Flocculation Harvesting Techniques for Microalgae: A Review

Self-flocculation behaviour of cellulose-based bioflocculant synthesized from sewage water grown Chlorella sorokiniana and Scenedesmus abundans

The global energy crisis has spurred a shift from conventional to clean and sustainable energy sources. Biomass derived from microalgae is emerging as an alternative energy source with diverse applications. Despite the numerous advantages of microalgae, large-scale biomass harvesting is not economical and convenient. Self-flocculation is considered an effective phenomenon facilitated by extracting the flocculating substances from microalgae that assist aggregation of algal cells into flocs. A novel cellulose-based bioflocculant has been synthesized from sewage water grown Chlorella sorokiniana and Scenedesmus abundans for harvesting application. The produced bioflocculant amounted to 38.5% and 19.38% of the dry weight of S. abundans and C. sorokiniana, respectively. Analysis via FTIR, XRD, and FESEM-EDX revealed the presence of cellulose hydroxyapatite (HA) in algae-derived cellulose. Harvesting efficiencies of 95.3% and 89.16% were attained for S. abundans and C. sorokiniana, respectively, at a dosage of 0.5 g/L. Furthermore, the bioflocculant was recovered, enabling its reuse with recovery efficiencies of 52% and 10% for S. abundans and C. sorokiniana, respectively. This simple and efficient approach has the potential to replace other harvesting methods, thereby contributing to the economic algal biofuel production.

Cultivation of microalgae in food processing effluent for pollution attenuation and astaxanthin production: a review of technological innovation and downstream application

Valorization of food processing effluent (FPE) by microalgae cultivation for astaxanthin production is regarded as a potential strategy to solve the environmental pollution of food processing industry and promote the development of eco-friendly agriculture. In this review paper, microalgal species which have the potential to be employed for astaxanthin in FPE were identified. Additionally, in terms of CO2 emission, the performances of microalgae cultivation and traditional methods for FPE remediation were compared. Thirdly, an in-depth discussion of some innovative technologies, which may be employed to lower the total cost, improve the nutrient profile of FPE, and enhance the astaxanthin synthesis, was provided. Finally, specific effects of dietary supplementation of algal astaxanthin on the growth rate, immune response, and pigmentation of animals were discussed. Based on the discussion of this work, the cultivation of microalgae in FPE for astaxanthin production is a value-adding process which can bring environmental benefits and ecological benefits to the food processing industry and agriculture. Particularly, technological innovations in recent years are promoting the shift of this new idea from academic research to practical application. In the coming future, with the reduction of the total cost of algal astaxanthin, policy support from the governments, and further improvement of the innovative technologies, the concept of growing microalgae in FPE for astaxanthin will be more applicable in the industry.

Cultivation of microalgae-bacteria consortium by waste gas-waste water to achieve CO2 fixation, wastewater purification and bioproducts production

The cultivation of microalgae and microalgae-bacteria consortia provide a potential efficient strategy to fix CO2 from waste gas, treat wastewater and produce value-added products subsequently. This paper reviews recent developments in CO2 fixation and wastewater treatment by single microalgae, mixed microalgae and microalgae-bacteria consortia, as well as compares and summarizes the differences in utilizing different microorganisms from different aspects. Compared to monoculture of microalgae, a mixed microalgae and microalgae-bacteria consortium may mitigate environmental risk, obtain high biomass, and improve the efficiency of nutrient removal. The applied microalgae include Chlorella sp., Scenedesmus sp., Pediastrum sp., and Phormidium sp. among others, and most strains belong to Chlorophyta and Cyanophyta. The bacteria in microalgae-bacteria consortia are mainly from activated sludge and specific sewage sources. Bioengineer in CBB cycle in microalgae cells provide effective strategy to achieve improvement of CO2 fixation or a high yield of high-value products. The mechanisms of CO2 fixation and nutrient removal by different microbial systems are also explored and concluded, the importance of microalgae in the technology is proven. After cultivation, microalgae biomass can be harvested through physical, chemical, biological and magnetic separation methods and used to produce high-value by-products, such as biofuel, feed, food, biochar, fertilizer, and pharmaceutical bio-compounds. Although this technology has brought many benefits, some challenging obstacles and limitation remain for industrialization and commercializing.

Microbial gas fermentation technology for sustainable food protein production

The development of novel, sustainable, and robust food production technologies represents one of the major pillars to address the most significant challenges humanity is going to face on earth in the upcoming decades - climate change, population growth, and resource depletion. The implementation of microfoods, i.e., foods formulated with ingredients from microbial cultivation, into the food supply chain has a huge potential to contribute towards energy-efficient and nutritious food manufacturing and represents a means to sustainably feed a growing world population. This review recapitulates and assesses the current state in the establishment and usage of gas fermenting bacteria as an innovative feedstock for protein production. In particular, we focus on the most promising representatives of this taxon: the hydrogen-oxidizing bacteria (hydrogenotrophs) and the methane-oxidizing bacteria (methanotrophs). These unicellular microorganisms can aerobically metabolize gaseous hydrogen and methane, respectively, to provide the required energy for building up cell material. A protein yield over 70% in the dry matter cell mass can be reached with no need for arable land and organic substrates making it a promising alternative to plant- and animal-based protein sources. We illuminate the holistic approach to incorporate protein extracts obtained from the cultivation of gas fermenting bacteria into microfoods. Herein, the fundamental properties of the bacteria, cultivation methods, downstream processing, and potential food applications are discussed. Moreover, this review covers existing and future challenges as well as sustainability aspects associated with the production of microbial protein through gas fermentation.

