# A Cyanobacteria-Based Biofilm System for Advanced Brewery Wastewater Treatment
## 1 Introduction  
The cultivation of photosynthetic microorganisms (microalgae and cyanobacteria) to produce a wide range of high value products such as pigments, proteins, polysaccharides, vitamins, and fatty acids [1], as well as electricity in bioelectrochemical systems [2], has been investigated extensively. Wastewater treatment systems utilizing photosynthetic microorganisms have recently emerged as a promising alternative to conventional biological processes such as activated sludge [3]. However, the treatment of agro-industrial, industrial and domestic wastewaters with algae/cyanobacteria has gathered significant research interest over the last decade and produced promising results in both organic and inorganic pollutant removal [4; 5; 6; 7; 8; 9]. This method offers several advantages, such as reduced operational cost as photosynthesis replaces mechanical aeration requirements, decreased CO\({}_{2}\) emissions [10], and the high cost of using chemical fertilizers for microalgal growth is avoided since microorganisms use the nutrients contained in the wastewaters [11]. Additionally, the produced biomass can be utilized as a platform for liquid (i.e., biodiesel and bioethanol) and gaseous (i.e., methane and biohydthane) biofuels [11; 12; 1]. Brewerywastewaters are produced in high quantities (3-10 L per L of beer produced) from beer brewing processing industries worldwide [13] and in 2019 the total global beer production amounted to about 1.91 billion hectoliters [14]. Brewery wastewaters are characterized by significant organic (2000-6000 mg\(\cdot\)L\({}^{-1}\) of chemical oxygen demand, COD), nitrogen and phosphorous loadings (25-80 and 10-15 mg\(\cdot\)L\({}^{-1}\), respectively) and they have been traditionally treated aerobically [13; 15]. Until today, a limited number of studies on brewery wastewater treatment using axenic microalgal cultures [6; 16] have been conducted in suspended cultivation systems. Recently, brewery wastewater was treated by our research group using a cyanobacterial-bacterial consortium dominated by the filamentous cyanobacterium _Leptolyngbya_ sp. either without pre-treatment [4; 5] or after pre-treatment with electrocoagulation [4; 5]. Biomass harvesting/processing is often a challenging task. Photosynthetic culture cell densities are typically low (as low as 0.5 g\(\cdot\)L\({}^{-1}\) in conventional open pond photobioreactors [17]) and separation methods such as chemical flocculation, gravitational sedimentation, filtration, and centrifugation can be complex, time-consuming, or expensive [3; 18]. Techno-economic analyses indicate that the cost of algal/cyanobacterial biomass harvesting and dewatering can amount to 20-30% of the total cultivation expenses in high-rate algal ponds [17; 19]. The introduction of attached cultivation (biofilm) systems could be a step towards addressing these issues. Biofilms are microbial communities living in a self-produced matrix of extracellular polymeric substances (EPSs) that provide structural integrity and mediate their adhesion to surfaces [20]. Under nutrient limitation and sufficient irradiation/carbon, cyanobacteria tend to over exceed their typical carbohydrate content in the form of EPSs, instead of channeling energy towards cell synthesis (\"carbon overflow\" metabolism) [20; 21]. Along with lower quantities of secreted proteins and nucleic acids, EPSs allow high cell density colonization of abiotic and biotic surfaces through hydrophobic or hydrophilic cell-cell and cell-surface interactions [20]. These properties enable efficient biomass harvesting by scraping [22], as biofilms have a similar consistency to centrifuged biomass [23]. Algal/cyanobacterial biofilms also show high potential in wastewater treatment applications. The biofilm matrix is typically dominated by synergistic micro-consortia and protects microbial populations from exogenous environmental stresses (dehydration, antibiotics, chemicals, heavy metals, UV radiation, protozoan predators, etc.) [20; 24].