Flotation Harvesting Research Articles

Application of microbubble air flotation to harvest Microcystis sp. from agriculture wastewater: The regulation and mechanisms.

The harvesting of microalgae is the main bottleneck of its large-scale biomass production, and seeking an efficient, green, and low-cost microalgae harvesting technology is one of the urgent problems to be solved. Microbubble air flotation has been proven to be an effective measure, but the mechanisms of microbubbles-algal cell attachment are still unclear. In this study, microbubble air flotation was used as a harvesting method for Microcystis cultured in agricultural wastewater. The process mechanism of microbubble air flotation harvesting microalgae in wastewater was fully revealed from three aspects (the design of bubble formation, the adhesion law, and the recovery rate of microalgae under different working conditions). The results show that the length of the release pipe is the main factor affecting the proportion of microbubbles with a particle size of less than 50 μm. In the process of adhesion, when the particle size of microbubbles is 0.6-1.7 times the size of Microcystis, the adhesion efficiency of microbubbles to Microcystis is the highest. Under the conditions of pressure 0.45 MPa, gas-liquid ratio 5%, and release pipe length 100 cm, the harvesting performance of Microcystis was the best. Microbubble air flotation has better harvesting performance (63.5%, collection rate) of Microcystis with higher density. By understanding the mechanism of microbubble flotation, the technical parameters of microbubble flotation for harvesting energy microalgae are optimized to provide support for the development of efficient and low-cost devices and equipment for collecting microalgae.

A foam column system harvesting freshwater algae for biodiesel production: An experiment and process model evaluations

The purpose of this study was to examine the application of the mathematical model of drift flux to the experimental results of the effect of cationic trimethyl-ammonium bromide (CTAB)-aided continuous foam flotation harvesting on the lipid content in Chlorella vulgaris microalgae. An experiment was conducted to determine the effect of the operating conditions on the enrichment factor (EF) and percentage recovery efficiency (%RE), where the flow rates at the inlet and bottom outlet remained constant. Data for the binary system (without algae) and ternary system (with algae) in an equal-area foam column show that the EF decreases linearly with increasing initial CTAB concentrations ranging from 30 to 75 mg/L for three levels of the studied air volumetric flow rate range (1–3) L/min. The percentage harvesting efficiency increased with increasing initial CTAB concentration and air volumetric flow rate to 96 % in the binary systems and 94 % in the ternary systems. However, in the foam column with the riser used in the three systems, a lower volume of liquid foam in the upward outlet stream resulted in a lower RE% than that of the column without the riser. The objective function of EF for the system with algae increased when the initial CTAB concentration was increased from 30 to 45 mg/L in the foam column with a riser for all air flow rates, and after 45 mg/L, a sudden drop in the microalgae EF was observed. In the comparison between the foam column with and without the riser for the system with algae, the optimum EF was 145 for the design of the column with the riser and 139 for the column without the riser.

Fundamental investigation of bio-surfactants-assisted harvesting strategy for microalgae

The lack of energy-sufficient harvesting technologies has dramatically impeded the commercialization of microalgae-derived bio-products. To address this challenge, a foam flotation harvesting approach using bio-surfactants has been previously developed for microalgae. However, most studies have applied chemical surfactants in the foam flotation, which is in opposition to the concept of sustainability. Thus, in this study, two bio-surfactants including rhamnolipid and Bovine Serum Albumin (BSA) were applied in foam flotation-assisted harvesting for Chlorella vulgaris. Initially, the ability of foam formation and foam stability in different microalgal culture media including distilled water, MiracleGro™, and Bold's Basal Medium (BBM) were evaluated. Following this, the microalgal cell adhesion mechanism was elucidated using FTIR analysis, zeta potential, and conductivity. The results showed that a higher degree of foam stability was observed when using BSA as the bio-surfactant than rhamnolipids. No big difference in the zeta potential was observed for BSA in all tested culture media, and the value of zeta potential was approx. 20 mV whereas, a difference was observed between rhamnolipid solution and rhamnolipid in culture media at all tested conditions. On the contrary, in the conductivity analysis, the use of rhamnolipids led to a higher conductivity at all investigated conditions compared to BSA. FTIR analysis suggested that the addition of either rhamnolipids or BSA did not affect the functionality. Overall, this fundamental study regarding the bio-surfactant-assisted foam flotation harvesting strategy for microalgae lays the foundation for process development.

