Synergy between membrane filtration and flocculation for harvesting microalgae

The novel chitosan-amphoteric starch dual flocculants for enhanced removal of Microcystis aeruginosa and algal organic matter

A novel flocculation strategy for simultaneously removing Microcystis aeruginosa and algal organic matter (AOM) was proposed using chitosan-amphoteric starch (C-A) dual flocculants in an efficient, cost-effective and ecologically friendly way, providing new insights for harmful algal blooms (HABs) control. A dual-functional starch-based flocculant, amphoteric starch (AS) with high anion degree of substitution (DS_A) and cation degree of substitution (DS_C), was prepared using a cationic moiety of 3-chloro-2-hydroxypropyltrimethylammonium chloride (CTA) coupled with an anion moiety of chloroacetic acid onto the backbone of starch simultaneously. In combination of the results of FTIR, XPS, ¹H NMR, ¹³C NMR, GPC, EA, TGA and SEM, it was evidenced that the successfully synthesized AS with excellent structural characteristics contributed to the enhanced flocculation of M. aeruginosa. Furthermore, the novel C-A dual flocculants could achieve not only the removal of >99.3 % of M. aeruginosa, but also the efficacious flocculation of algal organic matter (AOM) at optimal concentration of (0.8:24) mg/L, within a wide pH range of 3-11. The analysis of zeta potential and cellular morphology revealed that the dual effects of both enhanced charge neutralization and notable netting-bridging played a vital role in efficient M. aeruginosa removal.

Comprehensive analysis of the combined flocculation and filtration process for microalgae harvesting at various operating parameters

The combined process of flocculation and filtration can improve algae harvesting performance by combining the benefits of both and overcoming the drawbacks. The entire process was thoroughly examined in this study, considering technical and economic feasibility under a variety of operating situations. Dead-end filtration was performed to evaluate the harvesting performance, the removal of extracellular organic matter and the changes of flocs. Cross-flow filtration was then carried out to explore the effect of operating parameters on permeate flux and assess the technical and economic feasibility. The optimum operating condition was to use 5 mg/L cationic polyacrylamide with 25 μm pore size and 0.1 m/s cross-flow velocity, under which a high harvesting efficiency of 95.2 %, a high average permeate flux of 55.5 m³/(m² h) and a volumetric reduction factor of 118.9 were achieved. Algal floc analysis revealed that flocs formed by ferric chloride and polyaluminium sulfate tended to partially deconstruct into smaller pieces during the filtration process. In contrast, flocs formed by cationic polyacrylamide tended to aggregate into bigger flocs, which, when paired with the effect of flocculant dosage and membrane pore size, could explain the difference in filtration performance and membrane permeance. No negative effect on downstream technology was observed for the combined process. A significantly lowered estimated total cost of 0.139 $/kg under optimum operating condition was obtained compared to filtration without flocculation assisted (0.206 $/kg).

Industry chain and challenges of microalgal food industry-a review

Currently, the whole world is facing hunger due to the increase in the global population and the rising level of food consumption. Unfortunately, the impact of environmental, climate, and political issues on agriculture has resulted in limited global food resources. Thus, it is important to develop new food sources that are environmentally friendly and not subject to climate or space limitations. Microalgae represent a potential source of nutrients and bioactive components for a wide range of high-value products. Advances in cultivation and genetic engineering techniques provide prospective approaches to widen their application for food. However, there are currently problems in the microalgae food industry in terms of assessing nutritional value, selecting processes for microalgae culture, obtaining suitable commercial strains of microalgae, etc. Additionally, the limitations of real data of market opportunities for microalgae make it difficult to assess their actual potential and to develop a better industrial chain. This review addresses the current status of the microalgae food industry, the process of commercializing microalgae food and breeding methods. Current research progress in addressing the limitations of microalgae industrialization and future prospects for developing microalgae food products are discussed.

A review on biodiesel production from microalgae: Influencing parameters and recent advanced technologies

Microalgae are the important part of carbon cycle in the nature, and they could utilize the carbon resource in water and soil efficiently. The abilities of microalgae to mitigate CO₂ emission and produce oil with a high productivity have been proven. Hence, this third-generation biodiesel should be popularized. This review firstly introduce the basic characteristics and application fields of microalgae. Then, the influencing parameters and recent advanced technologies for the microalgae biodiesel production have been discussed. In influencing parameters for biodiesel production section, the factors of microalgae cultivation, lipid accumulation, microalgae harvesting, and lipid extraction have been summarized. In recent advanced technologies for biodiesel production section, the microalgae cultivation systems, lipid induction technologies, microalgae harvesting technologies, and lipid extraction technologies have been reviewed. This review aims to provide useful information to help future development of efficient and commercially viable technology for microalgae-based biodiesel production.

