# Microalgal Biomass Generation via Electroflotation: A Cost-Effective Dewaring Technology
## 2 Overview of Microalgal Biosystems
### Development of Microalgal Culture System
#### 2.1.1 Open Pond  
Open ponds or raceway ponds, including shallow big and circular pond tanks, are the simplest and one of the most intensively investigated microalgae cultivation systems, since the 1950s, due to their lower capital costs [30]. Raceway (open) pond is a shallow pond (between 0.2 and 0.5 m deep) to allow appropriate illumination and to attain high growth rate of microalgae [38]. Besides, biomass productivity of 60-100 mg/(L day\({}^{-1}\)) and biomass concentration of up to 1000 mg/L are achievable at those water depths [39]. This system is commonly applied at an industrial-scale for microalgae, and cyanobacteria cultivation, such as _Anabena_ sp., _Arthospira platensis_, _Chorella_ sp., \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
\begin{tabular}{c} High substrate cost; \\ contamination risk; high \\ equipment cost \\ \end{tabular} \\ \hline \hline \end{tabular}
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Table 1: Characteristics of different microalgae cultivation conditions [18; 19]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Species** & **Cultivation System** & **Features** \\ \hline _Arthropir platensis_ & Flat photobioreactor & \begin{tabular}{c} A suitable growth system to generate denser cultures. It has been \\ demonstrated to yield a denser algal culture, which has 4.5-times higher \\ performance indices than less dense cultures [32] \\ \end{tabular} \\ \hline _Chorella_ sp. & Open pond, photobioreactor & \begin{tabular}{c} A suitable system to produce algal culture in tropical conditions. It can \\ yield 35.5-36 g/day/m\({}^{2}\) of algal biomass during summer [33] \\ \end{tabular} \\ \hline _Chlorella synckhirian_ & Vertical photobioreactor & \begin{tabular}{c} This system has been used to produce 1.3 g of biomass per mol of \\ photobiorestructible activity active radiation (PAR) (400-700 nm), and showed a \\ better growth compared to a horizontal photobioreactor (0.85 g/mol) [34] \\ \end{tabular} \\ \hline _Dunaliella salina_ & Open pond, photobioreactor & \begin{tabular}{c} This system demonstrated a maximum specific growth rate of algae \\ (16 \(\pm\) 0.3/day) with a 8.1 \(\pm\) 1.2/day production rate of \(\beta\)-carotene [35] \\ \end{tabular} \\ \hline _Haematococcus plurvials_ & Open pond, photobioreactor & \begin{tabular}{c} This system produces a biomass concentration of 50–90 g/m\({}^{2}\) with a \\ 9-13 g/m\({}^{2}\)/d productivity [34] \\ \end{tabular} \\ \hline _Isechrysis guluna_ & Horizontal photobioreactor & \
\begin{tabular}{c} This system produced 4.6 g/L dry algal mass in summer with a \\ productivity of 1.6 g/L/d [36] \\ \end{tabular} \\ \hline _Porphyriulin currentum_ & Tubular photobioreactor &
\begin{tabular}{c} This system demonstrated a 7% productivity increase and 34% increase in \\ carbon dioxide bioconversion at steady state [37] \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Microalgae species and cultivation system [31]. _Phaeodactylum tricornotum_, _Dunaliella salina_, _Pleurochrysis carterae_, and _Nannochloropsis_[40]. Further, paddle wheels are used to ensure homogenous mixing of microalgae medium around the circulate channels. For instance, by increasing the speed of the paddle wheel from 1.4 to 2.1 rad s\({}^{-1}\) for an indoor open pond system using _Nannochloropsis_, [PERSON] et al. (2012) observed an increase in the biomass and lipid content of the microalgae medium [41]. The lower velocity of the paddle wheel leads to an inherent light reduction regime in raceway ponds, with microalgae cells near the bottom of the ponds being in the dark zone [42]. This layout eventually affects the biomass and lipid productivity of microalgae [43]. Hence, continuous mixing in the raceway ponds is essential to reduce the residence time of microalgae that may, otherwise, have remained in the dark regime. However, additional energy costs may be incurred since an increment in the mixing speed results in a cubical increment in the power consumption of the paddle wheel [42].