Naoufel Cheikhrouhou
email: naoufel.cheikhrouhou@hesge.ch

Taha Vafaeenezhad

Reza Tavakkoli-Moghaddam

Multi-objective mathematical modeling for sustainable supply chain management in the paper industry

Keywords: Supply chain management, Multi-objective programming model, Chemical process, Sustainability, ε-constraint method

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Introduction

The main objective of a supply chain master planning (SCMP) problem is to determine purchase, production and distribution quantities for facilities in a supply chain. Whereas for many years, minimizing the total cost or maximizing the revenue was the major aim of supply chains, responsibility for the environmental and social impacts of SC processes and products, the safety and health of their employees and the entire community are still lacking attention. Moreover, some governments have enacted legislations in favor of the environment, such as regulations for greenhouse gases (GHGs) reduction in European Union, Australia and Canada [START_REF] Devika | Designing a sustainable closed-loop supply chain network based on triple bottom line approach: A comparison of metaheuristics hybridization techniques[END_REF].

Sustainable SCM (SSCM) is defined as considering the environmental and social impacts of SC operations as well as its economic performance in management of information, material and capital flow [START_REF] Seuring | From a literature review to a conceptual framework for sustainable supply chain management[END_REF]. A multi-objective optimization problem (MOP) includes optimizing objective functions in a situation that there is more than one objective function while typically conflicting objective functions. MOPs arise in a variety of real word applications and the need for efficient and reliable methods is increasing [START_REF] Mavrotas | Effective implementation of the ε-constraint method in multi-objective mathematical programming problems[END_REF]. There are differences between single objective and multi-objective optimization, the most difference is that for multi-objective optimization, no single optimal solution exists; however, a set of equally good alternative solutions exist with various trade-offs, known as Paretooptimal solutions. These solutions are feasible solutions that cannot be enhanced in just one objective function without worsening their performance in at least one of the rest. In the absence of any other information, none of these solutions can be considered as better than the others. The decision maker usually needs to provide additional preference information to identify the most preferred solution (Mavrotas et al. 2013).

There is an actual necessity to develop a more appropriate and detailed comprehending of sustainability pointedly in relation to supply chains while there is an obviously scientific identification of the need to unify economic, environmental and social sustainability, given the extensive essence of these fields [START_REF] Ashby | Making connections: a review of supply chain management and sustainability literature[END_REF]. For considering all the economic, environmental and social dimensions simultaneously and in an integrated way, mathematical models enable the effective application of sustainable development principles to SC components [START_REF] Boukherroub | An integrated approach for sustainable supply chain planning[END_REF].

According to the current considerations, this paper firstly addresses the issue of multiechelon, multi-commodity, multi-period SCMP including suppliers, plant and warehouses. Secondly, dimensions of SSCM combine the economic objective of the model with environmental and social impacts that has been extracted from the gaps among the previous studies. Finally, an augmented ε-constraint (AUGMECON2) method is applied to a multiobjective linear programming (MOLP) model that optimizes supply chain operations, environmental and social impacts. The model including six objective functions is solved by the Augmecon method, which finds a set of solutions (i.e., Pareto solutions) authorizing the decision maker to opt the one following the best her/his priorities with respect to environmental, economic and social performances.

This paper is organized as follows. The related previous studies are reviewed in Section 2. In Section 3, an integrated mathematical model is presented leading to the application of the proposed paradigm. Section 4 introduces a case study. Section 5 presents our solution methodology, and Section 6 represents initial results and experiments. Finally, we present the main conclusion and future research in Section 7.

Literature review

Considering the works addressing multi-objective models in this section, sustainability is the most common issue for adding to the basic economic models [START_REF] Brandenburg | Quantitative models for sustainable supply chain management: Developments and directions[END_REF]. A few papers, add non-economic objectives (i.e., minimizing transportation time or lead time) or SC risks (e.g., financial risks). In the context of the sustainable supply chain, environmental impacts have been addressed in several papers and in different fields, such as the food industry (Validi et al. 2014a[START_REF] Validi | A solution method for A two-layer sustainable supply chain distribution model[END_REF]and Soyal et al. 2014). However, social impacts have rarely used instead. In evaluating environmental impacts, GHG emissions have been considered in the most studies and then the life cycle assessment (LCA) metric, such as [START_REF] Chaabane | Design of sustainable supply chains under the emission trading scheme[END_REF]. [START_REF] Luthra | Green Supply Chain Management -Review.pdf[END_REF] provided an outline of the several subjects related to Green Supply Chain Management (GSCM). They pointed out six key themes of GSCM in their study, but not mentioned the operations research (OR) models. [START_REF] Awudu | Uncertainties and sustainability concepts in biofuel supply chain management: A review[END_REF] mentioned different issues for modeling uncertainties and sustainable concepts in biofuel SCM. They discussed the evolution of biofuels, the three types of decision-making levels and uncertainties and how to model uncertainties with sustainable concepts simultaneously. However, they did not address the optimization models.

