*Review* **A Review on Textile Recycling Practices and Challenges**

**Jeanger P. Juanga-Labayen <sup>1</sup> , Ildefonso V. Labayen <sup>2</sup> and Qiuyan Yuan 3,\***


**Abstract:** The expansion of clothing and textile industry and the fast fashion trend among consumers have caused a rapid global increase in textile waste in the municipal solid waste (MSW) stream. Worldwide, 75% of textile waste is landfilled, while 25% is recycled or reused. Landfilling of textile waste is a prevalent option that is deemed unsustainable. Promoting an enhanced diversion of textile waste from landfills demands optimized reuse and recycling technologies. Reuse is the more preferred option compared with recycling. Various textile reuse and recycling technologies are available and progressively innovated to favor blended fabrics. This paper aims to establish reuse and recycling technologies (anaerobic digestion, fermentation, composting, fiber regeneration, and thermal recovery) to manage textile waste. Improved collection systems, automation of sorting, and discovering new technologies for textile recycling remains a challenge. Applying extended producer responsibility (EPR) policy and a circular economy system implies a holistic consensus among major stakeholders.

**Keywords:** textile waste; reuse and recycling; municipal solid waste; composting; sustainability

**1. Introduction**

Population growth, improvement of living standards, an increasing assortment of textile materials, and the decreasing life cycle time of textile products contributed to global fiber consumption that generates a significant amount of post-industrial and post-consumer fiber waste [1,2]. Globalization has made the apparel industry produce more clothing at lower costs, and many consumers have adapted a 'fast fashion' trend that considers clothing to be a disposable product [3]. Fast fashion characterized by mass production, variety, agility, and affordability has brought about a surge of apparel consumption [4].

The rising cost associated with textile manufacturing in terms of energy, raw materials, and waste management is putting pressure on businesses across the globe. The textile industry accounts for about 10% of total carbon emissions [5] and has been identified as the fifth largest contributor of carbon emissions [6,7]. In this regard, it is crucial to understand that 20th-century approaches in meeting 21st-century demands are not affordable for sustainable development [8]. It is essential to consider the efficient use and management of natural resources by reducing the raw material consumption through reuse and recycling of textile products regarded as waste, which would offer a sustainable approach for textile waste management. To improve the current behavior of clothing consumption and waste generation, an environmentally and financially sound long-term national program should be established [9].

Globally, approximately 75% of textile waste is disposed of in landfills, 25% is reused or recycled, and less than 1% of all textile is recycled back into clothing [10,11]. In this respect, advancing reuse and recycling technologies for textile waste in diverting waste

**Citation:** Juanga-Labayen, J.P.; Labayen, I.V.; Yuan, Q. A Review on Textile Recycling Practices and Challenges. *Textiles* **2022**, *2*, 174–188. https://doi.org/10.3390/ textiles2010010

Academic Editor: Philippe Boisse

Received: 6 February 2022 Accepted: 9 March 2022 Published: 16 March 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

from landfill is crucial. More importantly, closed-loop recycling of fabric is highly promoted. There have been several reinforced global actions integrating many expert stakeholders addressing both economic and environmental challenges that the clothing industry faces; among them are the Textile Exchange, Council for Textile Recycling, Sustainable Apparel Coalition, and the Boston Consulting Group, among others. For instance, Textile Exchange commits to reducing CO<sup>2</sup> emissions by 30% from textile fibers and material production by 2030 and fosters the role of the circular economy as a powerful instrument for mitigating impacts and contributing to the urgent need for climate action [11]. Hence, textile reuse and recycling are vital in promoting this innovative act. This paper determines the existing textile waste reuse and recycling technologies and the status of textile waste generation and management in some leading economies. from landfill is crucial. More importantly, closed-loop recycling of fabric is highly promoted. There have been several reinforced global actions integrating many expert stakeholders addressing both economic and environmental challenges that the clothing industry faces; among them are the Textile Exchange, Council for Textile Recycling, Sustainable Apparel Coalition, and the Boston Consulting Group, among others. For instance, Textile Exchange commits to reducing CO<sup>2</sup> emissions by 30% from textile fibers and material production by 2030 and fosters the role of the circular economy as a powerful instrument for mitigating impacts and contributing to the urgent need for climate action [11]. Hence, textile reuse and recycling are vital in promoting this innovative act. This paper determines the existing textile waste reuse and recycling technologies and the status of textile waste generation and management in some leading economies.

