*6.2. Fermentation of Textile Waste for Ethanol Production*

Investigation of cotton gin waste as feedstock for ethanol production started in 1979 at Texas Tech University; however, limited studies investigated the efficacy of textile waste for ethanol production [66]. The effect of alkali pretreatment to enhance ethanol production was evaluated using polyester/cotton blend (polycotton) textile. The maximum ethanol yield by simultaneous saccharification and fermentation was 70% after the pretreatment with NaOH/urea at −20 ◦C, which was considered the most desirable [67]. Moreover, the cotton part of the waste blue jeans (40% polyester/60% cotton) was investigated for ethanol production, which involves the process of enzymatic hydrolysis and fermentation [63]. Enzymatic hydrolysis converts cellulose to fermentable sugars [58]. The effect of corona

pretreatment of non-mercerized and mercerized cotton fabrics enhanced the glucose and ethanol yields. The cotton fabric demonstrated its potential as an alternative feedstock for bioethanol production [68]. Table 3 summarizes the optimum operating conditions for ethanol production based on the reviewed literature.


**Table 3.** Optimum operating conditions for ethanol production using cotton wastes.

#### *6.3. Composting of Textile Waste*

Composting is a natural phenomenon of biodegradation of organic waste, such as cotton waste, into a valuable soil supplement. Composting is a low technology, biooxidative process that reduces the volume of organic waste by up to 50% over the active phase of composting [66]. Composting utilized various microorganisms, including bacteria and fungi, to convert complex organic matter into simpler substances in the presence of air. Cotton waste poses a significant waste disposal problem nowadays, and composting was viewed as an alternative in preventing the direct landfill disposal of cotton trash. Composted and vermicomposted cotton trash could be an excellent long-term nutrient source [69].

Vermicomposting is a biotechnological composting process that uses earthworms to convert waste into compost with improved soil fertility that significantly exceeds conventional compost [69]. Using cotton waste substrate, the number of bacterial diversity in compost and vermicompost samples was similar. However, the vermicompost samples contain a rich density of bacterial isolates when compared with compost samples which produce better humus [70].

Vermicomposting of cotton textile waste in the form of willow waste from ginning factories was investigated. Willow waste is undesirable for textile application and is just disposed into landfill. The collected willow waste was mixed with cow dung slurry, cellulase, and amylase enzymes (isolated from cow dung), and an effective microorganism solution. The mixture was turned and sprinkled with water periodically. After 20 days, the waste was wholly decomposed, and earthworms were introduced. The vermicomposting process was ended when the waste mixture turned light brown or dark brown after 14 days. The resulting vermicompost was then used to grow plants in pots and revealed that the plants grown using the vermicompost made from willow waste had an excellent growth rate in root length, shoot length, and leaf area index compared to the control pot [71].

Furthermore, cotton gin waste cannot be directly reused on-farm due to farm hygiene risks, and composting of cotton gin waste is an accepted method [66]. Cotton gin waste was used as a bulking agent for pig manure composting under two different proportions of 4:3 and 3:4 of pig slurry:cotton gin waste [72]. This study concluded that the thermal properties of the bulking agent were responsible for the temperature development and aeration demand. The gaseous emissions were related to the organic matter degradation process. The compost with the higher proportion of pig slurry (4:3) had greater organic matter humification and higher nutrient concentrations.

Furthermore, since the 1980s, the waste cotton substrate was utilized for oyster mushroom cultivation. More than 90% of oyster mushroom growers utilized waste cotton substrate for cultivation [73]. Cotton waste with fermented poplar sawdust exhibited the highest yield on fruit bodies of oyster mushroom, equivalent to 742 g per 4 kg of substrate [73]. A new cotton waste composting technology to cultivate oyster mushrooms shows a higher mushroom yield of 65.1% over substrate dry weight when compared to a traditional natural fermentation technology with a 43.6% yield [74]. The process involves adjusting cotton waste moisture content to 65%, after which it was pre-composted for two days by soaking in a lime solution. Then, the cotton substrate was sprayed with the previously prepared Ctec2 enzyme under optimal enzymatic activity conditions (pH 5, 50 ◦C, 60 h, and enzyme to substrate ratio of 0.45%) and then inoculated in pure culture of fungus. Then spawning, caring of the bed, and harvesting was conducted [74].

#### *6.4. Fiber Regeneration from Textile Waste*

Since the 'export for reuse option' is no longer a sustainable option for second-hand clothing in many developing countries, virgin cotton fiber production demands the use of extensive resources. Fiber regeneration by recycling cotton waste garments is a closedloop upcycling technology for cotton waste garments [75]. Fiber regeneration involves transforming the waste cotton fabrics into pulp, dissolving the pulp using a solvent, and spinning into fibers. The N-methylmorpholine N-oxide (NMMO) solvent can dissolve cellulose completely without any degradation and is environmentally safe to use. Pulp reclaimed from cotton-based waste garments can be blended with wood pulp to make fibers similar to lyocell [76].

Furthermore, phosphoric acid pretreatment was applied to waste textiles to recover polyester and glucose. The four pretreatment conditions investigated were the phosphoric acid concentration, pretreatment temperature, time, and the textiles to phosphoric acid ratio. The results showed that 100% polyester recovery was achieved with a maximum sugar recovery of 79.2% at the optimized conditions of 85% phosphoric acid at 50 ◦C for 7 h and the ratio of textiles and phosphoric acid of 1:15 [77]. The feasibility of cellulase production and textile hydrolysis using fungal cellulase vs. commercial cellulase via submerged fungal fermentation (SmF) using textile waste was investigated. The study demonstrated that glucose recovery yields of 41.6% and 44.6% were obtained using fungal cellulase and commercial cellulase, respectively. Thus, the proposed process has great potential in treating textile waste for the recovery of glucose and polyester as value-added products [52].

