Insight into recent advances in microalgae biogranulation in wastewater treatment

Microalgae-based technology is widely utilized in wastewater treatment and resource recovery. However, the practical implementation of microalgae-based technology is hampered by the difficulty in separating microalgae from treated water due to the low density of microalgae. This review is designed to find the current status of the development and utilization of microalgae biogranulation technology for better and more cost-effective wastewater treatment. This review reveals that the current trend of research is geared toward developing microalgae-bacterial granules. Most previous works were focused on studying the effect of operating conditions to improve the efficiency of wastewater treatment using microalgae-bacterial granules. Limited studies have been directed toward optimizing operating conditions to induce the secretion of extracellular polymeric substances (EPSs), which promotes the development of denser microalgae granules with enhanced settling ability. Likewise, studies on the understanding of the EPS role and the interaction between microalgae cells in forming granules are scarce. Furthermore, the majority of current research has been on the cultivation of microalgae-bacteria granules, which limits their application only in wastewater treatment. Cultivation of microalgae granules without bacteria has greater potential because it does not require additional purification and can be used for border applications.

Obtaining more contaminant-resistant variants from a native Chlorella vulgaris strain

Cildáñez stream (in Matanza-Riachuelo basin, Buenos Aires) is one of the most polluted watercourses of Argentina, containing a mixed contamination from agricultural and industrial wastes. The application of water bioremediation processes for this kind of effluent will require microorganisms with a high tolerance to contamination. In this sense, obtaining higher contaminant-resistant microalgae lines is widely desired. In this study, adaptive laboratory evolution (ALE) and random mutagenesis were used to obtain Chlorella vulgaris LMPA-40 strains adapted to grow in polluted water from the Cildáñez stream. The ALE process was performed by 22 successive subcultures under selective pressure (Cildáñez wastewater alone or with the addition of phenol or H2O2) while random mutagenesis was performed with UV-C radiation at 275nm. Not all the cell lines obtained after ALE could adapt enough to overcome the stress caused by the Cildáñez wastewater, indicating that the process is quite random and depends on the stressor used. The best results were obtained for the Cildáñez wastewater adapted cells (Cild 3 strain) that were more resistant than the original strain. The concentration of protein, Chlorophyll A, Chlorophyll B, and carotenoids in the Cild 3 ALE evolved strain was higher than that of the control strain. However, this strain exhibited half of the lipid content compared to the same control strain. Interestingly, these alterations and the acquired tolerance may be reversed over time during storage. These findings suggest that the acquisition of novel cell lines could not be permanent, a fact that must be considered for future trials.

Growth Parameters of Various Green Microalgae Species in Effluent from Biogas Reactors: The Importance of Effluent Concentration

The use of liquid waste as a feedstock for cultivation of microalgae can reduce water and nutrient costs and can also be used to treat wastewater with simultaneous production of biomass and valuable products. This study applied strategies to treat diluted anaerobic digester effluent (ADE) as a residue of biogas reactors with moderate (87 ± 0.6 mg L^-1; 10% ADE) and elevated NH₄⁺-N levels (175 ± 1.1 mg L^-1; 20% ADE). The effect of ADE dilution on the acclimatization of various microalgae was studied based on the analysis of the growth and productivity of the tested green algae. Two species of the genus Chlorella showed robust growth in the 10-20% ADE (with a maximum total weight of 3.26 ± 0.18 g L^-1 for C. vulgaris and 2.81 ± 0.10 g L^-1 for C. sorokiniana). The use of 10% ADE made it possible to cultivate the strains of the family Scenedesmaceae more effectively than the use of 20% ADE. The growth of Neochloris sp. in ADE was the lowest compared to other microalgal strains. The results of this study demonstrated the feasibility of introducing individual green microalgae into the processes of nutrient recovery from ADE to obtain biomass with a high protein content.

Recent Trends in Biogenic Gas, Waste and Wastewater Fermentation

In recent years, the optimization of bioprocesses for the removal of pollutants from industrial biogenic gas emissions, waste and wastewater has been the focus of intensive research. Recently developed technologies not only aim to remove such pollutants, but also to valorize them, whenever possible, through their bioconversion into useful added-value products. In this domain of progressive research, lab-, pilot-, and demonstration-scale studies are dealing with the fermentation of biogenic gases (e.g., CO2, CO, and CH4), waste or wastewater to produce a range of biofuels and valuable products, based on the activity of pure or mixed cultures of native or recombinant aerobic and anaerobic bacteria, algae, or yeasts as biocatalysts. Waste can also be converted to syngas, which can subsequently be fermented as well. A broad range of bioproducts can be obtained, e.g., biofuels and several other platform chemicals and products. This environmentally-friendly biorefinery approach addresses the need to build modern societies according to the concept of a circular economy, and yields products of commercial interest. Different examples of such approaches are described in this collection of scientific reports.