Modelling of Harvesting Techniques for the Evaluation of the Density of Microalgae

A better estimation of the density of cells has great relevance in the design of harvesting units. In the case of microalgae, the density is a function of the internal composition, which in turn is affected by external environmental conditions. The density of microalgae is often regarded as a constant or a generic value is retrieved from literature. This study proposes a procedure to evaluate the density of Chlorococcum sp. with simple sedimentation and centrifugation experiments coupled with the population balance equation (PBE), which is solved numerically. The density of cells is not constant; instead, it is a function of the size of particles, which in turn changes with the cells' phase of their life cycle. The calculated cellular density ranged between 1000 and 1100 kg m^-3 in function of the cell size in both the sedimentation and centrifugation tests. The method can be extended to other microalgae species as well as to other types of cells.

Tunable particle/cell separation across aqueous two-phase system interface by electric pulse in microfluidics

HYPOTHESIS

Separations of particles and cells are indispensable in many microfluidic systems and have numerous applications in chemistry and biomedicine. The interface of aqueous two-phase system (ATPS) can act as a liquid filter. Under electric field stimuli, the selective transfer of targets across the liquid-liquid interface are expected for particles and cells separation.

EXPERIMENTS

The separations of particles and cells based on ATPS electrophoresis in a microfluidic chip were investigated. A systematical study of the mechanism of ATPS electrophoresis was performed first by employing polystyrene (PS) particles. Subsequently, the separations of particles and microalgae cells were demonstrated.

FINDINGS

The electrophoretic transfer of particles across the interface of ATPS is determined by multi-parameters, including the strength of electric pulse, particle size, zeta potential, and hydrophobicity of the particle. The continuous separations of particles/cells can be achieved through the controllable transfer of target particles/cells across the interface under electric pulses in a microfluidic chip. By simply turning the magnitude of the applied electric pulse, the technique is suitable for different purposes, for example, the separations of particles and cells, purification of cells, and viability identification of cells. This tunable separation approach opens opportunities in multidimensional particle and cell sorting for the fields of seed selection of microorganisms, environmental assessment, and biomedical research.

Membrane-Based Harvesting Processes for Microalgae and Their Valuable-Related Molecules: A Review

The interest in microalgae production deals with its role as the third generation of feedstock to recover renewable energy. Today, there is a need to analyze the ultimate research and advances in recovering the microalgae biomass from the culture medium. Therefore, this review brings the current research developments (over the last three years) in the field of harvesting microalgae using membrane-based technologies (including microfiltration, ultrafiltration and forward osmosis). Initially, the principles of membrane technologies are given to outline the main parameters influencing their operation. The main strategies adopted by the research community for the harvesting of microalgae using membranes are subsequently addressed, paying particular attention to the novel achievements made for improving filtration performance and alleviating fouling. Moreover, this contribution also gives an overview of the advantages of applying membrane technologies for the efficient extraction of the high added-value compounds in microalgae cells, such as lipids, proteins and carbohydrates, which together with the production of renewable biofuels could boost the development of more sustainable and cost-effective microalgae biorefineries.

Combining patterned membrane filtration and flocculation for economical microalgae harvesting

Membranes have a lot of potential for harvesting microalgae, but mainly membrane fouling and high harvesting costs linked to low fluxes are hampering their breakthrough. Patterned membranes can reduce fouling by enchancing local turbulences close to the membrane surface on one hand, and by increasing the active area per m² of installed membrane on the other. Flocculation can further increase membrane permeance by increasing microalgal partical size and reducing the fraction of free organic matter in the feed. In current study, the effect of polyethylene glycol (PEG) in the casting solution of patterned polysulfone membranes was investigated to better tune the performance of the patterned membranes, together with the effects of cross-flow velocity and chitosan dosage on membrane fouling. The energy consumption and total harvesting cost, extrapolated to a full-scale microalgal harvesting, were then estimated. The patterned membrane prepared with a 28w% PEG concentration showed the highest clean water permeance (900±22 L/m² h bar) and membrane permeance in a microalgal suspension (590±17 L/m² h bar). Patterned membranes showed a lower filtration resistance (15% permeance decline at the end of filtration) than flat membranes (72%) at a cross-flow velocity of 0.0025 m/s. Increasing cross-flow velocity could increase membrane permeance in most cases. The highest stable membrane permeance (110±17 L/m² h bar) and the lowest filtration resistance were achieved when combining patterned membrane filtration with flocculation at optimized chitosan dosage. A very low energy consumption (0.28 kWh/kg) and harvesting cost (0.16 €/kg) were achieved under these conditions.