To model environmental decision making in the reviewed papers, there are four main ways, namely LCA, GHG emissions, quantity of waste generated, and energy consumption. The LCA is a metric for evaluating environmental impacts in all of the components of a supply chain from raw materials to a final product. Executing a methodology (e.g., LCA) is not possible ever for a lot of reasons. Environmental concerns notify in companies slowly, therefore evaluating just a subdivision of environmental factors may be considered as a middle stage on the way complete integration. When gaining environmental data and modeling the entire supply chain is too though, environmental factors could be evaluated partially [START_REF] Luthra | Green Supply Chain Management -Review.pdf[END_REF]. The other three metrics are suitable for considering the partial assessment of environmental impacts. Table 1 shows the assessing metrics for environmental impacts found in the reference papers.

{Please insert Table 1 about here.} But, the social metrics of sustainability has been considered rarely in the reviewed papers as a part of mathematical models. [START_REF] Boukherroub | An integrated approach for sustainable supply chain planning[END_REF] considered "local development" as the main social impact in their mathematical model, whereas the concept of "Work Conditions" has been used by [START_REF] Saffari | Multi-objective robust optimization model for social responsible closed-loop supply chain solved by nondominated sorting genetic algorithm[END_REF], [START_REF] You | Optimal design of sustainable cellulosic biofuel supply chains: Multiobjective optimization coupled with life cycle assessment and input -output analysis[END_REF] and [START_REF] Yue | Sustainable design and operation of cellulosic bioelectricity supply chain networks with life cycle economic, environmental, and social optimization[END_REF]. [START_REF] Boukherroub | An integrated approach for sustainable supply chain planning[END_REF] proposed an integrated approach for applying sustainable development axioms to supply chain planning models. They used GHG emissions as environmental metrics, local employment and employment stability as social metrics. They used a weighted goal programming method to solve their mathematical model and finally mentioned involving OR decision-making tools for the future studies. [START_REF] Devika | Designing a sustainable closed-loop supply chain network based on triple bottom line approach: A comparison of metaheuristics hybridization techniques[END_REF] considered the three aspects of sustainability in the network design problem and developed a mixed-integer programming (MIP) model for the problem. They assumed environmental impacts, environmental benefits of using EOL products and harm caused by products as environmental metrics, fixed and variable job opportunities and work's damage as social metrics. However, they do not address a real case study as a field for application.

A tactical supply chain planning model presented by [START_REF] Fahimnia | A tradeoff model for green supply chain planning: A leanness-versus-greenness analysis[END_REF]. This model is suitable for inquiring trade-offs between costs and environmental diminishing made up with energy consumption, carbon emissions and waste generation. The proposed model also combines other features of supply chains in real world, such as lot sizing, multiple transportations and flexible holding volume of warehouses. They supposed the following environmental metrics:

• Carbon pollution in manufacturing, inventory holding and transportation.

• Consumed energy and generated wastes in manufacturing and inventory holding in warehouses and plants. Their suggestions include modeling the problem at the operational level and the investigation of the affection of organizational decisions on sustainability of communities and industries. [START_REF] Sazvar | A biobjective stochastic programming model for a centralized green supply chain with deteriorating products[END_REF] developed a stochastic mathematical model and proposed a new replenishment policy in a centralized supply chain for deteriorating items. The optimal solutions, in terms of types of transportation vehicles to be used, are determined as a tradeoff between environmental impact and financial costs/benefits. [START_REF] Jindal | Multi-objective fuzzy mathematical modelling of closedloop supply chain considering economical and environmental factors[END_REF] designed a multi-objective closed-loop supply chain taking the environmental and economic factors in to account with uncertainty in the parameters. They considered carbon footprints of transportation as the environmental metric and solved the model by an interactive epsilon constraint method. A multi-objective model was presented by Entezaminia et al. (2016), in which they consider as well a closed loop green system to be planned on the basis of a multi-site and multi-product supply chain. Mousazadeh et al. (2014) investigated the design and plan of problems in green and reverse logistics in a fuzzy environment. They considered the total CO2 equivalent emissions as the environmental metric. However, they did not consider social aspects in the sustainability concept and did not combine operational and tactical planning subjects into the present strategic models. Palak et al. (2014) analyzed the affections of carbon regulatory mechanisms on replenishment decisions and utilized mathematical models. For determining emissions as a function of load weight, distance traveled and transportation mode applied, they used relevant methods.

