Technologies for harvesting the microalgae for industrial applications: Current trends and perspectives

Optimizing calcium and magnesium in seawater medium with high bicarbonate concentration for efficient growth and self-flocculation harvesting of Chlorella sp

Microalgae cultivation using high bicarbonate concentration in seawater medium triggers CaCO3 precipitation due to pH elevation, contaminating the photobioreactor and reduce biomass productivity. It is necessary to control Ca2+ and Mg2+ concentration in appropriate range to allow microalgae rapid growth without precipitation, followed by effective self-flocculation for harvesting. This study optimized this concentration range as 0.5-1.0 mmol L-1 for Ca2+ and 2.5-5.0 mmol L-1 for Mg2+. With this concentration, subsequent pH adjustment to 11 induced > 90 % self-flocculation efficiency. Also, outdoor cultivation in floating photobioreactors utilized treated seawater with optimized Ca2+ and Mg2+ concentration achieved productivity of 9.36 g m-2 day-1, which is 112 % higher than untreated seawater (4.42 g m-2 day-1). The optimized process reached the goal of higher biomass productivity without precipitation and efficient harvesting. On the other hand, microalgae-induced precipitation may serve as potential carbon sink, contributing to ocean-negative carbon emissions.

Fungal - assisted microalgae flocculation and simultaneous lignocellulolytic enzyme production in wastewater treatment systems

The study investigates the application of white rot fungi for reactor-scale microalgae harvesting and explores the mechanisms underlying the algal-fungal interactions and their impact on biomass composition. Enzymatic analysis and microscopy revealed that the formation of algal-fungal complexes and successful harvesting are coupled with fungal cellulose-degrading enzyme production and hydrolytic processes of microalgae cells. Fluorescence intensity decreased by over 80 % in cells stained with Calcofluor-white after interaction with white rot fungi, indicating the reduction in cellulose content in microalgal cells caused by fungal enzymatic activity. These enzymes also caused significant cell damage and more than 50 % decrease in microalgae cell size. The presence of cellulolytic enzymes broadens the potential application of the resulting biomass in various biotechnological applications. Moreover, reactor-scale bioflocculation resulted in over 95 % T. obliquus and almost 85 % C. vulgaris harvesting efficiency from secondary wastewater within less than 24 h, demonstrating the method's scalability and industrial applicability.

Relationship Between Harvesting Efficiency and Filament Morphology in Arthrospira platensis Gomont

Arthrospira platensis, a filamentous cyanobacterium, exhibits morphological variability influenced by biotic and abiotic factors. We investigated the effect of sodium ion concentration on filament length, growth, and harvest efficiency. Increasing the sodium concentration from 0.2 M to 0.4 M (using NaHCO3 or Na2CO3) led to a significant increase in filament length, from 0.3393 to 0.7084 mm, and longer filaments had increased auto-flotation efficiency (from 87% to 94%) within 3 h. The linear filaments, obtained via spontaneous morphological conversion, also had increased photosynthetic activity and growth rates compared to coiled filaments, and we speculate this was due to decreased self-shading and increased light penetration. However, linear filaments also had poor auto-flotation efficiency (10% after 24 h) and decreased buoyancy, and this likely limits their survival in natural ecosystems. These findings provide insights into optimizing the cultivation of A. platensis for biomass harvesting.

Innovative microalgae technologies for mariculture wastewater treatment: Single and combined microalgae treatment mechanisms, challenges and future prospects

The discharge of aquaculture wastewater, comprising nitrogen, phosphorus, heavy metals, and antibiotics from large-scale aquaculture, poses a significant threat to marine ecosystems and human health. Consequently, addressing the treatment of marine aquaculture wastewater is imperative. Conventional physicochemical treatment methods have various limitations, whereas microalgae-based biological treatment technologies have gained increasing attention in the field of water purification due to their ability to efficiently absorb organic matter from mariculture wastewater and convert CO₂ into biomass products. Microalgae offer potential for highly efficient and cost-effective mariculture wastewater treatment, with particularly noteworthy advancements in the application of combined microalgae technologies. This paper explores the research hotspots in this field through bibliometric analysis and systematically discusses the following aspects: (1) summarizing the current pollution status of mariculture wastewater, including the types and sources of pollutants in various forms of mariculture wastewater, treatment methods, and associated treatment efficiencies; (2) analyzing the factors contributing to the gradual replacement of single microalgae technology with combined microalgae technology, highlighting its synergistic effects, enhanced pollutant removal efficiencies, resource recovery potential, and alignment with sustainable development goals; (3) exploring the mechanisms of pollutant removal by combined microalgae technologies, focusing on their technical advantages in bacterial-algal coupling, immobilized microalgae systems, and microalgal biofilm technologies; (4) discussing the challenges faced by the three main categories of combined microalgae technologies and proposing future improvement strategies to further enhance their application effectiveness. In conclusion, this paper offers a detailed analysis of these emerging technologies, providing a forward-looking perspective on the future development of microalgae-based mariculture wastewater treatment solutions.