Phycobiliproteins from microalgae: research progress in sustainable production and extraction processes

Phycobiliproteins (PBPs), one of the functional proteins from algae, are natural pigment-protein complex containing various amino acids and phycobilins. It has various activities, such as anti-inflammatory and antioxidant properties. And are potential for applications in food, cosmetics, and biomedicine. Improving their metabolic yield is of great interest. Microalgaes are one of the important sources of PBPs, with high growth rate and have the potential for large-scale production. The key to large-scale PBPs production depends on accumulation and recovery of massive productive alga in the upstream stage and the efficiency of microalgae cells breakup and extract PBPs in the downstream stage. Therefore, we reviewed the status quo in the research and development of PBPs production, summarized the advances in each stage and the feasibility of scaled-up production, and demonstrated challenges and future directions in this field.

A novel biomineralization-inspired flocculation approach for harvesting high quality microalgal biomass: Dual action of cationic polyelectrolytes and nanosilica

This study posed a novel biomimetic flocculation approach, aiming to efficiently harvest high-quality biomass of Scenedesmus quadricauda cultured with anaerobic digestate. Here, that poly(diallyldimethylammonium chloride) (PDADMAC) could serve as mimetic silicified proteins to spontaneously incorporate nanosilica onto microalgal cells, imparting diatom-like characteristics to S. quadricauda. Compared to the exponential growth phase (day 3), the highest harvesting efficiency (93.49%) was obtained at a lower dosage of PDADMAC (5 mg/g) in the stationary phase (day 6), which was attributed to changes in properties and composition of microalgal LB-EPS. On day 6, the hydrophobic functional groups in LB-EPS provided more binding sites during the flocculation process and formed a network structure of microalgal cells-flocculants-nanosilica. The resulting larger and more stable biomimetic silica shell promoted microalgal flocculation and sedimentation. Compared to conventional harvesting methods (centrifugation, polyacrylamide, alkaline flocculation), this method had the minimal negative impact on harvested biomass, with 9.95% of cell membranes damaged.

Microalgae-material hybrid for enhanced photosynthetic energy conversion: a promising path towards carbon neutrality

Photosynthetic energy conversion for high-energy chemicals generation is one of the most viable solutions in the quest for sustainable energy towards carbon neutrality. Microalgae are fascinating photosynthetic organisms, which can directly convert solar energy into chemical energy and electrical energy. However, microalgal photosynthetic energy has not yet been applied on a large scale due to the limitation of their own characteristics. Researchers have been inspired to couple microalgae with synthetic materials via biomimetic assembly and the resulting microalgae-material hybrids have become more robust and even perform new functions. In the past decade, great progress has been made in microalgae-material hybrids, such as photosynthetic carbon dioxide fixation, photosynthetic hydrogen production, photoelectrochemical energy conversion and even biochemical energy conversion for biomedical therapy. The microalgae-material hybrid offers opportunities to promote artificially enhanced photosynthesis research and synchronously inspires investigation of biotic-abiotic interface manipulation. This review summarizes current construction methods of microalgae-material hybrids and highlights their implication in energy and health. Moreover, we discuss the current problems and future challenges for microalgae-material hybrids and the outlook for their development and applications. This review will provide inspiration for the rational design of the microalgae-based semi-natural biohybrid and further promote the disciplinary fusion of material science and biological science.

Microbial astaxanthin: from bioprocessing to the market recognition

The attractive biological properties and health benefits of natural astaxanthin (AXT), including its antioxidant and anti-carcinogenic properties, have garnered significant attention from academia and industry seeking natural alternatives to synthetic products. AXT, a red ketocarotenoid, is mainly produced by yeast, microalgae, wild or genetically engineered bacteria. Unfortunately, the large fraction of AXT available in the global market is still obtained using non-environmentally friendly petrochemical-based products. Due to the consumers concerns about synthetic AXT, the market of microbial-AXT is expected to grow exponentially in succeeding years. This review provides a detailed discussion of AXT's bioprocessing technologies and applications as a natural alternative to synthetic counterparts. Additionally, we present, for the first time, a very comprehensive segmentation of the global AXT market and suggest research directions to improve microbial production using sustainable and environmentally friendly practices. KEY POINTS: • Unlock the power of microorganisms for high value AXT production. • Discover the secrets to cost-effective microbial AXT processing. • Uncover the future opportunities in the AXT market.

Enhanced growth and auto-flocculation of Scenedesmus quadricauda in anaerobic digestate using high light intensity and nanosilica: A biomineralization-inspired strategy

Coupling municipal anaerobic digestate (MAD) treatments with microalgal cultivation can concomitantly achieve nutrient removal and microalgal bioenergy production. However, the high cost caused by dilution water and microalgal harvesting is a great challenge. In this study, Scenedesmus quadricauda was screened as the most appropriate algae strain due to its potential for growth and auto-flocculation, and the MAD diluted 5-fold with WWTP effluent was demonstrated as an ideal medium for S. quadricauda growth. Moreover, inspired by naturally generated silica shells of diatoms, a low-cost and biomimetic auto-flocculation strategy that combined high light intensity induction and microalgal silicification was proposed to accelerate the auto-flocculation process. Compared with low light intensity groups, this strategy imparted diatom-like features to S. quadricauda cells, and contributed to 3.07-fold higher auto-flocculation efficiency within 30 min. It was attributed to the fact that the high light intensity of 150 μmol·m ^- 2·s ^- 1 stimulated the extracellular polymeric substances (EPS) secretion and induced the variation in property and composition of EPS, especially the protein secondary structures, which allowed silica nanoparticles to spontaneously attach onto S. quadricauda cells in the presence of viscous EPS. Furthermore, this strategy significantly increased microalgal biomass yield to a dry weight of 1.37 g·L ^- 1, accompanied by 93.78%, 96.39% and 91.36% removals of NH₄⁺-N, TP, and COD, respectively. The productivity of valuable by-products, including lipid, carbohydrate, protein, and pigment, reached 56.30, 101.35, 30.39 and 11.28 mg·L ^- 1·d ^- 1, respectively. Overall, this study supplies a novel approach for low-cost microalgal bioenergy production from MAD and energy-efficient microalgae harvest by auto-flocculation.