For designing a sustainable medical supply chain network in an uncertainty environment, [START_REF] Pishvaee | An accelerated Benders decomposition algorithm for sustainable supply chain network design under uncertainty: A case study of medical needle and syringe supply chain[END_REF] proposed a multi-objective possibilistic programming model and considered damage to workers' health, consumer risk, created job opportunities and local development as social metrics and life cycle assessment-based methods as environmental metric. However, they did not consider operational and tactical supply chain decisions under a full sustainability approach. Moreover, they did not include the development of social impact assessment methods.

Taking into account forward and reverse flows altogether via supply chain, [START_REF] Saffari | Multi-objective robust optimization model for social responsible closed-loop supply chain solved by nondominated sorting genetic algorithm[END_REF] promoted a supply chain network design model. They used three distinct objectives: total cost, social impacts like recruitment and justice in creating job opportunities and environmental impacts like CO2 emissions. The model looks for optimizing the facility location problem and to determine type of technology, network flows, and volume of manufacturers. Not only the work did not consider multi-period or multi-product related issues in the problem, but there was only one type of transportation mode tackled. They pointed out the need to consider other environmental and social factors such as energy consumption level, local suppliers' priority, industrial centers adjacency, and facilities' distance related to providing new job opportunities. Yu andSolvang (2016a and2016b) addressed the design of a reverse supply chain with a multi-objective modelling approach in the bio-chemical industry. The latter was interesting for some researchers due to the high environmental impact that could lead to efficient trade-off between the decision criteria of the three pillars of sustainability (economic, environmental and societal). [START_REF] You | Optimal design of sustainable cellulosic biofuel supply chains: Multiobjective optimization coupled with life cycle assessment and input -output analysis[END_REF] developed an MOMILP model including all aspects of sustainability. The environmental objective is calculated by the life cycle GHG emissions, economic objective is calculated by the total cost, and the social objective is calculated by the number of increased local jobs. However, the work lacked considering efficient optimization algorithms for analysis of nationwide and considering the time-dependent volume expansion plans.

For designing and operating sustainable bioelectricity supply chain networks, Yue et al. ( 2014) presented a multi-objective optimization model, which considers all three dimensions namely environmental, economic and social factors simultaneously. They considered GHG footprint as an environmental metric and the local job opportunities as a social metric.

As the main conclusion in the context of a sustainable supply chain, it has been deducted that environmental impacts have been mentioned a lot in the published papers; however, social impacts rarely used instead. To discuss about the model types of papers, MOMILP is more used. A few papers have incorporated fuzziness or uncertainties in terms of a specific characteristic of the model types. There are several methods to solve multi-objective models. Such as weighted sum of objectives, epsilon constraint, metaheuristics, goal programming, multi-criteria decision analysis, fuzzy approaches and hybrid methods. For NP-hard conditions, a variety of methods have been created or modified, especially for metaheuristics. The categories of application of the papers and type of their experiments: biomass (biofuel), chemical processes, industrial goods, consumer goods and public sector. The scientific contributions of this paper are as follows:

• Combining all aspects of sustainability on the basis of the triple bottom line principle in a large scale supply chain considering master planning • Extend previous models to multi-period, multi-product and considering inventory costs • Considering more real case constraints according to the novel case study from the paper industry • Supposing a new chemical process application that covers such rareness in applications with respect to the literature review findings • Addressing operational and tactical supply chain planning decisions under the sustainability paradigm.

Problem description and integrated mathematical model

This problem considers in a sustainable supply chain management in all dimensions simultaneously. As mentioned before, sustainable development has three main pillars. By considering environmental and social objectives for each supply chain in addition to economic objective, it would be a sustainable supply chain. In this paper, we consider three environmental objectives, namely GHG emissions, consumed energy and generated wastes. Two social objectives are considered as the total travel distance of employees and total number of hires and lay-offs. For the economic objective, we have to integrate three subset of planning problems, namely (1) choosing suppliers and order lot-sizing. (2) planning the firm's production, and (3) planning the distribution procedure of final products.

Our objective is to look for the best decisions of planning for the following subjects: • Purchasing plan: Determining the quantity of each raw material from each supplier that should be bought in each period.

• Supplying plan: Determining the quantity of each item from each supplier that should be supplied in each period.

• Producing plan: Determining the quantity of each final product that should be produced in each period.

• Distributing plan: Determining the quantity of each final product that should be transferred to each warehouse in each period.

Assumptions

The main model assumptions are listed below:

• Based on the contract, Nashtarood is self-responsible for shipping costs of purchased woods to MWPI. • There isn't any unusable wood between purchased forest woods and imported ones.

• Every unit of woods, pulps and papers are ton.

• Every supplier has a portion of total company supplying constantly.

• The capacity of supplying wood from forests is limited.

• The imported woods are not debarked in wood and chip handling plant.

