# A Cyanobacteria-Based Biofilm System for Advanced Brewery Wastewater Treatment
## 3 Results & Discussion
### Pollutant Removal  
Although primary aerobic wastewater treatment is generally effective at removing organic pollutants, it often does not reduce inorganic pollutant concentrations (especially phosphorus) to a level that allows safe disposal or reuse and, therefore, requires secondary physicochemical treatment [40]. The post-treatment biofilm reactor introduced in this study aims to effectively remove residual pollutant concentrations for safe effluent disposal. Figure 3a presents the results of NO3-N, NH4+-N, and PO43-Premoval over the course of the 15-day cultivation period. Similar inorganic pollutant removal rates and no statistically significant differences were observed for both surface materials used. NH4+-N is usually the preferred nitrogen substrate for cyanobacterial cultivation as its assimilation is less energy demanding and, in this study, was rapidly consumed after two days of cultivation [41]. However, it should be noted that under the alkaline pH conditions of these experiments (pH rose from 8.5 to 9.45) a large fraction of the initial ammonium concentration evaporated in the form of ammonia gas [41]. NO3-N gradually reduced by 67.0% (glass) and 67.6% (polyurethane) within the first ten days of cultivation, while TKN concentration followed a similar trend as, in both cases, its concentration dropped below 5 mg.L-1. It is worth noting that NO2-N concentrations did not exceed 0.05 mg.L-1 at any point during the experiments. The microbial consortium was also effective at removing phosphorus compounds. PO43-P Concentrations reduced by 76.2% and 75.5% for the glass and polyurethane growth surfaces, respectively. After post-treatment, all pollutant concentrations were found to comply with European regulations for effluent disposal (Articles 4 and 5 of the Urban Wastewater Directive, 91/271/EEC) [42]. Figure 2: Percentage of total biomass that attached to the biofilm growth surfaces (glass and polyurethane) over time. In absence of an external mechanical CO\({}_{2}\) feed, the microbial consortium utilized organic pollutants as its primary carbon source. In the case of the glass growth surface, 71.6% of the initial d-COD was removed, while for polyurethane the corresponding removal percentage was 76.2% (Figure 3b). Note that statistically significant differences in d-COD removal rates were not observed between the two surface materials. _Leptolyngbyya_ sp. is known to be a mixotrophic cyanobacterium and for this reason it has been utilized for treatment of wastewaters containing high concentrations of organic pollutants in past research [4, 5, 43, 44]. [PERSON] et al. investigated the performance of a _Leptolyngbya_-based mixotrophic culture in the treatment of various agro-industrial wastewaters (cheese whey, poplar sawdust, and grass hydrolysates) containing d-COD concentrations of up to 3660 mg:L\({}^{-1}\) and recorded removal percentages of up to 93% without the provision of an additional inorganic carbon source [44]. Table 2 presents pollutant removal percentages of the present study compared to those obtained by [PERSON] et al., who investigated post-treatment of the same wastewater using a suspended _Leptolyngbya_-based culture under identical pH, light, and temperature conditions [4]. The biofilm photobioreactor manages to equal or exceed all the removal percentages recorded in suspended cultivation. The greatest deviation between the two cultivation systems is in d-COD removal. Specifically, in attached cultivation 70-83% of the initial d-COD was removed within nine (9) days, in comparison to 24% in suspended cultivation over the same period. This behavior can be interpreted if the secretion of EPSs is considered. Under nitrogen limitation, which occurred throughout the experiments, cyanobacteria produce exopolysaccharideadaries and other slowly biodegradable macromolecules [20, 21] that add to the existing organic loading of the wastewater, thus reducing observed d-COD removal rates as cultivation progresses. Namely, [PERSON] et al.