## 7 The Concept of Microalgae Biorefineries
This is followed by further fractionation, such as the isolation of carbohydrates from water-soluble proteins [53] (Figure 3). In the concept of the microalgae biorefinery, there are advantages that integrate it into the principles of the circular economy (a production scheme where renewable materials are used as raw materials for the production of fuels and chemicals, and all byproducts are fully degradable). The biorefinery approach offers a solution to profitably exploit the full potential of microalgae through the recovery and separation of biomass components, as well as by minimizing waste generation. In this light, the microalgae biorefinery not only comprises sequential steps for separation of biomass fractions, but also a succession of functional units that groups laboratory-level research with culture development and subsequent processing stages, with little to no waste. Laboratory-scale research involves the selection of the specific strain of microalgae based on the desired product to be produced and the sources and availability of low-cost nutrients for microalgae culture. The cultivation stage concerns the construction, operation, and optimization of a cultivation system with the following characteristics:  
It best matches the requirements of the selected microalgae strain in terms of mixing with the culture medium and light penetration;  
It reduces investment and operating costs;  
It offers sufficient biomass concentration and productivity for efficient post-processing. Subsequent processing steps include the following:  
Biomass harvesting;  
Extraction of intracellular components through cell lysis;  
Fractional separation of ingredients to maximize commercial value products that can be obtained [54]. The microalgae biorefinery has so far not been a profitable activity. In commercial biotechnology products, the cost of post-processing amounts to 20-40% of the total production cost, while in multi-product microalgae biorefineries (a biorefinery production scenario that aims not only at one product, but at full utilization of all biomass components), it is significantly higher and amounts to 50-60%. This difference is mainly due to two main factors:  
Low biomass concentration in microalgae cultures (<3 g/L);  
The difficulty in applying mild techniques that provide the different desired biomass fractions without destroying other fragile substances present in biomass, to optimize and recover all valuable components (proteins, carbohydrates, and lipids) [53]. Disadvantages and difficulties exist at all the above stages. Figure 3: Simplified flow diagram of a multi-product microalgal biorefinery (modified from ’t [PERSON] et al. [53]). * At the laboratory level, it is the selection of the appropriate strains, their engineering, and the nutritional approach that leads to the overaccumulation of the desired product;
* At the cultivation stage, and especially in industrial photobioreactors, there are high costs and low concentrations of biomass, reaching about 3 g/L, while reactors should process biomasses with at least ten times the concentration for their efficient operation and utilization;
* Finally, in later processes, designs are made for a single main product and the rest is often \"waste\" that needs to be disposed of, which adds to the cost [54]. Notable improvements, which enhance the concept of the biorefinery, have been made to the overall picture of the microalgae technology sector. The development of technology in cultivation systems (open and closed) has led to thinner crops, where light penetration was improved, resulting in increased photosynthesis and, consequently, an increase in biomass produced. These cultivation systems offer satisfactory concentrations of biomass produced, thus facilitating the processes that follow (harvesting, cell lysis, and product fractionation).