## 4 Insights into Bacterial-Algal Biomass in Different Operational Configurations
### High-Rate Ponds (HRPs)  
The treatment of wastewater combined with the controlled production of microalgae in high-rate ponds is an adaptation of stabilization ponds, a model developed by [PERSON] in 1988 [107]. HRPs consist of an open, shallow lagoon with a system of rotors to move the liquid mass. They can be considered open photobioreactors that combine wastewater treatment and microalgae growth [108]. However, finding an economically viable alternative for harvesting the biomass remains challenging, and little is known about the behavior of the aggregate [19; 54]. This capacity depends on the microorganisms' sedimentation rate; however, the construction of a decanter can become economically and technically unfeasible--for example, for the production of microalgae targeting high-value-added compounds, harvesting efficiencies must be above 99% [109]. [PERSON] et al. (2018) [110] treated anaerobic lagoon effluent in HRPs with an HRT of 8 days. The biomass was then sent to decanters with a solids retention time (SRT) of 9 h, and a harvesting efficiency of 4-12% was observed due to the low flock formation. Thus, the authors concluded that the 8-day HRT was considered insufficient. [PERSON] et al. (2011) [111] indicated the retention time for the separation of the HRT from the solids. In other words, it is necessary to recirculate the biomass to promote better sedimentation with shorter hydraulic retention times; in this study, the harvesting technique consisted of coagulation followed by flocculation and sedimentation [109]. In addition to efficient harvesting, HRPs must also produce biomass in large quantities, and area productivity is a parameter used to determine the most productive system [112]. Furthermore, operational and environmental factors can also influence production. [PERSON] et al. (2018) [113] varied the water depth (0.32-0.55 m) and observed higher productivity in the 0.32 m pond (28.3 gSS/m\({}^{2}\)d). Similarly, [PERSON] et al. (2017) [112] varied the height between 0.15 m and 0.30 m and found the highest productivity (26 gSS/m\({}^{2}\)d) in the 0.30 m deep pond. Other factors that can influence productivity are solar radiation and temperature. [PERSON] et al. (2015) [114] varied solar radiation (387.1 to 1087.3 \(\mu\)E/m\({}^{2}\)d) and found no significant difference in area productivity. However, when area productivity was determined in terms of chlorophyll-a, the lowest values were obtained in the systems with the highest radiation, possibly due to the photoinhibition than can occur with higher solar radiation. As for the temperature factor, studies have shown that biomass production increases in warmer seasons [113; 115]. Radiation and temperature are also parameters that can influence the performance of HRPs. [PERSON] et al. (2015) [114] worked with various levels of radiation and observed that, although it had an effect on biomass productivity, it had no effect on nitrogen assimilation. Regarding temperature, great efficiencies have been observed in warmer seasons [114; 115]. As for HRT, unlike the behavior of microalgae biomass productivity, higher values favor sewage treatment performance [116]. Hydraulic retention time (HRT) is also a parameter related to biomass production. It is known that as the HRT increases, the ratio of microalgae: bacteria decreases. It has been reported that reducing the HRT from 8 to 4 days can lead to a 24.5% increase in microalgae production [111; 116]. [PERSON] et al. (2020) [117] recommend an HRT of 4 to 7 days for HRPs. Similarly, [PERSON] et al. (2011) [111] reported an 85% increase in microalgae biomass when the HRT was reduced from 8 days to 4 days. The injection of CO\({}_{2}\) into HRPs is reported to be one of the factors responsible for increasing biomass production [8]. In addition to controlling pH, it serves as a carbon source for microalgae. It has been reported that CO\({}_{2}\) assimilation occurs more efficiently in shallower HRPs [118]. Another aspect identified as relevant to productivity is the predisinfection of sewage in the lagoons.