A continuous flocculants-free electrolytic flotation system for microalgae harvesting

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

Microalgae are emerging as a promising source for augmenting the supply of essential products to meet global demands in an environmentally sustainable manner. Despite the potential benefits of microalgae in industry, the high energy consumption for harvesting remains a significant obstacle. This review offers a comprehensive overview of microalgae harvesting technologies and their industrial applications, with particular emphasis on the latest advances in flocculation techniques. These cutting-edge methods have been applied to biodiesel production, food and nutraceutical processing, and wastewater treatment. Large-scale harvesting is still severely impeded by the high cost despite progress has been made in laboratory studies. In the future, cost-effective microalgal harvesting will rely on efficient resource utilization, including the use of waste materials and the reuse of media and flocculants. Additionally, precise regulation of biological metabolism will be necessary to overcome algal species-related limitations through the development of extracellular polymeric substance-induced flocculation technology.

Lead-imprinted polyvinylidene fluoride membrane for selective removal of lead from contaminated water: material fabrication, filtration application, and mechanism study

The drinking water has become contaminated with lead in many countries across the world. In this study, a novel lead-imprinted polyvinylidene fluoride (PVDF) membrane was successfully fabricated for selective decontamination of lead from water. First of all, the membrane fabrication process was explored and optimized. The physical and chemical properties were then studied for a better understanding of the features of the membrane. The performance of lead removal by the adsorptive membrane was evaluated by systematic batch adsorption experiments, including pH effect, kinetics, isotherm, selectivity, and regeneration studies. The results indicated that the adsorptive membrane showed a high adsorption capacity of 40.59 mg Pb/g at the optimal pH of 5.5, fast kinetics of 2 h, high selectivity towards lead, and outstanding regeneration performance. The Langmuir equation fitted the isotherm better than the Freundlich equation, while the pseudo-second-order model and pore diffusion model well described the kinetics. The adsorptive membrane showed high selectivity towards lead in the lead/zinc binary solution. In the continuous filtration study, a small piece of adsorptive membrane could treat 3.75 L of lead solution. The XPS studies revealed that the lead uptake was mainly due to the complex reaction between lead and carboxyl and hydroxyl in the membrane.

Harvesting Chlorella vulgaris by electro-flotation with stainless steel cathode and non-sacrificial anode

As a promising method for efficiently harvesting microalgae, electro-flotation's performance is related to various factors including electrode design and process operating parameters. In this paper, bubble generation behavior on stainless-steel cathodes, with wire diameters of 0.8 mm, 0.2 mm and 0.05 mm, was studied. The results show that the bubble size increased with the increasing diameter of the electrode wire. Over 90 % harvesting efficiency was achieved using non-sacrificial anode. Extracellular polymeric substance is the main reason keeping bubbles from bursting.

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.

The Biological Performance of a Novel Electrokinetic-Assisted Membrane Photobioreactor (EK-MPBR) for Wastewater Treatment

Developing an effective phycoremediation system, especially by utilizing microalgae, could provide a valuable approach in wastewater treatment for simultaneous nutrient removal and biomass generation, which would help control environmental pollution. This research aims to study the impact of low-voltage direct current (DC) application on Chlorella vulgaris properties and the removal efficiency of nutrients (N and P) in a novel electrokinetic-assisted membrane photobioreactor (EK-MPBR) in treating synthetic municipal wastewater. Two membrane photobioreactors ran in parallel for 49 days with and without an applied electric field (current density: 0.261 A/m2). Mixed liquid suspended soils (MLSS) concentration, chemical oxygen demand (COD), floc morphology, total phosphorus (TP), and total nitrogen (TN) removals were measured during the experiments. The results showed that EK-MPBR achieved biomass production comparable to the control MPBR. In EK-MPBR, an over 97% reduction in phosphate concentration was achieved compared to 41% removal in the control MPBR. The control MPBR outperformed the nitrogen removal of EK-MPBR (68% compared to 43% removal). Induced DC electric field led to lower pH, lower zeta potential, and smaller particle sizes in the EK-MPBR as compared with MPBR. The results of this novel study investigating the incorporation of Chlorella vulgar is in an electrokinetic-assisted membrane photobioreactor indicate that this is a promising technology for wastewater treatment.

