Urbanization, industrialization and other human-related activities discharge various inorganic and organic toxic compounds into the environment. Many physical, chemical and biological methods have been practiced, to treat contaminated wastewater: among these, the biological method of wastewater treatment by utilizing algae has been reviewed widely. However, the removal efficacy of algae monoculture is low, as compared to the algae consortium systems. The presence of microorganisms such as fungi or bacteria in wastewater can establish various relationships, such as mutualism or symbiosis with algae, which help in the removal of various organic and inorganic compounds from wastewater, thus acting as a wastewater treatment system. Heterotrophic microorganisms can segregate natural organic matter, which is released by algae in the form of dissolved organic carbon, and releases carbon dioxide, which is utilized by algae for photosynthesis. In accordance with existing studies, microalgal consortiums with bacteria or fungi occurring naturally or crafted artificially can be utilized for wastewater treatment; therefore, the present review provides an outline of the symbiotic relationships between algae and other microorganisms, and their applications in wastewater treatment. Various mechanisms—such as mutualism, commensalism and parasitism—for the removal of different pollutants from wastewater by consortium systems have been elucidated in this review; moreover, this review addresses the challenges that are restricting large-scale implementation of these consortiums, thus demanding more research to enable enhanced commercialization.

In modern energy, various technologies for absorbing carbon dioxide from the atmosphere are being considered, including photosynthetic microalgae. An important task is to obtain maximum productivity at high concentrations of CO₂ in gas–air mixtures. In this regard, the aim of the investigation is to study the effect of light intensity on the biomass growth and biochemical composition of five different microalgae strains: Arthrospira platensis, Chlorella ellipsoidea, Chlorella vulgaris, Gloeotila pulchra, and Elliptochloris subsphaerica. To assess the viability of microalgae cells, the method of cytochemical staining with methylene blue, which enables identifying dead cells during microscopy, was used. The microalgae were cultivated at 6% CO₂ and five different intensities: 80, 120, 160, 200, and 245 μmol quanta·m⁻²·s⁻¹. The maximum growth rate among all strains was obtained for C. vulgaris (0.78 g·L⁻¹·d⁻¹) at an illumination intensity of 245 µmol quanta·m⁻²·s⁻¹. For E. subsphaerica and A. platensis, similar results (approximately 0.59 and 0.25 g·L⁻¹·d⁻¹ for each strain) were obtained at an illumination intensity of 160 and 245 µmol quanta·m⁻²·s⁻¹. A decrease in protein content with an increase in illumination was noted for C. vulgaris (from 61.0 to 46.6%) and A. platensis (from 43.8 to 33.6%), and a slight increase in lipid content was shown by A. platensis (from 17.8 to 21.4%). The possibility of increasing microalgae biomass productivity by increasing illumination has been demonstrated. This result can also be considered as showing potential for enhanced lipid microalgae production for biodiesel applications.

The influence of elevated CO₂ concentrations on the growth and viability of various microalgae strains was studied. Arthrospira platensis, Chlorella ellipsoidea, Chlorella vulgaris, Gloeotila pulchra, and Elliptochloris subsphaerica were tested. The cultivation of microalgae was carried out at constant CO₂ concentrations (0.04, 3, 6, or 9%—sequentially from lower to higher concentrations), under constant (24 h·day⁻¹) illumination with an intensity of 74.3 µmol quanta·m⁻²·s⁻¹, and a constant temperature of 23.5 ± 0.5 °C. The optical density of the microalgae biomass, pH, and the chemical composition of the culture medium were measured. Microscopy (including the cytochemical microscopic method) was conducted to monitor the state of the microalgae. The highest biomass growth rate (0.37 g·L⁻¹·day⁻¹), among all experiments, was achieved for Chlorella vulgaris at CO₂ = 3% and for Chlorella ellipsoidea at CO₂ = 6 and 9%. The lowest growth rate (0.12 g·L⁻¹·day⁻¹) was achieved for Arthrospira platensis at CO₂ = 3 and 9%. The microscopy results showed the absence or a minimum number of dead cells of the strains under selected conditions. The ability to maintain the viability of cultures up to significant concentrations of CO₂ = 9% was due to adaptation (gradual increase in CO₂ concentrations in the experiments).

Direct study of CO₂ capture efficiency during microalgae Arthrospira platensis cultivation at high CO₂ concentrations was carried out. Microalgae were grown in a 90 L photobioreactor on Zarrouk’s medium prepared with distilled water. Three 15-day experiments were carried out with different initial CO₂ concentrations: 1, 5, and 9 vol.%. During the experiments, both the change in the optical density of the microalgae suspension and the direct change in the CO₂ concentration in the chamber were measured. The maximum decrease in CO₂ concentration due to the growth of microalgae was 0.10 vol.% (CO₂)/day in the experiment with an initial CO₂ concentration of 5 vol.%. Growth rate of biomass density was 79.4, 76.3, and 48.4 (mg/L)/day at 1, 5, and 9 vol.% CO₂ concentrations, respectively. During the experiment with initial CO₂ concentrations of 1 and 5 vol.%., pH of the culture medium was increased, but pH was decreased from 9.2 to 8.8 at 9 vol.%. In general, good viability (high quality of biomass and high rate of its growth) of Arthrospira platensis was established at 1 and 5 vol. (CO₂)%, while massive death of Arthrospira platensis cells was observed in the experiment with 9 vol. (CO₂)%. Biochemical analysis of the resulting biomass revealed a decrease in the content of lipids and proteins with an increase in CO₂ concentration.

This article presents a comparative evaluation of pyrolysis and hydrothermal liquefaction (HTL) for obtaining biofuel from microalgal biomass (MAB). The research was carried out using biomass of a stable microalgae-bacteria consortium based on Arthrospira platensis. A. platensis was chosen because of its simple cultivation and harvesting. Pyrolysis was carried out at temperatures of 300, 400, 500, and 600 °C with a constant rate of temperature change of 10 °C/min; HTL was carried out at temperatures of 270, 300, and 330 °C. The bio-oil yield obtained by HTL (38.8–45.7%) was significantly higher than that of pyrolysis (up to 21.9%). At the same time, the bio-coal yields using both technologies were almost the same—about 27%. Biochar (bio-coal) can be considered as an alternative strategy for CO₂ absorption and subsequent storage since it is 90% geologically stabilized carbon.

Ball-milled hydroreactive powders of Mg-Al scrap with 20 wt.% additive (Wood’s alloy, KCl, and their mixture) and with no additives were manufactured. Their hydrogen yields and reaction rates in a 3.5 wt.% NaCl aqueous solution at 15–35 °C were compared. In the beginning of the reaction, samples with KCl (20 wt.%) and Wood’s alloy (10 wt.%) with KCl (10 wt.%) provided the highest and second-highest reaction rates, respectively. However, their hydrogen yields after 4 h were correspondingly the lowest and second-lowest percentages—(45.6 ± 4.4)% and (56.0 ± 1.2)% at 35 °C. At the same temperature, samples with 20 wt.% Wood’s alloy and with no additives demonstrated the highest hydrogen yields of (73.5 ± 10.0)% and (70.6 ± 2.5)%, correspondingly, while their respective maximum reaction rates were the lowest and second-lowest. The variations in reaction kinetics for the powders can be explained by the difference in their particle sizes (apparently affecting specific surface area), the crystal lattice defects accumulated during ball milling, favoring pitting corrosion, the morphology of the solid reaction product covering the particles, and the contradicting effects from the potential formation of reaction-enhancing microgalvanic cells intended to induce anodic dissolution of Mg in conductive media and reaction-hindering crystal-grain-screening compounds of the alloy and metal scrap components.