Typically, conventional processes for secondary treatment (activated sludge) take place through the remediation with microorganisms, generally heterotrophic bacteria and this process depends on O
2. The microorganisms, in the presence of O
2, carry out the biological oxidation of the organic substance
[1][31], since oxygen allows nitrification by bacteria
[32]. Generally, O
2 is produced by electromechanical blowers with high energy intensity. This supply of O
2 for wastewater treatment has high costs. It consumes approximately 1 to 3% of the total electricity generated in developed nations, of which 40 to 60% is expended on supplying air to the aeration basin.
[33]. Since mechanical aeration causes up to 50% of the operating costs of wastewater treatment plant (WWTP). To solve this problem, the cocultivation of microalgae and bacteria could lead to economic and environmental benefits
[13][30][31]. Obviously, the benefits are not always clear and effective when moving from laboratory prototypes to an industrial scale
[19]. This synergy between the two microorganisms promotes both the growth of microalgae and bacteria. The latter remove COD through heterotrophic growth, producing CO
2, and at the same time the microalgae assimilate CO
2 and the nutrients generated by bacteria, converting these substances into algal cell material through photosynthesis and producing O
2 that stimulates bacterial activity
[10][21][34]. This type of interaction is not always successful because in some cases the microalgae do not release enough oxygen to the bacteria for the degradation of COD. This, if not completely eliminated, could stimulate the mixotrophic metabolism of microalgae, thus reducing the net availability of oxygen for bacteria
[34]. Further advantages generated by this interaction are the exchange of cofactors such as growth-promoting compounds, vitamins, organic compounds and extracellular matrix. The latter provides attachment sites for bacteria and allows flocculation and subsequent harvesting of microalgal biomass.
[24][34][35]. However, cocultivation may be limited by competition between microalgae and bacteria for nutrients. Bacteria can inhibit the growth of microalgae by modifying the culture broth and secreting toxins. Microalgae can also negatively affect bacteria by inhibiting or suppressing bacterial activity by increasing the pH
[36][34][37].