Microalgae harvesting and subsequent biodiesel conversion

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).

Fe3O4-PEI Nanocomposites for Magnetic Harvesting of Chlorella vulgaris, Chlorella ellipsoidea, Microcystis aeruginosa, and Auxenochlorella protothecoides

Magnetic separation of microalgae using magnetite is a promising harvesting method as it is fast, reliable, low cost, energy-efficient, and environmentally friendly. In the present work, magnetic harvesting of three green algae (Chlorella vulgaris, Chlorella ellipsoidea, and Auxenochlorella protothecoides) and one cyanobacteria (Microcystis aeruginosa) has been studied. The biomass was flushed with clean air using a 0.22 μm filter and fed CO₂ for accelerated growth and faster reach of the exponential growth phase. The microalgae were harvested with magnetite nanoparticles. The nanoparticles were prepared by controlled co-precipitation of Fe²⁺ and Fe³⁺ cations in ammonia at room temperature. Subsequently, the prepared Fe₃O₄ nanoparticles were coated with polyethyleneimine (PEI). The prepared materials were characterized by high-resolution transmission electron microscopy, X-ray diffraction, magnetometry, and zeta potential measurements. The prepared nanomaterials were used for magnetic harvesting of microalgae. The highest harvesting efficiencies were found for PEI-coated Fe₃O₄. The efficiency was pH-dependent. Higher harvesting efficiencies, up to 99%, were obtained in acidic solutions. The results show that magnetic harvesting can be significantly enhanced by PEI coating, as it increases the positive electrical charge of the nanoparticles. Most importantly, the flocculants can be prepared at room temperature, thereby reducing the production costs.

Use of ultrasound for aiding lipid extraction and biodiesel production of microalgae harvested by chitosan

In this work, chitosan, a biodegradable flocculant, was investigated to determine its utility in flocculating microalgae, its effect on cell integrity, and its impact on lipid extraction and the conversion to fatty acid methyl ester (FAME). Results showed that chitosan adequately performed flocculation on Chlorella vulgaris microalgae and achieved a high harvesting efficiency of 96.35 ± 1.96% when implemented under the following conditions: chitosan dose = 120 mg/L^-1, pH = 5, mixing speed = 150 rpm for 20 min, followed by 10 min of settling time. Moreover, scanning electron microscope (SEM) combined with transmission electron microscope (TEM) demonstrated that chitosan protected the cells' structure from morphological damage. Finally, the highest lipid extraction yield and biodiesel production was obtained from the chitosan-harvested biomass when the microalgae were pretreated with ultrasound.

Insight on Extraction and Characterisation of Biopolymers as the Green Coagulants for Microalgae Harvesting

This review presents the extractions, characterisations, applications and economic analyses of natural coagulant in separating pollutants and microalgae from water medium, known as microalgae harvesting. The promising future of microalgae as a next-generation energy source is reviewed and the significant drawbacks of conventional microalgae harvesting using alum are evaluated. The performances of natural coagulant in microalgae harvesting are studied and proven to exceed the alum. In addition, the details of each processing stage in the extraction of natural coagulant (plant, microbial and animal) are comprehensively discussed with justifications. This information could contribute to future exploration of novel natural coagulants by providing description of optimised extraction steps for a number of natural coagulants. Besides, the characterisations of natural coagulants have garnered a great deal of attention, and the strategies to enhance the flocculating activity based on their characteristics are discussed. Several important characterisations have been tabulated in this review such as physical aspects, including surface morphology and surface charges; chemical aspects, including molecular weight, functional group and elemental properties; and thermal stability parameters including thermogravimetry analysis and differential scanning calorimetry. Furthermore, various applications of natural coagulant in the industries other than microalgae harvesting are revealed. The cost analysis of natural coagulant application in mass harvesting of microalgae is allowed to evaluate its feasibility towards commercialisation in the industrial. Last, the potentially new natural coagulants, which are yet to be exploited and applied, are listed as the additional information for future study.

A rapid inoculation method for microalgae biofilm cultivation based on microalgae-microalgae co-flocculation and zeta-potential adjustment

Due to the small size, similar density to water, cells inoculating onto the solid carrier is a major challenge for microalgae biofilm cultivation. To reduce biofilm inoculation time, A. falcatus with long stripe were chosen as the bond linking with the main microalgae cells forming microalgae-microalgae co-flocculation by bridging and twining. The optimal matching species were S. obliquus and A. falcatus with the volume ratio of 4-1. By changing the zeta-potential of the microalgae-microalgae co-flocculation to positive and negative through pH regulating, the inoculation time was significantly shorted from 4 h to 1.5 min due to the charge neutralization. Fortunately, the added A. falcatus and pH regulation has no negative effects on biofilm growth. Inversely, the porous microstructure of microalgae-microalgae co-flocculation improve the transfer efficiency of nutrients, resulting a 90.15% increase on biomass productivity (229.15 g m^-2) comparing to pure microalgae species.