*Textiles* **2022**, *2*, FOR PEER REVIEW 2

#### **2. Textile Production 2. Textile Production**

Clothing and textiles contributed 6% to the world exports of manufactured goods in 2017 (Figure 1); China and the European Union (EU) are the two leading regions for clothing and textile exports [12]. The worldwide volume production of textile fibers in 1975 was about 23.9 million metric tons (MMT), in 2017 it reached 98.5 MMT [13], and it increased further to about 111 MMT in 2019 [11]. For many years, cotton fiber demand dominated polyester; however, in 2002, polyester demand surpassed cotton fiber and has continued to grow at a faster rate than cotton fiber [14]. Polyester and cotton are the most common fibers used worldwide [14,15]. Moreover, the global fiber consumption in 2017 consists of 60% synthetic fibers or polyester/cotton blend (polycotton) and 40% cellulosic, which is the typical example of most textiles [16]. Nevertheless, the global fiber market in 2019 was dominated by polyester and cotton (Figure 2). From these figures, it is apparent that textile waste management is a critical issue that presents enormous challenges for the textile industry, policymakers, and consumers. Clothing and textiles contributed 6% to the world exports of manufactured goods in 2017 (Figure 1); China and the European Union (EU) are the two leading regions for clothing and textile exports [12]. The worldwide volume production of textile fibers in 1975 was about 23.9 million metric tons (MMT), in 2017 it reached 98.5 MMT [13], and it increased further to about 111 MMT in 2019 [11]. For many years, cotton fiber demand dominated polyester; however, in 2002, polyester demand surpassed cotton fiber and has continued to grow at a faster rate than cotton fiber [14]. Polyester and cotton are the most common fibers used worldwide [14,15]. Moreover, the global fiber consumption in 2017 consists of 60% synthetic fibers or polyester/cotton blend (polycotton) and 40% cellulosic, which is the typical example of most textiles [16]. Nevertheless, the global fiber market in 2019 was dominated by polyester and cotton (Figure 2). From these figures, it is apparent that textile waste management is a critical issue that presents enormous challenges for the textile industry, policymakers, and consumers.

**Figure 1.** Percentage share of world exports of manufactured goods in 2017 [12].

**Figure 2.** Global fiber production share in 2019 [11].

**Figure 2.** Global fiber production share in 2019 [11].

came more competitive in the United States (US) market [25].

Furthermore, many people in China have easy access to low-cost fashion clothing with a short service life. Roughly 45% of the textile produced in China is wasted. Approximately 26 million tons (MT) of garments are left untreated and dumped annually, while

Textile waste is considered as discarded or unwanted material from the production and use of fiber, textile, and clothing, which can be categorized into three types, pre-consumer, post-consumer, and industrial textile waste [8,17]. The pre-consumer textile waste is viewed as 'clean waste', as a by-product during the manufacturing process of fibrous materials. The post-consumer textile waste consists of discarded garments or household textiles (sheets, towels, and pillowcases) that are worn-out, damaged, and outgrown of no value to consumers after their service life [18]. Industrial textile waste is deemed as 'dirty waste' generated from commercial and industrial textile applications. The expansion of the clothing and textile industry and the consumer's fast fashion trend have caused a rapid global increase in textile wastes. The increased consumption of fashion textiles generates a growing amount of waste. As fashion textiles, are almost 100% recyclable, nothing in the textile and apparel industry should be wasted in an ideal scenario. Furthermore, more than 60% of all recovered clothes could be reused, 35% could be converted into wipers and fiber recycling, and only 5% would need to be discarded [19]. However, in the real world, a significant portion of textile waste is disposed of in landfills. As a result, it is critical to comprehend the challenges that leading economies face when it comes to textile production and waste management. In terms of textile exports, the leading economies considered in this study are China, The European Union, The United States, and Canada. China has the largest economy in clothing and textiles exports globally, yet the industry faces unprecedented crises [20,21]. The country's dominance as a textile provider across the globe is challenged by the loss of competitive advantages in terms of low labor costs as wages are rising. China attempts to maintain its dynamic advantage in labor-intensive textile products by encouraging the relocation of Chinese textile production bases to poorer Chinese provinces and neighboring least developed countries (LDCs). Simultaneously, China's global competitiveness was upgraded through technological advancement, implementing sound policies to develop capital-intensive textile goods, launching niche products and international brands [21,22]. The Chinese textile industry sector has experienced consistent economic growth over the last decade and is primarily focused on the production of apparel made of synthetic fabrics. Furthermore, China produces approximately 31% of the global ratio of synthetic fibers required by the modern textile industry [23] and produces nearly 65% of the world's clothing [24]. When China started imposing strict environmental standards on textile production, China's cloth products be-