Microalgae-derived biolubricants: Challenges and opportunities

Lubricants are indispensable in the modern economy for controlling friction and wear across many industries. Traditional lubricants are derived from petroleum crude and can cause significant ecological impact if released into the environment. Microalgae have emerged as a potential alternative to petroleum crude for producing renewable and environmentally friendly biolubricants. This review systematically assesses recent developments in microalgal-based biolubricant production, including tribological performance, microalgae selection, cultivation, harvesting, lipid and polysaccharide extraction and conversion to biolubricants, and market development. Compared to petroleum-based lubricants in terms of tribological properties, biolubricants are compatible with most emerging applications, such as electric vehicles and wind turbines. Nevertheless, they are less thermally and chemically stable, thus, may not be suitable for some traditional applications such as internal combustion engines. Literature data corroborated in this study reveals an urgent need for further research to scale up microalgae production and lower the cost of biomass harvesting. While technologies for converting microalgae-derived lipids to biolubricants appear to be well established, additional work is necessary to also utilize polysaccharides as another key ingredient for producing biolubricants, especially for low-temperature applications. Extraction methods are well established but further research is also needed to reduce the ecological impact, especially to utilize green solvents and reduce solvent consumption. Additionally, future research should delve into the use of nanoparticles as effective additives to obtain microalgae-based biolubricants with superior properties. Finally, it is essential to standardize the labeling system of biolubricants to establish a global market.

A Study of Theoretical Analysis and Modelling of Microalgal Membrane Photobioreactors for Microalgal Biomass Production and Nutrient Removal

This study presents a theoretical and mathematical analysis and modelling of the emerging microalgal membrane photobioreactors (M-MPBRs) for wastewater treatment. A set of mathematical models was developed to predict the biological performances of M-MPBRs. The model takes into account the effects of hydraulic retention time (HRT), solid retention time (SRT), and the N/P ratio of influent on the biological performance of M-MPBRs, such as microalgal biomass production and nutrient (N and P) removals. The model was calibrated and validated using experimental data from the literature. This modelling study explained that prolonged SRT could promote biomass production and nutrient removal, while prolonging HRT exhibited a negative effect. Furthermore, biomass production could be improved by augmenting nutrient loading, and nutrient removal would be limited under insufficient conditions. The modelling results demonstrated that the best performance was achieved at HRT = 1 d and SRT = 40 d for typical municipal wastewater with an influent N concentration = 40 mg/L. The modelling results are in good agreement with the experimental results from the literature. The findings suggest that the proposed models can be used as a powerful mathematical tool to optimize these parameters to improve the removal of nutrients (N and P), as well as the productivity of biomass in M-MPBRs. This study provides new insights into the use of mathematical models for the optimal design and operation of the emerging M-MPBRs for sustainable wastewater treatment.

Interference of extracellular soluble algal organic matter on flocculation-sedimentation harvesting of Chlorella sp

Extracellular soluble algal organic matter (AOM) significantly interferes with microalgae flocculation. This study investigated the effects of various AOM fractions on Chlorella sp. flocculation using ferric chloride (FeCl3), sodium hydroxide (NaOH), and chitosan. All flocculants achieved high separation efficiency (87-99 %), but higher dosages were required in the presence of AOM. High molecular weight (>50 kDa) AOM fraction was identified as the primary inhibitor of flocculation across different pH levels, whereas low/medium molecular weight (<3 and <50 kDa) AOM had minimal impact. Compositional analysis revealed that the inhibitory AOM fraction is a glycoprotein rich in carbohydrates, including neutral, amino, and acidic sugars. The significance of this study is in identifying carboxyl groups (-COOH) from acidic monomers in >50 kDa AOM that inhibit flocculation. Understanding AOM composition and the interaction dynamics between AOM, cells, and flocculants is crucial for enhancing the techno-economics and sustainability of flocculation-based microalgae harvesting.

A review of emerging membrane-based microalgal-bacterial processes for wastewater treatment: Process configurations, biological and membrane performance, and perspectives

Microalgal-bacterial (MB) consortia create an excellent eco-system for simultaneous COD/BOD and nutrients (N and P) removals in a single step with significant reduction in or complete elimination of aeration and carbonation in the biological wastewater treatment processes. The integration of membrane separation technology with the MB processes has created a new paradigm for research and development. This paper focuses on a comprehensive and critical literature review of recent advances in these emerging processes. Novel membrane process configurations and process conditions affecting the biological performance of these novel systems have been systematically reviewed and discussed. Membrane fouling issues and control of MB biofilm formation and thickness associated with these emerging suspended growth or immobilized biofilm processes are addressed and discussed. The research gaps, challenges, outlooks of these emerging processes are identified and discussed in-depth. The findings from the literature suggest that the membrane-based MB processes are advanced biotechnologies with a significant reduction in energy consumption and process simplification and high process efficiency that are not achievable with current technologies in wastewater treatment. There are endless opportunities for research and development of these novel and emerging membrane-based MB processes.

