# Flocculation Harvesting Techniques for Microalgae: A Review
## 6 Electrochemical Flocculation
### sacrificial Electrode
[127] compared aluminum and iron electrodes in batch mode harvesting of _C. vulgaris_. Further, they investigated several operating parameters including current density, processing time, stirring speed, initial pH, and inter-electrode distance. Optimal conditions for electrochemical harvesting of _C. vulgaris_ were reported to be aluminum electrodes, 60 min treatment with 2.9 mA/cm\({}^{2}\) of current density, pH 4, stirring speed of 250 rpm, and an inter-electrode distance of 1 cm. Compared with the cases for aluminum, the algae harvesting performance using iron electrodes was generally lower due to the low current efficiency [129; 135]. The flocculation capability of iron hydroxide formed from the iron electrode was also lower than that of the aluminum hydroxide formed from an aluminum electrode [136]. The salt content in the medium affects the electrical conductivity of the algal culture broth. Accordingly, significant difference in electrochemical harvesting efficiency between fresh and marine algal species has been reported. The minimum power required for effective harvesting of the freshwater alga _C. vulgaris_ was 2.1 kWh/kg at a current density of 1.5 mA/cm\({}^{2}\). While, the power could be significantly reduced to 0.2 kWh/kg at 0.6 mA/cm\({}^{2}\) under salty conditions in case of the marine alga _Phaeodactylum tricornutum_[132]. In the same context, the addition of 1.5 g/L NaCl to freshwater alga _C. vulgaris_ medium has been recommended to improve the harvesting performance of the ECF process using aluminum electrodes [127]. At the same time, the additional NaCl cost should be properly considered for large-scale applications. Temperature is one of the factors that can significantly affect ECF harvesting efficiency. Electrical conductivity increased with increasing temperature, which allowed for stronger current flow through the system thereby enhancing biomass recovery. For instance, the ECF harvesting efficiency of the two marine algae _Chlorococcus_ sp. and _Tetraselmis_ sp. were as low as 5% and 68%, respectively, at a temperature of 5 \({}^{\circ}\)C. However, when the operating temperature was increased to 60 \({}^{\circ}\)C, the harvesting efficiency of the former and latter species were significantly increased to 96% and 94%, respectively, as indicative of power requirements of 9.16 and 4.44 kWh/kg after treatment for 15 min [133]  
Current density is another important factor that affects both harvesting efficiency and energy consumption [58]. Increasing the current density usually shortens processing time but increases the energy consumption for electrochemical flocculation [130]. Therefore, a proper balance should be maintained between time and energy cost [58]. Medium acidity should also be properly considered for operation of the ECF process. For example, at an acidic and neutral pH range of 4 to 7, negatively charged algal cells efficiently react with aluminum oxides, whereas under alkaline conditions over pH 8, formation of algal flocs might be inhibited by undesirable monomeric hydroxo-aluminum ions [58]. It should be noted that the iron electrode-based ECF process can induce formation of a brown-colored algae-iron slurry [58]. Al\({}^{3+}\) ions from the aluminum electrode can also contaminate the algal biomass and supernatant effluent, which might seriously limit its large-scale application [64; 134]. Therefore, there is a need to develop effective strategies to reduce environmental and health toxicity as well as alternative efficient, environmentally-friendly materials to replace the current metal electrodes.