New Angled Twin-layer Porous Substrate Photobioreactors for Cultivation of Nannochloropsis oculata

An angled twin-layer porous substrate photobioreactor (TL-PSBR) using LED light was designed to cultivate Nannochloropsis oculata. Flocculation and sedimentation by modification of pH to 11 were determined as the optimal method for harvesting the N. oculata inoculum. The following optimised parameters were found: tilt angle 15°, Kraft 220 g m^-2 paper as substrate material, initial inoculum density of 12.5 g m^-2, 140 µmol photons m^-2 s^-1 light intensity, and a light/dark cycle of 6:6 (h). Test cultivation for 14 days was performed under optimised conditions. The total dried biomass standing crop was 75.5 g m^-2 growth area with an average productivity of 6.3 g m^-2 d^-1, the productivity per volume of used culture medium was 126.2 mg/L d^-1, total lipid content 21.9% (w/w), and the highest productivity of total lipids was 1.33 g m^-2 d^-1. The dry algal biomass contained 3% eicosapentaenoic acid (w/w), 3.7% palmitoleic acid (w/w), and 513 mg kg^-1 vitamin E. The optimisation of N. oculata cultivation on an angled TL-PSBR system yielded promising results, and applications for commercial products need to be further explored.

Cell disruption and lipid extraction from Chlorella species for biorefinery applications: Recent advances

Chlorella is a promising microalga for CO₂-neutral biorefinery that co-produces drop-in biofuels and multiple biochemicals. Cell disruption and selective lipid extraction steps are major technical bottlenecks in biorefinement because of the inherent robustness and complexity of algal cell walls. This review focuses on the state-of-the-art achievements in cell disruption and lipid extraction methods for Chlorella species within the last five years. Various chemical, physical, and biological approaches have been detailed theoretically, compared, and discussed in terms of the degree of cell wall disruption, lipid extractability, chemical toxicity, cost-effectiveness, energy use, scalability, customer preferences, environment friendliness, and synergistic combinations of different methods. Future challenges and prospects of environmental-friendly and efficient extraction technologies are also outlined for practical applications in sustainable Chlorella biorefineries. Given the diverse industrial applications of Chlorella, this review may provide useful information for downstream processing of the advanced biorefineries of other algae genera.

Time-Resolved Kinetic Measurement of Microalgae Agglomeration for Screening of Polysaccharides-Based Coagulants/Flocculants

Time-resolved monitoring of microalgae agglomeration facilitates screening of coagulants/flocculants (CFs) from numerous biopolymer candidates. Herein, a filtering-flowing analysis (FFA) apparatus was developed in which dispersed microalgal cells were separated from coagulates and flocs formed by CFs and pumped into spectrophotometer for real-time quantification. Polysaccharides-based CFs for Microcystis aeruginosa and several other microalgae were tested. Cationic hydroxyethyl cellulose (CHEC), chitosan quaternary ammonium (CQA) and cationic guar gum (CGG) all triggered coagulation obeying a pseudo-second-order model. Maximal coagulation efficiencies were achieved at their respective critical dosages, i.e., 0.086 g/g_M.a. CHEC, 0.022 g/g_M.a. CQA, and 0.216 g/g_M.a. CGG. Although not active independently, bacterial exopolysaccharides (BEPS) aided coagulation of M. aeruginosa and allowed near 100% flocculation efficiency when 0.115 g/g_M.a. CQA and 1.44 g/g_M.a. xanthan were applied simultaneously. The apparatus is applicable to other microalgae species including Spirulina platensis, S. maxima, Chlorella vulgaris and Isochrysis galbana. Bio-based CFs sorted out using this apparatus could help develop cleaner processes for both remediation of harmful cyanobacterial blooms and microalgae-based biorefineries.

Fungal Contamination in Microalgal Cultivation: Biological and Biotechnological Aspects of Fungi-Microalgae Interaction

In the last few decades, the increasing interest in microalgae as sources of new biomolecules and environmental remediators stimulated scientists’ investigations and industrial applications. Nowadays, microalgae are exploited in different fields such as cosmeceuticals, nutraceuticals and as human and animal food supplements. Microalgae can be grown using various cultivation systems depending on their final application. One of the main problems in microalgae cultivations is the possible presence of biological contaminants. Fungi, among the main contaminants in microalgal cultures, are able to influence the production and quality of biomass significantly. Here, we describe fungal contamination considering both shortcomings and benefits of fungi-microalgae interactions, highlighting the biological aspects of this interaction and the possible biotechnological applications.

Low energy harvesting of hydrophobic microalgae (Tribonema sp.) by electro-flotation without coagulation

Microalgae have great potential for biofuel production and wastewater treatment, but the high cost of harvesting hinders their practical application. In this study, economical harvesting of hydrophobic microalgae by electro-flotation without coagulation was assessed. The harvesting performance of this method for selected species of freshwater microalgae with different degrees of hydrophobicity (Tribonema sp., highly hydrophobic; Scenedesmus sp., moderately hydrophobic; and Pandorina sp., hydrophilic) were compared. It was found that microalgal hydrophobicity played a critical role in electro-flotation. Under the same condition (current 0.3 A, velocity gradient 200 s^-1, biomass concentration 1 g/L), Tribonema sp. could be effectively harvested (96.2 ± 0.4%) after 20 min of electro-flotation, while the harvesting efficiency decreased significantly with Scenedesmus sp. (70.1 ± 5.2%, 20 min) and Pandorina sp. (<10%, 1 h). The influences of current, electrolysis time, mixing intensity (velocity gradient) and biomass concentration on Tribonema sp. (hydrophobic) harvesting were further investigated. Increasing the current within a certain range (0.1 A-0.4 A) was beneficial to harvesting, while it's further increase decreased floating velocity, which was similar to the effect of the velocity gradient. Under the optimal condition, the harvesting efficiency of Tribonema sp. was 96.3% and the energy consumption (0.19 kWh/kg biomass) was much lower than other harvesting techniques, indicating that electro-flotation is a time-saving and economical approach for hydrophobic microalgae harvesting.