Continuous foam flotation harvesting with enhanced drainage: Overcoming the recovery-enrichment paradox

Microalgae are promising sources of sustainable biofuels, however high harvesting and enriching costs hamper economic competitiveness. Foam flotation––a low-cost, low-energy adsorptive bubble separation technique––may bridge this gap, yet its efficiency and adoption is compromised by a perceived trade-off (based on batch operation) between two key performance indicators: recovery percentage (R%) and concentration factor (biomass enrichment ratio; CF). Previously, we challenged this ‘recovery-enrichment paradox’ under continuous operation (R% 95, CF 173), but performance fell short of expectation. Here, we present a redesign based on the concepts of process intensification, by incorporating a drainage riser, delivering concomitantly high R% (91) and CF (722), retaining low power consumption (0.052 kWh m−3 of algae culture) with high total suspended solids yield (14.6%). This compares very favorably with other dewatering/harvesting techniques, overcoming the ‘recovery-enrichment paradox’. Our significant intensification of the foam flotation process will accelerate the advent of economically sustainable microalgae derived biofuels.

Towards a better understanding of microalgae natural flocculation mechanisms to enhance flotation harvesting efficiency.

In microalgae harvesting, flocculation is usually a compulsory preliminary step to further separation by sedimentation or flotation. For some microalgae species, and under certain growth conditions, flocculation can occur naturally. Natural flocculation presents many advantages as it does not require the addition of any flocculants to the culture medium and shows high efficiency rate. But because natural flocculation is so specific to the species and conditions, and thanks to the knowledge accumulated over the last years on flocculation mechanisms, researchers have developed strategies to induce this natural harvesting. In this review, we first decipher at the molecular scale the underlying mechanisms of natural flocculation and illustrate them by selected studies from the literature. Then we describe the developed strategies to induce natural flocculation that include the use of biopolymers, chemically modified or not, or involve mixed species cultures. But all these strategies need the addition of external compounds or microorganism which can present some issues. Thus alternative directions to completely eliminate the need for an external molecule, through genetic engineering of microalgae strains, are presented and discussed in the third part of this review.

Optimization of thermal pre‐flocculation treatment for effective air flotation harvesting of microalgae

AbstractBACKGROUNDThe production of microalgal biomass as a potential biofuel source is limited by expensive harvesting costs. However, air flotation has been applied as an effective method for harvesting microalgal cells from diluted liquid suspensions. In this study, Chlorella vulgaris and Scenedesmus obliquus harvesting experiments were performed with thermal pretreatment prior to air flotation.RESULTSAn orthogonal test showed that the harvesting efficiency was affected by the heating temperature and pH but not by the microalgal concentration or stirring rate. Under a heating temperature of 90 °C and pH 3, the optimal heat‐aided air flotation efficiency achieved for harvesting C. vulgaris was 26.5%, with a concentration factor of 5.5. For harvesting S. obliquus, the optimal harvesting efficiency was 93.6% and the concentration factor was 25.0 under a heating temperature of 70 °C and pH 3.CONCLUSIONHigh temperature and low pH increased the zeta potential value of the microalgal cell surface, and high temperature promoted the secretion of extracellular organic matter, causing microalgal cells to gather and form flocs. The smaller flocs and higher zeta potential value of S. obliquus led to its higher harvesting efficiency. These conditions provided the possibility of waste acidic gas and thermal energy from industries replacing flocculants for the harvesting process. © 2019 Society of Chemical Industry

Gemini surfactant: A novel flotation collector for harvesting of microalgae by froth flotation

Froth flotation has been proved to be a promising approach for commercial scale harvesting of microalgae. However, all the surfactants used in the microalgae flotation harvesting process are conventional monomeric surfactants contain a single similar hydrophobic group in the molecule, which results in a low harvesting efficiency. In this work, a novel Gemini surfactant, N,N′-bis(cetyldimethyl)-1,4-butane diammonium dibromide (BCBD) was prepared, and originally recommended as a collector for froth flotation harvesting of Chlorella vulgaris from culture medium. The performance of BCBD was compared with the results acquired using its conventional monomeric surfactant cetyl trimethyl ammonium bromide (CTAB). The bench-scale flotation results showed that BCBD had excellent collecting power for Chlorella vulgaris. Achieving the obviously superior flotation harvesting performance (flotation recovery increased by 21.4% and enrichment ratio increased by 22.9), the dosage of Gemini type BCBD collector is five times less than that of monomeric CTAB collector.