#### **3. Textile Waste Generation and Management in Leading Economies**

Textile waste is considered as discarded or unwanted material from the production and use of fiber, textile, and clothing, which can be categorized into three types, pre-consumer, post-consumer, and industrial textile waste [8,17]. The pre-consumer textile waste is viewed as 'clean waste', as a by-product during the manufacturing process of fibrous materials. The post-consumer textile waste consists of discarded garments or household textiles (sheets, towels, and pillowcases) that are worn-out, damaged, and outgrown of no value to consumers after their service life [18]. Industrial textile waste is deemed as 'dirty waste' generated from commercial and industrial textile applications. The expansion of the clothing and textile industry and the consumer's fast fashion trend have caused a rapid global increase in textile wastes. The increased consumption of fashion textiles generates a growing amount of waste. As fashion textiles, are almost 100% recyclable, nothing in the textile and apparel industry should be wasted in an ideal scenario. Furthermore, more than 60% of all recovered clothes could be reused, 35% could be converted into wipers and fiber recycling, and only 5% would need to be discarded [19]. However, in the real world, a significant portion of textile waste is disposed of in landfills. As a result, it is critical to comprehend the challenges that leading economies face when it comes to textile production and waste management. In terms of textile exports, the leading economies considered in this study are China, The European Union, The United States, and Canada.

China has the largest economy in clothing and textiles exports globally, yet the industry faces unprecedented crises [20,21]. The country's dominance as a textile provider across the globe is challenged by the loss of competitive advantages in terms of low labor costs as wages are rising. China attempts to maintain its dynamic advantage in labor-intensive textile products by encouraging the relocation of Chinese textile production bases to poorer Chinese provinces and neighboring least developed countries (LDCs). Simultaneously, China's global competitiveness was upgraded through technological advancement, implementing sound policies to develop capital-intensive textile goods, launching niche products and international brands [21,22]. The Chinese textile industry sector has experienced consistent economic growth over the last decade and is primarily focused on the production of apparel made of synthetic fabrics. Furthermore, China produces approximately 31% of the global ratio of synthetic fibers required by the modern textile industry [23] and produces nearly 65% of the world's clothing [24]. When China started imposing strict environmental standards on textile production, China's cloth products became more competitive in the United States (US) market [25].

Furthermore, many people in China have easy access to low-cost fashion clothing with a short service life. Roughly 45% of the textile produced in China is wasted. Approximately 26 million tons (MT) of garments are left untreated and dumped annually, while only 3.5 MT of the collected textile waste was recycled and reused in 2017 [24]. China's textile waste generation is estimated to range from 20 to 26 MT per year, with a low utilization rate [26]. The Chinese government is encouraging businesses to recycle their own brand clothing through mechanical and chemical recycling. China recognized the two-fold benefits of donating textile waste as it gives clothes a second life while generating revenue for charity. However, in the absence of effective recycling practices, used clothing is sent to wasteto-energy (WTE) incinerators [24]. In 2013, China's State Council mandated that textile manufacturers create a circular value chain to promote environmental sustainability in the disposal of post-consumer textiles [26].

The EU textile industry generates approximately 16 MT of waste annually. European consumers discard 5.8 MT of textiles per year, where only 26% is recycled, while a significant fraction of this waste is disposed of into landfills or incinerated [4,27]. The disposal cost of textile waste into landfills is about €60/ton in some countries in Europe, including France [28]. The European Waste Framework Directive (2008/98/EC) established the fundamental waste management principle and requires the EU member states to adopt a waste management hierarchy (prevention, reuse, recycling, and disposal) in waste management plans and waste prevention programs [29]. Furthermore, the European

Council (EC) promoted sustainability by substituting the Waste Framework Directive with a Circular Economy Package, which set a target for the municipal solid waste (MSW) recovery to 70% and limits the fraction to be landfilled to 10% by 2030 [30]. The extended producer responsibility (EPR) policy was essential in achieving such targets. The EPR holds the producers responsible for collecting, processing, and treatment, including recycling and disposal of products at the post-consumer stage of a product's life cycle [31]. The EPR policy has led to an average annual increase of 13% in post-consumer textile collection [4]. Furthermore, the EPR policy encourages waste prevention at source, promotes green product design, and encourages public recycling [31]. The financial responsibility of the producer, as well as separate collection and recycling agencies, are critical to the success of EPR-based environmental policies [32].

