# An Innovative Low-Cost Equipment for Electro-Concentration of Microalgal Biomass
## 3 Results  
From all the experiments, it was possible to retrieve the concentrated biomass. However, there were significant differences between the experiments. Table 2 presents the results for biomass concentration and temperature of the media. There was a considerable increase in temperature (\(>\)35 \({}^{\circ}\)C) in those experiments with efficiencies below 40%. This increase in the temperature of the medium can have adverse effects on the stability and quality of the biomass. However, Test 5 showed efficiency values below 20%, but with a temperature below 30 \({}^{\circ}\)C; this is due to the short time of exposure to the process in the experiment (6.6 min). According to [15], the distance between electrodes affects the overall energy consumption in the process. Our findings showed that not only the distance but the number of electrodes used could increase energy consumption (fewer electrodes, higher energy consumption). The experimental data concerning the number of electrodes, mixing rate and time on the concentration efficiency of the alga was fitted to two different models: linear (L) and quadratic (Q). The Pareto analysis (Figure 1a) illustrates that (\(p=0.05\)) the number of electrodes and agitation were the variables that most affected the concentration process. These results are consistent with the results presented by [16], where they demonstrated that the number of electrodes and agitation were critical variables for increasing efficiency since they allowed for increasing the active area of contact with the media and decreasing the electrical consumption. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Level** & **\# of Electrodes** & **Mixing (rpm)** & **Time (min)** \\ \hline \(-1\) & 2 & 100 & 10 \\
0 & 4 & 150 & 15 \\
1 & 6 & 200 & 20 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Variables for electroflotation of algal biomass. By analyzing the interaction between mixing and the number of electrodes (Figure 1b), Equation (2) was obtained, where X is the number of electrodes and Y is the mixing. These operating conditions are consistent with the results obtained by [16], where mixing had a direct relationship with the time of concentration since at speeds between 150 and 210 rpm, the biomass is aggregated in less time, with an efficiency of 90%. However, they recommended the use of agitation close to 150 rpm to save energy and maintain higher efficiencies. \[\begin{array}{l}Z=(113.95380840859-19.831858022329\times x)+\left(1.6873545265 373\times  
As the volume of algae increased, the efficiency was substantially reduced. Another factor that can affect this is the geometry of the reactor in which the experiments were carried out. However, neither of these two variables (volume and geometry) has been previously reported in the literature. Loss of efficiency due to increased volume is a problem that can hamper the viability of electroflotation for algae harvesting. Therefore, to demonstrate its stability, we carried out a couple of experiments, increasing the number of electrodes (with volume constant at 2 L) until obtaining the desired efficiency. Figure 2b shows the results for these experiments. The verification of the proposed operating conditions for the efficient concentration of the biomass was tested. The new operating conditions (11 electrodes, 150 rpm, 20 min) were tested, and a piece of innovative equipment was designed and built for the project. This new equipment has a built-in, Arduino-based magnetic stirrer at the bottom, with a working volume of 2L (Figure 3). The design of each section, the size of the electrodes and the electrical blueprint of the magnetic stirrer can be found in Appendix A, Appendix B, Appendix C. The stability of the electrodes was evaluated during 20 concentration cycles using 2 L of _Scenedesmus_ sp. After 15 min, the process reached an efficiency of 100%, with no increase in the temperature of the media (Figure 3). This process was repeated 20 times.