## 7 The Concept of Microalgae Biorefineries  
A biorefinery is a sustainable biomass processing facility that combines low-impact chemical and biological technologies to produce a variety of high-value products and energy. Biorefineries encourage the production of goods and energy from renewable sources, which helps to achieve the Sustainable Development Goals. A biorefinery can be a concept, a manufacturing process, a factory, or even a collection of facilities, according to the definition. The fundamentals involve converting biomass into diverse product streams and integrating different technologies and processes in the most environmentally friendly manner possible. One of the most promising feedstocks for the production of biofuels, such as biodiesel, biomethane, biohydrogen, and bioethanol, is microalgae biomass. Additionally, it offers substitutes for the production of goods used in the food, feed, cosmetic, and health sectors, as well as for the synthesis of soil nutrients and bioplastics. In contrast to the energy crop and oil industries, the microalgae biorefinery is a relatively new technology. However, with the aid of suitable genetic modifications, the addition of a novel technology combination, the application of nanotechnologies, the utilization of several valuable microalgae bioproducts, and the adaptation and improved comprehension of their cell biology, the commercialization and efficiency of the production of value-added products from microalgae biorefineering become feasible [21]. More specifically, microalgae biomass can be the raw material for the production of many bio-based products, such as fuels, chemicals, animal feed, personal care items, and food supplements. The most important advantages of microalgae biomass over traditional raw materials are as follows:  
* They are usually produced 10-100 times more than ground crops;
* They capture carbon at high yields;
* They contain lipids or carbohydrates at high concentrations that may be useful for biodiesel or bioethanol creation;
* They may be cultivated in seawater, brackish waters, or even sewage;
* They offer the possibility of utilizing non-arable land. Microalgae biomass can form a continuous biofuel production chain, similar to traditional oil refineries, because it can be harvested all year. Furthermore, on-site processing of the biomass generated and microalgae cultivation facilitate integrated and sequential production of multiple products while lowering logistical and administrative expenses at biorefinery facilities. Nevertheless, producing commercially viable biofuels from microalgae presents substantial technological obstacles, including the following:  
* Development of adapted microalgae strains at high biomass yields;
* Effective methods of cultivation and crop protection;
* Advancement of harvesting and lipid production methods;
* Optimization of biofuel conversion and production processes and other byproducts [52]. Value-added products like astaxanthin and beta-carotene are already produced by microalgae. To produce specialized products, microalgae like _Spirulina platensis_, _Haematococcus pluvialis_, _Chloella vulgaris_, and _Dunaliella salina_ are cultivated. One particular product, such as astaxanthin or beta-carotene, is produced, and the other important components, such as proteins and carbohydrates, are wasted or remain unutilized. The overall benefits of microalgae cultures could be greatly increased if multi-product production were achieved. In this way, biomass would be fractionated and exploited, and this would lead to economic viability, a method that is also applied in classic refineries of petrochemical or petroleum products. This type of concept is called a multi-product biorefinery, and it is a production scheme that does not aim at a single product but at the full utilization of all biomass fractions. Such a typical installation initially includes the stages of harvesting, cell wall rupture, and extraction of products.