Carbon footprint and water footprint assessment of virgin and recycled polyester textiles

Abstract

Recycled polyester textile fibers stemming from waste polyester material have been applied in the textile industry in recent years. However, there are few studies focusing on the evaluation and comparison of the environmental impacts caused by the production of virgin polyester textiles and recycled polyester textiles. In this study, the carbon footprint and water footprint of virgin polyester textiles and recycled polyester textiles were calculated and compared. The results showed that the carbon footprint of the virgin polyester textiles production was 119.59 kgCO₂/100 kg. Terephthalic acid production process occupied the largest proportion, accounting for 45.83%, followed by polyester fabric production process, ethylene production process, paraxylene production process, ethylene glycol production process and polyester fiber production process. The total carbon footprint of waste polyester recycling was 1154.15 kgCO₂/100 kg, approximately ten times that of virgin polyester textiles production. As for the water footprint, it showed that virgin polyester fabric production and recycled polyester fabric production both had great impact on water eutrophication and water scarcity. Chemical oxygen demand caused the largest water eutrophication footprint, followed by ammonia-nitrogen and five-day biochemical oxygen demand. The water scarcity footprint of virgin polyester fabric production and recycled polyester fabric production was 5.98 m³ H₂O_eq/100 kg and 1.90 m³ H₂O_eq/100 kg, respectively. The comprehensive evaluation of carbon footprint and water footprint with the life cycle assessment polygon method indicated that the polyester fabric production process exhibited greater environmental impacts both for virgin polyester and recycled polyester.

Keywords

Recycled polyester textile carbon footprint water footprint environmental impact comprehensive comparison

Polyester fibers have dominated the global textile industry and have long been popular with people because of their excellent properties, such as good wrinkle and abrasion resistance, spinnability and dyeability.¹ According to statistics, polyester fibers account for about 50% of the fibers market and the share is expected to continue to grow in the next few years.² The raw materials of virgin polyester fiber are derived from crude oil which is a typical non-renewable material. Only a small percentage of the virgin polyester is recycled, leaving most of it to be buried or burned.³ A considerable amount of energy, fresh water and textile chemicals are involved in polyester textile production, particularly in the highly energy-demanding and water-consuming dyeing and finishing processes. For example, once polyester is synthesized as flakes or pellets, the material is further extruded and spun by fusion at a high temperature of about 260°C, which is a major energy-consuming process in the polyester production chain. Approximately 30–50 litres of fresh water are required for dyeing a kilogram of polyester textile.^4–6 Owing to our dependence on polyester textiles for coordination with the latest fashions, there is no doubt that a series of severe environmental problems would result from the production of polyester textiles as well as their waste treatment.^7–10

Through the physical or chemical recycling process, the recycled polyester has a wide range of potential applications similar to virgin polyester, serving as the raw material of textiles.¹¹ Are the recycled polyester textiles more environmentally friendly than virgin polyester textiles? There are some researchers focusing on the comparison of the environmental impacts caused by the production of virgin polyester materials and recycled polyester materials. Woolridge et al. calclulated the net energy saving of recycling of 1 tonne of garments from virgin polyester garments. The results indicated that there was a net energy saving of 89,811 kWh.¹² Shen et al. revealed that recycled polyester fiber produced by mechanical recycling caused lower environmental impacts than virgin polyester fiber on energy use and global warming potential.¹³ Additionally, a calculation by a research group in Austria illustrated that the carbon footprint (CF) of recycled polyester showed a sharp reduction of around 79% compared with the virgin polyester.¹⁴ Furthermore, a report stated that a saving of fossil fuel energy of 2789 million MJ and a reduction of CO₂ emission of 89,699 tonnes could be obtained by producing 2.9 billion recycled polyester bottles.¹⁵ These researches only revealed specific environmental impact of these two kinds of polyester, especially focusing on energy consumption and CO₂ emission. Other environmental impacts, such as water scarcity and water pollution, were seldom involved.

