# A Review on the Use of Microalgae for Sustainable Aquaculture
## 3 Microalgae-Assisted Aquaculture
### Microalgae-Based Wastewater Remediation
#### 3.2.3 Microalgae Cultivation Systems
Therefore, in the application of a raceway pond for a microalgae-based aquaculture wastewater treatment, research interests are gradually moving from the bacterial contamination control to the establishment of cooperation between microalgae and bacteria for nutrients recovery. The use of a raceway pond system for an aquaculture wastewater treatment has a low investment cost and low operation cost, but this system is more likely to be influenced by the external environment. In addition, after algae cultivation in a raceway pond system, the harvesting process of biomass is time-consuming and energy-intensive. Revolving Algal Biofilm (RAB) System  
The RAB system, which was developed to grow microalgae on a film, was considered as a potential technology to improve land utilization efficiency and simplify the harvesting process [51; 52]. As shown in Figure 1, revolving algal biofilm is a system consisting of a microalgal biofilm, drive unit, and open pond with wastewater. Since the biofilm is established vertically on the open pond, theoretically, the RAB system has higher land utilization efficiency and biomass productivity than the standard raceway pond system [51]. Besides, compared with conventional harvesting methods, such as centrifugation and chemical-floculation, it is more cost-saving and eco-friendly to harvest biomass attached on film by using a scraper [53]. In a real-world application, to construct biofilm with a high biomass density, previous studies compared a couple of film materials and found that cotton is a good material for biofilm construction, yielding a biomass at 16.20 g\(\cdot\)m\({}^{-2}\)[52]. In addition to the research in the lab, a pilot-scale RAB system performed well in a wastewater treatment and biomass yield. According to the studies of [PERSON] and [PERSON] (2014) and [PERSON] and [PERSON] (2012), in the pilot-scale RAB system, nutrients removal rates were 2.1 and 14.1 g\(\cdot\)m\({}^{-2}\)-day\({}^{-1}\) for total dissolved phosphorus (TDP) and total dissolved nitrogen (TDN) and biomass productivity could reach 31 g\(\cdot\)m\({}^{-2}\)-day\({}^{-1}\)[51; 53]. Another advantage of the RAB system is that the harvesting process does not reply on any chemicals, thus, the harvested biomass has a high safety level for aquaculture use. Therefore, from the perspectives of nutrients recovery and biomass reuse, the RAB system is a promising technology for the wastewater remediation and resource recycle in aquaculture. As the RAB system is exposed to the atmosphere during operation, bacteria or fungi would grow together with microalgae on the biofilm. In spite of the competition on nutrients, the cooperation between microalgae and other microbes in the biofilm formation and nutrients uptake merits more attention. It was reported that extracellular polymeric substances (EPS) released by the bacteria function as \"glue\" to promote the formation of biofilm and extracellular enzymes produced by bacteria convert high-molecular-weight organics to low-molecular-weight organics, which could be assimilated by microalgae more efficiently [54; 55; 56] (Figure 1). However, to our knowledge, until now, mechanisms related with biofilm formation, nutrients conversion and assimilation, and algal-bacterial interaction in the RAB system have not been fully understood [57]. In the coming future, with the wide use of RAB system for aquaculture wastewater remediation, those scientific questions and technical problems currently bothering researchers will be addressed. The most important advantages of the RAB system is that this system integrates algae cultivation with biomass harvesting. Besides, compared with the raceway pond system, the RAB system has better performance in land utilization and harvesting cost control. However, the RAB system has a much higher investment cost than the raceway pond system. In a real-world application, the appropriate microalgae cultivation system should be selected according to the actual conditions.