Typically, conventional processes for secondary treatment (activated sludge) take place through the remediation with microorganisms, generally heterotrophic bacteria and this process depends on O
2. The microorganisms, in the presence of O
2, carry out the biological oxidation of the organic substance
[1][31][1,90], since oxygen allows nitrification by bacteria
[32][91]. Generally, O
2 is produced by electromechanical blowers with high energy intensity. This supply of O
2 for wastewater treatment has high costs. It consumes approximately 1 to 3% of the total electricity generated in developed nations, of which 40 to 60% is expended on supplying air to the aeration basin.
[33][58]. Since mechanical aeration causes up to 50% of the operating costs of wastewater treatment plant (WWTP). To solve this problem, the cocultivation of microalgae and bacteria could lead to economic and environmental benefits
[13][30][31][13,89,90]. Obviously, the benefits are not always clear and effective when moving from laboratory prototypes to an industrial scale
[19]. This synergy between the two microorganisms promotes both the growth of microalgae and bacteria. The latter remove COD through heterotrophic growth, producing CO
2, and at the same time the microalgae assimilate CO
2 and the nutrients generated by bacteria, converting these substances into algal cell material through photosynthesis and producing O
2 that stimulates bacterial activity
[10][21][34][10,21,49]. This type of interaction is not always successful because in some cases the microalgae do not release enough oxygen to the bacteria for the degradation of COD. This, if not completely eliminated, could stimulate the mixotrophic metabolism of microalgae, thus reducing the net availability of oxygen for bacteria
[34][49]. Further advantages generated by this interaction are the exchange of cofactors such as growth-promoting compounds, vitamins, organic compounds and extracellular matrix. The latter provides attachment sites for bacteria and allows flocculation and subsequent harvesting of microalgal biomass.
[24][34][35][24,49,59]. However, cocultivation may be limited by competition between microalgae and bacteria for nutrients. Bacteria can inhibit the growth of microalgae by modifying the culture broth and secreting toxins. Microalgae can also negatively affect bacteria by inhibiting or suppressing bacterial activity by increasing the pH
[36][34][37][48,49,92].
2.2. Secondary Treatment with Microalgae with Mixotrophic Metabolism
Microalgae with mixotrophic metabolism could be used for secondary wastewater treatments. With this type of treatment, the microalgae are able to oxidize organic matter in wastewater and to assimilate the resulting CO
2 for photosynthesis
[38][82]. The classic WWT is a two-step process that involves high energy consumption related to the supply of O
2 (for the oxidation of organic matter) and CO
2 (for photosynthesis). The advantage of the treatment with mixotrophic metabolism microalgae over traditional WWT systems is the fact that it is able to obtain the N, P and C discharge standards with a single treatment, also avoiding the energy consumption related to the supply of O
2 and CO
2 [28][39][87,93]. Generally, most of the studies concerning the WWT with microalgae are carried out with monospecific cultures on a laboratory scale in controlled environments. However, in large-scale outdoor cultivations microalgae are often contaminated with other microorganisms; this represents an important problem because it inhibits microalgae growth and lowers the quality of the bioproducts derived from them
[40][94]. A possible solution to this problem is represented by the extremophilic red alga
Galderia sulphuraria. This unicellular alga is able to live in acidic environments (pH between 1–5). It is one of the most adapted eukaryotes to acidic conditions and is predominant in extreme conditions where it is difficult for other organisms to grow
[39][40][93,94]. Further,
Galdieria sulphuraria can grow both in autotrophy, mixotrophy and heterotrophy
[41][95]. Finally, its efficacy in wastewater treatment is well-documented
[42][43][96,97]. This ability makes it very versatile and suitable for the removal of organic carbon from wastewater in a single phase instead of the two-step classic treatment. Regarding energy consumption, as reported by Oswald
[44][98], in the conventional activated sludge secondary treatment, the removal of 1 kg of BOD is associated with the consumption of 1 kW/h for aeration and the emission of 1 kg CO
2 equivalent. On the contrary, 1 kg of BOD removed in a mixotrophic algal system does not require energy inputs, and in theory, the microalgae biomass obtained could anaerobically generate methane for 1 kW/h of electricity. For all these reasons, large-scale treatment with mixotrophic metabolism microalgae could represent a valid solution, especially for lower energy consumption
[29][30][88,89]. However, it remains crucial to identify the conditions that allow maximum oxidation of BOD coupled with photosynthetic CO
2 fixation in order to maximize the net energy yields from wastewater treatment with microalgae with mixotrophic metabolism.
3. Conclusions
Microalgae can be grown on raw urban wastewater, treated urban wastewater and other wastewater such as digestate and centrate, deriving from the anaerobic digestion of activated sludge. However, the cultivation of microalgae on wastewater depends on several factors such as the selection of the species suitable for cultivation on wastewater, the type of wastewater, the technology used, the climatic factors (such as sunlight and temperature) and availability of CO2 that affects its yield. Through this approach, it is possible to obtain a production of microalgae biomass combined with the bioremediation of wastewater (mainly through the removal of substances such as nitrogen and phosphorus). This suggests that wastewater contains all the essential nutrients necessary for the cultivation and production of algal biomass, which can be used in different fields: (i) energy for the production of biofuels (biodiesel, biogas, biohydrogen, etc.), (ii) pharmaceutical products (derived from high-value biomolecules extracted from microalgae), (iii) foodstuffs (feed and food supplements), (iv) bioplastics and (v) chemicals (fertilizers and biochar). Microalgal biomass grown on wastewater is not yet completely safe, due to the possible presence of contaminants. However, it would be worthwhile to implement the knowledge and technologies in order to be able to allocate the microalgal biomass also to these purposes to expand their range of use. The use of microalgae provides a potential and valuable alternative to the conventional WWT, with the advantage of pursuing the goal of water treatment with less operational and energy costs and obtaining a resource such as microalgae biomass. A further advantage is associated with the fact that through this type of approach it is possible to use the CO2 released by other industrial plants for the cultivation of microalgae, subtracting such CO2 from emissions to the atmosphere. It should not be disregarded that this type of approach is representative of studies performed in the laboratory on a small scale, and the transition to a large-scale reality may present various obstacles such as high energy consumption and possible contamination by other microorganisms. A valid solution to this can be represented by the cultivation in mixotrophy of the extremophilic red alga Galdieria sulphuraria. The main bottlenecks that must be addressed are mainly the search for less expensive harvesting methods, both from an environmental and economic point of view, and the need for greater technological development of cultivation systems such as HRAP and PBR, in order to make the process feasible on large scale.