To fill the gap in comprehensive evaluation of environmental impacts caused by the production of virgin polyester textiles and recycled polyester textiles, this paper: (a) calculated and analyzed the CF and water footprint (WF) of the production processes of six key semi-products (i.e. ethylene, paraxylene (PX), ethylene glycol (EG), terephthalic acid (PTA), polyester fiber, polyester fabric) for the virgin polyester fabric production and two key semi-products (i.e. polyester fiber, polyester fabric) for the recycled polyester fabric production; (b) assessed the comprehensive environmental impacts incorporating CO₂ emission along with water consumption and water pollution caused by the production of the two kinds of polyester textiles based on the life cycle assessment (LCA) polygon method; and (c) further conducted a comprehensive comparison of environmental impacts of production processes of virgin and recycled polyester textiles.

Methods and data

In the past few decades, great efforts have been made to facilitate sustainable energy and water management in the textile industry. In order to quantify the environmental impact caused by carbon emissions, CF was introduced as one of the useful indicators for environmental impact evaluation. CF was defined as the amount of CO₂ or CO₂ equivalent emitted directly and indirectly over the life of an activity, a product (or service), or a geographical area.^16,17 Another footprint indicator, WF, was also proposed for quantifying and evaluating the impact caused by water resources consumption and water pollutants discharges.¹⁸ The methodologies employed for CF and WF evaluation in this research were referred to PAS 2050: 2011 and ISO 14046, respectively. The functional unit for the CF and WF analysis was 100 kg of polyester fibers.

System boundary description

The system boundary is a vital and deterministic part of CF and WF assessment. According to the characteristics of the production processes of virgin polyester textiles and recycled polyester textiles, the industrial production chain can be divided into four steps: polyester production process, spinning process, weaving process, and dyeing and finishing process. The consumption of energy, raw materials and fresh water and the discharge of wastewater and pollutants (e.g. oil, paraxylene (PX), chemical oxygen demand (COD), five-day biochemical oxygen demand (BOD₅), ammonia-nitrogen (NH₃^-N), and suspended solids (SS)) within the system boundary are shown in Figure 1.

Figure 1.

System boundary of the carbon footprint (CF) and water footprint (WF) on virgin and recycled polyester textiles.

Carbon footprint calculation method

In this section, the CF was calculated based on the terminal energy consumption and the corresponding CO₂ emission factor per 100 kg of polyester fiber. The CF calculation method is as follows¹⁶

CF = \sum_{n = 1}^{m} E_{i} \times f_{i}

(1)

where CF is in kgCO₂/100 kg; E_i (in kg) is the consumption of energy i in the production processes. f_i (in kgCO₂/kgsce) is the CO₂ emission factor for energy i;¹⁶ and m is the type of energy. The relevant data mentioned above are listed in Table 1 and Table 2.

Table 1.

Conversion factor from physical units to coal equivalent units

Energy	Average low calorific value	Conversion factor
Fuel oil	41,816 kJ/(10,000 kcal)/kg	1.4286 kgsce/kg
Diesel	42,652 kJ/(10,000 kcal)/kg	1.4571 kgsce/kg
Electricity	3600 kJ/(10,000 kcal)/kg	0.1229 kgsce/kg
Natural gas	32,238 kJ/(10,000 kcal)/m³	1.1000 kgsce/m³

sce: standard coal equivalent.

Table 2.

CO₂ emission factors of different energies

E_i	f_i
Fuel oil	2.219 kgCO₂/kgsce
Diesel	2.167 kgCO₂/kgsce
Electricity	6.113 kgCO₂/kgsce
Natural gas	1.162 kgCO₂/kgsce

sce: standard coal equivalent.

Water eutrophication footprint calculation method

The water eutrophication footprint (WF_eutr) is one of the evaluation methods for the water degradation footprint. It is used to evaluate the water eutrophication impact caused by discharged phosphorus and nitrogen pollutants. WF_eutr is calculated as follows¹⁹

{W F}_{eutr} = \sum_{i = 1}^{n} ({N P}_{eutr, i} \times M_{i})

(2)

where WF_eutr is in kg PO₄³⁻eq/100 kg; NP_eutr,i (in kgPO₄³⁻eq/kg) is the characterization factor of pollutant i; M_i (in kg) is the mass of pollutant i.

