## 3 Results and Discussion
### Evaluation of PHA Production Processes Using CO2 as Feedback  
With the results of this study, three production processes using CO2 as feedstock can now be assessed:  
* the autotrophic-autotrophic process
* the heterotrophic-autotrophic process
* autotrophic-heterotrophic-heterotrophic process  
Our calculations indicated that the indirect conversion of acetic acid from CO2 is an interesting alternative for the direct production of PHB from CO2 in terms of CO2 fixation, H2 consumption and substrate cost (Table 3). Furthermore, the bioconversion of CO2 to acetic acid is not paired with the use of potentially explosive gas mixtures, as is the case for H2 and O2. Also, in terms of process performance, the indirect production of PHB from CO2 via acetic acid is promising, as shown in Table 4. In this table, a summary of the literature results for PHB production obtained in fed-batch fermentations using _C. necator_ from CO2 at CDM between 10 and 20 g/L at the onset of nitrogen limitation (phase 2) is given. The best process performance in terms of PHB concentration and content was obtained in this study. [PERSON] et al. obtained a higher productivity in their process, but the gas composition during the autotrophic stage lies within the gas-explosion range [43]. With respect to PHBV production, a trade-off between PHBV concentration and 3 HV fraction exists (Table 5). The highest concentration of PHBV was obtained in this study, while the highest 3 HV fraction was generated using the heterotrophic-autotrophic cultivation system in the study of [PERSON] et al. [9]. For both systems, biopolymer content and productivities were comparable (Table 5). By introducing an intermediate phase to enable a low residual biomass growth rate for maintaining an active metabolism, PHBV concentration and productivity could probably be increased further for the heterotrophic-autotrophic cultivation process. However, at higher biomass concentrations, mass transfer limitations could occur [8]. The indirect conversion of acetic acid from CO2 towards biopolymers looks promising. The production process should be optimised further by delaying nitrogen limitation to reach a higher residual biomass concentration and providing the nitrogen separately from the carbon feed via an additional pump. The latter would make it possible to use more concentrated carbon feeding solutions. PHA production from acetic acid also requires a highly concentrated feedstock to avoid dilution effects. Dedicated research should focus on the integrated process and its design. Furthermore, a techno-economic assessment should be performed to elucidate the viability of the process. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Cultivation Process** & **Feedstock** & **kg CO2/kg PHB** & **kg H2/kg PHB** \\ \hline Autotrophic-autotrophic & CO2 & 2.84 & 0.96 \\ Heterotrophic-autotrophic & Glucose/CO2 & 1.58 & 0.77 \\ Autotrophic-heterotrophic-heterotrophic & CO2/acetic acid & 2.84 & 0.42 \\ Heterotrophic-heterotrophic & Glucose & 0 (emits 2.81) & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of processes in terms of CO2 and H2 consumption based on mass balances (Equations (1)–(6)).