Optimization of Air Flotation and the Combination of Air Flotation and Membrane Filtration in Microalgae Harvesting

On account of its small size and poor sedimentation performance, microalgae harvesting is restricted from a wider application. Air flotation is an efficient and fast solid–liquid separation technology, which has the potential to overcome the impediments of microalgae harvesting. In this study, factors influencing microalgae harvesting by air flotation were investigated. The results illustrated that bound extracellular organic matter (bEOM) had a greater effect on microalgae harvesting by air flotation, compared with dissolved extracellular organic matter (dEOM). Microalgae harvesting by air flotation in different growth stages proceeded, and the effect of air flotation in the heterotrophic stage was better than the autotrophic stage. The molecular weight distributions demonstrated that after air flotation, the proportion of high MW substance increased, while the proportion of low MW substance decreased, regardless of whether dEOM or bEOM. Membrane filtration was carried out for the algal solutions before and after air flotation. The membrane of pre-flotation algal solution had a higher critical flux of 51 L/m2·h than that of no-pre-flotation (24 L/m2·h), and, thus, pre-flotation had an active effect on membrane filtration in microalgae harvesting. Moreover, the combination of air flotation and membrane filtration provided an efficient technology for microalgae harvesting.

Global market and economic analysis of microalgae technology: Status and perspectives

Microalgae have been a promising alternative source of high-value compounds to replace the non-sustainable fossil fuels resource. The recent research development of algae-based bioproducts has remarkable impact various industries section for its renewability, efficiency, and environmentally friendly crops over those synthetic-made product. However, by utilizing microalgae biomass toward their full potential is still limited due to lack of research funding, social acceptability and challenges in policy implementation. This present review highlights the various microalgae biotechnology with consideration of economical aspect for the global potential of algae market, comparison between the microalgae market in Malaysia and international countries. In addition, the cultivation technologies and feasibility of microalgae biomass production globally, followed by insightful challenges and future development of microalgae industry are mentioned. The current study will contribute to the understanding of upstream and downstream of microalgae processing along with technical economical understandings for the successful commercialisation of microalgae products.

Cercosporin-bioinspired photoinactivation of harmful cyanobacteria under natural sunlight via bifunctional mechanisms

Harmful cyanobacterial blooms (HCBs), mainly caused by eutrophication, have deleterious impacts on water resources and pose a great threat to human health and natural ecosystems. Thus, an environmentally-friendly method to inhibit HCBs is urgently needed. Learning from nature, herein, natural product cercosporin, produced by the fungi Cercospora to damage plant cells under natural sunlight, was developed as a powerful photosensitive algicidal reagent to inhibit HCBs. Microcystis aeruginosa could be severely inactivated by 20 μM cercosporin in 36 h with 95% inhibition ratio under 23 W compact fluorescent light irradiation. Further mechanism investigation showed that algal cell walls and membranes along with the antioxidant and photosynthetic systems were damaged via two mechanisms, those being, reactive oxygen species generation and cell adsorption. More importantly, the practical applicability of cercosporin was demonstrated by its effectiveness in a 2 L-scale photoinactivation experiment using cyanobacterial blooms from Taihu Lake, China under natural sunlight with a lower dosage of cercosporin (7.5 μM). This study established the bifunctional mechanisms by which cercosporin inactivates HCBs, opening design possibilities for the development of novel photosensitive algicidal reagents to control HCBs.

Cyanobacterial biorefinery: Towards economic feasibility through the maximum valorization of biomass

Cyanobacteria are well known for their plethora of applications in the fields of food industry, pharmaceuticals and bioenergy. Their simple growth requirements, remarkable growth rate and the ability to produce a wide range of bio-active compounds enable them to act as an efficient biorefinery for the production of valuable metabolites. Most of the cyanobacteria based biorefineries are targeting single products and thus fails to meet the efficient valorization of biomass. On the other hand, multiple products recovering cyanobacterial biorefineries can efficiently valorize the biomass with minimum to zero waste generation. But there are plenty of bottlenecks and challenges allied with cyanobacterial biorefineries. Most of them are being associated with the production processes and downstream strategies, which are difficult to manage economically. There is a need to propose new solutions to eliminate these tailbacks so on to elevate the cyanobacterial biorefinery to be an economically feasible, minimum waste generating multiproduct biorefinery. Cost-effective approaches implemented from production to downstream processing without affecting the quality of products will be beneficial for attaining economic viability. The integrated approaches in cultivation systems as well as downstream processing, by simplifying individual processes to unit operation systems can obviously increase the economic feasibility to a certain extent. Low cost approaches for biomass production, multiparameter optimization and successive sequential retrieval of multiple value-added products according to their high to low market value from a biorefinery is possible. The nanotechnological approaches in cyanobacterial biorefineries make it one step closer to the goal. The current review gives an overview of strategies used for constructing self-sustainable- economically feasible- minimum waste generating; multiple products based cyanobacterial biorefineries by the efficient valorization of biomass. Also the possibility of uplifting new cyanobacterial strains for biorefineries is discussed.