Continuous electrocoagulation of Chlorella vulgaris in a novel channel-flow reactor: A pilot-scale harvesting study

The most frequently used method to harvest microalgae on an industrial scale is centrifugation, although this has very high energy costs. To reduce these costs, a continuous electrocoagulation process for harvesting Chlorella vulgaris was developed and tested using a pilot-scale 111 L working volume device consisting of an electrolyser with iron electrodes, aggregation channel and lamellar settler. The flow rate of the microalgal suspension through the device was 240 L/h. When using controlled cultivation and subsequent electrocoagulation, a high harvesting efficiency (above 85%), a low Fe contamination in the harvested biomass (<4 mg Fe/g dry biomass, a harvested biomass complied with legislative requirements for food) and significant energy savings were achieved. When comparing electrocoagulation and subsequent centrifugation with the use of centrifugation alone, energy savings were 80 % for a biomass harvesting concentration of 0.23 g/L. Electrocoagulation was thus proven to be a feasible pre-concentration method for harvesting microalgae.

Recent progress in flocculation, dewatering, and drying technologies for microalgae utilization: Scalable and low-cost harvesting process development

Microalgal research has made significant progress in terms of the high-value-added industrial application of microalgal biomass and its derivatives. However, cost-effective techniques for producing, harvesting, and processing microalgal biomass on a large scale still need to be fully explored in order to optimize their performance and achieve commercial robustness. In particular, technologies for harvesting microalgae are critical in the practical process as they require excessive energy and equipment costs. This review focuses on microalgal flocculation, dewatering, and drying techniques and specifically covers the traditional approaches and recent technological progress in harvesting microalgal biomass. Several aspects, including the characteristics of the target microalgae and the type of final value-added products, must be considered when selecting the appropriate harvesting technique. Furthermore, considerable aspects and possible future directions in flocculation, dewatering, and drying steps are proposed to develop scalable and low-cost microalgal harvesting systems.

Electrocoagulation reduces harvesting costs for microalgae

Centrifugation is the most commonly used method for harvesting autotrophically produced microalgae, but it is expensive due to high energy demands. With the aim of reducing these costs, we tested electrocoagulation with iron electrodes for harvesting Chlorella vulgaris. During extensive lab-scale experiments, the following factors were studied to achieve a high harvesting efficiency and a low iron content in the harvested biomass: electric charge, initial biomass concentration, pH, temperature, agitation intensity, residual salt content and electrolysis time. A harvesting efficiency greater than 95% was achieved over a broad range of conditions and the residual iron content in the biomass complied with legislative requirements for food. Using electrocoagulation as the pre-concentration step prior to centrifugation, total energy costs were reduced to 0.136 kWh/kg of dry biomass, which is less than 14% of that for centrifugation alone. Our data show that electrocoagulation is a suitable and cost-effective method for harvesting microalgae.

Microalgal Biomass Generation via Electroflotation: A Cost-Effective Dewatering Technology

Microalgae are an excellent source of bioactive compounds for the production of a wide range of vital consumer products in the biofuel, pharmaceutical, food, cosmetics, and agricultural industries, in addition to huge upstream benefits relating to carbon dioxide biosequestration and wastewater treatment. However, energy-efficient, cost-effective, and scalable microalgal technologies for commercial-scale applications are limited, and this has significantly impacted the full-scale implementation of microalgal biosystems for bioproduct development, phycoremediation, and biorefinery applications. Microalgae culture dewatering continues to be a major challenge to large-scale biomass generation, and this is primarily due to the low cell densities of microalgal cultures and the small hydrodynamic size of microalgal cells. With such biophysical characteristics, energy-intensive solid–liquid separation processes such as centrifugation and filtration are generally used for continuous generation of biomass in large-scale settings, making dewatering a major contributor to the microalgae bioprocess economics. This article analyzes the potential of electroflotation as a cost-effective dewatering process that can be integrated into microalgae bioprocesses for continuous biomass production. Electroflotation hinges on the generation of fine bubbles at the surface of an electrode system to entrain microalgal particulates to the surface. A modification of electroflotation, which combines electrocoagulation to catalyze the coalescence of microalgae cells before gaseous entrainment, is also discussed. A technoeconomic appraisal of the prospects of electroflotation compared with other dewatering technologies is presented.

Microwave assisted flocculation for harvesting of Chlorella vulgaris

Microalgae harvesting is a major hindrance for the development of the microalgae industry. In this paper, short microwave treatment was used to assist the flocculation of Chlorella vulgaris with three flocculants, Fe³⁺ (FeCl₃), chitosan, and Ca²⁺ (CaCl₂). A microwave irradiation time of 20 s, and a pH of 10 was found to be the optimum condition. The harvesting efficiency could be significantly increased by 43.2%, 49.5% and 39.6%, respectively for Fe³⁺, chitosan, and Ca²⁺ assisted by microwave under these conditions. Microwave treatment did not cause any damage to the algal cells, and had no obvious influence on the lipid extraction. Microwave treatment decreased the concentration of the flocculants in culture medium after flocculation; this treatment enabled the reuse of the supernatant. This study provides a new and promising method of improving the flocculation efficiency for microalgae harvesting, by using microwave energy.