Water scarcity footprint calculation method

The water scarcity footprint (WF_scar) is used to evaluate the impact on water shortage caused by fresh water consumption. The method used for WF_scar calculation is as follows²⁰

{W F}_{scar} = \sum_{i = 1}^{m} \sum_{j = 1}^{n} (\frac{{WSI}_{j}}{{WSI}_{nat}} \times Q_{i})

(3)

where WF_scar is in m³H₂Oeq/100 kg; WSI_j (dimensionless) is the water stress index of area j (0.01 < WSI _j < 1); WSI_nat (dimensionless) is the nationally averaged WSI; Q_i (in m³) is the consumption of freshwater per functional unit in area j; i is the production process; and j is the geographical position.

LCA polygon method

To present the LCA results in a less sophisticated way as a tool to aid the comprehensively comparative appraisal of products or processes according to their environmental performance, the LCA polygon method was introduced owing to its simplicity and flexibility.²¹ This method makes it possible to convert multiple values into a single value as a general estimate of the total environmental impact and allow for optimal selection over the life cycle.²²

As mentioned above, multiple values for different impact categories are given for the corresponding axes, forming a n-sided polygon in a radial shape, which is called the LCA polygon.²³ The total impact of each product or process is then concluded by comparing the combined areas of the different polygons. The larger the area is, the more serious the impact of the product or process. The LCA polygon has an irregular n-sided shape. Its area can be calculated by the sum of the areas of the n tri-angles formed. The calculation formulas used for the areas are as follows

A_{i} = \frac{1}{2} R_{i} R_{i + 1} \sin (\frac{360 °}{n}) for i = 1, 2, \dots, n - 1

(4)

A_{n} = \frac{1}{2} R_{n} R_{1} \sin (\frac{360 °}{n})

(5)

A = \sum_{1}^{n} A_{i} = \frac{1}{2} \sin (\frac{360 °}{n}) (R_{n} R_{1} + \sum_{i = 1}^{n - 1} R_{i} R_{i + 1})

(6)

where A_i is the area of triangle i; A is the sum of these triangular areas; R_i is the side of triangle i; and i is the number of triangles formed in the n-sided polygon.

Equation (7) determines the area of an LCA polygon under a random arrangement of impact categories in the radial system of axis. However, it is worth noting that the arrangement of the n axes in the polygon influences the total area, owing to the different values presented by different products R_i R_i₊₁ in equation (6).²³ To solve this crucial problem and make the results more accurate, the average areas of all the possible triangles for the different impact category arrangements needs to be calculated. The number of the triangles combined with sides R_i and R_j (i = 1, 2, …, n−1, j = 2, 3, …, n) is [n(n−1)]/2. The average area of the LCA polygon is calculated by equation (7).

A_{a v}^{pol} = n A_{a v}^{t r} = \frac{1}{2} \sin (\frac{360 °}{n}) \{n [\frac{2 \sum_{i, j = 1, i < j}^{n} R_{i} R_{j}}{n (n - 1)}]\}

(7)

where

A_{a v}^{pol}

is the average area of the LCA polygon and

A_{a v}^{t r}

is the average area of all possible triangles.

It should be pointed out that the premise of area calculating of LCA polygons is a further normalization, transforming these ecological parameters represented by different axes to a single value. It is needed with the aim of eliminating the difference among units. A relative grade is calculated as follows²²

P_{b, p} = \frac{{E P}_{b, p}}{{E P}_{b, \max}}

(8)

where, P_b,p is relative grade of the ecological parameter b for the product p. EP_b,p is the ecological parameter b of the product p, and EP_b_,max is the largest ecological parameter b.

Data collection

In this study, 100 kg of polyester fibers was taken as the functional unit. To produce 100 kg of virgin polyester fiber, 86.80 kg of refined terephthalic acid and 33.7 kg of ethylene glycol were required. According to the ratio of terephthalic acid to ethylene glycol and the molecular weight of the corresponding petroleum products, the corresponding petroleum product consumption was obtained. It was calculated that 55.43 kg of paraxylene and 15.23 kg of ethylene was consumed for each 100 kg of polyester fiber.

