## 4 Discussion
### Cultivation
Finally, in contrast to laboratory scale, the DO could not be maintained at the setpoint throughout the fermentations (Figures S25-S28). The DO dropped below the setpoint after 6 h (strain V194) or 9 h (strain V134), remaining below the setpoint throughout the fermentations. The unusual geometry of the small pilot scale reactor (height-to-diameter ratio of 1:0.17) may be another factor contributing to decreased oxygen transfer in small pilot scale compared to laboratory scale. Similar effects were observed when scaling the V134 process further up to the 500 L scale. The SCO content and SCO yield were comparable to the smaller scale processes, but the fermentation duration increased, and lipid productivity decreased. This is explained by a further decrease in the inoculation volume from 4% of the starting volume to 0.9%. Additionally, the DO also dropped and remained below the setpoint after 8 h of fermentation (Figure S29). As in small pilot scale, it is assumed that the geometry of the bioreactor and the configuration of the stirrer resulted in a lower OTR. In addition to being a potential influence on volumetric lipid productivity, limited oxygen uptake has also been reported to decrease the share of unsaturated fatty acids in the produced SCO [45]. Since there was not a sufficient air supply to maintain the DO setpoint, it is likely that such effects occurred in the described processes. Nevertheless, the ratio of saturated fatty acids (SFAs) to unsaturated fatty acids (UFAs) remained comparable at the different scales (see Figure 2). However, compared to the laboratory scale process an increased proportion of C18:0 in the lipids was observed in the pilot-scale fermentations, while the proportion of C17:0 decreased at approximately the same rate (see Figure 2). Since the ratio SFA/UFA remained essentially constant, and as the presence of HPr has been shown to result in the production of C17:0 fatty acids [23], the described effect can be primarily attributed to the reduction in the HPr/HAc ratio in the feed. While this change complicates the interpretation of the data, the feed was nevertheless adapted to better reflect the composition of actual VFA mixtures derived from the AD of food wastes [16]. Apart from the described effect, the FA composition of the lipids produced at the various scales is quite comparable. When comparing the DCW, SCO content, and lipid yield, no clear difference between the various scales could be observed. A slight change in the fatty acid pattern is observed, but, as described above, this can be primarily attributed to a deliberate change of the used feed composition. The described limitations regarding aeration are a point that should be addressed in the future, but otherwise, no major challenges in scaling the process could be identified. The decreased productivity in larger scale can potentially be overcome by using customized bioreactors with suitable oxygen transfer and the use of seed fermenters for inoculum preparation. Considering that the three types of bioreactor used for this study differed in their geometries and configurations depending on the scale, the results suggest a robust, scalable fermentation process.