# Flocculation Harvesting Techniques for Microalgae: A Review
## 5 Particle-Based Flocculation
### Multi-Functional Particle  
Production of biodiesel from algal biomass requires several subsequent post-harvesting downstream processes such as cell disruption, lipid extraction, and conversion to fatty acid methyl ester (FAME) [5]. Therefore, particle-based flocculants have recently been developed for additional functions required for those subsequent steps. Their integrated use can be categorized into three types: harvesting/cell disruption, harvesting/lipid extraction, and harvesting/lipid conversion (Figure 6).  
Figure 6: A schematic illustration of utilization of multi-functional nanoparticles in algal biorefinery process development. Reprinted from Reference [122] with permission from Springer International Publishing.  
To couple harvesting and disruption of algal cells for biofuel production, it is essential to use flocculant composites that have dual cationic and photocatalytic properties. For instance, chitosan-coated Fe\({}_{3}\)O\({}_{4}\)-TiO\({}_{2}\) composites were able to electrostatically attract algal cells causing floc formation [113]. In addition, this composite showed sufficient photocatalytic activity (due to the TiO\({}_{2}\) shell) in disrupting the cell wall of harvested algae after exposure to UV irradiation or visible light. The dosage of 0.07 g particle/g biomass from chitosan-coated Fe\({}_{3}\)O\({}_{4}\)-TiO\({}_{2}\) resulted in a harvesting efficiency of over 98% for _C. minutissima_ within 2 min (due to the magnetic property of Fe\({}_{3}\)O\({}_{4}\)). This dosage was significantly lower than that reported for chitosan only (0.11 g particle/g biomass) under the conditions of that study [113]. High percentage of lipid recovery at 96% and 97% could be obtained after exposure to UV for 2 h and visible light for 3 h, respectively, relative to lipid recovery in 10 min of ultrasonic treatment. In another study, [PERSON] et al. [100] used Mg-AC conjugated with TiO\({}_{2}\) for simultaneous harvesting and wet-disruption of algae. They reported that Mg-AC played a role in algae flocculation while TiO\({}_{2}\) facilitated the direct disruption of cells under UV radiation. Around 85% of the cells could be harvested within 10 min of injection of the Mg-AC-TiO\({}_{2}\) composite into the _Chlorella_ culture (1.5 g/L). Subsequently, the harvested biomass was irradiated by 365 nm UV light for 3 h, leading to 95% cell disruption.  
In terms of harvesting and lipid extraction, the multi-functional properties of Fe\({}_{3}\)O\({}_{4}\)-embedded carbon microparticles could be utilized for algae flocculation and hydrophobic-interaction-based lipid extraction. These composites have cationic and lipophilic properties derived from carbonized polyvinylpyrrolidone as well as magnetic properties due to the embedded Fe\({}_{3}\)O\({}_{4}\) nanoparticles. A dosage of 2 g/L was able to harvest 99% of _Chlorella_ within 1 min via electrostatic interaction. Moreover, with a high dosage (10 g/L) of Fe\({}_{3}\)O\({}_{4}\)-embedded carbon microparticles, the extracted lipid droplets could be recovered after 10 min of ultrasonic treatment [109]. Integration of algae flocculation harvesting and conversion of lipid into biodiesel have been effectively performed using Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) nanoparticles functionalized with a strong base, such as triazabicycloocene. This nanocomposite has been reported to effectively flocculate _C. vulgaris_ within 1 min (by the adsorption attraction between algal cells and the nanoparticles) and stimulate the conversion of algal lipid into FAME with a high conversion efficiency of 97.1%. This was an outstanding yield compared to those obtained using other catalysts such as H\({}_{2}\)SO\({}_{4}\), AlCl\({}_{3}\), Amberlyst-15, and CuCl\({}_{2}\)[123]. Despite the high manufacturing costs for magnetic particle-based flocculants, their multi-functional applications, including cell disruption and lipid extraction/conversion as well as flocculant recovery, might render them suitable for large-scale applications.