Furthermore, the EU establishes new waste management rules, with a focus on closedloop recycling from production to waste management, with the goal of making economies more sustainable and environmentally friendly [33]. The closed-loop system reduces waste by a repeated process of recycling and reusing materials until they become biodegradable waste. The system can address the fashion industry's intensive use of finite land, water, and energy resources in a sustainable manner [34]. The EU member states' reuse and recycling targets for municipal waste have been set at 55% by 2025, 60% by 2030, and 65% by 2035. By January 2025, a separate collection of textiles and hazardous waste from households will be implemented [33]. Across the European countries, only 18% of clothing is reused and recycled, while 30% is incinerated and a significant fraction of 70% goes to landfills [16]. In France, 40% of the post-consumer textiles collected are exported to African countries for reuse. As of 2017, France is the only European country that globally introduced EPR for textiles, household linen, and shoes [4]. European companies are innovative in formulating sustainability targets where the raw materials, design and development, manufacturing, and end-of-use are the priority on the agenda [34].

In the US, the majority of textile waste in the MSW stream is discarded apparel. However, other sources were identified such as furniture, carpets, tires, footwear, as well as other non-durable goods such as towels, sheets, and pillowcases [35,36]. Textile waste generation and the fraction of textile waste in MSW is increasing with time. In 2010, an estimated 13.2 MT of textile waste were generated, which is equivalent to 5.3% of total MSW stream. While in 2015 and 2017, the generated textile waste increased to 16.1 MT and 16.9 MT, accounting to 6.1% and 6.3% of the total MSW generation, respectively (Figure 3). Approximately 85% of all textiles in the US end up in landfills, and only 15% is donated or recycled [37]. The United States Environmental Protection Agency (USEPA) estimated that textile waste occupies nearly 5% of landfill space [37]. Among the leading economies in the textile industry, the US has the highest share of landfilling textile waste, amounting to 29.3 kg/ca in 2016 (Figure 4), and the estimated cost of textile waste sent to landfills is \$45/ton [38]. Since landfilling keeps the largest share in textile waste management in the US, promoting recycling technologies to many textile industries is crucial. Composting is not a common method of managing textile waste. Nevertheless, incineration and recycling are gaining popularity in textile waste management (Figure 5).

amounting to 29.3 kg/ca in 2016 (Figure 4), and the estimated cost of textile waste sent to landfills is \$45/ton [38]. Since landfilling keeps the largest share in textile waste management in the US, promoting recycling technologies to many textile industries is crucial. Composting is not a common method of managing textile waste. Nevertheless, incinera-

amounting to 29.3 kg/ca in 2016 (Figure 4), and the estimated cost of textile waste sent to landfills is \$45/ton [38]. Since landfilling keeps the largest share in textile waste management in the US, promoting recycling technologies to many textile industries is crucial. Composting is not a common method of managing textile waste. Nevertheless, incinera-

amounting to 29.3 kg/ca in 2016 (Figure 4), and the estimated cost of textile waste sent to landfills is \$45/ton [38]. Since landfilling keeps the largest share in textile waste management in the US, promoting recycling technologies to many textile industries is crucial. Composting is not a common method of managing textile waste. Nevertheless, incinera-

tion and recycling are gaining popularity in textile waste management (Figure 5).

tion and recycling are gaining popularity in textile waste management (Figure 5).

tion and recycling are gaining popularity in textile waste management (Figure 5).

**Figure 3.** Textile waste generation in the US [39]. **Figure 3.** Textile waste generation in the US [39]. **Figure 3.** Textile waste generation in the US [39]. **Figure 3.** Textile waste generation in the US [39].

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**Figure 4.** Annual generation of landfilled textiles (in kg/ca) in 2016 [4]. **Figure 4.** Annual generation of landfilled textiles (in kg/ca) in 2016 [4]. **Figure 4.** Annual generation of landfilled textiles (in kg/ca) in 2016 [4]. **Figure 4.** Annual generation of landfilled textiles (in kg/ca) in 2016 [4].

