Water footprint assessment of viscose staple fi ber garments

The viscose fiber industry forms a large part of the textile industry and is a typical water consumption and wastewater discharge industry. As a tool to quantify environmental impacts in terms of water resources, the water footprint assessment (WFA) is a control method for the textile and apparel industry to measure water consumption and wastewater discharge. In this study, the water footprints of viscose staple fiber blouses and blended men’s suits were comprehensively evaluated based on the ISO 14046 standard and the life cycle assessment (LCA) polygon method. The WFA results from our study indicate that the production stage of viscose staple fiber garments has the most significant water resource environmental load. Specifically, the water footprint related to the production of viscose staple fiber for three types of clothing accounted for more than 50% of the total water footprint, with men’s 100% viscose staple fiber suits having the largest impact on water resources and the environment. Furthermore, our results indicate that the water alkaline footprint is primarily influenced by the viscose staple fiber production as well as the dyeing and finishing processes. NaOH and Na2CO3 are the main pollutants that caused the water alkaline footprint. In addition, the water ecotoxicity footprint was the major driving factor of water resource environmental load. Zn2þ is the main pollutant that caused the water ecotoxicity footprint.


INTRODUCTION
According to data from the China Statistical Yearbook, in 2017, China's per capita water supply was only approximately 25% of the world's average supply, highlighting severe water shortages in the country. In the same year, the proportion of industrial water consumption to China's total water consumption (including agricultural, industrial, domestic, and ecological water usage) measured 23%-24%, and the proportion of industrial wastewater discharge to the total national wastewater discharge (including industrial and domestic wastewater discharge) was 27%-47% (NBS a, b, c). China's environmental situation in terms of water resources is a cause for concern, with water consumption and wastewater discharge from industry being one of the main reasons. The textile and garment industry is a traditional pillar industry in China, and viscose, including both viscose staple fiber and viscose filament yarn, has become an important raw material for the production of textiles and garments. In 2017 and 2018, China's viscose production was 3.70 and 3.97 million tons, respectively, with viscose staple fiber production accounting for approximately 95% thereof (Ji ). The market share of viscose staple fiber is larger than that of viscose filament, and China is the world's largest consumer of viscose staple fiber, with the domestic consumption of viscose staple fiber increasing from 2.8 million tons in 2011 to 3.4 million tons in 2018 (CBG ). Viscose staple fiber is mainly used for the manufacturing of textiles and garments due to advantages in comfort and moisture absorption, and its proportion in the mid-to-high-end fabric market continues to increase. It is often used in two major garment categories, namely shirts and suits. According to statistics, approximately 65 tons of fresh water is used for the production of one ton of viscose staple fiber, and approximately 110 tons of fresh water is used for the dyeing and finishing of one ton of textiles.
Alkali, carbon disulfide, sulfuric acid, and other chemicals used in the production process, in total approximately 3.3 tons/ton viscose staple fiber, remain in the wastewater and are discharged together with the wastewater (Yang ).
Therefore, the viscose staple fiber industry and its associated textile and garment industries consume a large amount of water and are responsible for serious wastewater pollution, which has multiple environmental impacts on water resources.
Water footprint assessment (WFA) is an effective tool for analyzing the correlation between human activities or manufacturing of specific products and water shortages and pollution problems. It aims to quantify the water footprint of production processes, products, producers, or consumers and also evaluates the sustainability of water resources and the environment. The water footprint network (WFN) defines water footprint as a comprehensive evaluation indicator to measure the level of water resource utilization, including the blue, green, and gray water footprints (Hoekstra et al.  The WFA method has been widely applied across various organizational, regional, service, and product levels, especially for agricultural products, such as cotton (Chico et  Water scarcity and degradation footprints are used as indicators to quantify water use in terms of total water resource supply capacity and wastewater discharge pollution in terms of the total pollution capacity of the water environment, respectively. Although the WFA is already used in the textile and clothing industry, it is not yet possible to fully quantify all the environmental impacts caused by the production of textiles and garments. In particular, there is no report on the environmental impacts of alkaline wastewater from viscose fiber production. In addition, water scarcity and degradation footprints can be quantified and evaluated from either a water consumption (volume) or a water pollution perspective (environmental impacts). Notably, the different evaluation methods for volume and quality do not reflect the overall environmental load in terms of water resources, making it difficult to horizontally compare and evaluate across products and processes.
Within this context, our study aims to (a) calculate the water footprints of typical viscose garment products from fiber production through to clothing consumption, and (b) conduct a comprehensive assessment and comparison of products and processes using the life cycle assessment (LCA) polygon method.

METHODS AND DATA COLLECTION
For our study, the WFA method was used based on the ISO 14046 standard and the comprehensive LCA polygon evaluation method. The water alkaline footprint was used to quantify the water alkalization impact caused by pollutants in wastewater discharge, thereby improving the WFA method. Furthermore, we quantified the environmental impact of viscose staple fiber blouses and blended men's suits on water resources; specifications of the blouses (size: 160/84A for the national garment size series standard in China) and men's suits (size: 170/92A for the national garment size series standard in China) are presented in Table 1. The functional unit of the water footprint calculations was set to 1,000 pieces of viscose staple fiber and chemicals) as well as outputs (e.g., wastewater discharge) was based on this unit.

