## 1 Introduction  
The concept of circular economy (CE) is an exploitative industrial model, characterized as a regenerative system with a circular flow of materials and use of raw materials and energy through multiple phases [1]. The circular model is an alternative to the linear developmental model of the 20 th century, where natural resources were used without limits to manufacture products, a process coupled with generation of waste without proper recycling [2], which led to heavy industrialization. Based on the CE concept, environmental friendly policies have been influenced and shaped worldwide, due to the rising global concerns and ecological awareness [3]. In the frame of a circular economy, converting wastes into value added products can increase their valorization, providing alternative routes of exploitation and environmental benefits from their management. Considering the CE concept, crude glycerol and decomposed organic matter, which are wastes produced from two distinct processes, could be used as the main input for microalgae production, which is a high value product. In more detail:  
* Crude glycerol is generated during industrial scale biodiesel production by acid or alkali catalyzed transesterification of triglycerides. These include vegetable oil, animal fats and waste cooking oils [4; 5]. The annual production within the countries of European Union was 12 million tons in 2017 [6]. For each ton of biodiesel produced, 100 kg of crude glycerol is generated [7]. Crude glycerol has low economic value. Its purification [8] for pharmaceutical and cosmetic applications has a significant cost [9], while its disposal can be harmful for the environment. An alternative method of crude glycerol valorization is its use as an organic carbon source for microalgae fermentation and anaerobic digestion [10]. Other methods include syngas production (gasification) [11], bio-char production (pyrolysis) [12] and bio-oil production (liquefaction) [13]. * Decomposed organic matter (or digestate) is generated during biogas production by the anaerobic microbial decomposition of crop residues from arable farming, food waste and manure livestock farming [14; 15]. The digestate is composed primarily of biomethane (60-70%), carbon dioxide and minor traces of ammonia hydrogen, sulfide and water vapor [16]. Nitrogen is primarily present in the form of ammonium (NH\({}_{4}\)+) derived from complex organic nitrogen compounds mineralization [17], whereas phosphorus in the form of organic and inorganic phosphates both contained in the liquid phase [18]. With regard to potassium, it retains its cationic form consequently being available in both liquid and solid phase [19]. Phycromidiation is a process where the digestate is utilized as a source of nutrients in microalgae cultures [20]. Several researchers implemented this process to reduce to a substantial amount, ammonia and phosphate levels in digestates derived from different manure types [21; 22; 23]. The digestate can be also utilized in agriculture crop fertilization due to its high nutritional value, since it is rich in nitrogen, phosphorous, potassium and micronutrients. However, uncontrolled land applications can induce eutrophication due to excess nutrient leeching [20]. The biomass produced from microalgae can serve as feed stock for renewable energy in the biofuel industry (biodiesel and bioethanol) [24; 25], a promising step in order to decouple the dependency of the biofuel industry from the primary production sector. Furthermore, it can be used for the production of other high value products such as antioxidants and vitamins [26], phytochemicals, food colorants [27], protein sources for farmed animals and aquaculture [28] and _n_-3 polyunsaturated fatty acids (PUFA) [29]. In relation to the microalgae production, some marine microorganisms from the family of thraustochytrids have attracted a vast interest in the scientific community during the last two decades, due to their high lipid productivity and docosahexaenoicenoic acid (DHA) content [30].