporation. GAMS, A User Guide; Washington, DC, 2005. (20) Lim, S. R.; Park, J. M. Synthesis of an environmentally friendly water network system. Ind. Eng. Chem. Res. 2008, 47 (6), 1988−1994. (21) McGhee, T. J. Water Supply and Sewerage; McGraw-Hill: New York, 1991.

2388


Eco-Design of a Wastewater Treatment System Based on Process

Recommend Documents