# Flocculation Harvesting Techniques for Microalgae: A Review
## 4 Chemical Flocculation
### Organic Flocculant
The addition of autoclaved rice starch (120 mg/L) has been reported to reduce the amount (up to 54%) of aluminum salts required for _Chlorella_ harvesting [89]. In addition, the residual starch components after harvesting can be used as organic nutrients for subsequent algae cultivations. Alginate is an anionic polysaccharide that is extracted mainly from brown seaweed [90]. Similar to unmodified starch, this material can be used also as a second flocculant to improve flocculation efficiency and reduce the dosage of the main chemical flocculant. [PERSON] et al. [40] observed a low harvesting efficiency of less than 70% for _S. obliquus_ when 10 mg/L chitosan was used alone at pH 6. Interestingly, after adding 10 mg/L alginate to the chitosan-treated algal solution (called the dual flocculation process), the flocculation efficiency could be significantly improved to over 85%, even under slightly alkaline conditions. This improvement might be attributable to the formation of efficient polyelectrolyte complexes in the presence of both chitosan and alginate. Soluble poly-L-lysine (PLL) is a cationic, linear biopolymer charged with hydrophilic amine moieties, and is secreted by various _Streptomycesetaceae_ bacteria as well as few filamentous fungi [91; 92]. This biopolymer is reportedly highly efficient for algae harvesting and can be used as an antimicrobial agent for reducing biomass contamination [93]. Application dosage and its efficiency are mainly dependent on its molecular weight (MW). PLLs with MWs of 4-15 and 70-150 kDa have been applied to harvest _C. ellipsoida_. A dosage of 5 mg/L of low MW PLL resulted in a harvesting efficiency of 89%, but only 1 mg of high MW PLL was required to achieve a very high harvesting efficiency of 98.9%. The investigators claimed that the quality and shelf-life of the PLL-treated algal biomass might have been enhanced by the prevention of external microbial contamination [83]. Tannins (plant phenols) are less-toxic, biodegradable, anionic branched polyelectrolytes, which can be extracted from many plants such as _Acacia mearnsii_. To use tannin as a flocculant for negatively charged particles such as algal cells, cationic moieties should be added to its original structure via chemical modifications such as the Mannich reaction [80]. A high harvesting efficiency of 99% was recorded for the marine alga _N. oculata_ at pH 6 by using only 10 mg/L of Tanfloc, a commercial product obtained by cationization of natural tannin, under bench-scale experimental conditions [82]. Remarkably, its good algae harvesting performance could be maintained in a pilot-scale process. However, efficacy of Tanfloc efficacy is species-specific. A higher dosage of 210 mg/L was required to achieve a 96.7% harvesting efficiency for freshwater alga _Scenedesmus_ sp. at a pH of 7.8 [81]. Inulin (a plant-based polysaccharide) has also been tried as a flocculant after the cationization reaction, using etherifying reagents such as 3-chloro-2-hydroxypropyl trimethyl ammonium chloride. This flocculant showed a harvesting efficiency of 88.6% for _Botryococcus_ sp. within 15 min, at a dosage of 60 mg/L. Bridging between particles was posited as the main flocculation mechanism here [72]. Synthetic polyelectrolyte (cationic and anionic) polymers in linear or branched forms including acrylamide, acrylic acid, diallyl dimethyl ammonium chloride, and epichlorohydrin disappropolamine dimethylamine have been reported for harvesting of both freshwater and marine algae [24; 79]. For example, high harvesting efficiency of over 90% was observed for _Scenedesmus_ sp. using only 8 mg/L polyamine polymer. In comparison, relatively high amounts of chitosan (80 mg/L) and alum (250 mg/L) were required to reach similar harvesting efficiencies [79]. However, it should be noted that synthetic polymers cannot be regenerated and have adverse effects on harvested biomass and aquatic environments due to their petroleum origins [94].