## 3 Insights into Algal-Bacterial Biomass
### Obtaining Value-Added By-Products from MABAs  
The recovery of by-products from cultivating pure cultures of microalgae is a widely discussed topic, as it promotes the substitution of fossil fuels with sustainable biodiesel,  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Biomass** & **Size (mm)** & **SV (m.h\({}^{-1}\))** & **SVI (mL.g\({}^{-1}\))** & **Sedimentation (\%)** & **References** \\ \hline AGS & 0.2–16 & 18–130 & \textless{}80 & - & [49] \\ \hline AS & 0.0005–1 & 0.6–15 & 100–150 & 84.44 & [13; 14; 49] \\ \hline MABAs & 0.1–5 & 36–360 & 42.55 & 42.37 & \\  & & 8.3 & 57 & \textgreater{}90 & \\ \hline MA & 0.005–0.05 & 0.001–0.026 & - & 13 & [13; 54] \\ \hline \hline \end{tabular}  
* AGS: aerobic granular sludge; AS: activated sludge; MABAs: microalgae– bacteria consortia; MA: microalgae; SV: settling velocity; SV: sludge volumetric index.  
\end{table}
Table 1: Size and sedimentation properties of aerobic biomass used in wastewater treatment.  
contributing to the circular economy. Microalgae store lipids intracellularly for their metabolic functions, and their content can vary depending on stress conditions imposed on the microorganisms [71], such as nutrient availability, lighting conditions, and cultivation medium [72; 73]. [PERSON] et al. (2011) [74] reported that nitrogen deficiency increases lipid production in the pure species Chlorella zofingiensis. The lipid content under the conditions of 0.04 and 1 g NaNO\({}_{3}\)/L was 54.5% and 27.3%, respectively. The following year, the same authors confirmed this finding and inferred that phosphate deficiency also affects lipid productivity, but to a lesser extent than nitrogen deficiency.  
In addition, algae-bacteria interaction in wastewater treatment systems hold promise in the field of biotechnology. Obtaining by-products from microalgal-bacterial aggregates can significantly contribute to the development of sustainable products. There are various possibilities for products to be recovered, including biodiesel [19], biopolymers [75], phosphorus [76], and biochar [77], among others, with potential applications across various economic sectors (Figure 3). [PERSON] et al. (2021) [75] successfully recovered phosphorus and alginate-like exopolysaccharide (ALEs) in bacterial granular sludge and algal-bacterial granular sludge. The authors observed a higher ALE content in the granularity of the algae-bacteria consortium compared to the bacteria-only one. However, the recovered phosphorus content was similar in both biomasses. These findings are consistent with the work of [PERSON] et al. (2021) [78], who evaluated the phosphorus content recovered in conventional AGS and algal-bacterial AGS, reporting similar results (more than 83% of phosphorus recovered) for both systems.  
The sub-product content and recovery is influenced by the operating conditions of the treatment system, and several studies are underway to assess which stress conditions are best suited for greater by-product production, such as light intensities and hydraulic retention time (HRT). [PERSON] et al. (2019) [79] evaluated the effect of different light intensities on lipid production in sequential batch reactors (SBRs) and reported that the ideal light intensity range for lipid production is between 45 and 135 \(\upmu\)mol/m\({}^{2}\).s. [PERSON] and [PERSON] (2019) [80] compared different hydraulic detention times (2, 4, 6, 8, and 10 h) in reactors treating domestic sewage and observed that this parameter is directly related to the potential for lipid production: The higher the HRT, the higher the system's lipid production. In addition, [PERSON] and [PERSON] (2016) [19], working with MABAs developed in high-rate  
Figure 3: Products with added value generated from MABAs.  
ponds, showed variations in methane production potential based on the hydraulic retention time. For an HRT of 10 h, the biomass formed produced 347.9 \(\pm\) 2.0 mL CH\({}_{4}\)/g.VS, while for an HRT of 6 h, the production was 290.4 \(\pm\) 12.1 mL CH\({}_{4}\)/g.VS.