**Figure 5.** Textile waste management in the US [40]. **Figure 5.** Textile waste management in the US [40].

**Figure 5.** Textile waste management in the US [40]. **Figure 5.** Textile waste management in the US [40]. In Canada, an estimated 500,000 tons of apparel waste is disposed of annually [41]. The average Canadian discards between 30 [42] and 55 [43] pounds of textiles annually [44]; almost 95% of those clothes could be reused or recycled [45]. Globally, textile waste has increased dramatically due to the rise in clothing consumption and production [45]. In Ontario, approximately 1.2 million people dispose their unwanted clothes into the waste bin at a rate of roughly 45,000 tons annually [46]. In the Metro Vancouver Regional District, an estimated 30,000 tons of textile waste are annually landfilled, accounting for 5% of the

annual total waste volume in 2016 [47]. In Toronto, a survey was conducted to determine if participants donated and/or disposed of their unwanted clothing [46]. According to the findings, 17% of participants consider "disposal" to be the most convenient (10%) and fastest (7%) method of getting rid of unwanted textile waste. In Manitoba, textile and carpet waste materials are under the Canadian Council of Ministers of the Environment (CCME) National Action Plan for EPR of the Waste Management Task Group [48]. Unwanted clothing items that could be donated are usually dropped off at city drop-off bins or collected by non-profit charitable organizations and municipal programs. Due to their poor condition, some donated textiles are frequently discarded in landfills [49].

#### **4. Textile Reuse and Recycling**

Generally, textile reuse and recycling could reduce environmental impact because it could potentially reduce virgin textile fiber production and avoid processes further downstream in the textile product life cycle. Moreover, textile reuse and recycling are more sustainable when compared to incineration and landfilling. However, reuse is considered more beneficial than recycling, mainly when sufficiently prolonging the reusing phase [50]. Textile reuse encompasses various means for extending the useful service life of textile products from the first owner to another [51]. This is commonly practiced by renting, trading, swapping, borrowing, and inheriting, facilitated by second-hand stores, garage sales, online and flea markets, and charities. On the other hand, textile recycling refers to reprocessing pre-consumer and post-consumer textile waste for use in new textile or non-textile products.

Textile recycling is typically classified as mechanical or chemical recycling. Mechanical recycling degrades waste into a decoration, construction, agricultural, and gardening use. Chemical recycling involves a process where polymers are depolymerized (polyester) or dissolved (cotton and viscose). Chemical recycling can produce fibers of equal quality compared to virgin materials [24,50]. The sorted textile waste could be chemically treated to extract resources such as protein-based fibers to produce wood panel adhesives; and cellulosic fibers for bioethanol production [27].

The textile recycling route can be classified based on the nature of the processes involved or the level of disassembly of the recovered materials [50]. Fabric recycling consists in recovering and reusing of a fabric into new products. Meanwhile, fiber recycling involves disassembling of fabric but preserving the original fibers. Polymer/oligomer recycling consists of disassembling of fibers while preserving the polymers or oligomers. Moreover, monomer recycling consists of disassembling of polymers or oligomers, while preserving the monomers [50].

Moreover, textile recycling can be classified into upcycling, downcycling, closed-loop, and open-loop recycling. If the product made from recycled material is of higher quality or value than the original product, it is termed 'upcycling'; the opposite of this is known as 'downcycling'. Closed-loop recycling involves recycling of a material from a product and reusing it in a more or less identical product. In contrast, open-loop recycling consists of recycling of a material from a product and reusing it in another product. Figure 6 shows the classification of various forms of reuse and recycling. The closed-loop recycling approach recovers the raw material used to produce a polymer product and then reprocess it into the same product of equivalent quality as that from the virgin material [50,52].

Furthermore, recycling technologies for fibers can be typically divided into primary, secondary, tertiary, and quaternary approaches. Primary approaches involve recycling industrial scraps. Secondary recycling involves the mechanical processing of a post-consumer product. Tertiary recycling involves pyrolysis and hydrolysis, converting plastic waste into chemicals, monomers, or fuels. Quaternary recycling refers to burning the fibrous solid waste and utilizing the heat generated [53].

**Figure 6.** Classification of textile reuse and recycling routes, reprinted with permission from [50]. Copyright 2018 Elsevier. **Figure 6.** Classification of textile reuse and recycling routes, reprinted with permission from [50]. Copyright 2018 Elsevier.

