# A Cyanobacteria-Based Biofilm System for Advanced Brewery Wastewater Treatment
## 3 Results & Discussion
### Biomass Production  
The sum of suspended and attached biomass per reactor working volume and reactor growth surface over the course of the 15-day cultivation period is presented in Figure 4. Similar growth rates were observed for both growth surface materials tested, although the use of a glass growth substratum appears to slightly enhance biomass growth in comparison to polyurethane. The corresponding maximum growth rates were 66 mg:L\({}^{-1}\)-d\({}^{-1}\) (glass) and 59 mg:L\({}^{-1}\)-d\({}^{-1}\) (polyurethane). After an exponential growth phase during the first 6-10 days of cultivation, total biomass concentration stabilized at around 406 mg:L\({}^{-1}\) for glass and 392 mg:L\({}^{-1}\) for polyurethane. In terms of attached biomass per biofilm surface area, the respective calculated values were 13.1 and 12.8 g:m\({}^{-2}\).  
The work of [PERSON] et al. [4] provides a reference point to assess the performance of the biofilm reactor introduced in the present study. Both biofilm setups (glass/polyurethane surfaces) significantly enhanced biomass growth rates compared to suspended cultivation. Specifically, on the ninth day of cultivation the suspended culture produced 360 mg:L\({}^{-1}\), i.e., 11% less than the biofilm cultivation experiments. Biofilms have been reported to enhance biomass growth in many studies [28]. For example, [PERSON] et al. studied synthetic secondary effluent treatment using CO\({}_{2}\)-aerated attached and suspended growth membrane bioreactors and reported a significant increase in biomass productivity and nutrient removal in the case of the biofilm reactor [26]. Similarly, [PERSON] et al. compared the performance of a membrane biofilm photobioreactor to a suspended growth  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{3}{*}{**Substance**} & \multicolumn{3}{c}{**Removal (\%)**} \\ \cline{2-4}  & \multicolumn{2}{c}{**Attached Cultivation**} & \multicolumn{1}{c}{**Suspended**} \\ \cline{2-4}  & **Glass** & **Polyurethane** & **Cultivation 1** \\ \hline NO\({}_{3}\)–N & 67.2 & 76.5 & 50.0 \\ NH\({}_{4}\)–N & 100 & 100 & 100 \\ PO\({}_{3}\)–P & 66.0 & 65.4 & 57.0 \\ d-COD & 83.2 & 70.7 & 24.0 \\ \hline \hline \end{tabular}  
* 1 Adapted by [PERSON] et al. (2020) [4].  
\end{table}
Table 2: Pollutant removal percentages after nine (9) days of attached cultivation (glass and polyurethane growth surfaces) and suspended cultivation 1.  
Figure 4: Total biomass concentration per working reactor volume (left \(y\)-axis) and reactor surface area (right \(y\)-axis) over cultivation time for glass and polyurethane growth surface materials.  
setup utilizing the same BG-11 cultivation medium and reported a 30% increase in biomass productivity when using biofilm reactors [45]. In this study, it is suspected that the thin water film flowing above the biofilm surface enables more efficient CO2 transfer between the gas (atmosphere) and liquid phases thus providing an additional inorganic carbon source. On the contrary, in cultures that are fully immersed in the liquid medium CO2 diffusion can be a slow process and limit photosynthetic growth [18]. Cyanobacteria microtophy is known to enhance growth rates and photosynthesis efficiency compared to strictly autotrophic or heterotrophic conditions [46]. Moreover, the thinness of the liquid film allows more efficient light transmission. The brewery wastewater treated in this study has a brown/yellow color and subsequently absorbs part of the available light before it radiates the cyanobacterial cells. Radiation absortivity is proportional to the distance light travels in a material (Beer-Lambert Law) [47]. Thus, reducing the light path offers a significant advantage to the biofilm reactor in comparison to suspended systems where light travels several centimeters through the liquid phase.