Wastewater sparged with CO2 provides a conducive growth medium for microalgae, enabling faster production rates and reduced nutrient levels in treated wastewater (Table 6), decreased harvesting costs, and increased lipid production [123]. Therefore, coupling of the production of biofuel-directed microalgae with bioreme­diation of wastewaters is considered an important strategy for successful deploy­ment [16, 123,215].

Microalgae are efficient removers of chemical and organic contaminants, heavy metals, and pathogens from wastewater [150], which provide a pathway for combating eutrophication in conjunction with the production of microalgae as energy resource [228]. This characteristic enhances the sustainability potential of the production process, through potential savings on requirements for chemical remediation of waste­water [218], and minimises the need for freshwater for algae production [118,195].

Microalgae have also exhibited significant potential for biological removal of hazardous or toxic compounds due to their negatively charged surfaces [106]. For example, they have demonstrated a strong absorption of polyvalent cations; an ion exchange capacity that is the basis for removal of heavy metals from wastewaters [149]. Another advantage is in the production of photosynthetic oxygen in wastewa­ter, which reduces the need for external aeration. The oxygen is also required by bacteria for biodegradation of pollutants such as polycyclic aromatic hydrocarbons (PAHs), phenolics and organic solvents [149]. However, some of these pollutants are potent inhibitors of photosynthesis in microalgae because they can induce mor­phological changes in the cells that lead to physiological incompatibility [106].