### *6.5. Building/Construction Material from Textile Waste*

Textile waste represents a source of raw materials for typical application in construction, such as insulation materials for noise and temperature and fillers or reinforcements of concrete [78]. The conversion of fibrous carpet waste into a value-added product as soil reinforcement demonstrated that fibrous inclusions derived from carpet wastes improve the shear strength of silty sands [79]. Moreover, textile reinforced concrete (TRC) is a composite concrete material that uses textile as reinforcement material used in various applications, including precast constructions, repair, rehabilitation, and structural strengthening of existing structures. This is innovated by the construction industry, which promotes sustainability in building material by utilizing waste from the textile industry. It combines fine-grained concrete and multi-axially oriented textiles which offers advantages such as thin size, good load-bearing capacity, resistance to corrosion, excellent ductility, no magnetic disturbances, and lightweight [80,81]. Furthermore, textile waste is used to produce thick ropes designed for slope protection against sliding and erosion. Scraps of insulating materials produced from poor quality wool and scraps of nonwoven produced from a blend of recycled fibers

were used to produce ropes. The results confirmed the usefulness of the technology for the protection of steep slopes [82].

#### *6.6. Thermal Recovery*

Incineration with the thermal recovery of unwanted textiles not suited for recycling (carpets or textiles with unknown fibers) is considered a viable alternative to landfilling. Carpet fibers have a high calorific value that can reduce the need for fuels, and the resulting ash becomes raw material for cement [1]. The advantage of the incineration option is that it can handle the most significant part of unsorted textile waste, and energy can be recovered from combustion. However, burning textiles alone can cause irregular temperature behavior, ignition rate, and weight loss percentage in the ignition propagation stage. For this, textile waste should be mixed with waste cardboard upon incineration to maintain a uniform burning behavior of textiles [83]. Incineration of 1 ton of household textile waste can recover 15,800 MJ of energy, and 27 kg of ash is generated [84,85].

#### **7. Textile Waste Management Challenges**

The global increase in clothing consumption and production has resulted in a significant increase in textile waste generation, posing alarming challenges in many leading countries. Textile waste is recognized as the fastest-growing waste stream in MSW across the globe. However, waste collection and economically viable sorting infrastructure remain a challenge. Sorting of textile waste involves intensive time and labor and complications by arising from variations in fiber blends pose a significant challenge. Automation for sorting and innovations in textile recycling are growing interests [4]. Textile reuse, the most preferred option, suffers a shrinking market due to banning imported used clothing in some countries. Textile reuse and recycling to produce new products should be driven by economic incentives to make it feasible for the operating industry. Sustainable blended materials made from recycled fibers are innovative to reduce environmental impact. Further work on the characterization of the structure and properties of cellulosic fibers regenerated from cotton-based waste is essential. Moreover, recycling technologies to sustainably manage other textile waste, such as man-made cellulosic fiber (MMCF) and other fibers (polyamide, wool, rayon, silk, acrylic, etc.), need to be investigated. MMCFs are a group of fibers derived primarily from wood and in other sources of cellulose, which constitute the third most commonly used fiber in the world, behind polyester and cotton. MMCF accounts for approximately 6.4% of total fiber production, with an annual production equivalent of about 7.1 MT [11,86].

Moreover, developing non-conventional fibers—such as bast fibers—and a chemicalfree binding technology promote sustainability. Natural fibers—such as bast fibers (among them hemp, flex, nettle, and jute)—can yield significant benefits due to a smaller environmental footprint when compared to conventional plant-based fibers. Innovations supporting the circular economy and closed-loop recycling systems include recycling technologies that can produce new fibers comparable to virgin fibers. Shifting from a current linear economy into a circular economy yields tremendous environmental benefits for the fashion industry while mitigating the effects of greater demand for garments due to a rising world population [34].

#### **8. Conclusions**

The global rise in population, industrial growth, and improved living standards have caused a global fiber consumption that generates an alarming amount of unwanted textiles. Economic and environmental sustainability should be incorporated into the longterm textile waste management program. Though the application of EPR policy in textile waste is still limited, it is considered essential in promoting a circular economy system. EPR makes the producers responsible for the overall textile waste management from the collection to the disposal at the end of the product's life cycle. Besides EPR, there is a holistic approach involving major stakeholders (industry, government, private agencies,

and consumers) who must work in unity to promote a dynamic circular system. The emerging economies in textile manufacturing should take the lead in shifting from a linear economy to a circular economy.

Textile reuse and recycling are more sustainable than incineration and landfilling, but reuse is more beneficial than recycling. For this, designing a textile product by prolonging the service life quality could promote reuse. In addition, it is essential to promote consumer awareness to foster an environmentally friendly consumption behavior on textile products. Leading economies should manage their textile waste in a closed-loop circular approach, mainly when exporting textile waste to developing countries is being outlawed. Various streams of textile recycling technologies are available and continue to innovate new ideas with biotechnology advancement. Applying holistic technologies, and not relying upon a single technology, to manage a complex textile waste is deemed essential.

**Author Contributions:** Conceptualization, Q.Y., J.P.J.-L.; Literature review, J.P.J.-L. and Q.Y.; Methodology, J.P.J.-L. and Q.Y.; Writing—original draft preparation, J.P.J.-L. and Q.Y.; Writing—review and editing, Q.Y., J.P.J.-L. and I.V.L.; Supervision, Q.Y.; Funding acquisition, Q.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research support was provided by the federal funds appropriated to the Graduate Enhancement of Tri-Council Stipends (GETS), the University of Manitoba and the Natural Sciences and Engineering Research Council of Canada (NSERC RGPIN-2014-05510).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
