# Sustainable Microalgae and Cyanobacteria Biotechnology
## 4 Other Important Applications of Microalgal Biotechnology
### Waste Management  
One of the most important applications of microalgae, such as _Chlorella vulgaris_, that has recently been developed is wastewater treatment [120]. The advancement of this technology allows microalgae to absorb various types of contaminants in water from municipal, industrial, agro-industrial, and livestock wastewaters, producing algal biomass and allowing the water to be safely disposed. It should be noted that the use of microalgal treatment plants has a positive effect because bioenergy is produced, allowing for a more effective, sustainable, simple, economical, and environmentally friendly remediation process [121, 122]. The advantage of using microalgae for wastewater treatment is that it allows the process of bioenergy production during remediation to be channeled through the following processes [123]: (1) biohydrogenesis, which involves converting waste to carbohydrates and ethanol; (2) complete solventogenesis, which involves converting waste to butyrate and butanol; (3) methanogenesis, which involves converting waste to simple sugars and methane; (4) bioelectrogenesis, which involves converting waste to simple sugars and CO2 (glycolysis, ATP production, and gluconeogenesis); (5) incomplete solventogenesis, in which wastes are converted into carbohydrates and ethanol; (6) anoxic respiration, in which wastes are converted into volatile fatty acids (VFA) and polyhydroxyalkanoates (PHA); and (7) lipogenesis, in which microalgal biomass and lipids are produced (fatty acids). Of course, the efficiency of the remediation method will be determined by the species of microalgae (microorganism partners) that are handled, in addition to the pretreatment [124]. For example, in wastewater treatment, the use of _Clostridium_ with acid pretreatments and a bacterial consortium (symbiotic processes) is highly efficient [125]. However, to achieve maximum metabolic efficiency, the remediation process with microalgae requires various pretreatments such as thermal, chemical (acidic and alkaline), oxygen, and infrared light, among others [126]. To improve the efficiency of the remediation process, several types of bioreactors have been developed, including suspension, embedded bed, biofilm, fluidized bed, reverse flow, anaerobic sludge bed, extended bed, and granular sludge, immobilized systems, and membrane-based systems [127]. Another benefit of using microalgae in wastewater treatment is the removal of toxic minerals such As, Br, Cd, Hg, Pb, Sc, and Sn ions, as well as the production of biomass (organic matter) that can be used in fermentation processes [128; 129]. During the degradation process, wastewater (domestic or industrial) acts as an electron donor (anolyte fuel) to generate reduced equivalents through the action of microalgal and bacterial consortia (microbial fuel cells, MFC) at a very high metabolic rate for extended periods of time. However, environmental variables such as pH, temperature, salinity, light, water flow rate, microelements, C:N:P ratio, etc., must be controlled [130; 131]. Biodiesel, on the other hand, can be produced from the production of microalgal biomass from wastewater treatment, where microalgae fix inorganic or organic CO\({}_{2}\) to produce fatty acids (biofixation) via the autotrophic/heterotrophic biosynthetic pathway of lipids, and it is converted into a biofuel via transesterification processes. _Scenedesmus_ microalgae, for example, were cultured in wastewater fermented with swine feces, allowing for high lipid production and waste removal [132]. Furthermore, several microalgal species, including _Chlamydomonas reinhardtii_ and _Chlorella vulgaris_, contain a high concentration of glycogen, starch, cellulose, agar, and other compounds that can be converted into bioethanol during fermentation [133; 134].