# Flocculation Harvesting Techniques for Microalgae: A Review
## 4 Chemical Flocculation
### Organic Flocculant  
Some organic polymers have been utilized for algae harvesting and wastewater treatment (Table 3). Generally, anionic polymers are not considered as ideal flocculants, due to poor bridging reactivity between them and negatively charged algal cells [76]. The cationic polymers are more suitable for inducing neutralization (or reduction) of the negative surface charges of algal cells, which results in the destabilization and flocculation of algal cells [75]. Natural biopolymers are generally regarded as renewable, less toxic, and environmentally friendly [86]. However, technical obstacles such as high production cost, narrow operating pH range, and low efficiency for some algal species (e.g., marine types) could limit commercial applications for algae harvesting [65]. Chitosan (deacetylated chitin) is a cationic polyelectrolyte polymer composed of linear poly-amino-saccharide chains. It is less toxic, biodegradable, and relatively inexpensive (e.g., when manufactured from abundant chitin waste such as crab and shrimp shells) [77; 78]. Accordingly, this polymer has been widely applied in the food, biomedicine, and cosmetics industries, and has been used as an immobilizing resin for bacteria and enzymes for various biocatalytic reactions. In addition, chitosan has been used for algae harvesting as well [87]. The flocculation capability of chitosan is attributed to various interactions between chitosan monomers and algal cells, such as adsorption, bridging, sweeping, and charge neutralization [61; 86]. Some commercial chitosan products such as ChitoVan TM are available elsewhere [86]. The efficiency and dosage of chitosan are largely dependent on various culture parameters such as pH, temperature, cell density, ionic strength, and growth stage as well as algal species [12; 77]. In particular, pH is regarded as the main factor affecting the polyelectrolyte property of the chitosan biopolymer. Under acidic conditions, chitosan tends to exist in a linear arrangement surrounded with positively charged deacetylated units, resulting in effective destabilization of algae, mainly by charge neutralization. Meanwhile, the structure of chitosan changes into a coiled arrangement under alkaline conditions and flocculation occurs primarily due to the bridging mechanism [61; 77]. Accordingly, chitosan flocculation operation is generally recommended for acidic conditions [78]. When the pH of chitosan solution was reduced below 7, a high harvesting efficiency of 99% was obtained for _C. sorokiniana_ at a very low dose of 10 mg chitosan/g biomass [76]. However, under alkaline conditions, relatively high chitosan dosages were required to achieve effective flocculation [12; 40]. Cationic starch can be synthesized through chemical modifications (e.g., etherification) from an abundant natural polymer, starch. The formulation degree of functional cationic groups is responsible for flocculation capability [73; 88]. Considering its relatively low cost and environmental safety, cationic starch has often been used for wastewater treatments and algae harvesting. The reported cationic starch concentrations for 1 g algal biomass recovery were 89 and 119 mg for _C. pyrenoidosa_ and _Botryococcus braunii_, respectively [75]. Harvesting efficiencies of 90 and 85% could be obtained for _S. obliquus_ using corn and potato-based cationic starches, respectively, at the same flocculant/algae ratio of 1.4:1 (_w/w_) [74]. Cationic starch can be used in combination with other biopolymers such as chitosan in order to increase harvesting efficiency [75]. Compared with chitosan flocculant, cationic starch has been reported to be cost-effective, particularly under alkaline conditions [22; 76]. Unmodified (original) starch has also been tested for algae harvesting but showed a very low efficiency (15% in case of _S. dimorphus_) [73]. To reduce the dosages of other chemical flocculants, original starch has been suggested as an auxiliary flocculant.