Mass cultivation and harvesting of microalgal biomass: Current trends and future perspectives

Microalgae are unicellular photosynthetic organisms capable of producing high-value metabolites like carbohydrates, lipids, proteins, polyunsaturated fatty acids, vitamins, pigments, and other high-value metabolites. Microalgal biomass gained more interest for the production of nutraceuticals, pharmaceuticals, therapeutics, food supplements, feed, biofuel, bio-fertilizers, etc. due to its high lipid and other high-value metabolite content. Microalgal biomass has the potential to convert trapped solar energy to organic materials and potential metabolites of nutraceutical and industrial interest. They have higher efficiency to fix carbon dioxide (CO₂) and subsequently convert it into biomass and compounds of potential interest. However, to make microalgae a potential industrial candidate, cost-effective cultivation systems and harvesting methods for increasing biomass yield and reducing the cost of downstream processing have become extremely urgent and important. In this review, the current development in different microalgal cultivation systems and harvesting methods has been discussed.

Optimization of heavy metal (lead) remedial activities of fungi Aspergillus penicillioides (F12) through extra cellular polymeric substances

Wastewater imposes a great threat to any ecosystem across the world, especially the aquatic one because of the different anthropogenic activities of human beings. The present study emphasizes the optimization of ecological parameters [pH, time (h) and temperature (°C)] employing Box-Behnken design (BBD) to achieve better bio-adsorption of a selected heavy metal [lead (Pb II)] from the wastewater through an extracellular polymeric substance (EPS) of a benthic fungus, Aspergillus penicillioides (F12) (MN210327). The relevant statistical analysis (ANOVA) has enabled to record of the optimized bio-adsorption (73.14 %) of lead (Pb II) by fungal EPS at pH (8.85) and temperature (32 °C) for a duration of 5.74 h. Besides that, at the concentration of 0.5 mg/L of EPS, the flocculating rate was noted to be highest (88.4 %) in kaolin clay and the 50 % emulsifying activity. This investigation has also opened up new vistas on the possibility of the development of an alternative method of eco-sustainable bioremediation of heavy metals by fungal EPS on an industrial scale.

Metal-based flocculation to harvest microalgae: a look beyond separation efficiency

Metal-based flocculants are commonly used for biomass harvesting in microalgae-based bio-refineries. Besides the high separation efficiency, additional aspects should be considered, related to the toxicity of metals for the algal biomass. Partitioning tests for commonly used flocculants (i.e., FeCl₃ and Al₂(SO₄)₃) showed that metals were mostly transferred to the solid phase with more than 95% of dosed metal ending up into the biomass, and low metal concentrations in the liquid effluent (lower than 0.4 mg L^-1 for both metals), thus allowing for water reuse. Photosynthesis inhibition was tested on microalgae and microalgae-bacteria cultures, using a standardized photo-respirometry protocol in which typical concentrations used during coagulation-flocculation were assessed. Modelling dose-response curves, concentrations corresponding to 50% inhibition (IC₅₀) were obtained, describing short-term effects. The obtained IC₅₀ ranged from 13.7 to 28.3 mg Al L^-1 for Al, and from 127.9 to 195.8 mg Fe L^-1 for Fe, showing a higher toxicity for the Al-based flocculant. The recovery of photosynthesis inhibition was also quantified, to evaluate the possibility of reusing/recycling the harvested biomass. The results highlighted that the residual photosynthetic activities, evaluated after 1 h and 24 h of exposure to metals were partially recovered, especially for Al, passing from 67.3% to 94.6% activity, respectively, while long-term Fe effects were stronger (passing from 64.9% to 77.6% activity). A non-toxic flocculant (cationic starch) was finally tested, excluding potential effects due to biomass aggregation, as the reduction of photosynthetic activity only reached 3.4%, compared to control. Relevant modifications to the light availability and the optical properties of algal suspensions were assessed, identifying a strong effect of iron which caused an increase of the light absorbance up to approximately 40% at high Fe concentrations. Possible implications of dosing metallic flocculants in MBWWT processes are discussed, and suggestions are given to perform inhibition tests on flocculating chemicals.

Novel approach for harvesting of microalgal biomass using electric geyser waste material deposit as flocculant in coupling with poultry excreta leachate

The aim of this work was to study the flocculation efficiency of algal biomass (Chlorella pyrenoidosa) in coupling with waste materials i.e. poultry excreta leachate by using other waste material which was obtained from deposition of scaling in electric geyser. Utilization of electric geyser waste material deposit (EGWMD) for flocculation is a novel approach because of various elements which are replica of chemical flocculants responsible for flocculation mechanism in culture medium. Flocculation process was optimized by response surface methodology and 98.21% flocculation efficiency was achieved with designed process parameters as temperature 32.5 °C, flocculant dose 275 mgL^-1, pH 5 and time 30 min. The reusability of spent medium was also analyzed at 70.2% and 32.5% flocculation efficiency with two successive steps. The cellular morphology of pre-harvested and post-harvested Chlorella pyrenoidosa was also observed. EGWMD is abundant and freely available that has no application till now and can alternate of chemical flocculants.

The use of natural organic flocculants for harvesting microalgae grown in municipal wastewater at different culture densities

Harvesting microalgae from liquid culture is a difficult issue to solve and is most commonly done through settling. However, settling is a slow process on its own and generally needs to be induced chemically or by introducing stress to the culture. Polymeric, cationic substances, such as cationised starch and chitosan, are often used for flocculation and settling. These large, positively charged molecules form large clusters with suspended particles in the liquid medium. In the present study, three natural organic flocculants (cationic starch, chitosan and acacia tannin S5T) were tested to harvest microalgal cultures grown in wastewater. Two microalgal species, one strain of Chlorella vulgaris and one strain of Scenedesmus obliquus, were cultured in municipal wastewater for different lengths of time, and settled using either cationic starch, chitosan or acacia tannin S5T. Results indicated that S5T worked with approximately the same efficiency in the two assayed species, although it requires a relatively high dosage to function (about 300 mg L^-1 ), while the other two flocculants varied from species to species.

