# Microalgal Biomass Generation via Electroflotation: A Cost-Effective Dewaring Technology
## 4 Conventional Dewatering Approaches in Microalgal Culture
### Flocculation  
Flocculation is a promising low-cost method that is considered as the initial economic step in dewatering and harvesting processes. Flocculation is also sometimes interchangeably referred to as coagulation. However, they are defined differently. Coagulation involves the adjustment of pH and addition of electrolyte, whereas flocculation is based on the cationic addition of polymers to the samples. However, both coagulation and flocculation are reported to perform in the same way [52]. Flocculation occurs by the interaction of the negatively charged surface of microalgae cells with the flocculants, and cells will further aggregate into larger particles. Microalgae cells exhibit negatively charged surface and are stabilized due to the presence of carboxylic (-COOH), phosphate (-PO4), and amine (-NH2) groups on their cell surface. The carboxyl group will dissociate at pH above 4-5, whereas amine will be uncharged at this pH, causing microalgae cells to pose net opposing surface charge above pH 4-5 [54]. Further, flocculation suits various microalgal strains of sizes between 5 and 5 microns [55], compared to other harvesting methods. Furthermore, several factors affect microalgae flocculation, such as pH (5 or 10), type, and dosage of the flocculants (e.g., aluminum sulfate at low dosage of \(<\)37 mg/L and high dosage of 70 mg/L), mixing intensity, size, and surface characteristics of algal cells (zeta potential), ionic strength (high ionic strength changes surface charge to positive), initial biomass concentration, the composition of the culture medium, and the motility of microalgae to some extent [56]. Additionally, microalgae contain algal organic matter (AOM), which also influences flocculating activity as well as in the harvesting of microalgae. These AOMs (10.6 and 58.5 mg/mL to 2.3 and 5.6 mg/mL for _Chlorella vulgaris_ and _Nannochloropysisulata_, respectively) are secreted mainly from the extracellular polysaccharide substances (EPS) by microalgae cells that produce proteins or polysaccharides. Later, these compounds are identified to react with flocculants, which can result in high flocculants demand and affect the dewatering process [57]. The impact on the environment, the reusability of the resultant supernatant or recycling of medium, price, and absence of harvested biomass contamination, post-usage of flocculants must be considered in selecting a suitable flocculant [52]. Organic flocculants are safer alternatives and environmentally friendlier choices of polymers or polyelectrolytes, compared to inorganic flocculants. In general, these polymers are categorized as cationic, anionic, and nonionic polymeric flocculants. Anionic flocculants may cause electro-repulsion with negatively charged microalgae cells. Thus, cationic polymers are more successful in flocculating microalgae cells [58]. Further, polymer flocculants can form a bridge that can physically bind with one or more particles together, rather than just neutralizing the surface charge [59]. Chitosan, an example of cationic polymers derived from chitin, is widely used to harvest microalgae (self-flocculating agent with about 22 \(\pm\) 2% efficiency in the stationary phase), due to its environmentally friendly nature, the reusability of the residual medium, and the further utilization of the dewatered biomass in field applications [60; 61]. Additionally, other plant-based flocculants such as _Moring oleifera_ seed extract (93% of _C. vulgaris_) and mung bean protein extract (90% of _Nannochloropysis_ sp.) possess microalgae dewatering potential [62; 63]. Furthermore, inorganic flocculants such as aluminum sulphate, ferric chloride, and ferric sulphate are commonly used. These, however, vary in effectiveness due to their ionic charge [52]. Aluminum sulphate is widely used to treat industrial wastewater and is highly effective compared to other metal salts in flocculating freshwater or marine species of microalgae.