• The yard on the whole is divided to six sections: unusable, non-standard and standard (Hornbeam, Beech, Mixed species and imported woods).

• PM 2 produces two kinds of fluting papers: scf-0 (30%) and scf-20 (70%).

• The scf-20 one of fluting just needs the recycled pulp.

• There are 5 warehouses in the whole country as distribution centers (i.e., Tehran, Tabriz, Mashhad, Ahvaz and Bandar Abbas).

Parameters and decision variables

Set: i index of supplied forest woods (i = 1, . . .,4) j index of purchased forest woods (j = 1, 2, 3) k index of imported forest woods (k = 1, 2) l index of supplied forest woods species (l = 1, 2, 3) m index of purchased forest woods species (m = 1, . . ., 6) n index of suppliers (n = 1, . . ., 9) p index of distribution centers (p = 1, . . ., 5) q index of final products (q = 1, 2, 3) t index of time periods (t = 1, . . ., 12) r index of regions (r = 1, 2, 3) shipping quantity of final product q to distribution center p in period t amount of employees live in region r and work in company in period t amount of employees live in region r hired by company in period t amount of employees live in region r laid-off by company in period t

u index of fluting paper (u = 1, 2) v index of importing pulp (v = 1,

Proposed mathematical model

Based on the above-mentioned assumptions and descriptions, we develop the following multi-objective linear mathematical model for a multi-echelon, multi-product, multi-period supply chain planning.

Objective functions

The formulation of all six objectives of the model is presented as follows:

Max OF1 ( )

TRS TCO TCP TCD = - + + (1) , , , , , , , , , , , , , , , , Max OF1 [
+ + + + + ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ , , , , , , , , , , , ( ) . .
+ + + ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ (2) , , , , , , Min OF2 .
Objective function OF1 (1) and ( 2) are to maximize the total profits of the company. The total profits of the company include the total selling revenues (TRS) and total cost of logistics.

The total costs of logistics include the total costs of ownership (TCO), total costs of production (TCP) and total costs of distribution (TCD). Objective function OF2 (3) minimizes total GHG emissions of the supply chain in production and transportation phases. Objective function OF3 (4) minimizes total consumed energy all over production stages. Objective function OF4 (5) minimizes total generated wastes throughout production plants. Objective function OF5 (6) minimizes total travel distance of employees with the aim of improving local development. The nearness of employees to firm indicates local employment plus employee welfare, e.g. by decreasing the time needed to spend in personal or public transportation mode. Objective function OF6 (7) in order to raise the employment consistency, minimizes total amount of hires and lay-offs. Objective function OF1 is an economic objective. Objective functions OF2, OF3 and OF4 are environmental objectives. Objective functions OF5 and OF6 are social objectives.

Constraint sets

(I) Inventory level constraints: All relevant balancing constraints of inventory at wood and chip handling plant are illustrates as follows:

, 1 . ij t jmnt ijt ijt ijt iln n t n I l X Y I CM NS -+ + -= + ∑ ∑ (8) 1 . t t t jmnt il t n n n F l X Y F NSF -+ + -= ∑ ∑ (9) , 1 knt k t kt kt n V Z V W -+ -= ∑ (10)
Constraint (8) represents a balancing equation for Hornbeam and Beech forest woods inventory. Constraint (9) represents balancing equation for mixed species and Persian Iron Wood forest wood inventory. Constraint (10) is about firewood inventory of every species and the last, for imported woods inventory.

(II) Yard of handling plant capacity constraints: The yard is limited to three generally sections for storing woods; standard, non-standard and unusable woods. The constraints summarized as follows:

.

.

jmnt ilnt ij n n e X f Y ysc + ≤ ∑ ∑ (11)
, , , ,

.

.

jmnt i l n j i m l n nt g X h Y ync + ≤ ∑ ∑ (12)
.

ilnt n i X yuc ≤ ∑ (13) knt k n Z yic ≤ ∑ (14)
Constraint ( 11) is the yard capacity for standard woods. Constraint ( 12) is the yard capacity for non-standard woods. Constraint (13) is the yard capacity for unusable woods. Constraint ( 14) is the yard capacity for imported forests' woods.

(III) Supplying woods proportion constraints: There are different portions for supplying woods included supply, purchase and import woods from every supplier. The constraints are summarized as follows:

.

ilnt ilnt n n X j X ≤ ∑ (15)
.

n jmnt jmnt n Y j Y ≤ ∑ (16)
.

n knt knt n Z j Z ≤ ∑ (17)
Constraint ( 15) represents the supply portion of the woods. Constraint ( 16) represents the purchase portion of the woods. Constraint (17) represents the import portion of the woods.

(IV) Importing budget constraint: We have a limited budget for importing woods and pulps in MWPI: , .

