# Flocculation Harvesting Techniques for Microalgae: A Review
## 6 Electrochemical Flocculation
### sacrificial Electrode  
The use of a sacrificial metal anode (usually iron and aluminum, or possibly magnesium, copper, zinc or brass) in an electrochemical system causes the release of positively charged metal cations. These dissolved metal cations are able to electrostatically attract negatively charged algal cells, resulting in flocculation [84; 126]. It should be noted that ECF processes do not produce counter ions such as chloride and sulfate (generally observed in the conventional chemical flocculation processes), since metal salts are not added here. In ECF, the water electrolysis reaction at the cathode can generate H\({}_{2}\) microbubbles that can attach to the algal cells and further promote electro-flotation (see Section 6.3. for details). The main disadvantages of the ECF process are power consumption, the short lifetime of electrodes due to their dissolution, and contamination of harvested biomass with metal oxides [127]. The performance of the ECF-based algae harvesting process is affected by various operating parameters such as electrode material, inter-electrode distance, current density, processing time, cultural pH, salt content, and temperature. Among various metals, aluminum is most commonly used as the sacrificial electrode because of its promising algae harvesting efficiency. For instance, ECF using aluminum electrodes for the harvesting of _Nannochloropsis_ sp. resulted in 97% harvesting efficiency at a current density of 8.3 mA/cm\({}^{2}\) when treated for 10 min. It should be noted that the properties related to biofuel quality (e.g., total lipid content and fatty acid composition) of the harvested algal biomass after ECF treatment did not show any considerable changes [131]. In ECF, the processing time required for harvesting could be reduced by increasing the current density. This was clearly observed when the current density was increased from 2.2 to 6.7 mA/cm\({}^{2}\) during ECF processing of _C. vulgaris_ with considerably decrease in processing \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Electrode**} & \multirow{2}{*}{**Alga (Cell Density)**} & \multicolumn{2}{c}{**Optimal Harvesting**} \\  & & **(Energy Requirement)** & **Ref.** \\ \hline Al & _C. pyrenoulosa_ & 95.8\% @ 1 min (0.3 kWh/kg cell) & [126] \\ \cline{2-4} Al & _C. vulgaris_ & 98\% @ 4 min (0.3 kWh/kg cell) & [129] \\ \cline{2-4} Al & _M. aeruginosa_ (1.3\(\times\) 10\({}^{9}\) cells/mL) & 100\% @ 45 min (0.4 kWh/m\({}^{3}\)) & [130] \\ \cline{2-4} Al & _Nannochloropsis_ sp. (2.5 g/L) & 97\% @ 10 min (0.06 kWh/kg cell) & [131] \\ \cline{2-4} Al & _Scendesmus_ sp. & -98.5\% @ 20 min (2.3 kWh/kg cell) & [128] \\ \cline{2-4} Al & _P. tricruntum_ & 80\% @ 30 min (0.2 kWh/kg cell) & [132] \\ \cline{2-4} Fe & _C. vulgaris_ & 80\% @ 30 min (2.1 kWh/kg cell) & [132] \\ \cline{2-4} Fe & _Chlorococcum_ sp. & 96\% @ 15 min (9.2 kWh/kg cell) & [133] \\ \cline{2-4} Fe & _Green algae mixture_ (_Scendesmus_, _Kirchneriella_, _and Microystis_) & -95.6\% @ 24 h (4.4 kWh/kg cell) & [134] \\ \cline{2-4} Fe & _Tetraselmis_ sp. & 94\% @ 15 min (4.4 kWh/kg cell) & [133] \\ \hline \multirow{2}{*}{Non-sacrificial} & C & _C. sorckiniana_ & -95\% @ 15 min (1.6 kWh/kg cell) & [124] \\ \cline{2-4} C & C & _C. pyrenoulosa_ & 79.2\% @ 1 min (0.3 kWh/kg cell) & [126] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison of sacrificial and non-sacrificial electrodes for algae harvesting. Figure 7: Electrochemical flocculation harvesting of microalgae using aluminum electrodes (Al), graphite electrodes (C), aluminum sulfate (Al\({}_{2}\)(SO\({}_{4}\))\({}_{3}\)), and graphite electrodes with aluminum sulfate (Al\({}_{2}\)(SO\(4\))\({}_{3}\) + C): (**a**) microalgae removal efficiency; and (**b**) photographs of the harvesting processes. Reprinted from Reference [128], distributed under the terms of the Creative Commons Attribution License. time from 7 to 4 min. The energy consumption under the reduced current density was estimated to be \(2.94\times 10^{-4}\) kWh/g biomass [129]. [PERSON] et al.