## 1 Introduction  
Water scarcity and water pollution are two of the major problems that humanity is going to face shortly [1]. Water is consumed mostly by agriculture, industry, and housing, which in turn release wastewater (WW) into the environment [2, 3]. These effluents are generally rich in organic matter, nitrogen (N), and phosphorus (P), and their release into the environment can bring to loss in river quality, sediment contamination, and even eutrophication. Even if deperuation is performed, WW treatment plants (WWTPs) are still considered a point source of pollution [4, 5, 6, 7]. The most commonly used deperuation process is called the activated sludge process (ASP), in which microorganisms degrade organic matter in an aerated bioreactor. In ASP, N is usually removed from WWs thanks to biological activities and alternating aerobic and anaerobic conditions. During the aerobic phase, ammonia is converted into nitrite and nitrate, which are then converted into molecular N\({}_{2}\) during the anaerobic phase. Molecular N\({}_{2}\) then leaves the medium, resulting in net nitrogen removal. Considering how important bioreactor aeration is for ASP, we should not be surprised that it can account for up to 70% of the total energy costs of the WWTP,also representing a major source of greenhouse gases [8]. As an alternative to the ASP, the anaerobic ammonium oxidation (Anammox) has been proposed [9]. In this process, ammonia and nitrite are converted into molecular nitrogen and nitrate simultaneously with denitrification under anaerobic conditions, thus reducing aeration costs. However, its applicability in full-scale WWTP is difficult because Anammox bacteria are easily susceptible to environmental conditions, such as low temperatures [9]. Moreover, sludge produced by both ASP and Anammox has a high water content (up to 98%), so other processes, e.g., thickening and de-watering, are needed [10; 11]. P biological removal is also possible, but far more complex, and chemical methods are usually preferred [12]. Chemical methods usually require the addition of magnesium salts that react under an alkaline environment, which can lead to cost increases, higher maintenance, and difficulty in recovery [13]. On the other hand, P biological removal is only possible using specific polyphosphate accumulating organisms (PAOs) that accumulate P in aerobic conditions and release P in anaerobic conditions. By alternating these two phases, it should be possible to have a net P removal from the WW [14]. These processes usually produce nutrient-rich effluents that must be depurated [10; 11; 15; 16]. Since some models suggest that municipal WW production will increase due to climate change, further research and improvement are needed [17]. As a green alternative or implementation to ASP, microalgae-based processes are gaining increasing attention for the N and P removal from WW [18] since microalgae can remove nutrients from the medium while producing nutrient-rich biomass [19; 20; 21]. Moreover, microalgae are photosynthesizing organisms that subtract CO\({}_{2}\) from the environment to perform photosynthesis, releasing O\({}_{2}\) that can be used by nitrifying bacteria to oxidize ammonia and organic matter [22]. Therefore, microalgae implementation in the ASP can both increment nutrient depletion and reduce CO\({}_{2}\) emission while reducing aeration costs [23; 24; 25; 26; 27]. Moreover, microalgae can accumulate P, producing biomass with a high content of this nutrient that can be used, for example, as an agricultural fertilizer [28; 29]. Moreover, the algal biomass resulting from WW depuration can also be used for biofuel production [30; 31; 32]. Considering that P removal from the effluent happens without the addition of other chemicals and that the algal biomass can be reused for other biotechnological applications, microalgae implementation in the WWTPs may help reduce maintenance costs [12; 33; 34].