# Microalgal Growth in Aquaculture Effluent: Coupling Biomass Valorisation with Nutrients Removal
## 1 Introduction  
A highly important issue nowadays concerns the environment and natural resources, namely water resources. Water pollution is mainly caused by the industrial and agricultural sectors and other activities, for instance, aquaculture. These sectors produce wastewaters that contain non-natural synthetic compounds and organic and inorganic substances. If the wastewater is not treated and discharged into the environment, the water quality and the surrounding ecosystems can be jeopardised [1]. Aquaculture farms typically use feed rich in protein and carbohydrates, and, to ensure high growth rates, nitrogen and phosphorus are usually added too. However, oversupply can occur, leading to an accumulation of these nutrients in the medium [2]. The abrupt enrichment of nutrients in water favours the growth of microalgae and aquatic plants that will put at risk the balance of the present ecosystem, overlapping the other organisms [3]. More specifically, algaegrowth on the water surface causes a reduction in oxygen levels. This deficit affects aquatic organisms, leading to the destruction of the ecosystem. A sudden increase in algae growth due to the enrichment of the medium with nutrients (nitrogen and phosphorous) is called eutrophication [4]. To prevent eutrophication, the European Union (EU) has taken steps to better control the levels of these nutrients in wastewaters. Accordingly, the limits for urban wastewater discharge are: (i) 10 or 15 mg\({}_{\text{N}}\) L\({}^{-1}\) for nitrogen, with 70-80% of minimum reduction; and (ii) 1 or 2 mg\({}_{\text{P}}\) L\({}^{-1}\) for phosphorus, with 80% of minimum reduction [5; 6]. To avoid the pollution of watercourses with uncontrolled discharges of effluents, it is necessary to apply effective treatment methods. The techniques commonly used in wastewater treatment plants to remove nitrogen and phosphorus, such as biological nitrification and denitrification and chemical precipitation, are energy-intensive, not eco-friendly and lead to the production of sludge and the release of greenhouse gases [7]. Microalgae are an environmentally friendly and low-cost alternatives that can be used as a solution for urban wastewater treatment and polishing. Microalgae are photosynthetic organisms whose dimensions vary between 2 and 30 \(\upmu\)m [8]. These microorganisms are found mostly in aquatic environments and have higher biomass productivity when compared to terrestrial plants [9]. Additionally, microalgal biomass is very rich in lipids (such as eicosapentaenoic and docosahexaenoic acids) and pigments. Some species even produce antioxidant pigments, while others contain vitamins and immunostimulants. Microalgal biomass also contains proteins and carbohydrates. The characteristics of microalgae have several advantages in their use. Since they are photosynthetic organisms, they can capture carbon dioxide (CO\({}_{2}\)) from the atmosphere or combustion gases [10]. Microalgal biomass can be further exploited for the production of biofuels and food supplements (for both humans and animals) and extracted pigments can be used in the cosmetic and pharmaceutical industries [11; 12]. Since microalgae can grow and survive in adverse aquatic environments and use the nitrogen and phosphorus in wastewater to grow, they are considered an economic wastewater treatment technique [1; 13; 14]. [PERSON] et al. [15] used _Chlorella vulgaris, Isochrysis galbana, Nannocloropsis salina, Scenedesmus obliquus_, and _Spiulina major_ to treat aquaculture wastewater. In batch mode, all the tested species were able to remove 100% of total nitrogen (TN) and total phosphorus (TP). Additionally, at least 72% of chemical oxygen demand (COD) was remediated by the microalgae. [PERSON] et al. [16] cultivated _Chlorella minutilisina_ in saline aquaculture wastewater and observed a decrease of 88% in TN and 99% in TP. In fact, the mass cultivation of microalgae, with application in wastewater treatment, has already captured global interest [17; 18].