Chlorella sp. is widely used for wastewater treatment, and has proven abilities of removing nitrogen, phosphorus, and COD, mixing with bacteria or not, which show their potentiality as tertiary biotreatment step (Figure 12) ^[1][29]. Microalgae of the genus Chlorella can be grown both in autotrophic and mixotrophic cultivation conditions, reaching high growth rates.

Lau et al. ^[2][30] reported that Chlorella vulgaris can reduce 86% of the inorganic nitrogen and 78% of the inorganic phosphates in primary settled wastewater. Instead, Colak and Kaya ^[3][28] reported that Chlorella sp. can remove 50.2% and 85.7% of these two elements from industrial wastewater.

Wang et al. ^[1][29] evaluated the ability of Chlorella sp. to remove nitrogen, phosphorus, COD, and metals on wastewaters sampled from four different points of the treatment process flow of a local municipal wastewater treatment plant: wastewater before primary settling, wastewater after primary settling, wastewater after activated sludge tank, and wastewater generated in sludge centrifuge. The results, reported in Table 12, demonstrate the efficiency in nutrient removal of Chlorella sp.

Baglieri et al. ^[4][34] tested the ability of C. vulgaris to remove contaminants from agricultural wastewater, considering two case studies: (i) the first on the growth rate of the species in wastewater from a hydroponic greenhouse cultivation, in order to evaluate the degree of removal of the main inorganic compounds; (ii) the second on microalgae ability to degrade five different active ingredients commonly used in agricultural practices (pyrimethanil, metalaxyl, iprodione, fenhexamid, and triclopyr). C. vulgaris demonstrated a good aptitude for the decontamination, removing about 99% of nitric nitrogen, 83% of the ammonia nitrogen, and 88% of phosphates. A reduction in the contents of other elements, such as iron, potassium, and total organic carbon, was also observed. The microalgae also showed ability to grow in the presence of all five active ingredients used in the trials, although in some cases, signals of suffering from a slightly toxic effect were observed. The dissipation of metalaxyl and fenhexamid provided the most interesting results, occurring faster in the presence of microalgae ^[4][34]. With regard to iprodione and triclopyr, the dissipation was less evident. Pyrimethanil showed a different behavior with respect to the other pesticides, resulting in more resistance to dissipation, although in the presence of C. vulgaris ^[4][34].

Rasoul-Amini et al. ^[5][27] tested two strains of Chlorella sp. (YG01 and YG02) for removal of nitrogen and phosphorus from municipal wastewater. The experiment confirmed that Chlorella sp. (YG01) can be considered an efficient nutrient remover in wastewaters of different origin, while in the other strains, a minor efficiency in the purification process was shown. All this evidence is summarized in Table 12.

Ankistrodesmus sp. is a green phototrophic microalgae that has a long crescent shape with a slight curve at both ends ^[13][35]. Mixotrophic conditions of growth of Ankistrodesmus sp. have shown the highest specific growth.

The phycoremediation ability of Ankistrodesmus sp. is reported only in a few studies available in literature. Among these, Ahmad Ansari et al. ^[7][15] focused on the potential strains, biomass-enhancement strategy, nutrient removal potential, and biochemical composition of the microalgae. In this study, Ankistrodesmus falcatus was grown using aquaculture wastewater. With regard to the removal efficiency, A. falcatus showed good performance (e.g., 80.85% of NO₃⁻, 98.52% of P, and 61% of COD), and the results are reported in detail in Table 12.

Also available in literature are some studies on the possible use of Ankistrodesmus sp. as an autoflocculating microalgae with a shape, and zeta potential that could have the ability to coagulate other microalgae species, as Chlorella sp., and so act as bioflocculant in harvesting biomass ^[13][35].

Scenedesmus sp. is one of the microalgae genera particularly interesting for wastewater treatment due to its efficiency of nutrient removal, rapid growth rate, and high biomass productivity ^[11][14][15][32,36,37]. Scenedesmus sp. can be grown under autotrophic, heterotrophic, and mixotrophic cultivation conditions.

Xin et al. ^[12][33] studied the properties of lipid accumulation and nutrient removal of Scenedesmus sp. LX1 in secondary effluent. With regard to the total nitrogen and total phosphorus contents, the results showed a notable removal efficiency, for both nutrients, of over 98% (Table 12).

Martinez et al. ^[11][32] studied the kinetics of N and P elimination as well as simultaneous growth of S. obliquus in the effluent from a secondary-sewage-treatment facility, under different conditions of stirring and temperature. The researchers chose as experimental conditions 20, 25, 30, and 35 °C, representing the range of average temperatures of wastewater in different seasons of a warm climate, and two levels of mixing: maximum (magnetic stirring and air bubbling in the culture medium) and minimum (absence of magnetic stirring), as reported in Table 12.

Many works are also available in literature about the cultivation process of microalgae to promote the degradation of inorganic compounds and pesticides in water. Baglieri et al. ^[10][31], as above reported on C. vulgaris in the same case studies, also evaluated the Scenedesmus quadricauda removal efficiency, showing in the wastewater of hydroponic greenhouse cultivation a consumption of about 99% nitric nitrogen, but only 5% of the ammonia nitrogen, and a remotion of 94% phosphates. S. quadricauda also showed to be able to grow in the presence of all five active ingredients (pyrimethanil, metalaxyl, iprodione, fenhexamid, and triclopyr) used in the trials, determining a reduction in their contents, and providing similar results to those above reported for C. vulgaris ^[10][31]. Another study in which the removal ability of active ingredients from agricultural wastewater by microalgae was evaluated was conducted by Kurade et al. ^[16][38]. The researchers screened S. obliquus for the removal of diazinon, an organophosphorus insecticide. The removal efficiency was evaluated in Erlenmeyer flasks containing 100 mL of BBM added to 20 mg diazinon L⁻¹. However, S. obliquus did not show high removal capacity of diazinon.

Although microalgae-based wastewater treatment is oriented towards efficient removal of nitrogen and phosphorus, not all contaminants can be eradicated ^[17][1].

In literature, other studies about microalgae species and cyanobacteria able to remove organic and inorganic compounds from wastewaters, of different origins, are reported. Rasoul-Amini et al. ^[5][27] evaluated the removal efficiency of nitrogen and phosphorus from municipal wastewater of the following species: two strains of Chlamydomonas sp. (YG04 and YG05), and one strain of Oocystis sp. (YG03). The results showed that Chlamydomonas sp. (YG04 and YG05) can act as efficient nutrient removers from wastewaters of different origin, while Oocystis sp. (YG03) showed a minor efficiency in the purification process, as reported in Table 12.

Zhu et al. ^[6][24] studied the nitrogen and phosphorus removal during the Anabaena flos-aquae biofilm growth, in two nutrient mediums, containing different nitrogen and phosphorus compounds. The results demonstrated that the nitrogen and phosphorus removal reached 94.9 and 96.8%, respectively, in the form of ammonium nitrogen, while 97.7% of phosphorus were removed in the form of orthophosphate phosphorous (Table 12).

Renuka et al. ^[9][26] tested the phycoremediation ability of four microalgae strains: Calothrix sp., Lyngbya sp., Chlorella sp., and Ulothrix sp. The researchers observed a different behavior of the strains, obtaining in all the cases a significant removal of NO₃–N (ranging from 57–78%) and PO₄–P (44–91%), as reported in detail in Table 12.

Hena et al. ^[8][25] evaluated the removal ability of Arthrospira platensis cultivated in dairy farm wastewater for biodiesel production. The results showed a good aptitude of A. platensis to remove the main pollutants.