# Impact of 50 Hz Electromagnetic Field on the Growth of _Chlorella vulgaris_
## 3 Results and Discussions
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Growth Time** [hours]} & \multicolumn{6}{c}{**Biomass Concentration \(c\) [gdw/L] and Stimulation Coefficient \(k\), [\%]**} \\ \cline{2-9}  & **Reference** & \multicolumn{2}{c}{**EMF-2 V/m**} & \multicolumn{2}{c}{**EMF-5 V/m**} & \multicolumn{2}{c}{**EMF-10 V/m**} & \multicolumn{2}{c}{**EMF-15 V/m**} \\ \cline{2-9}  & \(\epsilon_{\mathrm{eff}}\)* & \(\epsilon\) & \(k_{\mathrm{s}}\) & \(c\) & \(k_{\mathrm{s}}\) & \(c\) & \(k_{\mathrm{s}}\) & \(c\) & \(k_{\mathrm{s}}\) \\ \hline
48 & 0.76 \(\pm\) 0.037 & 0.78 \(\pm\) 0.014 & 2.6 \(\pm\) 0.14 & 0.82 \(\pm\) 0.07 & 7.9 \(\pm\) 0.78 & 0.96 \(\pm\) 0.093 & 26.3 \(\pm\) 2.85 & 1.02 \(\pm\) 0.03 & 34.2 \(\pm\) 1.95 \\
72 & 1.29 \(\pm\) 0.027 & 1.36 \(\pm\) 0.015 & 5.4 \(\pm\) 0.13 & 1.52 \(\pm\) 0.02 & 17.8 \(\pm\) 0.44 & 1.66 \(\pm\) 0.017 & 28.8 \(\pm\) 0.67 & 1.75 \(\pm\) 0.08 & 35.7 \(\pm\) 1.79 \\
96 & 1.77 \(\pm\) 0.11 & 1.88 \(\pm\) 0.046 & 6.2 \(\pm\) 0.42 & 2.12 \(\pm\) 0.023 & 19.8 \(\pm\) 1.25 & 2.29 \(\pm\) 0.011 & 29.4 \(\pm\) 1.83 & 2.41 \(\pm\) 0.06 & 36.2 \(\pm\) 2.42 \\ \hline \hline \end{tabular}
* \(\epsilon_{\mathrm{eff}}\) [gdw/L] concentration of reference; mean \(\pm\) SD (N = 12). Mean \(\pm\) SD (N = 3).  
\end{table} 
Table 1: The evolution of the biomass concentration (% dried mass) as a function of time at various intensities of stimulating fields. Figure 4: The evolution of the absorbance variation rate during the first 96 h of growth of the algal mass. from the air/CO\({}_{2}\) bubbles. Under these conditions, the OD from the algal suspension in liquid culture medium is a parameter that characterizes the speed of the process (2). The results of the recordings regarding the time evolution of dissolved oxygen OD in the culture medium under experimental conditions are presented in Figure 5.  
By analyzing Figure 5, it can be noticed that before the inoculation of the liquid culture medium, OD (given by the oxygen supply from the air/CO\({}_{2}\) bubbles) is approximately 7.64 mg/L. After the inoculation of the culture medium, due to the increase in the algal mass, following the development of the processes described by Equation (2), the OD increase is more pronounced in the first approximately 30 h; as it approaches the maximum solubility of oxygen under the given conditions, it shows a limiting trend. This limitation of OD is more pronounced in the case of algal mass suspension exposed to 15 V/m EMF (appears after approx. 24 h limitation to approx. 8.13 mg/L), explained by the maximum growth rate of algal mass in these conditions, as is also the case of the results from Figure 4. The bioreactor is open at the top. Thus, under the current experiment conditions, by injecting bubbles of an air/CO\({}_{2}\) mixture, a turbulent flow is generated throughout the reactor and at its surface, intensifying the oxygen transfer from the atmosphere to the culture environment. Thus, it is difficult to assume that the equilibrium is reached. The value of 8.13 mg O\({}_{2}\)/L is actually the result of the two opposing processes occurring in the reactor: algal photosynthesis that generates O\({}_{2}\) and the bubbling system that injects a mixture of air and 5% CO\({}_{2}\). In order to evaluate the consumption of nutrients from the culture medium according to the growth of the algal mass through specific chemical analyses, the initial concentrations \(c_{i}\) (before inoculation) and the final concentrations \(c_{f}\) after 96 h of growth of the algal mass for inorganic nitrogen (N-NO\({}_{3}\)) were determined, with total nitrogen (TN\({}_{\rm b}\)) and total phosphorus (Pt). The difference between total nitrogen TN\({}_{\rm b}\) and inorganic nitrogen N-NO\({}_{3}\) represents the amount of nitrogen retained by the algal mass (organic nitrogen).