# A Review on the Use of Microalgae for Sustainable Aquaculture
## 3 Microalgae-Assisted Aquaculture
### Technologies for Biomass Production
#### 3.3.1 Pretreatment of Wastewater  
Aquaculture wastewater contains some solid organics, which are regarded as sludge in some cases [27; 28; 55]. Without appropriate treatment, solid organics with a large size could not be assimilated by microalgae directly, and even hinder the photosynthesis of microalgae by increasing the turbidity of wastewater. The core principle of pretreating solid organics is to convert indigestible nutrients to digestible nutrients, such as volatile fatty acids, sugar, and carbon dioxide. Common pretreatment methods include anaerobic digestion and aerobic digestion. [PERSON] et al. (2010) that summarized the results of employing anaerobic digestion to treat aquaculture sludge revealed that the removal efficiency of a total solid (TS) could reach 80%-100%. By controlling relevant parameters of anaerobic digestion, volatile fatty acids, which are good carbon sources for microalgae metabolisms, could be produced [58]. In the treatment of aquaculture wastewater, which is not suitable to be anaerobically digested due to lower concentrations of nutrients, aerobic digestion is commonly applied to convert solid organics to carbon dioxide. Dissolved carbon dioxide could be further fixed by microalgae through photosynthesis. Thus, by appropriate pretreatment, conversion efficiency of solid organics to microalgae biomass is highly improved.  
Unbalanced Nutrients Profile  
According to the redfield ratio for phytoplankton, the ratio of C/N in culture medium or wastewater should be controlled around 6.1:1, which is much higher than the ratio of C/N in aquaculture wastewater [59]. A comparison between aquaculture wastewater and a commonly used mixotrophic medium also showed that carbon deficiency might be a barrier to the microalgae growth in aquaculture wastewater. For some ammonia-sensitive microalgal strains, a high concentration of ammonia could cause toxicity or even lead to the failure of microalgae growth [40]. Similar problemshave been reported in the application of microalgae to treat other sources of wastewater, such as food processing wastewater and livestock industry effluent [36; 60].  
To address the aforementioned problems in aquaculture wastewater, some methods used in other wastewater may be useful. First, the most straightforward method to balance the nutrients profile is adding certain nutrients in wastewater. It was verified that with the addition of glucose and acetic acids, nitrogen assimilation ability of microalgae cells was improved since carbon metabolism and nitrogen metabolism are closely connected through the TCA cycle and GS-GOGAT pathway [31]. In a real-world application, [PERSON] et al. (2016) found that mixed food processing wastewater from different sources have a more balanced nutrients profile and become suitable to microalgae growth. Thus, not only the biomass yield, but also nutrients recovery efficiency is dramatically improved by appropriate mixing. For example, the biomass yield of algae grown in mixed wastewater ranged between 1.32 g/L and 2.68 g/L while in non-mixed wastewater the biomass yields were less than 1.16 g/L. Second, [PERSON] et al. (2015) found that microalgae cells (_Chorella_ sp.) pretreated by nitrogen starvation performed amazingly in ammonia removal, assimilating NH\({}_{3}\)-N at 19.1 mg/L/d in wastewater with 160 mg/L NH\({}_{3}\)-N. Therefore, it is a possible way to quickly remove ammonia in aquaculture wastewater by appropriate nitrogen starvation of microalgae.