## 3 Results and Discussion
### Evaluation of Production Costs by Process Scenarios
Regarding production costs from closed systems, specifically in PBR, [PERSON] et al. [43] estimate biomass production at US $9.54/kg and [PERSON] et al. [24] report a cost of $4.15/kg. [PERSON] et al. [64] report the cost of lipid production at $18.10/gal. As we can see, there is a great variety in the estimates of the cost of biomass production, however, most are only evaluations of a specific scenario, which complicates the comparison with other production scenarios. Another relevant datum in the design of biomass production scenarios from the cultivation in PBR is to determine the consumption of fresh water. Several authors mention that fresh water consumption may be lower when biomass is grown in closed systems, compared to open systems. In this study, a consumption of 16,872 m\({}^{3}\)/year of fresh water was determined, only for the biomass cultivation stage, including the preculture. In a previous study [56], a consumption of 58,061 m\({}^{3}\)/year of fresh water was calculated for cultivation in the raceway pond (open system), considering 1 ha as the cultivation surface, the same as in this study. This indicates that up to 71% of the water needed for the culture can be saved if it is considered in PBR. To analyze the impact of fresh water consumption  
Figure 9: Total water consumption vs. total production cost per kg biomass produced. in the different scenarios, consumption was compared to the cost of production per kg of biomass produced (Figure 9). In Figure 9, four groups of scenarios are observed, (1) the scenarios with the highest fresh water consumption and highest production cost (located in the upper right corner, scenarios 3, 4, 5, 6, 7 and 8); (2) the scenarios with lower consumption of fresh water, but higher production cost (located in the upper left corner, scenarios 1 and 2); (3) the scenarios with the lowest production cost, but with high consumption of fresh water (located in the lower right corner, scenarios 11, 12, 13, 14, 15, 16, 19, 20, 21, 22, 23 and 24); and (4) the scenarios with lower consumption of fresh water and lower production cost (located in the lower left corner, scenarios 9, 10, 17 and 18). All scenarios require fresh water for the cultivation stage in PBR and in the secondary harvest, if they use the filtration technologies. The filtering technology consumes fresh water to wash the biomass and try to remove the largest amount of mineral residues that are accompanying the biomass. Group 1 scenarios have the characteristic of using NaOH as a flocculant, and as mentioned before, these scenarios are the ones that present the lowest biomass production compared to the rest of the scenarios, although their fresh water consumptions are similar. Group 2 scenarios use centrifugation for secondary harvest, saving fresh water consumption at this stage. The scenarios of group 3, where the majority are found, consider the filtration technologies for the secondary harvest, this causes the consumption of fresh water to increase. From this group, it is observed that scenarios 12, 16, 20 and 24 are the ones with the lowest water consumption and the lowest production cost. These scenarios use vacuum filter (scenarios 12 and 20) and pressure filter (scenarios 16 and 24) for secondary harvest. In group 4, there are the scenarios that consider centrifugation for the secondary harvest and, although they are not the ones with the lowest production cost, they are the scenarios with the lowest consumption of fresh water. These results allow to know estimates on energy consumption, fresh water consumption and the cost of production under different technological scenarios, when the purpose is to obtain biomass through photobioreactors.