## 5 Targeted Acidogenic Fermentation for Bioplastics Production
By carefully managing the process pH, it is possible to tailor the VFA profile to synthesize bioplastics with specific mechanical, thermal, and degradation properties, aligning the fermentation process with industrial and environmental demands. Apart from pH, HRT and OLR are pivotal operational parameters in anaerobic digestion processes, significantly influencing the production of specific VFA, which serve as precursors for bioplastics like PHA. Adjusting HRT impacts microbial growth and the quantity of organic matter introduced into the reactor. Extended HRTs facilitate the degradation of complex substrates by allowing for increased bacterial proliferation and the establishment of microbial communities capable of hydrolyzing feedstock components, thereby enhancing the availability of soluble organic matter for the acidogenic phase. However, prolonged HRTs, associated with low flow rates, may encourage the growth of slow-growing microorganisms such as methanogenic archaea, which are undesirable in this context. Therefore, shorter HRTs are often preferred to suppress methanogenic activity [28]. High OLRs, typically linked with short HRTs, can lead to process overloads that lower pH levels, thereby promoting the production of short-chain fatty acids and further inhibiting methanogenic activity. For instance, [PERSON] et al. [137] observed that HRTs shorter than 6 days enhanced VFA production, while [PERSON] et al. [138] reported a twofold increase in VFA production when increasing HRT from 19 to 41 days using untreated complex substrates. Additionally, short HRTs are generally associated with the production of shorter VFAs, such as acetic and propionic acids [19], whereas longer HRT with carbohydrate-rich feedstocks combined with acidic pH conditions tend to result in the accumulation of longer VFAs like butyric and capric acids [139]. Consequently, selecting appropriate operational conditions should consider the macromolecular composition of the feedstock and the desired VFA profile for subsequent processes. Given the substantial impact of HRT and OLR on process performance and VFA profiles, alternative reactor configurations to continuous stirred tank reactors are under active investigation. Depending on feedstock characteristics, systems such as up-flow anaerobic sludge blanket reactors [140], sequencing batch reactors [141], and anaerobic membrane bioreactors [142] are studied, as these designs allow for the decoupling of HRT and solid retention time. This decoupling enables the maintenance of high flow rates without necessitating significantly larger tank volumes, due to the high solid retention within the system, thereby enhancing the cost-effectiveness of the process. As already discussed for pH, the selection of operational parameters, including HRT and OLR, significantly impacts the VFA profile generated during anaerobic digestion, which subsequently determines the type of bioplastic that can be produced. For instance, a study on the acidogenic fermentation of wood mill effluents operated at pH 5.5 with HRT of 1.0 and 1.5 days, corresponding to OLR of 5.6 and 2.9 g COD/L-d, respectively, found that while acidification yields were similar, higher HRTs led to increased concentrations of longer-chain VFAs [143]. These longer-chain VFAs, such as butyric and valeric acids, are crucial precursors for the production of mcl-PHAs. Therefore, by adjusting HRT and OLR, it is also possible to steer the fermentation process toward the production of specific VFAs, thereby tailoring the synthesis of desired bioplastics. Another crucial parameter influencing VFA production during acidogenic fermentation is temperature. The optimal temperature range is determined by the microbial community, which is influenced by the composition and complexity of the feedstock. Since hydrolysis is often the limiting step in the anaerobic fermentation process, mesophilic and thermophilic conditions are commonly employed to enhance microbial enzymatic activities.