A review on microalgae cultivation and harvesting, and their biomass extraction processing using ionic liquids

The richness of high-value bio-compounds derived from microalgae has made microalgae a promising and sustainable source of useful product. The present work starts with a review on the usage of open pond and photobioreactor in culturing various microalgae strains, followed by an in-depth evaluation on the common harvesting techniques used to collect microalgae from culture medium. The harvesting methods discussed include filtration, centrifugation, flocculation, and flotation. Additionally, the advanced extraction technologies using ionic liquids as extractive solvents applied to extract high-value bio-compounds such as lipids, carbohydrates, proteins, and other bioactive compounds from microalgae biomass are summarized and discussed. However, more work needs to be done to fully utilize the potential of microalgae biomass for the application in large-scale production of biofuels, food additives, and nutritive supplements.

Critical processes and variables in microalgae biomass production coupled with bioremediation of nutrients and CO2 from livestock farms: A review

Development of renewable and clean energy as well as bio-based fine chemicals technologies are the keys to overcome the problems such as fossil depletion, global warming, and environment pollution. To date, cultivation of microalgae using wastewater is regarded as a promising approach for simultaneous nutrients bioremediation and biofuels production due to their high photosynthesis efficiency and environmental benefits. However, the efficiency of nutrients removal and biomass production strongly depends on wastewater properties and microalgae species. Moreover, the high production cost is still the largest limitation to the commercialization of microalgae biofuels. In this review paper, the state-of-the-art algae species employed in livestock farm wastes have been summarized. Further, microalgae cultivation systems and impact factors in livestock wastewater to microalgae growth have been thoroughly discussed. In addition, technologies reported for microalgal biomass harvesting and CO₂ mass transfer enhancement in the coupling process were presented and discussed. Finally, this article discusses the potential benefits and challenges of coupling nutrient bioremediation, CO₂ capture, and microalgal production. Possible engineering measures for cost-effective nutrients removal, carbon fixation, microalgal biofuels and bioproducts production are also proposed.

A novel algal biofilm photobioreactor for efficient hog manure wastewater utilization and treatment

This study developed a high-efficiency algal biofilm photobioreactor for microalgae by designing a special array of curtain membrane components. This paper also discusses the growth and nutrient composition of Chlorella vulgaris, and hog manure wastewater utilization and purification. It was found that after about 5 days of culture, the biomass of C. vulgaris on the membrane could reach as high as 7.37 g/m2 and the algae were easily harvested by mechanical scraping. The lipid content of C. vulgaris on the membrane structure was 10.17% while the lipid content of the algae in suspension was 14.29%. The light intensity showed a significant effect on the fatty acid composition. The C. vulgaris grew very well, and achieved deep purification of the hog manure wastewater; the COD, TP, TN, and NH4+-N removal rates reached 95.67%, 64.40%, 69.55%, and 91.24%, respectively.

Edible fungi-assisted harvesting system for efficient microalgae bio-flocculation

Conventional flocculants, commonly used to improve harvesting efficiency, can contaminate the broth and cause microalgae not suitable for food or feed production. In the present study, Pleurotus ostreatus, an edible fungal strain, was developed to improve the harvesting efficiency of microalgae. The results show that Pleurotus ostreatus pellets cultured under 100 rpm agitation resulted in higher harvesting efficiency than pellets cultured under 0 rpm and 150 rpm agitation. Lower pH of the Chlorella sp. suspension resulted in higher harvesting efficiency. The maximum recovery efficiency reached 64.86% in 150 mins. The above process could be used to achieve low cost, flocculant-free harvesting of microalgae as feedstock for feed or food production.

The kinetics of the polyacrylic superabsorbent polymers swelling in microalgae suspension to concentrate cells density

Different from current harvesting methods, the aim of this study was to concentrate microalgae by removing the medium with polyacrylic superabsorbent polymers (PSAPs). This method can concentrate freshwater microalgae Chlorella sp. at a relatively high biomass concentration of 90.23 g L^-1. Without further dewatering, the concentrated microalgae can be directly used to produce biofuels by oil extraction or fermentation. The kinetic characteristics of PSAPs swelling in different solutions were investigated. The results indicate that the negative influence on absorbency caused by ionic strength was greater than microalgae concentration. Compared with the diffusion part, water absorbed by the relaxation of PSAPs was dominant and accounted for over 97%. Equilibrium absorbed water equations under different microalgae concentration were fitted and could provide guide to quantifiably concentrate microalgae. Increasing temperature decreased the absorbency of PSAPs, while, the absorption and desorption rate were increased. Moreover, the absorbency remained at 91.45% after recycling three times.