.

knt n k ci Z k mb ≤ ∑ (18) , .
.

v vt k nt n k v ci Z cip A m b + ≤ ∑ ∑ (19)
Constraint ( 18) is the budget limitation of importing woods and Constraint ( 19) is the budget limitation of importing pulp.

(V) Supply capacity constraint: The supplying capacity of supplied forest woods from each supplier is limited:

ilt n ilnt X ssc ≤ ∑ (20)
Constraint (20) shows the limitation of supplying capacity of each supplier.

(VI) Chipping system capacity constraint: The chipping system capacity is limited: , , .

. ijt ij ijt i j CM qc CM = ∑ (24) , . ijt ij ijt i j NS qc NS = ∑ (25) . ut u ut u FF p FF ≤ ∑ (26)
Constraints ( 24) and ( 25) are mixture constraints. Constraint (26) represents the fluting papers' portions of production.

(IX) Balancing constraints: The balancing constraints in pulp plant are summarized as follows: , ( 1) .

i ijt i kt i j i k pf CM pf W qnp NP + + - = ∑ ∑ ∑ (27) , ( 1
) .

t ut i ijt i i j i k pf NS pf NSF qfp NF + + - = ∑ ∑ ∑ (28)
The balancing constraints in paper machines plant are summarized as follows:

. . . The balancing constraints in packaging plant are summarized as follows:

1 t pt p FP S = ∑ (31) 1 2 t pt p FF S = ∑ (32) 2 3 t pt p FF S = ∑ (33)
The balancing constraints for clearing the situation of some dependent variables are represented below: , , .

ilnt t i l n Wg pgw X = ∑ (34) , , . ilnt t i l n Wd pdw X = ∑ (35) , ,
.( )

t t ijt ijt i j i j Ld pwd CM NS NSF = + + ∑ ∑ (36)
.

t k t k L c p w c W = ∑ (37)
(X) Demand constraint: Demand constraint among different distribution centers and for all products in every month is represented as follows:

q p t q p t S d ≥ (38)
(XI) Social constraints: Total amount of employees laid-off should not transcend the amount of employees working within a period:

i n t 1 r r L F L ab ≤ (39) ,( 1) rt r t LF Lab - ≤ (40)
Total amount of employees lives in region r and work in company at period t:

in t 1 1 1 r r r r L ab L ab L H L F ≤ + - (41)
,( 1)

rt rt rt r t Lab Lab LH LF - ≤ + - (42)
The amount of employees live in region r and work in company at period t should not transcend the total amount of employees that are ready for work from that region:

rt r Lab LabCap ≤ (43)
The amount of employees hired from a region should not transcend the total amount of employees that are ready for work from that region:

rt r LH LabCap ≤ (44)
The amount of employees needed should supply the whole workload within each period:

= + + + + + + ∑ ∑ ∑ ∑ (45)

Case study

Mazandaran pulp and paper industry (MWPI) is the biggest firm producing pulp and paper in the Middle East. This company was founded in 1996 and supplied by the first class machines of paper and pulp. MWPI has the capacity of producing 175000 tons per year. This has two pulping production lines of chemical mechanical pulp (CMP) and neutral sulphite semi-chemical (NSSC) as well as two paper machines in width of 440 cm and 610 cm. MWPI produces newsprint, wood-container printing & writing, and semi-chemical fluting paper and board. It sustains great communication with customers in native and international markets, such as CIS countries, Africa, Middle East, and Far East with a strong focus on innovation and contiguous progress in quality and environment safekeeping.

MWPI is located in north of Iran, which supplied from a predesignated set of suppliers (e.g., West of Haraz, Talar, Tajan, Gorgan, Tonekabon, Nowshahr and Nashtarood). The final products are eventually delivered to different warehouses (i.e., Tehran, Tabriz, Mashhad, Ahvaz and Bandar Abbas) to satisfy their demands. The process of producing paper in the MWPI is simplified in Fig. 1. Woods from forest or some external suppliers store in plant 300. Then, they will be transferred to plant 400 for transforming to pulp and in plant 500 that they will be steamed. In plant 600 the provided pulp will transform to paper roles and will be cut and filled in packages finally in plant 650.

There are some assumptions with exact values due to the case study: • Standard wood measures:

Length: 2 -6 meter, diameter: 10 -80 centimetre • The maximum importing budget is 12 million dollars in each year that 67% of it is for woods.

• Based on historical data, 5% of any supplied forest woods is unusable; 2% grade 1 wood and 3% decayed wood.

• Based on historical data, 6% of supplied forest woods and 1% of purchased forest woods are oversize (longer than 6 meter or thicker than 80 centimetre or knotted woods).

• The imported woods are used by minimum 80% of birch and maximum 20% of aspen in wood and chip handling plant.

