## 3 Results
### Effect of Stirring Speed on Microalgae Growth  
Hydrodynamic stress is a factor that has not been studied in Raceway bioreactors because it is considered so low that it can be neglected [31]. However, no study has been conducted to confirm this assumption. Figure 5 represents the growth curves of the microalgae consortium with the same culture medium but different stirring speeds. The growth difference is evident at 45 rpm where the cell density increased more than six times. The increase in biomass measured in cell doublings at 35 and 40 rpm is similar, 1.762 and 1.782 times, respectively, which corresponds to 570, 417, and 626,250 cells/mL for each stirring speed. It is thought that this situation occurs due to the biomass quantification method, in which the number of cells present is counted, but the weight of biomass per unit volume is not determined since the microalgae have different sizes [32; 33]. However, the graph (Figure 5) shows that agitation favors the growth of microalgae. This has already been observed by [PERSON] et al. [34], who determined the growth of _Chorella vulgaris_ in a 20 L Raceway bioreactor at speeds of 13, 30, and 35 rpm, where they obtained 1.3, 1.8, and 1.85 g/L of final biomass for each stirring speed. Poor agitation leads to poor mixing and results in poor oxygen removal during photosynthesis, which is counterproductive for microalgae as it slows the growth and accelerates cell death [4]. The literature shows that, as a microalgae culture develops, the flow rate is reduced due to the increase in cell density, making it difficult to maintain a homogeneous suspension. This increases the sedimentation of microalgae and the stagnation of the flow, so it is necessary to double the stirrer speed to maintain a flow speed of 0.30 m/s [4]. Figure 6 shows the speed reduction as a function of the sum of the speed difference (\(\sum\Delta v\)) with respect to the cultivation time of the microalgae. The data were processed from the beginning of the stationary phase of culture that corresponds to day 10. The graph represents different speeds of agitation. The data indicate that, at 35 rpm, with an initial cell density of 323,750 cells/mL and a final of 570,417 cells/mL, the flow velocity is 0.3913 and 0.3271 m/s, which implies a decrease of 16.41%. At 40 rpm, the speed was reduced by 17.11%; the initial speed was 0.4494 m/s, and it decreased to 0.3725. In the two previous cases, the drop in speed presented a percentage difference of 0.7. However, at 45 rpm, the linear speed decreased by 20.13%, starting at 0.4990 m/s and ending at 0.3985 m/s. For the three stirring speeds, the speed reduction behaves linearly for the first three days (lag phase). The greatest effect on the decrease in speed occurs from the fourth day and up to the eighth (in the exponential phase) due to the increase in the number of cells. After day nine, the drop in speed began to slow down, which corresponds to a slower cell doubling (stationary phase). The velocity decrease curve can be used as an analogy to the growth curve of the microalgae consortium but in an inverted way. To maintain the same speed at the beginning  
Figure 5: Comparison of consortium growth measured in the sum of cell duplications at 35, 40, and 45 rpm. and end of the culture, the angular speed must be adjusted and must be proportional to the cell density and its increase. On the other hand, one of the effects of hydrodynamic stress on microorganisms is the change in the size of cell aggregates, causing microalgae to agglomerate to protect themselves [17]. This behavior was observed in the consortium under study. Figure 7 represents the evolution of the consortium at 35 rpm, where the cells agglomerated to form granules, where it is difficult to identify individual cells. Figure 6: Representation of the decrease in speed with respect to the cultivation time of the microalgae consortium at different agitation speeds.