Meat represents an important protein source, even in developing countries, but its production is scarcely sustainable, and its excessive consumption poses health issues. An increasing number of Western consumers would replace, at least partially, meat with alternative protein sources. This review aims at: (i) depicting nutritional, functional, sensory traits, and critical issues of single-cell proteins (SCP), filamentous fungi, microalgae, vegetables (alone or mixed with milk), and insects and (ii) displaying how fermentation could improve their quality, to facilitate their use as food items/ingredients/supplements. Production of SCP (yeasts, filamentous fungi, microalgae) does not need arable land and potable water and can run continuously, also using wastes and byproducts. Some filamentous fungi are also consumed as edible mushrooms, and others are involved in the fermentation of traditional vegetable-based foods. Cereals, pseudocereals, and legumes may be combined to offer an almost complete amino acid profile. Fermentation of such vegetables, even in combination with milk-based products (e.g., tarhana), could increase nutrient concentrations, including essential amino acids, and improve sensory traits. Different insects could be used, as such or, to increase their acceptability, as ingredient of foods (e.g., pasta). However, insects as a protein source face with safety concerns, cultural constraints, and a lack of international regulatory framework.

Wastewater (WW) treatment using microalgae has become a growing trend due the economic and environmental benefits of the process. As microalgae need CO₂, nitrogen, and phosphorus to grow, they remove these potential pollutants from wastewaters, making them able to replace energetically expensive treatment steps in conventional WW treatment. Unlike traditional sludge, biomass can be used to produce biofuels, biofertilizers, high value chemicals, and even next-generation growth media for “organically” grown microalgal biomass targeting zero-waste policies and contributing to a more sustainable circular bioeconomy. The main challenge in this technology is the techno-economic feasibility of the system. Alternatives such as the isolation of novel strains, the use of native consortia, and the design of new bioreactors have been studied to overcome this and aid the scale-up of microalgal systems. This review focuses on the treatment of urban, industrial, and agricultural wastewaters by microalgae and their ability to not only remove, but also promote the reuse, of those pollutants. Opportunities and future prospects are discussed, including the upgrading of the produced biomass into valuable compounds, mainly biofuels.

Urban wastewaters contain pharmaceuticals that are not appropriately removed in conventional wastewater treatments, limiting treated water reuse. Microalgae have been shown to remove pharmaceuticals from urban wastewater in laboratory trials, but few studies have been conducted under natural conditions. In this work, pharmaceutical removal was assessed in a pilot-scale microalgal tertiary wastewater treatment in real conditions. Even after secondary treatment, the water contained measurable amounts of pharmaceuticals (an average of 218.4 ng/L) that significantly decreased to 39.83 ng/L at the exit of the microalgal system. Pharmaceuticals’ average removal rates were slightly higher in the summer (79.1%) than in autumn (71.1%). Antibiotics and antipsychotics were better removed (88.8 and 86.4%, respectively) than antihypertensives (75.3%) and others (Bezafibrate and Diclofenac; 64.0%). Physicochemical characteristics of the wastewater influenced pharmaceutical removal; significant positive correlations were observed between anti-hypertensive drug removal and ammonium concentration (r = 0.53; p < 0.05), total nitrogen and total pharmaceutical removal (r = 0.46; p < 0.05), and total nitrogen and antipsychotic drug removal (r = 0.47; p < 0.05). The results demonstrate the effectiveness of microalgal tertiary treatment in the removal of pharmaceuticals.

The valorization of microalgal biomass produced during wastewater treatment has the potential to mitigate treatment costs. As contaminated biomass (e.g., with pharmaceuticals, toxic metals, etc.) is often generated, biogas production is considered an effective valorization option. The biomass was obtained from a pilot facility of photobioreactors for tertiary wastewater treatment. The pilots were run for one year with naturally formed microalgal consortia. The biogas was generated in 70 mL crimp-top vials at 35 °C, quantified with a manometer and the methane yield measured by gas chromatography. A maximum biogas production of 311 mL/g volatile solids (VS) with a methane yield of 252 mL/g VS was obtained with the spring samples. These rather low values were not improved using previous thermo-acidic hydrolysis, suggesting that the low intrinsic biodegradable organic matter content of the consortia might be the cause for low yield. Considering the total volume of wastewater treated by this plant and the average amount of methane produced in this study, the substitution of the current tertiary treatment with the one here proposed would reduce the energy consumption of the plant by 20% and create an energy surplus of 2.8%. The implementation of this system would therefore contribute towards meeting the ambitious decarbonization targets established by the EU.

Changes in nutritional and lighting conditions to obtain compounds of interest and biomass via microalgal cultures are among the main foci of studies in algal biotechnology. Growth medium supplementation using organic compounds, such as glycerol, is a promising approach for increasing biomass productivity and the viability of microalgal cultivation and adding value to byproducts of the biodiesel industry. In this study, the influence of crude glycerol on Spirulina sp. LEB 18 was investigated via culturing using different photoperiods, and its effect on biomass composition and cell growth was evaluated. The microalgae were subjected to three photoperiods (continuous light, 24:0; 12 h light and 12 h dark, 12:12; and no illumination, 0:24) and crude glycerol supplementation (2.5 g L⁻¹); better productivity and biomass concentrations were obtained in cultures with a 12:12 photoperiod (28.36 mg L⁻¹ h⁻¹ and 1.24 g L⁻¹, respectively). Under this condition, the highest protein yield was achieved (647.3 mg L⁻¹, 52.2% w w⁻¹), and the obtained biomass showed favorable characteristics for applications in animal feed enrichment.