# Microalgal Biomass Generation via Electroflotation: A Cost-Effective Dewaring Technology
## 2 Overview of Microalgal Biosystems
### Development of Microalgal Culture System
#### 2.1.3 Hybrid Cultivation System in Two-Stage Growth  
The two-phase hybrid cultivation system has been proposed to overcome weaknesses in both open and closed-culture systems. This system is designed to reduce the overall production and operation cost as well as to elevate biomass productivity of microalgae [38]. Besides, the hybrid system can increase lipid accumulation in microalgae [29] as it comprises two-stage cultures. The first phase of cultivation occurs in a photobioreactor, where the growth parameters are controlled to favor continuous cell division as well as to minimize contamination from other microorganisms [48; 49]. In the second phase, the growing microalgae cells from the photobioreactor are transferred to open ponds and subjected to nutrient depletion and certain additional environmental stresses. The rationale is to enhance the lipid accumulation within the microalgal cells and to achieve a fatty acid composition target for biodiesel \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Production System** & \multicolumn{2}{c}{**Advantages**} & \multicolumn{2}{c}{**Disadvantages**} \\ \hline \multirow{7}{*}{Tubular photobioreactor} & \(\bullet\) & High biomass productivity & \(\bullet\) & High initial investment costs \\  & \(\bullet\) & Applicable for outdoor & \(\bullet\) & Toxic accumulation of O\({}_{2}\) fouling \\  & \(\bullet\) & Low mutual shading effect & \(\bullet\) & Overheating \\  & \(\bullet\) & Cost-effective & \(\bullet\) & Dissolved oxygen builds up \\  & \(\bullet\) & Lower photo-inhibition & & \\ \hline \multirow{7}{*}{Flat-plate photobioreactor} & \(\bullet\) & High biomass productivities & \(\bullet\) & Difficult in temperature control \\  & \(\bullet\) & Large illumination surface area & \(\bullet\) & Short light penetration depth \\ \cline{1-1}  & \(\bullet\) & Small accumulation of & \(\bullet\) & Scale-up requires many support \\ \cline{1-1}  & \(\bullet\) & dissolved oxygen & \(\bullet\) & materials and large areas of land \\ \cline{1-1}  & \(\bullet\) & Less prone to contamination & \\ \cline{1-1}  & \(\bullet\) & Lower power consumption & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of tubular and flat-plate photobioreactors [47]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Open Pond (Raceway Pond)** & **Closed Photobioreactors** & **Hybrid System** \\ \hline Space required & High & Moderate & High \\ Setup cost & Low & High & Moderate \\ Contamination risk & High & Low & Low \\ Evaporation loss & High & Almost none & Moderate \\ CO\({}_{2}\)-losses & High & Almost none & Not given \\ Biomass quality & Not susceptible & Susceptible & Susceptible \\ Culture quality & Low & High & High \\ Growth rate & Low & High & High \\ Scaling-up feasibility & Easy & Difficuit & Moderate \\ Reproducibility of production & - & Possible within certain tolerances & - \\ parameters & - & Batch & Continuous \\ Operation type & Batch & High & Moderate \\ Energy input for mixing & Low & High & Moderate \\ Maintenance & Easy & Difficult & Moderate \\ Standardization & Not possible & Possible & - \\ Weather dependence & Absolute, cultivation is impossible & \begin{tabular}{c} Insignificant as closed \\ configurations allow cultivation \\ even during bad weather \\ \end{tabular} & - \\ Efficiency of treatment processes & \
\begin{tabular}{c} Low, time-consuming, large \\ volume flows due to low \\ concentrations \\ \end{tabular} &
\begin{tabular}{c} High, short time, relatively small \\ volume flows \\ \end{tabular} & - \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of open, closed, and hybrid microalgae photobioreactor facilitated cultivation systems [29; 39]. production [18; 49]. High inoculum is needed in the second phase to ensure that the microalgae population rather dominates the cultivation medium and not other microbes.