Impacts of applied voltage on forward osmosis process harvesting microalgae: Filtration behaviors and lipid extraction efficiency

Microalgae are promising source of biofuels, while harvesting process is the obstacle for the further development. Herein, a treatment system that combined electrochemical process with forward osmosis (FO) membrane filtration process was developed to achieve microalgae harvesting. The conductive FO membranes were used as both electrode materials and basic separation system. With -5 V electric field being applied, 57.6% of reduction in water flux loss was observed, while microalgae recovery efficiency increased by 17.3%. The lipid content also increased to nearly 38%. Meanwhile, the inevitable reverse diffusion of solutes in the FO process and the concentration process of the microalgae solution increased the salinity of the microalgae solution, which is generally regarded as an obstacle for the application of FO. However, in the electrically-assisted FO system, it not only improved the efficiency of the electrochemical process, but also can increase the lipid content. The lipid extraction efficiency of the -5 V electric field increased from 17.7% and 28.5% to 20.4% and 31.1%, respectively, with one and two times extractions. The synergistic effect of the reverse diffusion of Cl^- and electrochemical process was conducive for the improvement of the lipid extraction efficiency, and is expected to reduce the energy consumption of the lipid extraction process.

Simultaneous harvesting and extracellular polymeric substances extrusion of microalgae using surfactant: Promoting surfactant-assisted flocculation through pH adjustment

In this research, the use of four different types of surfactants on biomass harvesting and extracellular polymeric substances (EPS) extrusion of Chlorella sorokiniana sp was investigated. The synergy between cationic surfactants and pH was tested to improve flocculation efficiency through the combined mechanism of charge neutralization, bridging and sweeping. Zeta potential and microscopic images were used to gain mechanistic understanding. The harvesting efficacy correlated positively with the biomass zeta potential and the surfactants alkyl-chain length; i.e., CTAB (88%) > DTAB (66%) > triton X-100 (41%) > SDS (11%). When the pH increased from 8 to 12, the harvesting efficiency was improved 12% and 39% for CTAB and DTAB, respectively. More interestingly, pH adjustment dramatically reduced the optimal dosages of CTAB and DTAB from 400 to 50 and 1000 to 300 mg/L, respectively. All selected surfactants could successfully release high value components of EPS such as protein and polysaccharide.

Harvesting microalgal biomass using negatively charged polysulfone patterned membranes: Influence of pattern shapes and mechanism of fouling mitigation

Membranes have a lot of potential for harvesting microalgae, but membrane fouling is hampering their breakthrough. In this study, the effects of charge and corrugated surface on membrane filtration performance were investigated. The clean water permeance (CWP), the microalgae harvesting efficiency and the membrane flux for a microalgal broth were determined using patterned polysulfone (PSf) membranes with different shapes of the surface patterns and containing different charge densities by blending sulfonated polysulfone (sPSf). The flow behavior near the patterned membrane surface, as well as the interaction energy between membrane and microalgae were investigated using computational fluid dynamics (CFD) simulation and the improved extended "Derjaguin, Landau, Verwey, Overbeek" (XDLVO) theory, respectively. Membrane charge and pattern shape significantly improve the membrane performance. The critical pressures of all sPSf blend patterned membranes were higher than 2.5 bar. A 4.5w% sPSf blend patterned membranes with wave patterns showed the highest CWP (2300 L/m² h bar) and membrane flux in the microalgal broth (1000 L/m² h bar) with 100% harvesting efficiency. XDLVO analysis showed that sPSf blend patterned membranes prepared obtained the lowest interaction energy and highest energy barrier for microalgal attachment. CFD simulation showed a higher velocity and wall shear on the pattern apexes.