Microalgae harvesting techniques: A review

Flocculation of Chlorella vulgaris by shell waste-derived bioflocculants for biodiesel production: Process optimization, characterization and kinetic studies

Flocculants are foreign particles that aggregate suspended microalgae cells and due to cost factor and toxicity, harvesting of microalgae biomass has shifted towards the use of bioflocculants. In this study, mild acid-extracted bioflocculants from waste chicken's eggshell and clam shell were used to harvest Chlorella vulgaris that was cultivated using chicken compost as nutrient source. It was found that a maximum of 99% flocculation efficiency can be attained at pH medium of 9.8 using 60 mg/L of hydrochloric acid-extracted chicken's eggshell bioflocculant at 50 °C of reaction temperature. On the other hand, 80 mg/L of hydrochloric acid-extracted clam shell bioflocculant was sufficient to recover C. vulgaris biomass at pH 9.8 and optimum temperature of 40 °C. The bioflocculants and bioflocs were characterized using microscopic, zeta potential, XRD, AAS and FT-IR analysis. The result revealed that calcium ions in the bioflocculants are the main contributor towards the flocculation of C. vulgaris, employing charge neutralization and sweeping as possible flocculation mechanisms. The kinetic parameters were best fitted pseudo-second order which resulted in R² of 0.99 under optimal flocculation temperature. The results herein, disclosed the applicability of shell waste-derived bioflocculants for up-scaled microalgae harvesting for biodiesel production.

Improved Nylon 6,6 Nanofiber Membrane in A Tilted Panel Filtration System for Fouling Control in Microalgae Harvesting

The competitiveness of algae as biofuel feedstock leads to the growth of membrane filtration as one of promising technologies for algae harvesting. Nanofiber membrane (NFM) was found to be efficient for microalgae harvesting via membrane filtration, but it is highly limited by its weak mechanical strength. The main objective of this study is to enhance the applicability of nylon 6,6 NFM for microalgae filtration by optimizing the operational parameters and applying solvent vapor treatment to improve its mechanical strength. The relaxation period and filtration cycle could be optimized to improve the hydraulic performance. For a cycle of 5 min., relaxation period of ≤2 min shows the highest steady-state permeability of 365 ± 14.14 L m⁻² h⁻¹ bar⁻¹, while for 10 min cycle, 3 min. of relaxation period was found optimum that yields permeability of 402 ± 34.47 L m⁻² h⁻¹ bar⁻¹. The treated nylon 6,6 NFM was also used to study the effect of aeration rate. It is confirmed that the aeration rate enhances the steady-state performance for both intermittent and continuous mode of aeration. Remarkably, intermittent aeration shows 7% better permeability than the full aeration for all tested condition, which is beneficial for reducing the total energy consumption.

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.

Synergy of biofuel production with waste remediation along with value-added co-products recovery through microalgae cultivation: A review of membrane-integrated green approach

Development of advanced biofuels such as bioethanol and biodiesel from renewable resources is critical for the earth's sustainable management and to slow down the global climate change by partial replacement of gasoline and diesel in the transport sector. Being a diverse group of aquatic micro-organisms, algae are the most prominent resources on the planet, distributed in an aquatic system, a potential source of bioenergy, biomass and secondary metabolites. Microalgae-based biofuel production is widely accepted as non-food fuel sources and better choice for achieving goals of incorporation of a clean fuel source into the transportation sector. The present review article provides a comprehensive literature survey as well as a novel approach on the application of microalgae for their simultaneous cultivation and bioremediation of high nutrient containing wastewater. In addition to that, merits and demerits of different existing conventional techniques for microalgae culture reactors, harvesting of algal biomass, oil recovery, use of different catalysts for transesterification reactions and other by-products recovery have been discussed and compared with the membrane-based system to find out the best optimal conditions for higher biomass as well as lipid yield. This article also deals with the use of a tailor-made membrane in an appropriate module that can be used in upstream and downstream processes during algal-based biofuels production. Such membrane-integrated system has the potential of low-cost and eco-friendly separation, purification and concentration enrichment of biodiesel as well as other valuable algal by-products which can bring the high degree of process intensification for scale-up at the industrial stage.

Microalgae in Food-Energy-Water Nexus: A Review on Progress of Forward Osmosis Applications

Nowadays the world is facing vulnerability problems related to food, energy and water demands. The challenges in those subsystems are intertwined and thus require inter-discipline approaches to address them. Bioresources offer promising solutions of the dilemma. Microalgae biomass is expected to become a superfood and a favorable energy feedstock and assist in supplying clean water and treat wastewater. Efficient mass production of microalgae, both during upstream and downstream processes, is thus a key process for providing high quality and affordable microalgae biomass. This paper covers recent progress in microalgae harvesting and dewatering by using osmotic driven membrane process, i.e., forward osmosis. Critical factors during forward osmosis process for microalgae harvesting and dewatering are discussed. Finally, perspective on further research directions and implementation scenarios of the forward osmosis are also provided.

