# Microbial and Enzymatic Biodegradation of Plastic Waste for a Circular Economy
## 4 Biodegradable Plastic and the Circular Economy  
Plastic valorisation is the process of turning waste plastic into energy, useful chemicals, or other usable resources to reduce resource depletion and environmental impact. Enzymatic valorisation is an environmentally friendly approach when compared with traditional chemical or mechanical recycling because it operates under milder conditions, such as lower temperature and pressure, and typically does not require harmful chemicals [167]. High-purity product recovery is also possible since enzymes can be highly selective [168], resulting in the production of valuable byproducts upon degradation [168]. [PERSON] et al. reported biodegradation of PHA into 3-hydroxyalkanoates (3 HAs), which can be used to make new plastics [169]. Creating a closed loop re- and up-cycling system, where the degraded products from plastic are repolymerised into new materials and other products such as butanol or biofuels, is currently of significant interest [170]. To make bio-based valorisation more effective, bioreactors are employed for enzymatic degradation in controlled environments. The degradation of PLA in non-sterile soil environments is influenced by both abiotic and biotic factors. Microorganisms present in the soil can catalyse the hydrolysis of PLA. Therefore, the observed 16% degradation after 180 days is likely a combined effect of both intrinsic polymer properties and microbial activity in the soil environment (see Table 2). However, employing enzymatic degradation in a controlled environment, such as a bioreactor, could enhance degradation and favour product recovery. For example, enhanced PET waste conversion into monomers has been demonstrated by enzyme systems like PETase/MHETase (see Section 2.3), indicating the possibility for multienzyme systems (i.e., enzyme cocktails) which can break down PET and possibly polyolefins and other plastic additives [171]. An example of industry-scale enzymatic valorisation was commercialised by CARBIOS [172] for recycling PET by employing specifically engineered enzymes, effectively breaking PET down into ethylene glycol and terephthalatic acid. These monomers can then be repolymerised to produce high-value, food-grade PET [173]. Applying such models to biodegradable plastics can lead to more effective and sustainable valorisation strategies [174]. The potential for obtaining biodegraded products from mixed plastic is still challenging and currently requires plastic sorting and segregation prior to degradation. This \"sorting problem\" is an expensive part of the recycling process. However, promising methods are being developed, including the use of metal-promoted autoxidation to chemically oxidise mixed plastics such as HDPE, PET, and polystyrene [175; 176]. In this approach, the polymers are treated to become water-soluble intermediates, which are subsequently broken down for re- and up-cycling through enzymatic degradation by engineered _Pseudomonas putida_[177]. End-of-life biodegradable plastics can be valorised through a circular lifecycle concept, specifically by upcycling biodegradable plastics into chemical and molecular intermediates, which can then be converted into more valuable products. For example, in agricultural munching fields, plastic muld films are widely used for weed control, moisture retention, and soil temperature regulation. However, these films often persist in the environment after use. According to EU guidelines, all polymer products used in agriculture must be biodegradable [36]. Therefore, in the context of agriculture, increasing the rate of biodegradation of mulding films in situ will save time and provide organic fertiliser for the soil, leading to increased soil fertility. An interesting example of recycling plastic waste involves PLA, which is a biodegradable plastic that can be depolymerised into lactate monomers (see Figure 7). The hydrolytic depolymerisation of PLA into its monomeric units is catalysed by enzymes such cutinases, lipases, and proteinase K [178; 179].