## 4 Discussion
Despite close systems providing higher control under the abiotic culture conditions, the costs associated to illumination and sterilization hinders its industrial application [92; 93; 94]. Moreover, the suspended microalgal biomass harvesting is still expensive, so techniques have been being developed to tackle this problem, such as microalgae immobilization [94]. Furthermore, the added-value product can be valorized for different applications. For instances, microalgae cultivation in municipal wastewater can be used for bioactive compounds extraction if the safety and quality of the biomass is guaranteed. In fact, recent studies showed the potential of microalgae wastewater cultivation for polyhydroxyalkanoate extraction [95], which is a compound with several biomedical applications [96; 97; 98]. Another bioactive compound produced by microalgae is chlorophyll, a well-known antioxidant, and a pigment, that can be employed in the textile industry as a natural dye [55]. In addition, microalgae grown in municipal wastewater can contribute to human health through the production of metabolic by-products during the wastewater treatment process, making them renewable sources for sustainable agriculture, highlighting three main objectives: environment, economic feasibility, and socioeconomic net patrimony. Since microalgae produce high levels of important micronutrients and macronutrients for plant growth, they can be used as biofertilizers. Several studies have identified an association between higher nutrient absorption, higher biomass accumulation and higher cultivation yield through the incorporation of microalgae-based fertilizers [99; 100; 101]. In a study that examined the effects of algae extracts on plant growth, faster germination and greater rice seed growth were observed [102]. The use of the green microalga _Chloella_ sp. is reported in some studies of microalgae-based fertilizers spread application on agricultural land, for instance, liquid extract application of _C. vulgaris_ as a foliar spray has promoted the germination percentage, the radicle and plumule length of three plant species, _Lepidium sativum_ (garden cress), _Eruca sativa_ (arugula) and _Vigna radiata_ (mung bean) [103; 104; 105; 106]. Moreover, dried _C. vulgaris_ can be used as a soil conditioner and natural fertilizer, enriching the soil with nutrients, and promoting _Sea mays_ (maize) seedling [107]. Microalgae production in municipal wastewaters has been studied for energy production [108; 109]. Microalgal lipidic profile, particularly the triacylglycerlycerides (i.e., C14:0, C16:0, C16:1, C18:0, C18:2 and C18:3), through transesterfication have been shown to be suitable for bioteful production. For instance, researchers demonstrated that _Chloella_ sp. cultivated in municipal wastewater exhibited a 27.4% lipid content [110], while _C. vulgaris_ produced 18.6% [111] and _Auxencholorella producticoides_ (formerly _Chloella protothecoides_) presented a lipidic concentration of 33.4% [112]. After lipids extraction, the residual biomass still contains carbohydrates, which are also interesting for bioethanol production [113]. The content of lignin and cellulose that is synthesized by microalgae can be converted into methane-rich biogas through anaerobic digestion. For example, previous research demonstrated that through this method, _Chloella sorokiniana_ and _Chloella vulgaris_ were able to achieve a biogas yield up to 40-73% [114]. Another study points out that the biomass of _C. vulgaris_ cultured in urban sewage allowed the production of biogas, presenting a yield of 442 mL/g Volatile Solids (VS), in batch reactors [115]. Furthermore, _C. vulgaris_ can also be used as a feedstock for bio-hythane, an alternative to fossil fuels, by coupling dark fermentation with anaerobic digestion [60].