## 2 Wastewater Treatment by Microalgae
### Agricultural Wastewater
#### 2.2.3 Microalgae Treatment of Agricultural Wastewater  
As shown in Table 5, nitrogen, phosphate, and some metal ions are present in agricultural effluent. Wastewaters are dangers to environmental safety if left untreated or cleaned incompetently, resulting in eutrophication of nearby water rivers and greenhouse gas emissions. Lately, considerable research has been conducted on the culture of microalgae in agricultural wastewater for their excellent nutrient removal, energy resource and the other products extracted from microalgal biomass (Tables 7 and 8). The Figure 4 illustrates the removal efficiencies of total nitrate, total phosphate and chemical oxygen demand from agricultural wastewater by different species of microalgae. Swing wastewater treatment by microalgae was investigated in a recent study [79]. They evaluated the potential of seven strains which are _Chlorella_ sp. HQ, _Scenedesmus_ sp. LX1, _Chlorella vulgaris_ (FACHIB-8), _C. pyrenoidosis_ (FACHIB-5), _S. obliquus_ (FACHIB-417), _Selenastrum capricomutum_ (FACHB-271) and _Chlorella_ sp. HL, to remove pollutants from a ten-fold dilution of swine wastewater [79] and they reported that _Chlorella_ sp. HL had the highest potential for growth in swine wastewater and nutrient removal among the other species. This strain showed the greatest specific growth rate (\(\upmu\)) and cell density, both of which were 0.51 d\({}^{-1}\) and 2.43 \(\times\) 10\({}^{7}\) cells mL\({}^{-1}\) after nine days of culture, with the TP and COD removal rates of 69.13% and 72.95%, respectively. In addition, [79] suggested that _Chlorella_ sp. HL can be a suitable candidate for energy production due to the higher heating value of 18.25 MJ kg\({}^{-1}\) of _Chlorella_ sp. HL biomass. The author of [57] studied the aptitude of _Chlorella vulgaris_ MBFJNU-1 strain to reduce the pollutants of swine wastewater using a column photobioreactor in outdoor environments. At the same time, they evaluated the carbon dioxide fixation by this species. Their results suggested that the cultivation of _Chlorella vulgaris_ MBFJNU-1 at 3% carbon dioxide, provided the maximum of biomass productivity by about 36 mg L\({}^{-1}\) d\({}^{-1}\) and the best removal efficiency rate of TN, TP and COD by about 16.84 mg L\({}^{-1}\) d\({}^{-1}\), 1.40 mg L\({}^{-1}\) d\({}^{-1}\) and 8.47 mg L\({}^{-1}\) d\({}^{-1}\), respectively [57]. However, these two studies did not assess the biochemical composition of the studied species grown in swine wastewater, which is a critical parameter to be able to use these microalgae for both wastewater treatment and the generation of high-value products at the same time [57; 79]. Other research examined the microalgae immobilization method as an innovative technique to remove contaminants from swine wastewater [58; 61]. The author of [58] assessed the ability of three species of _Chlorella sorokiniana_ genera to grow and remove pollutants of a 2-fold diluted swine wastewater. They reported that _Chlorella sorokiniana_ AK-1 had the highest biomass and protein production rate compared to other species (5.45 g L\({}^{-1}\) and 0.27 g L\({}^{-1}\) d\({}^{-1}\), respectively) [58]. In the same study, they immobilized _Chlorella sorokiniana_ AK-1 cells using sponge, activated carbon, clay and alginate beads as solid carriers in a 2-fold diluted swine wastewater, and found that the sponge immobilization method improved the biomass concentration and protein productivity (8.08 g L\({}^{-1}\) and 0.272 g L\({}^{-1}\) d\({}^{-1}\), respectively) compared to the control and other solid carriers [58]. In addition, using the sponge as the solid carrier enhanced the removal efficiency of TN, TP and COD from 88.6% to 94.1%, 99.3% to 99.5% and from 84.1% to 91.6%, respectively, compared to the control [58]. A fixed-bed biofilm reactor, as another immobilization technique of algal cells, was used [61] to improve the productivity of the biofilm and the removal efficiencies of total nitrate, total phosphate and metal ion (Cu(II)) from swine wastewater.