Each year, 450 billion m³ of water are consumed globally for industrial and domestic use. Domestic use contributes to 70% of this consumption, and the consequent wastewater was to be used as a substrate for microalgae growth, about 23.5 billion tons of oil could be generated [6]. In addition to energy purposes, this algal biomass could also be used in human or animal nutrition, and owing to the presence of high-value molecules in cosmetics [6]. In Europe, improvements to wastewater management started with Directive 91/271/EEC, which deals with protecting the environment from eutrophication by establishing processes for wastewater treatment [7]. This was followed by Directive 98/15/EC which established discharge limits for total nitrogen and phosphorus (98/15/EC). According to this directive, for plants with over 100,000 personal equivalents, the maximum concentration of total phosphorus that can be released into the environment is 1 mg/L, while that of total nitrogen is 10 mg/L [8]. In the last 30–40 years, techniques for urban wastewater treatment have improved throughout most of Europe. In the 27 countries of the European Union, 69% of wastewater produced by the population undergoes tertiary treatment, while 13% undergoes only primary and secondary treatments. However, there are differences between the various European countries in the percentage of the population connected to urban wastewater treatment plants. Furthermore, a lack of data and the fact that not all of the population is connected to wastewater treatment plants does not allow for an exhaustive overall assessment [9]. Although wastewater treatment is a technology developed in the last century, some ancient civilizations around 4000 BC already adopted rudimentary water treatment methods such as filtration through coal, exposure to light and use of boiling water [10]. Nowadays, conventional secondary treatments of wastewater based on biological purification with activated sludge from microorganisms such as bacteria have some disadvantages, such as high energy consumption (related to the nitrification–denitrification process), high operating costs and the need for sludge disposal [11,12,13]. Cocultivation of microalgae and bacteria may be a valid alternative to conventional wastewater treatment (WWT) [14,15,16]. Microalgae can grow in many environments and on different substrates such as wastewater. When growing on wastewater, microalgae assimilate phosphorus and nitrogen, nutrients necessary for their growth. In addition, they can also assimilate heavy metals and pharmaceutical products from wastewater and capture atmospheric carbon dioxide (CO₂).

This, as well as favoring bioremediation of wastewater and protecting the environment from the risk of eutrophication, can also favor the removal of dangerous contaminants from wastewater and mitigate the negative effects caused by the excessive concentration of CO₂ in the atmosphere. Lastly, this type of treatment, in addition to recycling water, can also produce microalgae biomass that can be destined for different uses, such as food, energy and other products at lower costs [17,18]. The use of microalgae for wastewater treatment dates back to the 1950s, when Oswald devised this type of treatment as an alternative to traditional waste-stabilization ponds (WSP) [19]. However, research on this field has only increased in the last decade [20,21], also responding to an increasing global demand for microalgae, from 10.51 million tons in 2000 to 30.45 million tons in 2015. In 2016, the microalgae sector generated an added value of EUR 1.69 billion, and 14,000 workers were employed in the microalgae sector and associated production chain [22]. The major producers of algal biomass were Asian countries, which produced 97% of the total production in 2015. Europe is only the third largest producer, after USA. In Europe, production is still fluctuating, from 0.30 million tons in 2000 to 0.23 million tons in 2015. Norway, accounting for 65%, was the largest European producer of microalgae biomass in 2015 [23]. Cultivation upon alternative culture media such as wastewater could further reduce algal production costs, ranging from 20 to 200 USD/r kg depending on the case [24]. A large part of the production costs of microalgae (up to 20%) are attributable to the use of industrial fertilizers used for their growth [25,26]. In this sense, cultivation upon wastewater could lead to both economic and environmental benefits.

2. Secondary Treatment with Microalgae

2.1. Secondary Treatment with Cocultivation Microalgae–Bacteria

Typically, conventional processes for secondary treatment (activated sludge) take place through the remediation with microorganisms, generally heterotrophic bacteria and this process depends on O₂. The microorganisms, in the presence of O₂, carry out the biological oxidation of the organic substance [1,90], since oxygen allows nitrification by bacteria [91]. Generally, O₂ is produced by electromechanical blowers with high energy intensity. This supply of O₂ 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. [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 [13,89,90]. 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 CO₂, and at the same time the microalgae assimilate CO₂ and the nutrients generated by bacteria, converting these substances into algal cell material through photosynthesis and producing O₂ that stimulates bacterial activity [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 [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. [24,49,59]. 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 [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 CO₂ for photosynthesis [82]. The classic WWT is a two-step process that involves high energy consumption related to the supply of O₂ (for the oxidation of organic matter) and CO₂ (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 O₂ and CO₂ [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 [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 [93,94]. Further, Galdieria sulphuraria can grow both in autotrophy, mixotrophy and heterotrophy [95]. Finally, its efficacy in wastewater treatment is well-documented [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 [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 CO₂ 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 [88,89]. However, it remains crucial to identify the conditions that allow maximum oxidation of BOD coupled with photosynthetic CO₂ 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.