# Valorization of Agri-Food Waste into PHA and Bioplastics: From Waste Selection to Transformation
## 3 Production of Bioplastics from Volatile Fatty Acids
### Microorganisms Responsible for the Transformation of VFA into PHA  
#### 3.1.1 Phosphorus-Accumulating Organisms  
Phosphorus accumulating bacteria, or Phosphorus-Accumulating Organisms (PAOs), are a specialized group of microorganisms that are capable of storing significant quantities  
Figure 3: Schematic overview of the dark fermentation process and production of VFAs that can be consumed by different microorganisms to produce different bioproducts. Adapted from [PERSON] et al. [32].  
of phosphorus intracellularly, predominantly in the form of polyphosphates. Consequently, in wastewater treatment systems, PAOs play a pivotal role in the biological phosphorus removal process (BPRS), assisting in the reduction of phosphorus content in the final effluent and the prevention of its discharge into water bodies [130,131].  
PAOs operate within a specific anaerobic-aerobic cycle. The anaerobic phase is characterized by the absence of oxygen. PAOs release phosphorus into water by decomposing their polyphosphate reserves and, concurrently, assimilate volatile fatty acids, which they subsequently store as polyhydroxyalkanoates [132,133] in the subsequent aerobic phase. In the presence of oxygen, PAOs utilize PHAs as an energy source, thereby recovering phosphorus from the medium and rebuilding their intracellular polyphosphate reserves. This cycle enables the net accumulation of phosphorus in the system and its removal from the sludge in water treatment [134]. The optimal pH and temperature range for PAOs is near neutral (6.5-8.0) and moderate (15-30 \({}^{\circ}\)C) [118]. The growth of PAO and their PHA storage capacity is dependent on volatile fatty acids such as acetic and propionic acid. This enables efficient phosphorus processing [131,135].  
It is evident that these phosphorus-accumulating bacteria play a pivotal role in the removal of phosphorus in wastewater treatment systems. Additionally, they have the capacity to function as PHA producers from volatile fatty acids. PAO bacteria have metabolic characteristics and the ability to adapt to specific conditions that make them of great interest for use in PHA production processes:  
* PAO can utilize volatile fatty acids, such as acetic acid and propionic acid, found in sewage and other organic wastes. This allows the use of low-cost materials, creating a more economical and sustainable PHA production process, rather than relying on purified carbon sources such as glucose. This capability contributes to the circular economy concept by enabling the vaporization of waste [133,134].
* PHA production in an anaerobic-aerobic cycle. Under anaerobic conditions, storage cells integrate PHAs using VFAs as a carbon source and store it in their cells as an energy reserve. In the aerobic phase, they consume this PHA for phosphorus accumulation, a process specific to PAOs. This anaerobic-aerobic cycle has the advantage that PHA production does not require continuous aeration, thus reducing energy consumption compared to other microorganisms that produce PHAs only under aerobic conditions [132].
* Flexibility in the production of different types of PHA. PAOs can produce different types of PHA, such as poly(3-hydroxybutyrate) (PHB) or poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) [135]. This flexibility in the type of PHA allows the mechanical and thermal properties of the bioplastic to be tailored to specific applications, such as biodegradable packaging or medical materials products [136].
* Compatibility with wastewater treatment. Already found in biological wastewater treatment systems due to their role in phosphorus removal, PHA production systems could be integrated into water treatment facilities, making use of existing infrastructure and eliminating the need for separating production facilities. This would allow simultaneous nutrient removal and bioplastic production, maximizing system efficiency [137].