## 5 Challenges and Prospects  
Domestic, agricultural and industrial wastewaters have recently received increased attention from microalgae researchers in order to assess the capability of microalgae to reduce contaminants found in these various forms of wastewater, and furthermore, project a microalgal biorefinery system. Microalgae have demonstrated their ability to remove these pollutants, but this ability can be increased if the difficulties encountered during culture are resolved. Figure 10 depicts a number of challenges (economic and biological) associated with the microalgae-based wastewater treatment process as well as ideas in order to deal with these issues. Microalgae cell growth can be limited by low nutrient in wastewater, however this is one of the biological obstacles that can be solved by combining different wastewaters, adding inorganic compounds, fertilizers, and food waste [41; 74; 91; 105]. Using agricultural waste (poutlry litter) mixed with municipal wastewater to cultivate _Acutodesmus obliquus_ result in a biomass 74.67% higher than using agricultural waste as culture medium [41]. The effect of alcohol wastewater as an anaerobic digested starch wastewater supplement on the removal of pollutant load by the microalgae _Chlorella pyrenoidosis_ has been evaluated [105]. The strain was grown in various concentrations of anaerobic digested starch wastewater (ADS) and alcohol wastewater (A), but the proportion 1/0.053 of ADS/A wastewaters generated the highest results in terms of biomass (3.01 g/L), lipid production (127.70 mg/L/d), and pollutants removals (75.78%, 91.64 % and 90.74% of COD, TN and TP respectively) [105]. In contrast, researchers employed the dilutions approach to maximize the development of the microalgae in wastewater that had a high concentration of chemical oxygen demand and organic carbon addition to alleviate ammonia toxicity [162; 171; 172]. As reported, large amounts of ammonia can be toxic to cells by inhibiting development, photosynthesis and chlorophyll production, also destroying pH and C/N balances [145; 171; 173]. Temperature and pH both affect the presence of NH\({}_{3}\)-N, which predominates when pH is greater than  
Figure 10: Limitations in growing microalgae in wastewater and possible solutions. 9.25 [145]. Two processes through which ammonia is toxic to the growth of microalgae can be observed: free NH\({}_{3}\) diffusion in addition to NH\({}_{4}\)+ production in thylakoid under acidic conditions (reduces ATP to ADP conversion) and the promotion of NH\({}_{3}\) ligation (harms the oxygen-evolution complex subunit) [145]. Additionally, in microalgae culture system with a temperature between 20 and 25 \({}^{\circ}\)C and a pH of 9.5, the photosynthesis is reduced up to 90% only because of 2 to 3 \(\upmu\)M of NH\({}_{3}\), hence the need to minimize ammonia toxicity [174]. Several different strategies exist to reduce toxicity of ammonia, including dilution, chemical stripping and addition of organic carbon [171]. Indeed, using glucose as carbon source for three species of microalgae (_Chlorella sorokiniana_, _Coelastrum_ sp., and _Desmodesumumis_) was found to minimize toxicity in all species at ammonia concentrations ranging from 50 to 1000 mg/L [171]. Others biological obstacles, such as high content of suspended solids, turbidity, heavy metals and bacteria can be overcome using appropriate pre-treatment methods for each obstacle. The previous Tables 2, 6 and 9 describe a few pretreatment techniques that were employed in the literature prior to the cultivation of microalgae but most of them are not possible at large scale. One of the challenges could thus be to develop industrializable pre-treatments processes. In the field of microalgae growth in wastewater for biomass generation and nutrient removal, the development and design of low-cost microalgae culturing systems is now a critical topic [175]. Closed photobioreactors often cost a lot of money and have a difficult design.