System boundary description
The total life cycle of the viscose staple fiber blouses and blended men's suits included the following seven parts: cotton linters, viscose production, fabric production, dyeing and finishing, garment production, consumer care, and recycling ( Figure 1).
In this study, the system boundary of viscose staple fiber apparel included water usage and wastewater discharge from the production and processing of fibers, fabrics, and linings, as well as from the production and use of apparel products, but excluded consumption and emissions related to employees, transportation, as well as the maintenance and cleaning of machinery and equipment. As the amount of auxiliary materials (non-woven lining, buttons, etc.) was negligible, the processes related to their production were not included in our calculations. A summary of the system boundary is shown in Figure 2.

Framework for water footprint assessments
The WFA framework of the ISO 14046 standard includes the following guidelines: (a) determine the objectives and scope of the assessment, which should be consistent; (b) analyze the water footprint list, which should include the input and output of each process unit in the research system; (c) assess the impact of the water footprint, taking into account any potential environmental impacts caused by changes in water quantity and quality in the system, such as the water scarcity footprint and water degradation footprint; and (d) interpret the results in terms of the water footprint list and environmental sustainability (Bai et al. ).

Water scarcity footprint
The water scarcity footprint (WF SC ) is used to quantify the potential impact of the amount of water used on the local water supply capacity. In this study, we used water stress   used the following equation to calculate the WF SC : where WF SC (m 3 H 2 O eq) is the water scarcity footprint, Q j

Water eutrophication footprint
The water eutrophication footprint (WF EU ) is used to quantify the potential eutrophication impact of wastewater pollutants on the water environment, with phosphate (PO 4 3À ) used as a reference substance to measure the biomass-forming ability of different pollutants. The WF EU is calculated as follows (Heijungs et al. ): where WF EU (kg PO 4 3À eq) is the water eutrophication footprint, EUP i (kg PO 4 3À eq/kg pollutant) is the characteristic factor of eutrophication pollutant i, and M i is the emission of pollutant i.

Water acidification footprint
The water acidification footprint (WF AC ) is used to quantify the potential acidification impact of wastewater pollutants on the water environment, with sulfur dioxide (SO 2 ) used as a reference substance to measure the hydrogen ion (H þ )-releasing ability of pollutants. The WF AC is calculated as follows (Bai et al. ): where WF AC (kg SO 2 eq) is the water acidification footprint, ACP i (kg SO 2 eq/kg pollutant) is the characteristic factor of the acidification pollutant i, and M i is the emission of pollutant i.

Water ecotoxicity footprint
The water ecotoxicity footprint (WF AET ) is used to measure the potential ecotoxicity impact of wastewater pollutants on the water environment, with its characteristic factor based on the maximum tolerance concentrations (MTCs) determined according to the Environmental Protection Agency (EPA). Therefore, the WF AET is calculated as follows (Heijungs et al. ): where WF AET (m 3 H 2 O eq) is the water ecotoxicity footprint, where WF AL (kg OH À eq) is the water alkaline footprint, where S pol,av is the area of the LCA polygon, S av is the average area of all possible polygons, n is the number of possible polygons, and R is the side of the LCA polygon.

Data collection
The weight of 1,000 blouses and men's suits were calculated according to specifications, fabric, and linings. Following  Water consumption and wastewater pollutant discharge related to the cleaning of equipment used for textile processes were not included in our calculations. Blouse fabric was pre-treated before scouring, and reactive dyes were subsequently used for one-step dyeing in overflow dyeing machines; finally, the post-processing was softened. Data for water consumption and wastewater pollutant discharge related to the dyeing and finishing of blouse fabric was obtained from the monitoring data of chemical fiber printing and dyeing enterprises and accessed through an environmental data platform (www.ipe.org.cn). Men's suit fabric and lining was pre-treated according to fiber composition (e.g., degreased), followed by the reactive dye/acid dye or reactive dye/direct dye one-bath one-step method for neutral color dyeing using overflow dyeing machines; finally, the post-processing was shaped or softened. Data related to water consumption and wastewater pollutant discharge for

RESULTS AND DISCUSSION
The WF SC and the WF DE related to three types of typical viscose staple fiber garments were calculated using Equations (1)-(5), and LCA polygons were drawn based on the results obtained from water footprint analyses. The area of each LCA polygon was calculated based on Equation (6) to obtain a single integrated value for multiple indicator results.

Results of water scarcity footprint (WF SC ) analysis
The WF SC of three types of viscose staple fiber apparel as well as viscose staple fiber production is shown in Figure 6.
Results presented in Figure 6 show the WF SC of blouses and thus the whiteness index and oil rate can be improved for the scouring process (Shen & Patel ). The difference between the WF SC of blouses and men's suits was mainly due to differences in viscose staple fiber production and different washing processes by consumers. In total, 154.90 kg of viscose staple fiber was used for the production of 1,000 blouses, while 709.90 and 418.40 kg was used for the production of 100% viscose staple fiber and 55% viscose staple fiber blended men's suits, respectively. The main reason for the difference is that a large amount of water (225.57 m 3 , based on water usage for 50 machine washes of 1,000 blouses) was used by the washing of blouses by consumers, while for men's suits treated with dry cleaning, little water was consumed.

