**4. Biological recycling methods**

Polymers can be degraded by a synergetic combination of different degradation mechanisms present in nature. Enzymes or by products produced by microorganisms such as bacteria, yeasts, fungi, enzyemes, are responsible for the occurance of microbiological degradation. Also, mechanical, chemical or enzymic aging of polymers can be caused by macro-organisms that eat and, sometimes, digest polymers. The main steps of biological degradation are depolymerisation, down to oligomeric or monomeric fragments, and mineralization. Enzymes are the biological catalysts, which can cause huge increases in reaction rates in an environment that is not favorable for chemical reactions [72].

Biological recycling, considered as an improved form of chemical recycling by some researchers, is an emerging approach and expected to be relatively cleaner than conventional textile recycling approaches [31, 73]. During biological recycling, certain polymers can be converted into compost or other substances under specific process parameters (pressure, pH, etc) within the presence of microorganisms [74]. There are several bio-recycling methods developed to recover cotton and PET fiber from textile waste. Research on pretreatment and hydrolysis of cellulose (biodegradable part in textiles) has been carried out to convert it into fermentable glucose [75]. However, polyester in the cotton-based textiles restricts bioconversion of textile waste, as it obstructs the enzymes from reaching the cotton during hydrolysis. Hence, current researches mainly focus on optimizing the conditions for pretreatment and enzymatic hydrolysis, for effective and efficient bioconversion of cotton–polyester blends [76]. According to one such biological recycling method PET fibers could be recovered from polyester-cotton blended wastes by hydrolyzing cotton via enzymatic methods to obtain glucose. The remaining non-biodegradable component-polyester can be re-spun into fibers. The carbon components of most man-made polymers cannot be broken down by the enzymes of microorganisms and that is why such polymers are resistant to biological degradation. Despite this, a commercial approach has been succesfully developed by CARBIOS for enzymatic recycling of PET in various plastics or textiles [58, 77]. Accordingly, the process biologically recycles PET by using an enzyme capable of specifically depolymerizing the polymer to its monomers.

A few organizations, in collaboration with several clothing brands, are also developing sustainable solutions to effectively manage textile waste for obtaining bio-based raw materials. The Hong Kong Research Institute of Textiles and Apparel (HKRITA), for instance, has joined hand with the Hong Kong Polytechnic University to develop new bio-based textiles that combine the properties of Polyactic (PLA) and polyhydroxybutyrate-co-hydroxyvalerate (PHBV) to function as a green alternative to non-biodegradable synthetic polymers in the market. PLA is derived from renewable sources such as cassava roots, corn, and sugar cane,

#### *An Evaluation of Recycled Polymeric Materials Usage in Denim with Lifecycle Assesment… DOI: http://dx.doi.org/10.5772/intechopen.99446*

whereas PHBV is naturally produced by fermentation process of bacteria [78, 79]. Worn Again Technologies has, however, focused on converting polyester and polycotton blended textiles, and PET plastic back into circular raw materials using their special recycling technology which is stated to be able to separate, decontaminate and extract polyester and cellulose from textile waste, polyester bottles and packaging to produce dual PET and cellulose outputs [80].

Clearly, textile waste with high cellulose content, mainly from cotton, can also be used as an alternative feedstock for man-made cellulosic fibers' (MMCF) production by recycling of post-production, pre-consumer (e.g., samples or stock that cannot be sold) or post-consumer textiles. Promising natural fibers with high α-cellulose content and low hemi-cellulose content are also found in the fibers of banana, pineapple, and abaca leaves. Man-made cellulosic fibers (MMCF) producers are, therefore, intensifying their research and development (R&D) activities to focus on alternative feedstock for cellulose production [81]. Regarding that, Circulose® is one of the few commercially available products. It is "dissolving pulp" from 100% textile waste, such as worn-out jeans and production scraps, used to manufacture viscose, lyocell, modal, acetate other types of regenerated fibers [82]. SaXcell, an abbreviation of Saxion cellulose, is another regenerated virgin textile fiber made from chemical recycled domestic cotton waste. The end product is SaXcell, a regenerated virgin cellulose fiber that can be spun into yarns and turned into fabrics [83]. As may be seen from these few examples, the use of alternative fibers could become an interesting option for the production of man-made cellulosic fibers (MMCF), but they still face some economic, technological and social barriers to scalability.
