## 7 The Concept of Microalgae Biorefineries
The dominant microalgae biorefinery optimization approaches are developed at two levels:  
* At the first level, the strategy aims to maximize the production of the desired value-added product and the utilization of byproducts, through overaccumulation achieved in closed flat thin-surface photobioreactors. This includes the cultivation of Haematococcus sp. for astaxanthin production where high-yielding costly technology is used to optimize production. The remaining biomass after astaxanthin extraction contains mainly proteins, carbohydrates, and pigment residues. Neither the yields or quality of these substances correspond to the market for food additives, but they can be used in biofuel production or for feeding animals. * The second level of the approach aims at the production of multiple products in successive stages. The slow part of the exponential phase of crop growth is selected, where nitrogen depletion occurs and exploitable products accumulate. The priority of extraction of products is regulated by the fragility of substances (soluble proteins, dyes, lipids, and starch). In this case, robust and dynamic strains such as _Scenedesmus_, _Chlorella_, _Tetraselmis_, _Phaeadactylum_, and _Nannochloropsis_ spp. are used, resulting in a wide range of substances from medium value (PUFAs, functional proteins) to low value (starch or insoluble proteins). With a chain pattern of extractions, the water-soluble components (mainly proteins and hydrocarbons) are obtained first by gentle cell wall disruption and centrifugation. This is followed by lipid extraction and, in some cases, the separation and enrichment of specific lipids such as PUFA. The last stage examines the recovery and utilization of the starch fraction in the residue, for application in the production of bioplastics. In conclusion, it is proven that the maximum utilization of all components of microalgae biomass, such as water-soluble proteins that can be food additives instead of being used for animal feed, is a very important point of research direction. In addition, other substances may be present in the residual fraction after extraction of the main compounds, which need to be investigated for their possibility of producing new commercial products. Finally, the costs of developing and introducing complex and costly technologies for further recovery of biomass fractions should guarantee reciprocal economic benefits [54]. As the biggest challenge in the industrial production of biofuels from microalgae is the high cost, it is extremely important to use waste as a growing medium to achieve a reduction in biomass production costs. Moreover, the production of a single biofuel from microalgae is not economically viable. It is therefore imperative to link biofuel production with other activities for the energy balance. The use of waste or byproducts, where one form of energy is produced from the production of another form of energy, is an excellent way to achieve economic viability. An integrated biorefinery approach to maximize the utilization of microalgae biomass and the use of waste for microalgae cultivation as a growth medium is a viable and cost-effective solution for a greener future [55]. Essentially, integrating microbiomes production with waste treatment leads to environmental sustainability, as well as to improved economic performance. The main products are recycled water (purified waste water), biomass-derived products such as biofuels, and soil improvers as residues of the biofuel production process. The two main approaches in this direction are as follows:  
* The use of microalgae for the direct treatment of waste, with the effluents then disposed of for off-site use (i.e., the waste is used only once to produce the microalgae biomass);
* The use of treated or raw wastewater as a growing medium for microalgae biomass production, with wastewater then recycled after treatment [56].