Effect of shear rate on floc characteristics and concentration factors for the harvesting of Chlorella vulgaris using coagulation-flocculation-sedimentation

Coagulation-based separation has been increasingly applied to microalgal harvesting because of its competitive cost and high scalability. The characteristics of flocs formed during coagulation/flocculation are critical for efficient harvesting. However, few studies have been devoted to systematically investigating the structural characteristics of microalgal flocs and their influences on subsequent settling performance. In this paper, the dynamic mean size and fractal dimension, strength, regrowth and settling performance of Al³⁺ coagulated Chlorella vulgaris flocs were characterized at various flocculation shear rates. The influence of shear rate on floc characteristics was revealed. An appropriate shear rate (9 s^-1) produced more desirable microalgal flocs (in terms of size and compactness), with better settling performance and a higher concentration factor, than higher or lower shear rates, favoring their separation and subsequent harvesting. At this condition, the concentration factor reached 13.50, which was a 177.21% improvement over the 4.87 reached at a low shear rate.

Biofuel Application as a Factor of Sustainable Development Ensuring: The Case of Russia

Diffusion of the biofuels (BF) using is justified by opening up the opportunities for obtaining fuel and energy from previously inaccessible sources and by the existence of energy-deficient regions, in particular in Russia. Works of different scientists on the problems of creating and using BF were the methodological basis of this study. Information on the state and prospects of the development of renewable energy sources in Russian regions was collected from regulatory documents and was obtained by employing a questionnaire survey. For the study of the collected materials, the different methods of comparative analysis, and the methods of expert assessments were used. The results of the Status-Quo analysis of BF production in Russia have shown that the creation of BF performed relatively successfully. However, there are many more perspectives, connected with expanding the utilization of the different raw materials. Also, the analysis of organizational and economic mechanisms applied for production of BF and the obtained data on several organizations-producers allowed for proposing six indexes for the assessment of the BF production effectiveness. It is suggested that BF production in Russia will contribute to the sustainable development of a number of the country’s regions in the near future.

Flocculation Harvesting Techniques for Microalgae: A Review

Microalgae have been considered as one of the most promising biomass feedstocks for various industrial applications such as biofuels, animal/aquaculture feeds, food supplements, nutraceuticals, and pharmaceuticals. Several biotechnological challenges associated with algae cultivation, including the small size and negative surface charge of algal cells as well as the dilution of its cultures, need to be circumvented, which increases the cost and labor. Therefore, efficient biomass recovery or harvesting of diverse algal species represents a critical bottleneck for large-scale algal biorefinery process. Among different algae harvesting techniques (e.g., centrifugation, gravity sedimentation, screening, filtration, and air flotation), the flocculation-based processes have acquired much attention due to their promising efficiency and scalability. This review covers the basics and recent research trends of various flocculation techniques, such as auto-flocculation, bio-flocculation, chemical flocculation, particle-based flocculation, and electrochemical flocculation, and also discusses their advantages and disadvantages. The challenges and prospects for the development of eco-friendly and economical algae harvesting processes have also been outlined here.

Growth performance, biochemical composition and sedimentation velocity of Tetraselmis sp. CTP4 under different salinities using low-cost lab- and pilot-scale systems

Biomass harvesting is one of the most expensive steps of the whole microalgal production pipeline. Therefore, the present work aimed to understand the effect of salinity on the growth performance, biochemical composition and sedimentation velocity of Tetraselmis sp. CTP4, in order to establish an effective low-cost pilot-scale harvesting system for this strain. At lab scale, similar growth performance was obtained in cultures grown at salinities of 5, 10 and 20 g L^-1 NaCl. In addition, identical settling velocities (2.4-3.6 cm h^-1) were observed on all salinities under study, regardless of the growth stage. However, higher salinities (20 g L^-1) promoted a significant increase in lipid contents in this strain compared to when this microalga was cultivated at 5 or 10 g L^-1 NaCl. At pilot-scale, cultures were cultivated semi-continuously in 2.5-m³ tubular photobioreactors, fed every four days, and stored in a 1-m³ harvesting tank. Upon a 24-hour settling step, natural sedimentation of the microalgal cells resulted in the removal of 93% of the culture medium in the form of a clear liquid containing only vestigial amounts of biomass (0.07 ± 0.02 g L^-1 dry weight; DW). The remaining culture was recovered as a highly concentrated culture (19.53 ± 4.83 g L^-1 DW) and wet microalgal paste (272.7 ± 18.5 g L^-1 DW). Overall, this method provided an effective recovery of 97% of the total biomass, decreasing significantly the harvesting costs.

Biodegradable branched cationic starch with high C/N ratio for Chlorella vulgaris cells concentration: Regulating microalgae flocculation performance by pH

To improve the carbon to nitrogen (C/N) ratio of harvested microalgae biomass for better producing biogas by fermentation, biodegradable cationic starch with high C/N ratio were synthesized to harvest Chlorella vulgaris. The impact of pH was also studied as the zeta potential of both microalgae and cationic starch would change with pH. Results indicated the cationic starch can harvest above 99% of the microalgae and the C/N ratio can rise from 7.50 to 7.90. The zeta potential of microalgae always kept negative and presented a trend of descending firstly and then upgrade. The maximum microalgae biomass flocculation capacity of 1 g cationic starch was 8.62 g with the help of self-flocculation at pH 3. The concentration of flocs formed at pH 11 was 25.74 g L^-1 and the diameter was 0.553 mm which was much larger than the flocs formed at pH 3 (0.208 mm).