• The yard of storing woods is 80000 m 2 and the maximum height of storing woods in yard limited to three meters, because of transporting limitations.

• Maximum space for unusable woods in yard is 2% and for non-standard woods is 10%.

• The maximum input capacity of chippers is 190 cubic meters per hour; it's equivalent to 136 tons per hour.

• The wood and chip handling plant has 96.4% efficiency; 0.6% wastes for debarking system and 3% waste for chipping system. • The chipping system's capacity is 136 tons per hour.

• The maximum input capacity of the CMP process is 310 tons newsprint pulp in one day (12.92 ton per hour).

• The maximum input capacity of the NSSC process is 300 tons newsprint pulp in one day (12.50 ton per hour).

• Maximum 20% Aspen and minimum 80% Birch is mixed in CMP process.

• Based on BOM (Bill of Materials), sequentially 0.7, 0.3, 0.75, 0.25, 0.4, 0.6, 0.3, 0.7 mixture of Hornbeam, Beech, Mixed species and Persian Iron Wood -both log and cuttine -needed for CMP and NSSC processes incomes.

• The efficiency of the CMP and NSSC processes are 85% and 75%, respectively.

• It uses 106 kilograms liquor for per ton of output newsprint pulp.

• It uses 120 kilograms liquor for per ton of output fluting pulp.

• The efficiency of PM1 (Paper machine for producing newsprint paper, and writing and printing paper) is 89% and for PM2 (Paper machine for producing fluting paper) is 117% {Per one unit input in PM2 we'll have 1.17 unit output}.

• Based on BOM, sequentially 0.83 and 0.17 mixtures of CMP pulp and white imported pulp is needed for PM1 and for PM2, 0.95 and 0.05 of NSSC pulp and brown imported pulp is needed.

• The scf-0 one of fluting needs 40 kilogram imported pulp per unit and 30 kilogram recycled pulp per unit in PM 2. • There is a fixed time for production each month. Table 2 shows production time per month in hours. {Please insert Table 2 about here.}

Solution methodology

To solve multi-objective models, several approaches have been proposed in the literature as we have reviewed in prior sections. An improved version of the augmented ε-constraint method (AUGMECON2) is used for the proposed model. The ε-constraint method has a lot of benefits. The ε-constraint method is a generation method that all the efficient solutions of the problem are generated first and afterwards the decision maker is free to select among them, the most preferred one by her/his attitude. In case that decision maker is unavailable or scarcely available and interaction is tough, they are desirable as the decision maker will just participate in the second step. In addition, when none of the possible results has been left undiscovered, it fortifies confidence of decision maker to find the final decision.

The solution methodology as given by [START_REF] Mavrotas | An improved version of the augmented ε-constraint method (AUGMECON2) for finding the exact pareto set in multi-objective integer programming problems[END_REF] is summarized below:

In the original AUGMECON method given by [START_REF] Mavrotas | Effective implementation of the ε-constraint method in multi-objective mathematical programming problems[END_REF] the problem solved is formulated as follows:

1 2 2 3 3 M ax ( ( ) ( )) p p f x ep s S r S r S r + × + + + L s.t.
+ × + × + + × L
This modification is done in order to perform a kind of lexicographic optimization on the rest of the objective functions if there are any alternative optima. For example, with this formulation the solver will find the optimal for f 1 and then it will try to optimize f 2 , then f 3 and so on. With the previous formulation, the sequence of optimizations of f 2 -f p was indifferent, while we now force the sequential optimization of the constrained objective functions (in case of alternative optima).

For each objective function (2, …., p), we calculate the objective function range. Then we divide the range of the k-th objective function to q k equal intervals using (q k -1) intermediate equidistant grid points. Thus we have in total (q k + 1) grid points that are used to vary parametrically the RHS (e k ) of the k-th objective function. The total number of runs becomes (q 2 + 1) × (q 3 + 1) × … × (q p + 1). Let r k be the range of the objective function k (k = 2, …, p). Then the discretization step for this objective function is given by:

k k k step r q =
The RHS of the corresponding constraint in the t-th iteration in the specific objective function will be as follows:

min kt k k e f t step = + ×
where f min k is the minimum from the pay-off table and t is the counter for the specific objective function.

In each iteration, we check the surplus variable that corresponds to the innermost objective function. In this case, it is the objective function with p=2. Then we calculate the bypass coefficient as follows: where int () is the function that returns the integer part of a real number. When the surplus variable S 2 is larger than step 2 , it is implied that in the next iteration the same solution will be obtained with the only difference being the surplus variable which will have the value S 2step 2 . This makes the iteration redundant, and therefore we can bypass it as no new Paretooptimal solution is generated. The bypass coefficient b actually indicates how many consecutive iterations we can bypass.