A multicriteria decision analysis for the evaluation of microalgal growth and harvesting

Biomass obtained from microalgae research studies gained momentum in recent years because of their extensive application potential in multiple industries such as high-value nutraceuticals, bioproducts, cosmetics, animal feed industries, and biofuels while being a sustainable and environmentally friendly option. Although they have high biomass yields and rapid growth rates there are some limitations and challenges that remain for large-scale commercialized cultivation and harvesting methods of microalgae. Since there are multiple pathways related to efficient cultivation and harvesting methods to be viable, this study adopted, TOPSIS (Technique for Order Preference by Similarity to Ideal Solution), a multicriteria decision-making tool, to find the most acceptable alternative by using excel spreadsheets to evaluate the information that is derived from literature and pilot-scale studies. As a result, tubular (helical) and plate (flat panel) photobioreactors (PBRs) for cultivation and chemical harvesting (with chitosan) and bio-flocculation for harvesting were deemed suitable, while plastic bag PBR and suspended air flotation were deemed unsuitable.

Valorization of microalgae biomass into bioproducts promoting circular bioeconomy: a holistic approach of bioremediation and biorefinery

The need for alternative source of fuel has demanded the cultivation of 3rd generation feedstock which includes microalgae, seaweed and cyanobacteria. These phototrophic organisms are unique in a sense that they utilise natural sources like sunlight, water and CO₂ for their growth and metabolism thereby producing diverse products that can be processed to produce biofuel, biochemical, nutraceuticals, feed, biofertilizer and other value added products. But due to low biomass productivity and high harvesting cost, microalgae-based production have not received much attention. Therefore, this review provides the state of the art of the microalgae based biorefinery approach to define an economical and sustainable process. The three major segments that need to be considered for economic microalgae biorefinery is low cost nutrient source, efficient harvesting methods and production of by-products with high market value. This review has outlined the use of various wastewater as nutrient source for simultaneous biomass production and bioremediation. Further, it has highlighted the common harvesting methods used for microalgae and also described various products from both raw biomass and delipidified microalgae residues in order to establish a sustainable, economical microalgae biorefinery with a touch of circular bioeconomy. This review has also discussed various challenges to be considered followed by a techno-economic analysis of the microalgae based biorefinery model.

Optimization of Microalga Chlorella vulgaris Magnetic Harvesting

Harvesting of microalgae is a crucial step in microalgae-based mass production of different high value-added products. In the present work, magnetic harvesting of Chlorella vulgaris was investigated using microwave-synthesized naked magnetite (Fe₃O₄) particles with an average crystallite diameter of 20 nm. Optimization of the most important parameters of the magnetic harvesting process, namely pH, mass ratio (m_r) of magnetite particles to biomass (g/g), and agitation speed (rpm) of the C. vulgaris biomass-Fe₃O₄ particles mixture, was performed using the response surface methodology (RSM) statistical tool. Harvesting efficiencies higher than 99% were obtained for pH 3.0 and mixing speed greater or equal to 350 rpm. Recovery of magnetic particles via detachment was shown to be feasible and the recovery particles could be reused at least five times with high harvesting efficiency. Consequently, the described harvesting approach of C. vulgaris cells leads to an efficient, simple, and quick process, that does not impair the quality of the harvested biomass.

Innovative membrane photobioreactor for sustainable CO2 capture and utilization

The rising of greenhouse-gas emissions (GHG), during the last 200 years, is associated to the well known global warming phenomena. One of the main sources of CO₂-equivalent GHGs emissions are the environmental protection plants accounting for 1.57% of the global emissions and thus sustainable and effective technologies for their mitigation are strongly needed. The current paper presents and discusses the assessment of an innovative membrane photo-bioreactor (MPBR) whose aim was the promotion of CO₂ capture from conveyed flows, such as those from wastewater treatment plants (WWTPs), landfill and composting plants, for production and energy valorisation of algal biomass. Chlorella vulgaris microalgae strain was selected as photosynthetic platform for the abovementioned purposes. The influence of various operating parameters has been explored, including the photosynthetic photon flux densities (PPFD) (60 and 120 μmol m^-2 s^-1), liquid/gas ratio (L/G = 5, 10 or 15) and CO₂ concentration (5, 10 and 15%) in order to investigated their effects on carbon capture effectiveness and biomass production. The results demonstrated that the increasing of PPFD significantly enhanced the biomass production in terms of biomass productivity (P) and total dry weight (DW). The highest biomass concentration of 1.01 g L^-1 was achieved at PPFD of 120 μmol m^-2 s^-1 with a L/G of 15. Under the aforementioned conditions, carbon dioxide removal efficiency (RE) reached values up to 80%. In addition, the novel MPBR equipped with an innovative self-forming dynamic membrane (SFDM) showed a simultaneous biomass harvesting rate of 41 g m^-2 h^-1.

