## 1 Introduction  
Algae show continuous growth at minimum environmental requirements and irrespective of seasons [1]. Water, inorganic carbon, solar radiation and nutrients such as nitrogen(N), phosphorus (P) and potassium (K), temperature and pH are necessary for the growth of algae. Availability of nutrients is the most important factor for growing algae.  
_Chlorella_ spp. and _Scenedesmus_ spp. are the fastest-growing microalgae, and are rich in valuable constituents that can be used later for the production of many valuable products, e.g., food, pharmaceuticals, feed for animals, biofuels, bioplastics. _Chlorella vulgaris_ can be grown in autotrophic as well as heterotrophic conditions [2]. _C. vulgaris_ is known as one of the most useful microalgae with many potential applications, e.g., the high ability to remove pollutants [3].  
Algae can be cultivated in open ponds or closed systems in fresh or marine water. Algae could be cultivated in suspension in the solution or immobilized on alginate beads. Immobilization helps to achieve more density of algal cells as well, as it is easier to harvest immobilized algal biomass from wastewater. There are several chemical and organic substances that can be used for immobilization of algal cells, e.g., polyacrylamide gel, alginians, agar, carragens [4].  
The main challenge during cultivation of microalgae is utilization of fresh water and availability of nutrients. Wastewater--prior to being discharged to the sewage systems or recycled in industrial processes--has to be treated to reduce the load of contaminants.  
Microalgae can grow in N-rich wastewater and can assimilate ammonium nitrogen for biomass growth. The optimum concentration of nitrogen in the environment that can be assimilated by the most of the algal species is from 5 to 50 millimoles [4]. Different microalgal species can utilize different quantities of nitrogen and phosphorus, e.g., _Chloella vulgaris_ has 8.5% of nitrogen in dry mass [5; 6].  
The Circular Economy addresses the benefits to the environment, economy and society. Nowadays, there is an urgent need for providing technological solutions that can adopt the principles of the circular economy--eliminate as much as possible of wastes and pollution, circulate products and materials and regenerate nature. Sustainable circular bioeconomy is an element of the circular economy that is connected to all processes, products and technologies that are \"bio\", and aims at sustainability.  
Sources of clean water are limited, industrial wastewaters are available, and utilization of biomass as a renewable energy source is increasing; therefore, wastewater could be used as a medium for production of biomass. Such an approach is directly connected to a sustainable circular bioeconomy, because biomass production in an environmentally friendly way in wastewater could be one of the solutions.  
Industrial wastewater is usually characterized by many contaminants that are toxic to the environment and human health. Therefore, using industrial wastewater for cultivation of microalgae has not been previously investigated.  
The main aim of the present study is highly innovative and was to assess the possibility of cultivating _C. vulgaris_ and _S. armatus_ when suspended and immobilized on carriers in industrial wastewater with high concentrations of inorganic and organic contaminants. Microalgae was immobilized on alginate beads that are biopolimers made of algal extracts.  
There is no doubt that the results of these investigations are connected to sustainable development and can bring benefits to the environment, economy and society by fulfilling the sustainable circular bioeconomy principles.