The data for virgin polyester textile production were initially collected from enterprises in Shanghai Province and Anhui Province, China.^10,24 The data for recycled polyester textile were collected from enterprises in Shandong Province, China.²⁵ According to Google Earth, the national average WSI of China was 0.602.²⁶ The local WSI for Shanghai and Anhui was 0.9999 and 0.02818, respectively. Shandong’s WSI was 0.9948, which was calculated based on the ratio of water withdrawal to the availability of water resources.²⁷ The data relating the calculation of WF_eutr were derived from two Chinese national standards, GB 8978-1996 Integrated wastewater discharge standard and GB 4287-2012 Discharge standard of water pollutant for dyeing and finishing of textile industry. The characteristic factors of water pollutants refer to T/CNTAC 14-2018 General technical requirements for quantifying the water footprint of textile products, as shown in Table 3.

Table 3.

Characterization factors of water pollutants

Impact category	Substance	Characterization factor	Unit
Water eutrophication footprint	COD	0.022	kg PO₄³⁻eq/kg
	BOD₅	0.022
	NH₃⁻N	0.33

COD: chemical oxygen demand; BOD: biochemical oxygen demand.

Results

The CF and WF of polyester textiles were calculated according to equations (1), (2) and (3). A comparative assessment of the total environmental impact of different production processes was then conducted based on the LCA polygon method.

As can be seen from Figure 2, the CF of the production process of PTA synthesized by high-temperature oxidation was the largest, followed by the production processes of polyester fabric, ethylene, PX, EG, and polyester fiber in the virgin production process. With regard to the recycling step, the CF of recycled polyester fiber production process was three times larger than that of recycled polyester fabric production process. The total CF of recycling processes was almost 10 times larger than that of virgin production processes. It also worth noting that electricity was used in every semi-production process, contributing great environmental impact to the total impact.

Figure 2.

Carbon footprint of production processes of virgin polyester and recycled polyester textile per functional unit.

In both production chains, COD contributed the most to the WF_eutr, followed by NH₃⁻N and BOD₅ according to Figure 3. In the virgin polyester textile production process, the WF_eutr of the polyester fiber production was 0.037 kg PO₄³⁻_eq/100 kg. For the production of polyester fabric, WF_eutr was 0.156 kg PO₄³⁻eq/100 kg, which was almost five times larger than that of the polyester fiber production process. The production processes of EG, PTA, ethylene and PX generated smaller WF_eutr due to little eutrophication pollutants discharge. Recycled polyester fabric production had greater potential to lead to eutrophication of water bodies in the recycling stages. The total WF_eutr of virgin polyester textiles production was 0.197 kg PO₄³⁻eq/100 kg, which was almost three times larger than that of recycling processes.

Figure 3.

WF_eutr of each production process of virgin polyester and recycled polyester textile per functional unit.

It can be seen from Figure 4 that WF_scar of virgin production processes are as follows: polyester fabric > polyester fiber > PTA > EG > PX > ethylene. WF_scar of the recycled polyester fabric production process was larger than that of the recycled polyester fiber production process. The WF_scar of virgin polyester fabric production was 5.98 m³ H₂Oeq/100 kg and occupied the largest proportion. The WF_scar of the ethylene, PX, EG and PTA production processes were much smaller. The total WF_scar of the virgin production processes and recycling production processes was 6.32 m³ H₂Oeq/100 kg and 2.61 m³ H₂Oeq/100 kg, respectively.

Figure 4.

WF_scar of production processes of virgin and recycled polyester textile per functional unit.

In the virgin polyester textile production processes, as depicted in Figure 5, the polygon area can be arranged in the following descending order: polyester fabric > polyester fiber > PTA > EG > ethylene > PX. It should be pointed out that in the production of ethylene, PX and EG, the values for freshwater consumption and water pollution were relatively smaller compared with the CF per functional unit. Consequently, the polygon area was in the shape of a line, which indicates that the CO₂ emission was the most important contributing factor causing the environmental burden in these three production steps. In addition, it is apparent that the environmental impact of dyeing and finishing processes was much larger than those of the other steps. The LCA polygons of dyeing and finishing processes were 42 times larger than that of the polyester fiber production. Despite the large value of CF in the recycled polyester fiber process, the polygon area of recycled polyester fabric (0.635) with larger values of WF_scar and WF_eutr was much larger than that of recycled polyester fiber (0.135) as shown in Figure 6. In summary, the LCA polygon area of recycled polyester production (0.7696) was larger than that of virgin polyester production (0.7473).

Figure 5.