Experiments and numerical results

The aim of this section is to remark how the mentioned approach will aid the decision maker perceiving and making trade-offs regarding to the three sustainability pillars of the model. According to the previous studies and sections, we design our paper to consider six objective functions simultaneously as a novel approach in the context of sustainable supply chain management:

• Overall supply chain profit • Total generated GHG emissions • Total consumed energy • Total generated wastes • Total travel distance of employees • Total amount of employees would be hired and laid-off Our mathematical model is solved according to the wood and paper industries case presented in Section 3. By purpose of displaying the applicability of the presented model and methods of relevant references, the improved version of the augmented ε-constraint method is coded in the GAMS software to solve the complete case of the problem contained almost 2000 variables and 4000 constraints. GAMS (ver. 24.1.2) using the CPLEX solver is able to solve MILP models. The PC with Intel ® Core TM i7-4702| processor, 2.2 GHz, 6 GB of RAM, and the Windows 8 operating system is utilized as a technical platform.

Table 3 shows the instance solution with overall SC profit in Rials, GHG emissions in Kg, consumed energy in KW-h, generated wastes in unit, traveled distance of employees in Km, and hires and lay-offs of employees in a number of people. {Please insert Table 3 about here.} Fig. 2 shows the Pareto solutions in various iterations. The overall SC profit has a tendency to increase; however, the other objective functions have mixed tendencies, either to increase or decrease. As a result, it depends on the decision maker's attitude to select the best result in his/her point of view. In Fig. 2, the overall SC profit is multiplied by 10 -8 , consumed energy is multiplied by 10 -11 , generated wastes are multiplied by 10 -3 and travel distance of employees is multiplied by 10 -2 . Table 4 illustrates the corresponding results to Fig. 2 as well.

Based on the detailed results, the decision maker has a variety of choices for choosing the best solution. All the values of objective functions should be considered simultaneously whether they are maximum or minimum. For example, in iteration 18, the overall profit of the supply chain is 18135767600000 with 31474.921 kg of GHG emissions generated, 2132480000000 MW-h energy used, 14407958.10 unit wastes generated, 1672948.85 km travel distance by employees and 10597 number of hires and lay-offs of employees totally. {Please insert Fig. 2 about here.} {Please insert Table 4 about here.} In addition, Fig. 3(a,b,c) and 4(a, b, c) depict the values of the objective functions of Pareto solutions three by three. As a consequence, it can be realized that the results of operating diverse objective functions are not inevitably stable, and hence the objective functions should be taken in to account apart. Fig. 3(a) shows surface plot of three objective functions namely, consumed energy vs generated GHG emissions and SC profit. Fig. 3(b) shows surface plot of three objective functions namely, generated wastes vs generated GHG emissions and SC profit. Fig. 3(c) shows surface plot of three objective functions namely, hires and lay-offs vs travel distance and SC profit. Fig. 4(a) shows a contour plot of three objective functions namely, consumed energy vs generated GHG emissions and SC profit.

Conclusions

An integrated approach for applying sustainable development axioms to supply chain planning models has been proposed in this paper. Contrary to increasing interest in mathematical modeling of sustainable supply chains, comprehensive approaches considering the application of the three pillars (i.e., economy, environment and society) are very rare yet. Based on our detailed literature review, we created an approach that combines sustainability criteria with the supply chain decisions in the context of supply chain management and supply chain master planning. By linking this approach to mathematical programming, the environmental, economic and social impacts could all be simultaneously combined. Indeed, for a multi-product, multi-echelon, multi-period supply chain planning, we formulated a multi-objective linear programming model. Additionally, we considered and solved the wood and paper industry case by using an improved version of the augmented ε-constraint method (AUGMECON2). We showed by analyzing the results, how the Pareto solutions could be used by the decision maker to aid her/him in selecting the supply chain plans that collates best her/him aims and traits in terms of environmental, economic and social performances. In our future work, we aim to deepen our research on finding and defining new social factors that have significant roles in supply chains. Hires & Lay-offs Tables Table 1. Metrics used for the assessment of environmental impacts. Article LCA GHG emissions

  {Please insert Fig. 1 about here.} Plant 300: Wood and Chip Handling Plant Plant 400: Pulp Plant Plant 500: Steam Plant Plant 600: Paper Machine Plant Plant 650: Packaging Plant

  e 2 , e 3 , …, e p are the parameters for the RHS for the specific iteration drawn from the grid points of the objective functions 2, 3, …, p. The parameters r 2 , r 3 , …, r p are the ranges of the respective objective functions. S 2 , S 3 , …, S p are the surplus variables of the

  Fig. 4(b) shows a contour plot of three objective functions namely, generated wastes vs generated GHG emissions and SC profit. Fig. 4(c) shows a contour plot of three objective functions namely, hires and lay-offs vs travel distance and SC profit. {Please insert Fig. 3 about here.} {Please insert Fig. 4 about here.}