Microalgal bio-flocculation: present scenario and prospects for commercialization

The need for sustainable production of renewable biofuel has been a global concern in the recent times. Overcoming the tailbacks of the first- and second-generation biofuels, third-generation biofuel using microalgae as feedstock has emerged as a plausible alternative. It has an added advantage of preventing any greenhouse gas (GHG) emissions with simultaneous carbon dioxide sequestration. Dewatering of microalgal culture is one of the many concerns regarding industrial-scale biofuel production. The small size of microalgae and dilute nature of its growth cultures creates huge operational cost during biomass separation, limiting economic feasibility of algae-based fuels. Considering the recovery efficiency, operation economics, technological feasibility and cost-effectiveness, bio-flocculation is a promising method of harvesting. Moreover, advantage of bio-flocculation over other conventional methods is that it does not incur the addition of any external chemical flocculants. This article reviews the current status of bio-flocculation technique for harvesting microalgae at industrial scale. The various microbial strains that can be prospective bioflocculants have been reviewed along with its application and advantages over chemical flocculants. Also, this article proposes that the primary focus of an appropriate harvesting technique should depend on the final utilization of the harvested biomass. This review article attempts to bring forth the beneficial aspects of microbial aided microalgal harvesting with a special attention on genetically modified self-flocculation microalgae.

Evaluation of natural flocculant efficiency in the harvest of microalgae Monoraphidium contortum

This study assesses two parameters that can result in high efficiency in the recovery of the microalgae Monoraphidium contortum. The significant contribution of this paper is to test different coagulants in different conditions of concentration in the coagulation and flocculation processes followed by sedimentation to evidence the best coagulant and the best condition for harvesting of Monoraphidium contortum biomass. So the proposed methodology aimed to perform preliminary tests using a tannin-based cationic coagulant (TANFLOC SG®), FeCl2, and Al2 (SO4)3, where they were performed at concentrations of 0, 20, 40, 60, 80, and 100 mg L−1 at a fast mixing speed of 400 RPM. The tests determined 20 mg L−1 of Tanfloc SG® as the most efficient turbidity reduction in the preliminary test. The obtained results were used to construct a non-factorial central composite planning. Therefore, after a design of experiments, the study outcome shows the best turbidity removal range from the main tests came at 35 mg L−1 and 550 RPM of fast mixing speed.

Production of Microalgal Biomass in Photobioreactors as Feedstock for Bioenergy and Other Uses: A Techno-Economic Study of Harvesting Stage

The cultivation of microalgae has become a viable option to mitigate increase in CO2 in the atmosphere generated by industrial activities since they can capture CO2 as a carbon source for growth. Besides, they produce significant amounts of oils, carbohydrates, proteins, and other compounds of economic interest. There are several investigations related to the process, however, there is still no optimal scenario, since may depend on the final use of the biomass. The objective of this work was to develop a techno-economic evaluation of various technologies in harvesting and drying stages. The techno-economic estimation of these technologies provides a variety of production scenarios. Photobioreactors were used considering 1 ha as a cultivation area and a biomass production of 22.66 g/m2/day and a CO2 capture of 148.4 tons/ha/year was estimated. The production scenarios considered in this study have high energy demand and high operating costs (12.09–12.51 kWh/kg and US $210.05–214.59/kg). These results are mainly a consequence of the use of tubular photobioreactors as a biomass culture system. However, the use of photobioreactors in the production of microalgal biomass allows it to be obtained in optimal conditions for its use in the food or pharmaceutical industry.

One-step removal of harmful algal blooms by dual-functional flocculant based on self-branched chitosan integrated with flotation function

Harmful algal blooms induce severe environmental problems. It is challenging to remove algae by the current available treatments involving complicate process and costly instruments. Here, we developed a CaO₂@PEG-loaded water-soluble self-branched chitosan (CP-SBC) system, which can remove algae from water in one-step without additional instrumentation. This approach utilizes a novel flocculant (self-branched chitosan) integrated with flotation function (induced by CaO₂@PEG). CP-SBC exhibited better flocculation performance than commercial flocculants, which is attributed to the enhanced bridging and sweeping effect of branched chitosan. CP-SBC demonstrated outstanding biocompatibility, which was verified by zebrafish test and algae activity test. CaO₂@PEG-loaded self-branched chitosan can serve as an "Air flotation system" to spontaneous float the flocs after flocculation by sustainably released O₂. Furthermore, CP-SBC can improve water quality through minimizing dissolved oxygen depletion and reducing total phosphorus concentrations.

Importance of Downstream Processing of Natural Astaxanthin for Pharmaceutical Application

Astaxanthin (ASX) is a xanthophyll pigment considered as a nutraceutical with high antioxidant activity. Several clinical trials have shown the multiple health benefits of this molecule; therefore, it has various pharmaceutical industry applications. Commercial astaxanthin can be produced by chemical synthesis or through biosynthesis within different microorganisms. The molecule produced by the microorganisms is highly preferred due to its zero toxicity and superior therapeutic properties. However, the biotechnological production of the xanthophyll is not competitive against the chemical synthesis, since the downstream process may represent 70–80% of the process production cost. These operations denote then an opportunity to optimize the process and make this alternative more competitive. Since ASX is produced intracellularly by the microorganisms, high investment and high operational costs, like centrifugation and bead milling or high-pressure homogenization, are mainly used. In cell recovery, flocculation and flotation may represent low energy demanding techniques, whereas, after cell disruption, an efficient extraction technique is necessary to extract the highest percentage of ASX produced by the cell. Solvent extraction is the traditional method, but large-scale ASX production has adopted supercritical CO2 (SC-CO2), an efficient and environmentally friendly technology. On the other hand, assisted technologies are extensively reported since the cell disruption, and ASX extraction can be carried out in a single step. Because a high-purity product is required in pharmaceuticals and nutraceutical applications, the use of chromatography is necessary for the downstream process. Traditionally liquid-solid chromatography techniques are applied; however, the recent emergence of liquid-liquid chromatography like high-speed countercurrent chromatography (HSCCC) coupled with liquid-solid chromatography allows high productivity and purity up to 99% of ASX. Additionally, the use of SC-CO2, coupled with two-dimensional chromatography, is very promising. Finally, the purified ASX needs to be formulated to ensure its stability and bioavailability; thus, encapsulation is widely employed. In this review, we focus on the processes of cell recovery, cell disruption, drying, extraction, purification, and formulation of ASX mainly produced in Haematococcus pluvialis, Phaffia rhodozyma, and Paracoccus carotinifaciens. We discuss the current technologies that are being developed to make downstream operations more efficient and competitive in the biotechnological production process of this carotenoid.

