## 1 Introduction  
The recent water crisis underscores the urgency of investigating and studying water usage to ensure access to a clean water supply and efficient wastewater management compliance with current standards and legislation [1]. Biological aerobic treatment (BAT) has been extensively employed globally and emerged as a prominent method to convert organic matter from effluents into biomass and carbon dioxide [2; 3; 4; 5]. However, considering that traditional wastewater treatment plants are encountering technical limitations and high operational costs in removing nutrients, organic loads, and recovering biomass [6; 7], systems with microalgae-bacteria consortium offer several advantages, including lower maintenance costs, compact designs, ease of operation, and reduced energy consumption [5; 8].  
According to [PERSON] et al. (2023) [9], the water and sewerage sector is the largest electricity consumer in cities, consuming around 40% of the total energy consumption in urban areas.  
In addition, instead of releasing CO2 formed during the organic matter removal process, microalgae-bacteria consortium cultivation is gaining attention, since microalgae cultivation is effective in treating effluents and removing inorganic nutrients from wastewater [10], while also contributing to the reduction of greenhouse gas emissions [11]. Contrary to what happens in bacteria-dominant reactors via the nitrification-denitrification process, the consortium facilitates the incorporation of nitrogen into biomass through microalgae assimilation [12], especially in granular biofilm, where the microbiota is well defined. [PERSON] et al. (2018) [13] observed that bacteria's nitrification process in the symbiotic system facilitated the assimilation of oxidized nitrogen forms (NO2\({}^{-}\) NO3\({}^{-}\)) by the microalgae _Chlorella vulgaris_. Similarly, [PERSON] et al. (2016) [14] noted the proliferation of nitrogen-fixing bacteria in this symbiotic setup.  
In addition, microalgae offer promising prospects for the generation of value-added by-products, including combustible options like bioethanol, biogas, and biodiesel, as well as non-combustible alternatives such as feed, vitamins, ultraviolet protection, bioplastics, and nanoparticles [15; 16]. Moreover, this interaction has been associated with increased microbial community diversity [17], lipid production, sedimentation capacity [18], and biomass production for methane generation [19].  
Granulation occurs in four stages: initiation, maturation, maintenance, and disintegration [20], involving algae adherence to flocculent sludge surfaces, subsequent bacterial attachment to algae, and continued consortia growth until biomass attachment and detachment achieve equilibrium [21]. Nowadays, some strategies are being utilized to monitor the algal growth process, including single-molecule techniques [22] and nanomechanical characterization [23]. Also, the consortium benefits from the degradation of organic matter by heterotrophs to bacteria while producing extracellular polymeric substances (EPSs), which are responsible for bacteria, microalgae, and cyanobacteria aggregation. At present, there is a wide array of research investigating the microalgae-bacteria consortium across various reactor configurations. These encompass continuous flow reactors, photobioreactors in single-batch and sequential-batch mode, and high-rate ponds [24; 25; 26; 27].  
In this context, the microalgae-bacteria consortium is a low-cost alternative for reducing energy consumption and gas emissions in aerobic treatment systems. This review thus outlines the sustainable potential for biomass formation and by-product recovery, along with an assessment of current systems and operational parameters utilizing this consortium for wastewater treatment. The focus lies on examining physical and biological characteristics and applied engineering, aiding in informed decision-making for future applications.