## 3 Results and Discussion
### Technical and Economic Evaluation by Process Stage
#### 3.1.2 Technical and Economic Evaluation for the Primary and Secondary Harvest Stage  
After cultivation, it is necessary to collect the biomass. Harvesting was divided into two stages, primary harvest and secondary harvest, since the literature shows that in this way, energy consumption is lower compared to the use of a single-stage, resulting in an economical and effective harvesting process [59]. On the other hand, it is desirable to consider a harvest method that allows recycling the culture medium and thus reduces nutrient costs for the cultivation [53]. For the design of the primary harvest, flocculation was considered, which is induced by the addition of chemical substances that interact with the negatively charged cell surface of the microalgae [52]. In the case of C1, NaOH was considered to induce an increase in pH and cause auto-flocculation, in the case of C2 it was considered an iron salt (FeCl\({}_{3}\)) since it is an efficient and highly used flocculant. For the C3 case, Chitosan was considered since it is a natural biopolymer and it has been reported that it does not contaminate the biomass and also shows successful flocculating activity. For this stage, the results obtained by [PERSON] et al. [52] were taken as a reference, in which they evaluate the removal efficiencies of these flocculants in cultures of the _Chlorella_ sp. Regarding the primary harvest, it was determined that 2.59 tons/year of NaOH are necessary to flocculate the biomass (case C1), 0.63 tons/year of FeCl\({}_{3}\) for case C2, and 0.754 tons/year of Chitosan for case C3 (Table 3 summarizes the results of the primary harvest). In this stage, the objective is to concentrate the biomass to approximately 50 g/L and with the different removal efficiencies, an approximate recovery of 77.73 tons/year of biomass for C1 and 78.91 tons/year for C2 and C3 is achieved. The efficiency of the flocculant usually depends on the selected microalgae species, since each species has specific physicochemical characteristics that must be considered [54]. The most expensive flocculant is Chitosan, approximately 3 times the cost of FeCl\({}_{3}\) and 32% more than NaOH. However, the consumption of Chitosan is low compared to NaOH, which favors reducing the cost of production in the case C3. It has been reported that an alternative for alkaline flocculation is to replace NaOH with slaked lime (Ca(OH)\({}_{2}\)) which costs 50% less and is not toxic to biomass [60], since these chemicals help to provide a high basic pH (10-11 approximately) where the formation of flocs is favored [58]. The estimate of energy consumption shows similar values for the three cases analyzed since they operate under very similar conditions. There is no significant difference between the cases analyzed for this stage regarding energy consumption and production cost. However, this stage is decisive for the potential use of biomass due to the flocculant used, since it can be contaminated by the chemicals used for flocculation [31]. For example, the use of ferric salts can contaminate the biomass by the metals used. It has been shown to cause a brown-yellow coloration in microalgae, causing pigments to not be extracted, and it also makes it difficult to apply it as biomass for biofuels or animal feed [53]. The use of Chitosan with great effectiveness in flocculation and low contamination of the biomass obtained has been reported in the literature. A great advantage of using Chitosan as a flocculant is being able to recycle almost 90% of the culture medium after sedimentation [59]. However, its high cost prevents its implementation on a large scale, since it raises the operating costs of biomass production [61]. In the secondary harvest, the biomass production will depend on the feeding since the flocculation does not have the same removal efficiencies for all the evaluated cases. To evaluate the technologies of the second harvest, the output of case C1 was considered as feeding.