## 3 Results and Discussion  
### Assessment of C. vulgaris Growth in an Outdoor Photobioreactor  
Figure 1 presents the time course of evolution of the microalgal biomass concentration over 8 d of cultivation. _C. vulgaris_ grew exponentially at the initial stage of cultivation (0-4 d), with a specific growth rate of (0.455 \(\pm\) 0.09) d\({}^{-1}\). However, growth inhibition was observed subsequently as the culture reached the stationary phase. In the growth curve, different growth phases are defined for microalgal cultures: lag, exponential, stationary, and decline phase. In this work, the lag phase was not evidenced, considering that the microalgal culture comes from a growing matrix in the bank of LABAM. The stationary phase was observed between the fourth and eighth day of cultivation. The biomass was harvested on the eighth day. Harvesting the biomass in the stationary phase could help improve the coagulation/flocculation process since microalgae produce substances such as extracellular polymeric substances (EPSs) that increase the aggregation of microalgal cells. For instance, according to [PERSON] et al. [28], during the stationary phase, culture aging and an increase in EPS concentration can play a key role in the flocculation efficiency of _C. vulgaris_.  
The maximum biomass concentration was 1.28 g L\({}^{-1}\), while [PERSON] et al. [29] achieved a maximum concentration of 75 mg L\({}^{-1}\) in approximately 2 weeks of growing _Chorella_ in BG-11 medium with nitrogen and phosphorus contents of 45 and 4.5 mg L\({}^{-1}\), respectively, in an artificial climate chamber. [PERSON] et al. [30] obtained a specific growth rate of 0.21 d\({}^{-1}\) and biomass concentration of 0.99 g L\({}^{-1}\) for 30 d of cultivation of _C. vulgaris_. In  
Figure 1: Time-course evolution of the microalgal biomass concentration.  
this study, the culture was carried out at 20 \({}^{\circ}\)C with light-restricted conditions (incident irradiation of 400 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\)), although it had increased to 3 g L\({}^{-1}\) of NaNO\({}_{3}\) of the BG-11 medium to ensure full nitrogen supply to the biomass growth of _C. vulgaris_. By comparing the cultivation conditions of this work with that of [PERSON] et al. [30], the different results can be explained by the different experimental conditions, such as the lower concentration of nitrogen and phosphorus in the culture medium, since the BG-11 medium was diluted in the cooling water from the distiller, the shorter cultivation period, the use of solar radiation rather than artificial and consequent temperature variations. In other words, microalgae demonstrated efficiency in adapting to a culture system in an environment with nutrient restrictions and with high light intensity during the day to grow and develop.  
The growth rate allows for defining the growth rate of _C. vulgaris_; according to the literature, the growth rate should be between 0.550 and 1.872 d\({}^{-1}\)[31]. The average biomass productivity was 0.12 g L\({}^{-1}\) d\({}^{-1}\), consistent with the results of [PERSON] et al. [31]. In their study, microalgae grew slowly in the beginning (the maximum biomass productivity and the maximum specific growth rate were 0.192 g L\({}^{-1}\) d\({}^{-1}\) and 0.864 d\({}^{-1}\), respectively), due to an inadequate supply of carbon, as only CO\({}_{2}\) from atmospheric air was supplied to the _C. vulgaris_ culture.  
According to [PERSON] et al. [32], microalgae such as _Chlorella_ sp. and _Scenedesmus_ sp. are characterized by high growth rates when cultured outdoors under natural sunlight. In their work, the culture of C. _vulgaris_ achieved a specific growth rate of 1.48 d\({}^{-1}\) and biomass productivity of 0.297 g L\({}^{-1}\) d\({}^{-1}\), while, in this work, these kinetic parameters were lower. This reason may also be associated with nitrogen insufficiency and CO\({}_{2}\) limitation due to the aeration of atmospheric air.