# A Review on the Use of Microalgae for Sustainable Aquaculture
## 3 Microalgae-Assisted Aquaculture
### Microalgae-Based Wastewater Remediation
#### 3.2.3 Microalgae Cultivation Systems  
Although various microalgae cultivation systems have been developed for different purposes by previous studies, not all of them are suitable for microalgae-based aquaculture wastewater remediation. Generally, cultivation systems with high land utilization efficiency, a low investment cost, and high photosynthesis rate, have the potential to be used in aquaculture. Two systems, including a raceway pond and revolving algal biofilm (RAB), which meet the criteria mentioned above, have more advantages in the aquaculture industry. As shown in Figure 1, a raceway pond system consists of a closed circulation channel with a depth of 0.2-1.0 m and one or two paddle wheels drive the circulation of water body [41; 46]. As the raceway pond system has been intensively studied in both lab research and an industrial application,  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Animal Type** & **TN (mg/L)** & **NH\({}_{3}\)-N (mg/L)** & **TP (mg/L)** & **COD (mg/L)** & **Total Solids (g/L)** & **Reference** \\ \hline Shrimp & 361 & 90 & _NA_ & 1321 & _NA_ & [41] \\ NA 1 & 1023.84 & 28.08 & 239.76 & 904.2 & 21.6 & \\ NA & 777.87 & 50.25 & 383.91 & 348.8 & 20.1 & [28] \\ NA & 533.42 & 23.84 & 458.92 & 2494 & 14.9 & \\ Shrimp & \textgreater{}365 & 83.7 & _NA_ & 1593 & _NA_ & [42] \\ NA & 110.8 & 0.07 & _NA_ & 19.7 & _NA_ & [43] \\ Shrimp & \textgreater{}395 & 101.7 & _NA_ & 1201 & 13.1 & [27] \\ Rainbow trout & 1.18 & 0.27 & 0.19 & 17.6 & 0.01 & [44] \\ Crucian carp & 6 & 0.9 & \textgreater{}0.7 & _NA_ & _NA_ & [45] \\ Water eel & 12.4 & 4.6 & 5.2 & 48 & _NA_ & Our lab 2 [FOOTNOTE:2]Footnote 2: “NA” is short for “Not Available”. 1 [FOOTNOTE:2]Footnote 2: “NA” is short for “Not Available”. a couple of problems have been identified, including: (1) Contamination risks of microalgae biomass by bacteria and other microbes are high; and (2) microalgae grown in a raceway pond system might be limited by unfavorable conditions, such as temperature fluctuation and light deficiency. However, compared with closed glass photo-bioreactors, a raceway pond system has a much lower investment cost but higher volume, making it more suitable for the treatment of aquaculture wastewater [47]. In addition, a raceway pond system has lower annual operating expense ($42.65 M) and yields lipids at a lower cost ($13/gal) than a photo-bioractor ($62.80 M and $33/gal) [48]. A life cycle analysis (LCA) conducted by [PERSON] et al. (2015) indicated that it is a sustainable way to use microalgae-bacterial flocs in a raceway pond system for aquaculture wastewater remediation and recycle biomass for aquaculture feed. As a lot of efforts have been devoted into the fundamental research and parameters optimization, recent studies are heading towards the demonstration scale of microalgae cultivation for either wastewater remediation or value-added biomass production. Considering the turbidity and bacteria co-growth in aquaculture wastewater, in the demonstration scale of microalgae cultivation in the raceway pond system for aquaculture wastewater remediation, two critical factors should be considered. First, location and pond depth should be selected or designed reasonably to improve the light transmittance and photosynthesis rate. Second, the relationship between microalgae and bacteria in the raceway pond system should be fully understood. [PERSON] et al. (2014) reported that microalgae bacterial flocs contributed to the removal of 28% COD, 53% BOD, 31% TN, and 64% TP in aquaculture wastewater (12 m\({}^{3}\) raceway pond), suggesting that with the establishment of beneficial cooperation in microbial community, threat of bacteria to microalgae growth in raceway pond system could be reduced to a low level. A similar phenomenon was reported by the studies on the interaction between microalgae and bacteria or fungi in other wastewater sources [49; 50].