FiguresFig. 1 .Fig

 1 Figures

  EPH environmental impact of producing one unit output of chips & handling plant EPP environmental impact of producing one unit output of pulp plant EPR environmental impact of producing one unit output of paper machine plant EPT environmental impact of transportation one unit final product to the distribution centers PRH processing time of producing one unit output of chips & handling plant PRP processing time of producing one unit output of pulp plant PRM processing time of producing one unit output of paper machine plant ENH energy use per hour for producing one unit output of chips & handling plant ENP energy use per hour for producing one unit output of pulp plant ENM energy use per hour for producing one unit output of paper machine plant WAH mean generated waste for producing one unit output of chips & handling plant WAP mean generated waste for producing one unit output of pulp plant WAM mean generated waste for producing one unit output of paper machine plant distance between region r and company primary amount of employees live in region r and work in company in period t cap t maximum number of hours that an employee could work during period t amount of employees ready for work in region r in period t

	cin	unit inventory holding cost
	rsn q	unit price of product q
	rwg	unit price of grade 1 woods
	rwd	unit price of decayed woods
	rdl	unit price of debarking and chipping losses (supplied and purchased woods)
	rcl	unit price of chipping losses (only imported woods)
	l	portion of usable woods
	e	portion of standard supplied woods
	f	portion of standard purchased woods
	g	portion of non-standard supplied woods
	h	portion of non-standard purchased woods
	i	portion of unusable supplied woods
	j n	portion of supply/purchase/import woods from supplier n
	ysc ij	yard capacity for standard wood i of wood species j
	yic k	yard capacity for importing wood k
	ync	yard capacity for non-standard woods
	yuc	yard capacity for unusable woods
	k	portion of importing budget for wood
	mb	monthly budget for importing
	ssc ilt Decision variables: supplying capacity of every supplier for forest wood i of wood species l in period t
	cch X ilnt	chipping system capacity per hour supplying quantity of forest wood i of wood species l from supplier n in period
	mp t	production time per month t t
	qw k Y jmnt	required quality of k imported wood chips in the CMP process purchasing quantity of forest wood j of wood species m from supplier n in
	qc ij	required quality of i forest wood chips of j species in the CMP and NSSC period t
	Z knt	processes importing quantity of forest wood k from supplier n in period t
	pf i I ijt	portion of forest wood i chips usage in the CMP and NSSC processes ending inventory level of forest wood i of wood species j in period t
	qnp F t	quantity of chips required to produce one unit newsprint pulp ending inventory level of firewood in period t
	qfp V kt cmc CM ijt	quantity of chips required to produce one unit fluting pulp ending inventory level of imported forest wood k in period t 2) CMP process capacity per hour chips quantity of forest wood i of wood species j for the CMP process in period
	nsc Parameters: NSSC process capacity per hour t cs il pnp portion of newsprint pulp usage in PM1 NS ijt chips quantity of forest wood i of wood species j for NSSC process in period t unit cutting cost of forest wood i of wood species l cp jm pfp portion of fluting pulp usage in PM2 NSF t chips quantity of firewood for the NSSC process in period t unit purchasing cost of forest wood j wood species m ci pwp portion of white imported pulp usage in PM1 W kt chips quantity of imported forest wood k for the CMP process in period t unit importing cost of forest woods css ln pbp portion of brown imported pulp usage in PM2 A vt importing quantity of pulp v using for paper machine in period t unit shipping cost of supplied forests' woods of species wood l from supplier n cps mn qnsp quality of PM1 inputs to produce one unit newsprint paper Wg t quantity of grade 1 woods in period t unit shipping cost of purchased forests' woods of species wood m from supplier n cis n qflp quality of PM2 inputs to produce one unit fluting paper Wd t quantity of decayed woods in period t unit shipping cost of imported forests' woods from supplier n cso cip v unit importing cost of pulp v Ld t quantity of debarking losses in period t unit slicing cost of oversize woods (l > 2 or d > 80 or knotted woods) cdc p u portion of producing fluting paper u in PM2 Lc t quantity of chipping losses in period t unit debarking and chipping cost of woods cch d qpt demand of final product q at the distribution center p in period t NP t pulp quantity of newsprint in period t unit chipping cost of woods cpn pgw portion of grade 1 woods NF ut pulp quantity of fluting u pulp in period t unit producing newsprint pulp cost cpf u pdw portion of decayed woods FP t paper quantity of newsprint in period t unit producing fluting pulp u cost cpa pwd portion waste of debarking and chipping system FF ut paper quantity of fluting u in period t unit packing cost cds p unit shipping cost to distribution center p pwc portion waste of chipping system S qpt