## 5 PET Bottle Degradation Management Strategies
### Improving the Efficiency of Waste Management
#### 5.2.1 Biodegradation and Biological Recycling  
Microorganisms capable of degrading PET, which can even assimilate the products of degradation, represent one of the solutions to the problem of plastic pollution in the environment. Reduced human involvement in the PET biodegradation process is a significant advantage of this approach. This process is natural but usually very slow. Many microorganisms capable of breaking down PET have already been discovered, and some of them have been genetically modified to improve the efficiency of this process. One of the most recognizable microorganisms degrading PET is _Ideonella sakaiensis_. This gram-negative bacterium was isolated in 2016 in Sakai, Japan, from samples taken from a PET bottle recycling site [125]. This bacterium has a unique ability to break down PET using two enzymes: PETase and MHETase. PETase hydrolyzes PET into mono(2-hydroxyethyl)erephthalate (MHET), which is then converted by MHETase into terephthalate acid and ethylene glycol. These end products can be utilized by the bacteria but also harvested for further applications, such as in industry. Studies have shown that _Ideonella sakaiensis_ grows exclusively on amorphous regions of PET while areas with high crystallinity do not support its growth [126]. This indicates the significant impact of PET bottle crystallinity on their enzymatic degradation and recycling process. Most PET bottle surfaces used for water packaging are made of highly crystalline PET, which hinders their enzymatic breakdown. In contrast, low-crystallinity PET is much more susceptible to enzyme action [127]. To improve the efficiency of biodegradation and enzymatic recycling of PET bottles, it may be crucial to apply techniques that modify their crystallinity, such as rapid cooling of molten PET, leading to a more amorphous structure [126]. Protein engineering enables the design of modified enzymes that hydrolyze PET, significantly improving the efficiency of its degradation. An example is the engineering of PETase, with modifications aimed at increasing the enzyme's thermal stability and activity. The IsPETase triple mutant exhibits significantly higher thermal stability and degradation efficiency compared to the wild-type PETase [128]. Additionally, [PERSON] et al. [129] developed an improved variant of leaf-branch compost cutinase (LCC), which achieves at least 90% PET depolymerization into monomers within 10 h at a temperature of 72 \({}^{\circ}\)C. This process, based on the enzymatic depolymerization of PET waste, allows for the reuse of monomers to produce new bottles, thereby supporting the concept of a circular economy, where PET can be cyclically used in packaging production. Studies have shown that PET obtained through biological recycling exhibits the same properties as PET produced from petrochemical raw materials. One of the solutions to increase the efficiency of PET degradation could be enzyme engineering based on machine learning. Using this approach, [PERSON] and [PERSON] [130] designed a mutated PETase enzyme that exhibits activity at a higher optimal temperature and is characterized by thermostability, enabling more effective PET degradation. Additionally, [PERSON] et al. [37] applied machine learning to design a new PET hydrolase, named FAST-PETase. They found that this variant exhibits significantly higher hydrolytic activity towards PET water bottles compared to wild-type PETase and other modified variants. FAST-PETase is capable of almost completely degrading pre-treated PET bottles at a temperature of 50 \({}^{\circ}\)C in less than 2 weeks. Moreover, at this temperature, FAST-PETase can also depolymerize untreated, amorphous parts of commercial water bottles and whole bottles subjected to thermal treatment. One of the key challenges for enzymatic PET recycling is its variable crystallinity, the presence of contaminants, and additives in actual post-consumer waste streams.