Biocompounds from wastewater-grown microalgae: a review of emerging cultivation and harvesting technologies

Microalgae biomass has attracted attention as a feedstock to produce biofuels, biofertilizers, and pigments. However, the high production cost associated with cultivation and separation stages is a challenge for the microalgae biotechnology application on a large scale. A promising approach to overcome the technical-economic limitations of microalgae production is using wastewater as a nutrient and water source for cultivation. This strategy reduces cultivation costs and contributes to valorizing sanitation resources. Therefore, this article presents a comprehensive literature review on the status of microalgae biomass cultivation in wastewater, focusing on production strategies and the accumulation of valuable compounds such as lipids, carbohydrates, proteins, fatty acids, and pigments. This review also covers emerging techniques for harvesting microalgae biomass cultivated in wastewater, discussing the advantages and limitations of the process, as well as pointing out the main research opportunities. The novelty of the study lies in providing a detailed analysis of state-of-the-art and potential advances in the cultivation and harvesting of microalgae, with a special focus on the use of wastewater and implementing innovative strategies to enhance productivity and the accumulation of compounds. In this context, the work aims to guide future research concerning emerging technologies in the field, emphasizing the importance of innovative approaches in cultivating and harvesting microalgae for advancing knowledge and practical applications in this area.

Microalgae cultivation with recycled harvesting water achieved economic and sustainable production of biomass and lipid: Feasibility assessment and inhibitory factors analysis

This study was conducted to achieve economic and sustainable production of biomass and lipids from Chlorella sorokiniana by recirculating cultivation with recycled harvesting water, to identify the major inhibitory factors in recirculating culture, and to analyze accordingly economic benefits. The results showed that recirculating microalgae cultivation (RMC) could obtain 0.20-0.32 g/L biomass and lipid content increased by 23.1 %-38.5 %. Correlation analysis showed that the extracellular polysaccharide (PSext), chemical oxygen demand (COD) and chromaticity of recirculating water inhibited photosynthesis and induced oxidative stress, thus inhibiting the growth of C. sorokiniana. In addition, the economic benefits analysis found that circulating the medium twice could save about 30 % of production cost, which is the most economical RMC solution. In conclusion, this study verified the feasibility and economy of RMC, and provided a better understanding of inhibitory factors identification in culture.

Effect of external acetate added in aquaculture wastewater on mixotrophic cultivation of microalgae, nutrient removal, and membrane contamination in a membrane photobioreactor

The mixotrophic cultivation of microalgae in wastewater has attracted extensive attention due to its many advantages. In this study, acetate, which can be prepared by hydrolysis of aquaculture waste, was used as exogenous organic matter to promote the growth of Chlorella pyrenoidosa cultured in aquaculture wastewater. Microalgae cultivation was carried out in a membrane photobioreactor (MPBR) with continuous inflow and outflow mode. The results showed that exogenous acetate greatly promoted the mixotrophic growth of C. pyrenoidosa. When the dosage of acetate reached 1.0 g L-1, the relative growth rate of microalgae in the logarithmic stage reached 0.31 d-1, which was 4.4 times that of the control. As a result, exogenous acetate also promoted the removal of nutrients from aquaculture wastewater. During the stable operation stage of the MPBR with acetate added in the influent, an average of 87.41%-93.93% nitrogen and 76.34%-88.55% phosphorus was removed from the aquaculture wastewater containing 19.41 mg L-1 total inorganic nitrogen and 1.31 mg L-1 total inorganic phosphorus. However, it was worth noting that adding exogenous acetate also led to an increase in the membrane resistance of the membrane module in the MPBR. Membrane resistance was mainly composed of internal resistance (Ri) and cake resistance (Rc), and with the increase of acetate content in the influent, their proportion in the total resistance gradually increased. Ri contributed the major membrane resistance and was most affected by acetate dosage. Ri reached 32.04 × 1012 m-1 with 1 g L-1 acetate, which accounted for 69.49% of total resistance. Moreover, with the increase of influent acetate concentration of the MPBRs, both the number of insoluble contaminants and dissolved organic contaminants in the membrane modules increased. In addition, the composition of proteins, polysaccharides, and humus in dissolved organic contaminants was close to that in extracellular polymeric substances and soluble microbial products secreted by microalgae. These results suggested that the membrane fouling of membrane modules was closely related to the algal biomass content in the MPBRs. The above results provided a theoretical basis for reducing membrane fouling of MPBR.