# Application of Extracellular Polymeric Substances during Cultivation of Microalgae Biomass
## 1 Introduction  
EPS secreted by microorganisms are complex compositions of biopolymers with a high molecular weight, such as polysaccharides, proteins, humic substances, nucleic acids, and lipids [1]. EPS serve to shield cells from environmental stresses, such as dehydration, external heavy metals, and toxic compounds, and helps to maintain matrix stability [2]. Polymers included in EPS retain stable matrix structures and form a network for cells to interact with each other and mediate their adhesion to surfaces. One of the possible applications of extracellular polymers is the biofloculation of microalgae biomass. Technologies using microalgae biomass are solutions with significant potential that can be applied in broadly understood field of environmental engineering. Research carried out so far has proved that it is possible to use systems for the intensive cultivation of microalgae biomass in wastewater and leachate treatment, waste and sewagesludge management, carbon dioxide biosequestration, or flue gas purification. An important problem of microalgae cultivation on an industrial scale is the lack of simple and effective technology for the separation of the biomass from the medium. Centrifugation involves using a rotational force that accelerates the mobility and detachment of particles by different densities of the particle and the medium. This process, because of its high efficiency and simplicity, is the most commonly employed technology for industrial microalgae harvesting [3]. However, it is not recommended for certain species due to shear stress. The disc stack centrifuges enable the separation of microalgae of small sizes (3-30 mm) with low cell densities. However, the high capital costs and considerable energy requirements make this technology unprofitable [4]. Filtration is the separation of solid and liquid via a membrane with pores that allow microalgae medium to pass through while retaining cells [5]. Membrane fouling is the biggest issue with using this technology, and it usually requires various improvement strategies to increase its efficiency [6]. Flocculation is a process that requires the presence of flocculating agents inducing microalgal cells to aggregate, which facilitates easier and faster sedimentation. Flocculants can be inorganic, organic, or have biological origin. Currently, research has focused on developing integrated systems for simultaneous microalgae culture and cell harvesting, such as the co-culture of microalgae and fungi, the co-culture of microalgae and bacteria, using microalgae biofilms, etc. These systems show a high density of microalgae cells and efficient cell separation from the medium realized in the same reactor [7]. However, most of these approaches are still in the early stages of development, and future research should focus on improving and optimizing these integrated culture and harvesting systems. Biofloculation is initiated by extracellular polymers (EPS) produced by well-settling microorganisms (microalgae, bacteria, fungi, or yeast). The advantage of biofocculation is its non-toxic nature and potential as an economic method for the concentration of microalgae biomass. EPS added for biofocculation determine the settling properties and dewatering of flocs. The biofocculant produced by _Solibacilus silbestris_ was used to concentrate microalgae _Nantochlorovopsis oceanica_[8]. The biofocculant did not require the addition of other coagulants and was not toxic to microalgae cells. In addition, the research presented the possibility of its recovery after applying filtration and evaporation. The results indicate that the biofocculation efficiency of the recovered biofocculant dropped by only 3%, which significantly reduces the costs of biomass concentration. However, these results are only attractive for small-scale processes.