# Microalgae Water Bioremediation: Trends and Hot Topics
## 1 Introduction  
Water quality is a fundamental concern of the present century, considering the increasing scarcity of water resources. Degradation of water quality, either caused by anthropogenic activities (e.g., pollution; resources overexploitation) or natural phenomena (e.g., global warming; extreme climatic events) often leads to severe impacts on ecosystems, public health and economic growth, negatively affecting society and environment [1]. The improvement of water quality is a central issue included in the 2030 Agenda and on Sustainable Development Goals, recognizing the importance of good-quality water for a sustainable development of society and, therefore, the need to globally address this problem. The discharge of untreated or inadequately treated effluents into rivers, lakes, aquifers and coastal waters supplies the aquatic environment with a myriad of chemical compounds that can impact aquatic organisms directly, by triggering hazardous effects, and indirectly, by changing some physicochemical features of the medium (e.g., oxygen concentration, pH, redox potential and nutrient concentration) [2, 3]. Contaminants often result from domestic, agricultural and industrial effluents and their physical and chemical compositions differ according to their origins [4]. The most frequently found are metals [5], pesticides [6] and nutrients, such as nitrate [7] and phosphate [8]. Moreover, the occurrence of emerging organic and inorganic pollutants, such as microplastics [9], pharmaceuticals [10], flame retardants [11], personal care products [12], hazardous and noxious substances [13] has been increasingsince conventional wastewater treatment plants (WWTP) are not yet equipped and suitable to remove these new contaminants. The complexity of effluent composition is increasing as human activities intensify, so it is urgent to develop adequate wastewater treatment processes, which should be easily applicable, effective and eco-friendly, in order to prevent water quality degradation and to protect water resources. Physicochemical processes alone are proven to be inefficient regarding the treatment of effluents with complex composition [11]. A possible solution can be the combination of physicochemical and biological treatment technologies aiming at the development of sustainable treatment processes. Biological treatments often involve the cultivation of microalgae. The capacity of microalgae to remove nutrients [14], metals [4], pharmaceuticals [15; 16], radioactive minerals [17] or pathogenic organisms [18] from the medium has been highlighted in literature. Using microalgae to remove pollutants from wastewaters or effluents before their discharge or reutilization is named phycoremediation [6]. The basic mechanism behind this process is inherent to the algal metabolism, nevertheless, the removal of pollutants will be variable accordingly to different microalgae used and effluents' characteristics. Therefore, the design of the cultivation system is a critical parameter for microalgae production in order to achieve optimal growth rates and minimize costs [19]. For microalgae production, several factors are important to take into account, namely, the biology of the microalgae, cultivation land area, light, nutrient supply, labor, energy, carbon dioxide, pH, temperature and the type of the final product [20; 21; 22]. Several methodologies of microalgae production have been developed over time, from laboratorial systems under accurate controlled conditions, to open systems in the field under natural, uncontrolled and unpredictable conditions [20]. Table 1 summarizes the main systems, methods and strategies used. Microalgae are typically cultivated in open or closed culture systems. Furthermore, diverse strategies, namely in batch, continuous and semi-continuous, are applied around the world and which differ mainly on nutrient supply and operation modes [20]. Due to the high energy demand and operation costs of harvesting processes, solutions were investigated in order to simplify this process [23].