## 2 Wastewater Treatment by Microalgae
### Domestic Wastewater
#### 2.1.3 Microalgae Treatment of Domestic Wastewater
The COD content in kitchen wastewater was gradually reduced in the first four days, after which it was kept around the same value and varied in a modest range throughout the culture period. COD reduction efficiency was 32.14% on the final day of culture [12]. In this study, the color of the kitchen wastewater was black during microalgal culture, and they supposed that it was due to the high concentration of ammonia and high turbidity and then concluded that these conditions were not conducive to microalgal development. To solve this problem and make the kitchen wastewater acceptable for microalgal development, they diluted it by about 1:1 with water [12]. [PERSON] et al. 2021 [33] reported that septic tank wastewater can be utilized for microalgae cultivation. They tested diverse types of wastewaters for _Chloella pyrenoidosa_ (FACHB-9) cultivation and demonstrated that the toilet wastewater (TN: 203.6 mg L\({}^{-1}\); NH\({}_{4}\)-N: 157.5 mg L\({}^{-1}\); TP: 22.3 mg L\({}^{-1}\); COD: 5200 mg L\({}^{-1}\)) was the great medium for biomass (1.68 g L\({}^{-1}\)) and nutrient removal efficiencies (NH\({}_{4}\)-N: 90.8%; TP: 62.9%; COD: 61.3%). According to the same study, discharging kitchen or laundry wastewater decreased the biomass production by 50.5-79%, and should be isolated from toilet wastewater. [PERSON] et al. 2021 [35] compared the remediation capability of the marine diatom _Cheetoceros_ with the haptophyte _Isochrysis_ microalgae when grown in urban wastewater. Their results demonstrated that the two strains can successfully remediate NO\({}_{2}\)-N of 0.63% from 10% of domestic wastewater by _Chaetoceros_ and 5.57% from 30% of domestic wastewater by _Isochrysis_, as well as a total phosphorus removal of 83-84% and 84-94% by _Chaetoceros_ and _Isochrysis_, respectively. Chemical oxygen demand was reduced more efficiently in 40% of domestic wastewater by _Chaetoceros_ (157 mg L\({}^{-1}\)) and in 100% of domestic wastewater by _Isochrysis_ (93%). According to [35], the use of domestic wastewater as a culture medium for _Chaetoceros_ sp. and _Isochrysis_ sp. has shown their effectiveness as a sustainable food source and a source of biofuels due to their production of total polyunsaturated fatty acids (PUFAs) from 33.5% to 71.6% in the case of _Chaetoceros_ and from 20% to 63.4% in _Isochrysis_. The effectiveness of autotrophic and heterotrophic metabolisms in different microalgae species for wastewater bioremediation and the generation of sustainable microalgal products/coproducts was also evaluated using various culture regimes [27; 34; 39; 40]. For example, [PERSON] et al. 2022 [39] assessed the photoperiod-induced mixotrophic metabolism in _Chloella vulgaris_. They measured cell biomass growth and lipid content of _Chloella vulgaris_ and conducted organic nutrient removals from municipal wastewater (COD: 145 mg L\({}^{-1}\); NH\({}_{4}\)-N: 48 mg L\({}^{-1}\)) with distinct light: dark photoperiod cycles. In their results, the 16:8 (light: dark) photoperiod was shown to be appropriate for producing high biomass (0.89 g L\({}^{-1}\)) and lipid production (0.16 g L\({}^{-1}\)), while also removing chemical oxygen demand and ammonium-nitrogen from municipal wastewater with near-complete removal (>94%). The removal of nutrients from municipal wastewater by _Chloella vulgaris_ and _Nannochloropsis oculata_ was examined utilizing mixotrophic culture with glycerol (0 g L\({}^{-1}\) to 5 g L\({}^{-1}\)) [34]. The addition of 2 g L\({}^{-1}\) of glycerol in the municipal wastewater as a medium for _Chloella vulgaris_ enhanced the biomass production to 56 mg L\({}^{-1}\) d\({}^{-1}\) and had a total nitrate removal of 64%. However, for _N. oculata_, the best nitrogen removal (80.62%) was obtained with 3 g L\({}^{-1}\) of glycerol, with COD and TP removals of 96.3 and 60.72%, respectively, using 1 g L\({}^{-1}\) and 5 g L\({}^{-1}\) of glycerol. _C.