2.1 Wastewater treatment with microalgaeMicroalgae have great potential to solve energy and environmental challenges worldwide. Treating wastewater with microalgae is a more environmentally friendly approach to reducing nitrogen and phosphorus and removing heavy metals from wastewater. Microalgae can absorb a significant amount of nutrients because they require large amounts of nitrogen and phosphorus for proteins (45-60% of microalgae dry weight) and metals as micronutrients for growth. William Oswald was the first to develop the idea of ​​treating wastewater using microalgae and to achieve photosynthesis in wastewater treatment [29]. Figure 2.1 briefly describes the process involved in a high algae pond in which the algae play a dual role by assimilating nutrients from the wastewater and providing oxygen to the bacteria. Bacteria absorb oxygen and degrade organic matter present in wastewater, the same process that is used in activated sludge treatment [29].Figure 2.1. Process involved in a pond with a high concentration of algae [29]. The selection of algal strains for use in wastewater treatment is determined by their robustness to wastewater and their ability to grow and assimilate nutrients from wastewater [30]. Chlorella, Scenedesmus, Neochloris and Spirulina are the algae species widely used in experimental studies on wastewater treatment. The main advantages of using microalgae over conventional methods, summarized by De la Noüe (1992) [31], are: (a) nutrients can be removed more efficiently; (b) no generation of toxic by-products (sludge) (c) biofuels can be produced from harvested biomass (energy efficiency); (d) cost-effectiveness.2.2 Mechanisms for nutrient and heavy metal removal Nutrient removal by algae involves...... middle of paper ...... l density. The main advantage of autotrophic growth conditions is that they can reduce carbon dioxide to produce useful organic compounds. However, the limitation is that light penetration is inversely proportional to algae density. As algal density increases, light exposure of algal cells decreases, limiting nutrient removal from wastewater. During heterotrophic growth, no light is required and an organic carbon source such as glucose or glycerol acts as the carbon source. Algae use this organic carbon for their growth. High algal densities can be achieved in heterotrophic growth because growth is not limited by light. Mixotrophic growth is a combination of heterotrophic and autotrophic growth, where carbon dioxide and organic carbon are simultaneously assimilated and respiratory and photosynthetic metabolism operate simultaneously.. [49, 50].