Overcoming Microalgae Harvesting Barrier by Activated Algae Granules

The economic factor of the microalgae harvesting step acts as a barrier to scaling up microalgae-based technology designed for wastewater treatment. In view of that, this study presents an alternative microalgae-bacteria system, which is proposed for eliminating the economic obstacle. Instead of the microalgae-bacteria (activated algae) flocs, the study aimed to develop activated algae granules comprising the microalgae Chlorella sp. as a target species. The presence of the filamentous microalgae (Phormidium sp.) was necessary for the occurrence of the granulation processes. A progressive decrease in frequency of the free Chlorella sp. cells was achieved once with the development of the activated algae granules as a result of the target microalgae being captured in the dense and tangled network of filaments. The mature activated algae granules ranged between 600 and 2,000 µm, and were characterized by a compact structure and significant settling ability (21.6 ± 0.9 m/h). In relation to the main aim of this study, a microalgae recovery efficiency of higher than 99% was achieved only by fast sedimentation of the granules; this performance highlighted the viability of the granular activated algae system for sustaining a microalgae harvesting procedure with neither cost nor energy inputs.

Microalgae-microbial fuel cell: A mini review

Microalgae-microbial fuel cells (mMFCs) are a device that can convert solar energy to electrical energy via biological pathways. This mini-review lists new research and development works on microalgae processes, microbial fuel cell (MFC) processes, and their combined version, mMFC. The substantial improvement and technological advancement are highlighted, with a discussion on the challenges and prospects for possible commercialization of mMFC technologies.

Current progress and future prospect of microalgal biomass harvest using various flocculation technologies

Microalgae have been extensively studied for the production of various valuable products. Application of microalgae for the production of renewable energy has also received increasing attention in recent years. However, high cost of microalgal biomass harvesting is one of the bottlenecks for commercialization of microalgae-based industrial processes. Considering harvesting efficiency, operation economics and technological feasibility, flocculation is a superior method to harvest microalgae from mass culture. In this article, the latest progress of various microalgal cell harvesting methods via flocculation is reviewed with the emphasis on the current progress and prospect in environmentally friendly bio-based flocculation. Harvesting microalgae through bio-based flocculation is a promising component of the low-cost microalgal biomass production technology.

Comparison of harvesting methods for microalgae Chlorella sp. and its potential use as a biodiesel feedstock

Three methods for harvesting Chlorella sp. biomass were analysed in this paper--centrifugation, membrane microfiltration and coagulation: there was no significant difference between the total amount of biomass obtained by centrifugation and membrane microfiltration, i.e., 0.1174 +/- 0.0308 and 0.1145 +/- 0.0268 g, respectively. Almost the same total lipid content was obtained using both methods, i.e., 27.96 +/- 0.77 and 26.43 +/- 0.67% for centrifugation and microfiltration, respectively. However, harvesting by coagulation resulted in the lowest biomass and lipid content. Similar fatty acid profiles were obtained for all of the harvesting methods, indicating that the main components were palmitic acid (C16:0), oleic acid (C18:1) and linoleic acid (C18:2). However, the amounts of the individual fatty acids were higher for microfiltration than for centrifugation and coagulation; coagulation performed the most poorly in this regard by producing the smallest amount of fatty acids (41.61 +/- 6.49 mg/g dw). The harvesting method should also be selected based on the cost benefit and energy requirements. The membrane filtration method offers the advantages of currently decreasing capital costs, a high efficiency and low maintenance and energy requirements and is thus the most efficient method for microalgae harvesting.

Microalgae harvesting by flotation using natural saponin and chitosan

This study aims to investigate the harvesting of microalgae by dispersed air flotation (DiAF) using natural biosurfactant saponin as the collector and chitosan as the flocculant. Two types of microalgae, Chlorella vulgaris and Scenedesmus obliquus, were used in this study. It was observed that saponin was a good frother, but not an effective collector when used alone for flotation separation of algae. However, with the pre-flocculation of 5 mg/L of chitosan, separation efficiency of >93% microalgae cells was found at 20 mg/L of saponin. Removal efficiency of >54.4% and >73.0% was found for polysaccharide and protein, respectively at 20 mg/L of saponin and chitosan each. Experimental results show that DiAF using saponin and chitosan is effective for separation of microalgae, and algogenic organic matter (AOM). It can potentially be applied in the integrated microalgae-based biorefinery.