Enhancement of astaxanthin production by Haematococcus pluvialis using magnesium aminoclay nanoparticles

Improving the content and production of high-value ketocarotenoid pigments is critical for the commercialization of microalgal biorefineries. This study reported the use of magnesium aminoclay (MgAC) nanoparticles for enhancement of astaxanthin production by Haematococcus pluvialis in photoautotrophic cultures. Addition of 1.0 g/L MgAC significantly promoted cellular astaxanthin biosynthesis (302 ± 69 pg/cell), presumably by inducing tolerable oxidative stress, corresponding to a 13.7-fold higher production compared to that in the MgAC-untreated control (22 ± 2 pg/cell). The lipid content and cell size of H. pluvialis improved by 13.6- and 2.1-fold, respectively, compared to that of the control. Despite reduced cell numbers, the overall astaxanthin production (10.3 ± 0.4 mg/L) improved by 40% compared to the control (7.3 ± 0.6 mg/L), owing to improved biomass production. However, an MgAC dosage above 1.0 g/L inhibited biomass production by inducing electrostatic cell wall destabilization and aggregation. Therefore, MgAC-induced stimulation of algae varies widely based on their morphological and physiological characteristics.

Magnetophoretic Harvesting of Nannochloropsis oculata Using Iron Oxide Immobilized Beads

In this work, the harvesting of Nannochloropsis oculata microalgae through the use of nanosized Fe3O4 immobilized in polyvinyl alcohol (PVA)/sodium alginate (SA) as a flocculant (Fe3O4/PS) is investigated. Using the Fe3O4/PS immobilized beads could reduce the amount of soluble ferrous ions (Fe2+) released from naked Fe3O4 in acid treatment, leading to easy recovery. The characterization was performed under different dosages and pH values of Fe3O4/PS. The results show that the Fe3O4/PS, when applied to the algae culture (500 mg dry cell weight/L), achieves a 96% harvesting efficiency under conditions of a pH of 4 with 200 mT magnetic field intensity. Fe3O4/PS can be directly reused without adjusting the pH value. The recycled Fe3O4/PS shows stability in terms of its surface properties, maintaining more than 80% harvesting efficiency after five recycles. Magnetophoretic harvesting, using immobilized magnetic iron oxide as a particle-based flocculant, is a potential method to reduce challenges related to the cost-effective microalgae-harvesting method.

Morphological Change and Cell Disruption of Haematococcus pluvialis Cyst during High-Pressure Homogenization for Astaxanthin Recovery

Haematococcus pluvialis accumulates astaxanthin, which is a high-value antioxidant, during the red cyst stage of its lifecycle. The development of a rigid cell wall in the cysts hinders the recovery of astaxanthin. We investigated morphological changes and cell disruption of mature H. pluvialis cyst cells while using high-pressure homogenization for astaxanthin extraction. When treated with French-press-cell (pressure, 10,000–30,000 psi; passage, 1–3), the intact cyst cells were significantly broken or fully ruptured, releasing cytoplasmic components, thereby facilitating the separation of astaxanthin by ethyl acetate. Fluorescence microscopy observations using three different fluorescent dyes revealed that a greater degree of cell breakage caused greater external dispersion of astaxanthin, chlorophyll, lipids, proteins, and carbohydrates. The mechanical treatment resulted in a high cell disruption rate of up to 91% based on microscopic cell typing and Coulter methods. After the ethyl acetate extraction, the astaxanthin concentration significantly increased by 15.2 mg/L in proportion to the increase in cell disruption rate, which indicates that cell disruption is a critical factor for solvent-based astaxanthin recovery. Furthermore, this study recommends a synergistic combination of the fast instrumental particle-volume-distribution analysis and microscope-based morphologic phenotyping for the development of practical H. pluvialis biorefinery processes that co-produce various biological products, including lipids, proteins, carbohydrates, chlorophyll, and astaxanthin.

Harvesting of freshwater microalgae Scenedesmus sp. by electro-coagulation-flocculation for biofuel production: effects on spent medium recycling and lipid extraction

There is growing interest in recent times for microalgae as a sustainable energy source. However, efficient harvesting of microalgal biomass for various industrial applications is still considered a bottleneck. The present study attempts to evaluate microalgae Scenedesmus sp. harvesting using electro-coagulation-flocculation (ECF). Plackett-Burman design was exploited to explore the significant process parameters, whereas Taguchi's array design was employed for optimization. The optimal conditions were optimized as initial pH 5.0, electrolysis time 15 min, electrode distance 2 cm, sedimentation time 60 min, and current density 12 mA cm^-2 for complete harvesting. Under optimum conditions, the energy utilization and the operation cost of ECF process was estimated to be 2.65 kWh kg^-1 and USD 0.29 kg^-1, respectively. Thus, ECF-based microalgae harvesting was found as a low-cost technique. In addition, neutralizing pH and supplementing macro- and micronutrients enabled the flocculated medium to maintain an approximate growth yield in algal cultivation to that of the fresh BG11 medium. ECF did not affect the amount of microalgal lipids (28.6 ± 1.2, % wt.), chlorophyll a (8.3 ± 0.3 μg mL^-1), and fatty acid methyl ester composition (C15:0, C16:0, C17:0, and C18:0) as well. These results strongly recommend ECF as the most appropriate and promising method for harvesting Scenedesmus sp. for biofuel production.