The effects of surfactants (sodium dodecyl sulfate, triton X-100 and cetyl trimethyl ammonium bromide) on the dewaterability of microalgae biomass using pressure filtration

The application of pressure filtration in microalgae harvesting requires chemical pretreatment in order to reduce membrane fouling and to increase water flux. Surfactants have shown potential to enhance microalgae dewaterability by charge neutralization, bridging and releasing extracellular polymeric substances (EPS) and bound water. In this study, the effect of three surfactants including anionic sodium dodecyl sulfate (SDS), non-ionic triton X-100 and cationic cetyl trimethyl ammonium bromide (CTAB) on the dewaterability of Chlamydomonas sp. was investigated. Filtration fluxes and biomass concentrations were used to evaluate the microalgae dewaterability. Based on the results, SDS and Triton X-100 had a negative effect on the dewaterability of microalgae biomass. However, CTAB improved the dewaterability by decreasing the reversible and irreversible fouling resistance. The optimum dosage of CTAB was found to be 1500 mg/L, and resulted in 95.8% and 140% improvement on average water flux and biomass recovery efficiency, respectively.

Bioflocculation formation of microalgae-bacteria in enhancing microalgae harvesting and nutrient removal from wastewater effluent

Microalgal bacterial flocs can be a promising approach for microalgae harvesting and wastewater treatment. The present study provides an insight on the bioflocs formation to enhance harvesting of Chlorella vulgaris and the removal of nutrients from seafood wastewater effluent. The results showed that the untreated seafood wastewater was the optimal culture medium for the cultivation and bioflocculation of C. vulgaris, with the flocculating activity of 92.0 ± 6.0%, total suspended solids removal of 93.0 ± 5.5%, and nutrient removal of 88.0 ± 2.2%. The bioflocs collected under this optimal condition contained dry matter of 107.2 ± 5.6 g·L^-1 and chlorophyll content of 25.5 ± 0.2 mg·L^-1. The results were promising when compared to those obtained from the auto-flocculation process that induced by the addition of calcium chloride and pH adjustment. Additionally, bacteria present in the wastewater aided to promote the formation of bioflocculation process.

Advanced membrane bioreactors systems: New materials and hybrid process design

Membrane bioreactor (MBR) is deemed as one of the most powerful technologies for efficient municipal and industrial wastewater treatment around the world. However, low microbial activity of activated sludge and serious membrane fouling still remain big challenges in worldwide application of MBR technology. Nowadays, more and more progresses on the research and development of advanced MBR with new materials and hybrid process are just on the way. In this paper, an overview on the perspective of high efficient strains applied into MBR for biological activity enhancement and fouling reduction is provided first. Secondly, as emerging fouling control strategy, design and fabrication of novel anti-fouling composited membranes are comprehensively highlighted. Meanwhile, hybrid MBR systems integrated with some novel dynamic membrane modules and/or with other technologies like advanced oxidation processes (AOPs) are introduced and compared. Finally, the challenges and opportunities of advanced MBRs combined with bioenergy production in wastewater treatment are discussed.

Harvesting of Microalgae by Flocculation

Due to increasing demands for microalgal biomass and products originating from microalgae, large-scale production systems are necessary. However, current microalgal production technologies are not cost-effective and are hindered by various bottlenecks, one of which is the harvesting of microalgal biomass. Cell separation is difficult because of the low sedimentation velocity of microalgae, their colloidal character with repelling negative surface charges, and low biomass concentrations in culture broths; therefore, large volumes need to be processed in order to concentrate the cells. Flocculation is considered to be one of the most suitable methods for harvesting microalgal biomass. This article provides an overview of flocculation methods suitable for microalgal harvesting, their mechanisms, advantages and drawbacks. Special attention is paid to the role of surface charge in the mechanism of flocculation. The novelty of the review lies in the interconnection between the context of technological applications and physico-chemical surface phenomena.

Nutrient recovery from wastewaters by microalgae and its potential application as bio-char

The intensive agricultural practices are increasing the demand for chemical fertilizers, being currently produced from a non-environmental friendly way. Besides the environmental impacts, the nutrient uptake efficiency by the crops is very low, representing huge losses into the fields. Therefore, it is crucial to study alternatives for the current chemical fertilizers, which simultaneous improve nutrient efficiency and minimize environmental impacts. A sustainable solution is to recover nutrients from wastewater streams with microalgal cultures and the biomass conversion into bio-char for soil amendment. Wastewaters are loaded with nitrogen and phosphorus and can be used as culture medium for microalgae. Thus, nutrients can be recycled, reducing the requirement of chemical fertilizers. This paper aims to review nutrient recovery from wastewater using microalgae and the biomass conversion into bio-char. This process promotes nutrient recycling and the bio-char (when added to soil) improves the nutrient uptake efficiency by crops.