#### Furthermore, recycling technologies for fibers can be typically divided into primary, **5. Environmental Sustainability in Textile Recycling**

secondary, tertiary, and quaternary approaches. Primary approaches involve recycling industrial scraps. Secondary recycling involves the mechanical processing of a post-consumer product. Tertiary recycling involves pyrolysis and hydrolysis, converting plastic waste into chemicals, monomers, or fuels. Quaternary recycling refers to burning the fibrous solid waste and utilizing the heat generated [53]. **5. Environmental Sustainability in Textile Recycling** Reuse and recycling of textile waste offers environmental sustainability. Upcycling and closed-loop recycling are the potential recycling routes that maximize conservation of resources such as raw materials, water, and energy, with minimal environmental im-Reuse and recycling of textile waste offers environmental sustainability. Upcycling and closed-loop recycling are the potential recycling routes that maximize conservation of resources such as raw materials, water, and energy, with minimal environmental impact [8]. Moreover, textile reuse and recycling reduce environmental impact compared to incineration and landfilling, and reuse is more beneficial than recycling [50]. Applying ecological footprint in a textile tailoring plant revealed that the resources category has the highest ecological footprint, followed by the energy consumed [54]. Resources recovery can provide significant environmental gains by replacing products from primary resources [55]. For every kilogram of virgin cotton displaced by second-hand clothing and polyester could save approximately 65 kWh and 95 kWh, respectively [56].

#### pact [8]. Moreover, textile reuse and recycling reduce environmental impact compared to incineration and landfilling, and reuse is more beneficial than recycling [50]. Applying **6. Textile Recycling and Recovery Technology**

ecological footprint in a textile tailoring plant revealed that the resources category has the highest ecological footprint, followed by the energy consumed [54]. Resources recovery can provide significant environmental gains by replacing products from primary resources [55]. For every kilogram of virgin cotton displaced by second-hand clothing and Nowadays, various technologies can be chosen to promote textile waste recycling and recovery. Technologies such as anaerobic digestion, fermentation, and composting are among the biotechnology available for textile waste. The following sections also discuss thermal recovery and conversion of textile waste into insulation/building materials.

#### polyester could save approximately 65 kWh and 95 kWh, respectively [56]. *6.1. Anaerobic Digestion of Textile Waste*

**6. Textile Recycling and Recovery Technology** Nowadays, various technologies can be chosen to promote textile waste recycling and recovery. Technologies such as anaerobic digestion, fermentation, and composting are among the biotechnology available for textile waste. The following sections also discuss thermal recovery and conversion of textile waste into insulation/building materials. *6.1. Anaerobic Digestion of Textile Waste* Anaerobic digestion (AD) is widely used to treat a biodegradable fraction of organic waste for biogas production. Cotton was characterized by more than 50% cellulose, a po-Anaerobic digestion (AD) is widely used to treat a biodegradable fraction of organic waste for biogas production. Cotton was characterized by more than 50% cellulose, a potential substrate for biological conversion (Table 1). Over the last decade, studies have been conducted on AD using cotton waste to produce methane-rich biogas. Cotton wastes (cotton stalks, cottonseed hull, and cotton oil cake) can be treated anaerobically to produce biogas [57]. Cotton waste from spinning mills is a potential substrate for AD [58]. The AD of medical cotton industry waste under thermophilic condition with the use of cattle manure as inoculum demonstrated an improved biogas yield of approximately 92% [59]. Pretreatment methods enhance the biodegradation of complex organic matter in AD systems, resulting in an increase in biogas quality and production and improved biosolids quality

tential substrate for biological conversion (Table 1). Over the last decade, studies have

in reduced production [60,61]. Various pre-treatment technologies mainly mechanical, thermal, chemical, biological, and their integration can be chosen to enhance the digestion process [60,62]. Pretreatment prior to AD of waste jeans (60% cotton, 40% polyester) and pure cotton waste substrates using 0.5 M Na2CO<sup>3</sup> at 150 ◦C for 120 min generates a maximum methane yield of 328.9 and 361.1 mL CH4/g VS, respectively [63]. Furthermore, a comparable maximum methane production rate of 80% was obtained using single-stage and two-stage digestions in batch reactors utilizing viscose/polyester or cotton/polyester textiles with 20 g/L cellulose loading [64]. Table 2 summarizes the optimum operating conditions using batch process of anaerobic digestion from the reviewed literature.

**Table 1.** Characteristics of cotton waste [58].



