Microalgae dewatering based on forward osmosis employing proton exchange membrane

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.

Factors Affecting the Performance of Membrane Osmotic Processes for Bioenergy Development

Forward osmosis (FO) and pressure-retarded osmosis (PRO) have gained attention recently as potential processes to solve water and energy scarcity problems with advantages over pressure-driven membrane processes. These processes can be designed to produce bioenergy and clean water at the same time (i.e., wastewater treatment with power generation). Despite having significant technological advancement, these bioenergy processes are yet to be implemented in full scale and commercialized due to its relatively low performance. Hence, massive and extensive research has been carried out to evaluate the variables in FO and PRO processes such as osmotic membrane, feed solutions, draw solutions, and operating conditions in order to maximize the outcomes, which include water flux and power density. However, these research findings have not been summarized and properly reviewed. The key parts of this review are to discuss the factors influencing the performance of FO and PRO with respective resulting effects and to determine the research gaps in their optimization with the aim of further improving these bioenergy processes and commercializing them in various industrial applications.

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.

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.

Investigation of reverse ionic diffusion in forward-osmosis-aided dewatering of microalgae: A molecular dynamics study

This study aimed to investigate the transport mechanisms of ions during forward-osmosis-driven (FO-driven) dewatering of microalgae using molecular dynamics (MD) simulations. The dynamical and structural properties of ions in FO systems of varying NaCl or MgCl₂ draw solution (DS) concentrations were calculated and correlated. Results indicate that FO systems with higher DS concentration caused ions to have lower hydration numbers and higher coordination numbers leading to lower diffusion coefficients. The higher hydration number of Mg²⁺ ions resulted in significantly lower ionic permeability as compared to Na⁺ ions at all concentrations (p = 0.002). The simulations also revealed that higher DS concentrations led to higher accumulation of ions in the membrane. This study provides insights on the proper selection of DS for FO systems.

Optimisation of Tray Drier Microalgae Dewatering Techniques Using Response Surface Methodology

The feasibility of the application of a tray drier in dewatering microalgae was investigated. Response surface methodology (RSM) based on Central Composite Design (CCD) was used to evaluate and optimise the effect of air temperature and air velocity as independent variables on the dewatering efficiency as a response function. The significance of independent variables and their interactions was tested by means of analysis of variance (ANOVA) with a 95% confidence level. Results indicate that the air supply temperature was the main parameter affecting dewatering efficiency, while air velocity had a slight effect on the process. The optimum operating conditions to achieve maximum dewatering were determined: air velocities and temperatures ranged between 4 to 10 m/s and 40 to 56 °C respectively. An optimised dewatering efficiency of 92.83% was achieved at air an velocity of 4 m/s and air temperature of 48 °C. Energy used per 1 kg of dry algae was 0.34 kWh.

Recent developments and key barriers to advanced biofuels: A short review

Biofuels are regarded as one of the most viable options for reduction of CO₂ emissions in the transport sector. However, conventional plant-based biofuels (e.g., biodiesel, bioethanol)'s share of total transportation-fuel consumption in 2016 was very low, about 4%, due to several major limitations including shortage of raw materials, low CO₂ mitigation effect, blending wall, and poor cost competitiveness. Advanced biofuels such as drop-in, microalgal, and electro biofuels, especially from inedible biomass, are considered to be a promising solution to the problem of how to cope with the growing biofuel demand. In this paper, recent developments in oxy-free hydrocarbon conversion via catalytic deoxygenation reactions, the selection of and lipid-content enhancement of oleaginous microalgae, electrochemical biofuel conversion, and the diversification of valuable products from biomass and intermediates are reviewed. The challenges and prospects for future development of eco-friendly and economically advanced biofuel production processes also are outlined herein.

Species variation in the effects of dewatering treatment on macroalgae

Seaweeds can be a valuable resource for biorefinery and biotechnology applications, but their high water content is a recurrent problem and one of the key bottlenecks for their sustainable use. Treatments to increase dry matter content of the kelp Laminaria digitata were recently described by the authors. However macroalgae are an extremely diverse group of organisms and compositional variation between species may influence the effects of particular treatments. In this study, potential dewatering treatments including drying, osmotic media, and the application of both organic and mineral acids all followed by screw-pressing have been tested on two other species of kelp (Laminaria hyperborea and Saccharina latissima) and a red seaweed (Palmaria palmata). Conditions that dewatered these species were identified and the data have been combined with the previous results for L. digitata. There were significant differences between species across all the traits of interest. However dewatering was highly dependent on specific interactions with both treatment and season of collection. Nevertheless, the dry matter content of brown seaweeds was widely and successfully increased by air drying or acid treatment followed by screw-pressing. The results for P. palmata were quite different, particularly with regard to juice production. For this species, acid treatment did not result in dewatering, but dry matter content could be increased by screw-pressing immediately after harvest. Together the data presented here demonstrate that dewatering pre-treatments need to be specific for the type of seaweed to be processed; important knowledge for the future use of this sustainable biomass resource.