# A Review on the Use of Microalgae for Sustainable Aquaculture
## 3 Microalgae-Assisted Aquaculture
### Technologies for Biomass Harvesting
#### 3.4.3 Flotation and Modified Flotation  
Flotation has been assessed as one of the most economic technologies for microalgae harvesting [74]. The main process of flotation-based harvesting is generating fine air bubbles continuously in wastewater or a culture medium with microalgae. As the air bubbles attach on suspended microalgae cells, microalgae cells will rise to the surface of aqueous phase. Flotation can be considered as an inverted sedimentation, having a small footprint, low detention period, and high overflow rate [74].  
A negative repulsive charge on the surface of microalgae cells is the main reason for suspension of microalgae cells. In a real-world application, to improve the harvesting rate, flotation can be combined with flocculation, which partly neutralizes the negative charge on microalgae cells [75]. Previous studies have explored various types of flocculating agents, such as metal ion and polymer, for microalgae flocculation [66; 76; 77]. [PERSON] et al. (2012) optimized the addition content of aluminum ion in microalgae flocculation process and found that the harvesting efficiency could reach 82%. For aquaculture practice, to recycle the harvested microalgae as feed, biomass safety should be strictly controlled. Owing to the serious threats caused by metal accumulation in food chain, metal-ions  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Microalgae** & **Fungi** &
\begin{tabular}{c} **Harvesting** \\ **Efficiency** \\ \end{tabular} & **Conditions** & **Reference** \\ \hline \multirow{2}{*}{_Chlorella_ sp.} & \multirow{2}{*}{_Penicillium_ sp.} & \multirow{2}{*}{98.2\%} & Fungal pellets; 30–34 \({}^{\circ}\)C; pH: & \\  & & & 4.0–5.0; Agitation speed: & \\  & & & 120–160 rpm & [67] \\ \hline \multirow{2}{*}{_Chlorella_ sp.} & \multirow{2}{*}{_Penicillium_ sp.} & \multirow{2}{*}{99.3\%} & Fungal spores; 40 \({}^{\circ}\)C; pH: 7.0; & \\  & & & & Agitation speed: 160 rpm & \\ \hline \multirow{2}{*}{_Chlorella vulgaris_} & \multirow{2}{*}{_Aspergillus oryzae_} & \multirow{2}{*}{93\%} & Fungal spores; Heterotrophic & \\  & & & culture; 25 \({}^{\circ}\)C, Agitation & \\  & & & speed: 150 rpm; 3-day & \\ \hline \multirow{2}{*}{_Chlorella vulgaris_} & \multirow{2}{*}{_Aspergillus_ sp.} & Almost 100\% & 25 \({}^{\circ}\)C; pH: 5.0–6.0; Agitation & \\  & & & speed: 100 rpm; 2-day & \\ \hline \multirow{2}{*}{_Chlorella vulgaris_} & \multirow{2}{*}{_Aspergillus niger_} & \multirow{2}{*}{\(>\)60\%} & Fungal spores; 27 \({}^{\circ}\)C; pH: 5.0; & \\  & & & Agitation speed: 150 rpm; & \\  & & & 3-day & \\ \hline \multirow{2}{*}{_Chlorella vulgaris_} & \multirow{2}{*}{_Aspergillus fumigatus_} & \multirow{2}{*}{\(>\)90\%} & Fungal pellets; 28 \({}^{\circ}\)C; & \\  & & & & Agitation speed: 150 rpm; & \\ \hline \multirow{2}{*}{_Scenedesmus quadricauda_} & \multirow{2}{*}{_Aspergillus fumigatus_} & \multirow{2}{*}{\(>\)90\%} & \multirow{2}{*}{Agitation speed: 150 rpm;} & \\  & & & 2-day & \\ \hline \multirow{2}{*}{_Pyrocystis lunula_} & \multirow{2}{*}{_Aspergillus fumigatus_} & Around 30\% & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Fungi-assisted microalgae harvesting.  
based flocculation can not be considered as a safe and eco-friendly technology. Recently, some studies reported the coagulation effects of natural polymers, such as protein and polysaccharide, secreted by microorganisms [78; 79]. Generally, by natural-polymer based flocculation, the harvesting efficiency of microalgae could reach 90% [78; 80]. Therefore, it is suggested that in aquaculture, to harvest microalgae by flotation-flocculation, natural polymers can be used for safety purpose.