## 7 The Concept of Microalgae Biorefineries
The palm oil production process produces high pollutant load (POME) waste, which is discharged into the environment after being treated, to reach the required discharge limits [61]. The use of microalgae for processing POMEs in combination with biogas installations in palm oil plants can simultaneously remediate waste and reduce the cost of microalgae biomass production given the high concentrations of nutrients in them. In this direction, a microalgae biorefinery can be incorporated into the processing of POME, which will grow with the residue of anaerobic treatment, binding CO2 and producing lipids and energy through combustion, after harvesting and dehumidification [62]. Biomass is produced, which then, through solvent extraction of hexane and isopropanol, produces lipids, and by the transesterification process, biodiesel and glycerol are produced, while the residue is used for combustion for energy production. The results show that from a techno-economic point of view, this processing route produces the highest profit, which amounts to 7.73 \(\times\) 10.5 USD/year [62]. For the development of sustainable biofuel production processes, the selection of an appropriate extraction solvent is still a major obstacle, despite the fact that many different approaches have been reported. Lipid extraction uses conventional organic solvents, which are inexpensive and simple to use, and are derived from petroleum products. They require more processing steps, are toxic, and consume large amounts of time. Furthermore, they react with other substances to worsen the product's quality, and the majority of them are flammable. European directives such as REACH (2006/1907/EC) tightly control their use. The use of \"green\" solvents (green extraction technologies) is being researched to improve the properties of energy requirement, ecological friendliness, non-toxicity, and efficient lipid extraction in order to address the aforementioned challenges. Terpenes, for instance, are naturally occurring green solvents with exceptional chemical and technical qualities, and are extracted from citrus fruits and many other plants. With no change in the composition of fatty acid methyl esters (FAMEs), terpene application produced a higher yield in the biomass extraction of _Chlorella vulgaris_ when compared to hexane extraction. Recyclable, green solvents function as a solvent system that eschews recovery--a step that is typically taken in conventional processes. Therefore, in a variety of industrial extraction processes, these solvents may be used in place of conventional and other environmentally friendly solvents. This reduces post-processing, saves energy and time, and produces fewer or no byproducts to easily offset the high cost of green solvents. The economic feasibility of biofuels produced from microalgae could be enhanced through multi-product cogeneration. For instance, _Chlorella_ sp. and _Tetraselmis suecia_ hold great promise for clean and sustainable CO2 capture mechanisms in the future. They can also be used as feedstock for the anaerobic fermentation of biomass by the bacterium _Clostridium saccharoperbutyllacium_, which will produce acetic and butyric acid, as well as biofuels (ethanol, butanol, and acetone). Since additional extraction and separation processes could result in excessive costs for the biorefinery, virtually every biorefinery scheme needs to be evaluated separately to ensure that the highest level of sustainability is achieved both economically and environmentally. This evaluation should primarily focus on the choice of technologies, raw materials, and products [34]. According to [PERSON] et al. [63], _Chlorella vulgaris_ is the most preferable strain used for nano-catalytic conversion into biofuel, due to its high lipid content. In terms of waste, the creation of the perfect integrated system that combines the anaerobic digestion of waste with the cultivation of microalgae is thought to be essential for the achievement of microalgae biomass production at a reasonable cost.