## 2 Microalgae-Based Wastewater Treatment
### Removal of Medicinal/Pharmaceutical Substances  
The increase in global water pollution with a plethora of pharmaceutical substances (PCs), such as antibiotics, painkillers, steroids, antidepressants, and antipsychotics, used on a daily basis by the ever-growing world population, is nowadays a growing environmental issue due to their significant ecotoxicity and the resulting health consequences. These are complex chemicals that are transported by air, water, and soil, resulting through the food chain to accumulate in living beings and humans, due to their hydrophobic and important resistant property. Pharmaceuticals from urban wastewater have been demonstrated to be extracted by microalgae in both natural and experimental conditions [24]. Existing technologies for their removal in wastewater treatment systems are advanced oxidation processes (AOPs), absorption by activated carbon, biodegradation by bacteria, and fungi and enzyme processing. Increased cost, unsatisfactory efficacy, and production of process byproducts with a higher burden than the main active substances of drugs (APIs), enhance the search for new methods to remove these substances [25]. Using microalgae for the bioremediation of effluents contaminated from PCs attracts particular scientific interest, as their characteristics, such as their photosynthetic action by exploiting sunlight and capturing CO\({}_{2}\), easily manageable quantities in processing streams, and fast growth, are combined with simultaneous biomass production, leading to biofuels, soil improvers, and high-value products. In particular, mixed crops with their metabolic potential have the required flexibility to survive and thrive in extreme environments. The mechanisms of removal and metabolism of PCs by microalgae are bio-adsorption, bioaccumulation, and intra- and external cellular biodegradation. Bio-adsorption is an extracellular process that depends on the hydrophobic structure and active molecules of PCs and on the type of microalgae. The cell walls of microalgae have polymer lattices that resemble cellulose, pectins, hemicelluloses, proteins, and lignin, while they are negatively charged due to the presence of carboxylates, phosphates, and amines. For example, adsorption by various microalgae ranging from 0 to 16.7% of active substances, such as ibuprofen, metoprolol, diclofenac, and paracetamol, has been recorded, while progesterone and norgestrel cells of _Scenedesmus obliquus_ and _Chlorella pyrenoidosa_ have been found [25]. Bioaccumulation is a metabolic process in which microalgae absorb organic pollutants along with the nutrients they need for their growth. For example, _Desmodesmus subspicatus_ accumulates about 23% of radiolabeled 17\(\alpha\)-ethyleneestradiolol (14C-EE2) in 24 h, while _Chlamydomonas mexicana_ and _Scenedesmus obliquus_ accumulate carbamazepine [25]. Biodegradation is the most successful way for microalgae to minimize the organic load, as they form simpler molecules through catalytic degradation of the complex substances of their environment. An example of intracellular biodegradation is the action of _S. obliquus_ and _C. pyrenoidosa_, where, in an aqueous medium, they biodegradable 95% of the progesterone content. Biodegradation of 30-80% of ibuprofen, caffeine, and carbamazepine from microalgae in municipal wastewater has also been recorded [25]. Extracellular polymers (EPs) produced by microalgae and diffused into the environment are polysaccharides, proteins, and enzymes. These can create, through their interactions, a film that acts as an external mechanism of degradation of organic substances in the environment, such as PCs, whether they are dissolved in the medium or are in solid or colloidal form. With the accumulation of nutrients from negatively charged polysaccharides and proteins, xenobiotic substances are also absorbed, resulting in the neutralization of toxic substances present in the growth medium of microalgae, while at the same time internal cell degradation is affected through the produced byproducts [25].