Results of water degradation footprint analysis
The water degradation footprint (WF EU , WF AC , WF AL and WF AET ) of three types of viscose staple fiber apparel as well as viscose staple fiber production is shown in Figures 7-10.
Results from our WF EU analysis indicated that blouses had the highest WF EU footprint, followed by 100% viscose staple fiber men's suits (a) and 55% viscose staple fiber blended men's suits (b). In the production chain of the three garment types, the viscose staple fiber production as well as fabric dyeing and finishing processes were partly responsible for the WF EU of 1.58, 6.42, and 3.94 kg PO 4 3À eq/1,000 pieces, respectively, of which viscose staple fiber production accounted for more than 89%. The difference between the WF EU values of the three types of clothing was due to the washing of blouses by consumers (51.45 kg PO 4 3À eq/1,000 pieces), accounting for 77% of the total WF EU of blouses, and mainly caused by the presence of COD, BOD 5 , and NH 3 À N in the wastewater from washing machines. Although the dry cleaning of men's suits did not influence the overall WF EU , the lining dyeing and finishing process of 0.40 kg PO 4 3À eq/1,000 pieces did affect the overall WF EU . Furthermore, the WF EU caused by the production of viscose staple fiber was 8.25 kg PO 4 3À eq/ton, of which the presoaking process had the largest WF EU of 6.78 kg PO 4 3À eq/ton, accounting for 82% of the total WF EU of viscose staple fiber production. The main pollutants were COD, BOD 5 , and NH 3 À N, which are mainly from a large amount of organic impurities such as cellulose in the wastewater (black liquor) from the presoaking process, with COD concentrations of as high as 8,000 mg/L (Zhang ; Ke ). the wastewater discharged from the two-bath, acid-station, and scouring processes (Shen & Patel ). The WF AC of viscose staple fiber production was 77.98 kg SO 2 eq/ton, whilst the WF AC of the scouring process and the two-bath process was 40.94 kg SO 2 eq/ton and 29.64 kg SO 2 eq/ ton, respectively, accounting for 91% of the total viscose staple fiber production WF AC .
As seen in Figure 9, the WF AL of men's 100% viscose staple fiber suits (a) and men's 55% viscose staple fiber suits (b) were more than four and three times that of blouses, respectively. One reason for this is that men's (a) and (b) suits used approximately five and three times as much viscose staple fibers as blouses, respectively. A second reason is that the dyeing and finishing stages of men's suits included fabric and lining dyeing as well as finishing, while blouses only included fabric dyeing and finishing. The WF AL of blouses, men's (a) suits, and men's (b) suits was 11.55, 55.90, and 38.33 kg OH À eq/1,000 pieces, respectively. In summary, the WF AL related to the production of viscose staple fiber was 40.25 kg OH À eq/ton, with wastewater discharge WF AL from the presoaking process being the highest at 37.54 kg OH À eq/ton and accounting for 93% of the WF AL of viscose staple fiber production. Water alkalization caused by the main pollutants NaOH and Na 2 CO 3 originated mainly from the utilization of caustic soda in the viscose staple fiber production as well as dyeing and finishing process, functioning as a dissolution and dyeing auxiliary (Shen & Patel ; Wang ).     (c) The WF AC and WF AL had relatively smaller impact on the environmental load in terms of water resources related to the three types of garments compared with the two indicators mentioned above. The water acidification of the three types of clothing stemming from the production of viscose staple fiber was mainly due to H 2 SO 4 in the wastewater discharged from the twobath, acid-station, and scouring processes. Furthermore, it is the combined efforts of base input in the production of viscose staple fiber and fabric dyeing and finishing that caused water alkalization. Apparently, the WF EU had the smallest impact on the size of the LCA polygon area. The water eutrophication was caused by the chemicals used in the production and consumption processes.
Therefore, measures can be taken to reduce the use of chemicals and control pollution can effectively reduce the environmental load in terms of water resources by improving production and post-processing technology or adjusting product development strategies.
The LCA polygon of each production process of viscose staple fiber is shown in Figure 12, and the areas of presoak-

CONCLUSIONS
The following conclusions can be drawn from our investigations and results: (a) Sub-module calculations related to the water footprint of viscose fiber clothing showed good results in terms of calculation accuracy and the identification of various pollution factors and sources. Viscose staple fiber production showed the most significant environmental load in terms of water resources with its water footprint accounting for more than 50% of the total water footprints of the three types of clothing.
(b) In this study, the viscose staple fiber production as well as fabric dyeing and finishing processes showed the highest water alkaline footprint with NaOH and Na 2 CO 3 being the main pollutants. The water alkaline footprints related to blouses as well as men's (a) and (b) suits were 11.55, 55.90, and 38.33 kg OHeq/1,000 pieces, respectively.