Life cycle assessment polygon of environmental impacts of six production processes of virgin polyester textile.

Figure 6.

Life cycle assessment polygon of environmental impact of production processes of recycled polyester textile.

Discussion

Several managerial insights can be inferred from the aforementioned results. The total CF of recycling processes was much larger than that of virgin production processes. This was caused by a series of energy intensive procedures (e.g. crushing, high temperature cleaning and drying) involved in the spinning stage from waste polyester bottles to recycled polyester fibers.^13,28 In addition, with regard to the eye-catching CF of the PTA production process in the virgin process, this was mainly because the amount of electricity consumed for the PTA production stage was much greater than for the other processes due to the high temperature oxidization process. Therefore, one way to reduce the carbon emissions of the polyester textiles production process is to use clean electricity (e.g. wind power, solar power and water power) and improve polyester recycling technology.

As for the WF_eutr results, we found that the polyester fabric production process had the highest value of the virgin processes, as shown in Figure 3. This was mainly attributed to the usage of catalysts and other chemicals in the process of esterification and polycondensation. Furthermore, it was the consumption of dyes and auxiliaries during dyeing and finishing processes that led to the result that the WF_eutr of polyester fabric production was much larger than that of the polyester fiber production process. Cleaner production can be gained by reductions in levels of consumption of toxic functional textile chemicals with the help of chemical footprint methodology.

The polyester fabric process accounted for a considerable percentage of WFeutr in virgin and recycling production process. This was because of the fact that there were highly water-consuming processes, such as desizing, bleaching and mercerizing, in virgin polyester fabric pretreatment.^24,25 The WF_scar of virgin polyester fabric production occupied the largest proportion, mainly because the WSI of Shanghai was much larger than that of Anhui.

The LCA polygons of dyeing and finishing processes far outweigh that of the polyester fiber production. The main reason was that WF_eutr and WF_scar of the polyester fabric production process contributed significantly to the total environmental impact. In the recycling processes, the polygon area of recycled polyester fabric was much larger than that of recycled polyester fiber. There is no doubt that the use of indispensable chemical agents and consumption of larger quantities of fresh water contributed mostly to WF_eutr and WF_scar. The LCA polygon area of recycled polyester production was larger than that of virgin polyester production, leading to severe total environmental impact. In order to reduce the total environmental impacts of polyester textiles production, more attention should be paid to polyester fabric production, such as reducing water eutrophication impact and increasing the proportion of clean energy usage.

Conclusion

In this paper, the main aim is to calculate and comprehensively evaluate the environmental loads of the production process of virgin polyester textiles and recycled polyester textiles considering CO₂ emission, water scarcity and water degradation. Moreover, the novelty of this paper is that a comprehensive evaluation and comparison between virgin polyester textiles and recycled polyester textiles based on the LCA polygon method were conducted.

The results have demonstrated that the production processes of virgin polyester textiles caused greater environmental impact in terms of water consumption and water pollution. In contrast, CO₂ emission in recycling processes of waste polyester material caused more adverse impact than virgin polyester textiles production. In summary, the comparison of combined environmental impact caused by CO₂ emission as well as water consumption and water pollution has shown that the recycling processes exhibited greater environmental impact than virgin polyester textiles production. In addition, the following measures were found to be crucial for decreasing environmental impacts of polyester textiles production: (a) use clean energy to directly reduce the CO₂ emission; (b) improve polyester production and wastewater treatment technologies to reduce water consumption and water pollution; and (c) utilize environment-friendly textile dyes and auxiliaries to mitigate water eutrophication.

There are many kinds of environmental impacts caused by the production of polyester textiles. This paper focused on CF and WF and gave a comprehensive evaluation and comparison. Chemical footprint, a footprint methodology to evaluate the toxicity impacts caused by textile chemical pollution discharge, can be applied in the comprehensive evaluation of environmental impacts of polyester textiles production in future research.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by Natural Science Foundation of Zhejiang Province (LY20G030001), National Key R&D Program of China (2018YFF0215703), Excellent Graduate Thesis Cultivation Foundation (LW-YP2020052), Science and Technology Innovation Activities of University Students in Zhejiang Province (2020R406074), and National Innovation Training Program for College Students (202010338017).

ORCID iD

Laili Wang

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