Continuous harvesting of microalgae biomass using foam flotation

Biomass harvesting and dewatering are major operational costs that constrain the development and expansion of the industrial use of microalgae; particularly for low value biofuels. In this study we demonstrate, for the first time, a surfactant-aided foam flotation column that is designed and optimised for the continuous harvest of microalgae not only to improve the productivity of microalgae biomass but also as an attempt to overcome the trade-off between recovery efficiency and enrichment in batch and semi-batch foam flotation. The following operational parameters were optimised; surfactant concentration, air flow rate, feed flow rate, column height, liquid pool depth, and sparger type. Additionally, the effects of cell surface characteristics (hydrophobicity, zeta potential, and contact angle) were investigated on Chlorella vulgaris flotation performance. Hydrophobicity was enhanced using three surfactants; the cationic cetyltrimethylammonium bromide (CTAB), the anionic sodium dodecyl sulphate (SDS), and the non-ionic TWEEN®20; with CTAB producing the greatest enhancement. Surfactant concentration, column height, and air flow rate had the greatest effect on the algae concentration factor (CF) and recovery efficiency (RE). The optimised design (CTAB = 35 mg L−1, air flow rate = 1 L min−1, feed flow rate = 0.1 L min−1, column height = 146 cm, liquid pool depth = 25 cm, with a fine porous sparger) yielded RE of 95, 93, and 89% with 173, 271, and 143-fold biomass enrichments for freshwater C. vulgaris and marine Isochrysis galbana and Tetraselmis suecica microalgae respectively. Achieving high RE for freshwater and in the case of marine microalgae (irrespective of ionic strength) at moderate surfactant dosages, gives foam flotation the advantage of being a growth media independent harvesting process. The process had a very low power consumption (0.052 kWh m−3 of algae culture). Our findings demonstrate the potential for continuous, low cost, scalable flotation harvesting with particular relevance for the biofuels, water and wastewater treatment industries.

Buoy-bead flotation harvesting of the microalgae Chlorella vulgaris using surface-layered polymeric microspheres: A novel approach

To improve microalgae harvesting efficiency and to reduce the addition of chemicals in the buoy-bead flotation process, a novel buoy-bead flotation approach has been developed for harvesting Chlorella vulgaris, using surface-layered polymeric microspheres (SLPMs). Next, the detachment of microalgae cell-SLPM aggregates and the reusability of SLPMs were investigated. The experimental results showed that a maximum harvesting efficiency of 98.43% was achieved at a SLPM dosage of 0.7 g/L and a pH of 9, and harvesting efficiency quickly decreased with increasing ionic strength. A detachment efficiency of 78.46% and a concentration factor of 19.56 were achieved at an ionic strength of 700 mM and a mixing speed of 3000 rpm without changing the pH. Reused SLPMs can still reach an efficiency of 72.13% after five cycles. The presented results show that this method can potentially be applied for large-scale microalgae harvesting.

Towards a better understanding of the flocculation/flotation mechanism of the marine microalgae Phaeodactylum tricornutum under increased pH using atomic force microscopy

In the context of climate change, the interest for sustainable sources to produce energy is growing. One promising resource for biofuel production is microalgae, but their industrial use is limited by the lack of efficient harvesting techniques. In this study, we use a multi-scale approach to understand the magnesium hydroxide-mediated flocculation/flotation mechanism of Phaeodactylum tricornutum, an effective oil-producer diatom, under high pH. While flotation experiments give a population-scale quantification of the efficiency of flocculation/flotation using magnesium or calcium hydroxide, or at increased pH, AFM allows probing the mechanical properties of the cells at different pH values. Finally we develop an original strategy to functionalize AFM tips with hydroxide particles that we use in multiparametric imaging experiments to understand at the molecular scale the forces driving the adhesion of hydroxide particles to cells. Altogether, our results give a better understanding of the molecular mechanism underlying alkaline flocculation/flotation, paving the way towards the development of low-cost flocculant-free flotation harvesting processes.

Investigation on the role of surfactants in bubble-algae interaction in flotation harvesting of Chlorella vulgaris

In this work, a fundamental study was carried out on the role of surfactants in bubble-algae interaction to improve the understanding of how surfactants influence the flotation performance. Flotation tests for harvesting Chlorella vulgaris were first conducted using two surfactants, hexadecyltrimethyl ammonium bromide (C16TAB) and tea saponin. The effect of surfactants on harvesting efficiency was found to depend on their type and concentration. The present results also indicated that C16TAB exhibited higher harvesting efficiency than tea saponin. The adsorption experiments of surfactants onto C. vulgaris and the characterization measurements of algae surface were then carried out to reveal underlying interaction mechanisms between surfactants and algae in air flotation process. The results confirmed the adsorption process of surfactants onto C. vulgaris was feasible, spontaneous and endothermic. Subsequently, two mechanism models were proposed to qualitatively establish the interaction relationship among algae, surfactants and bubbles in the flotation. According to two models, C16TAB could neutralize the algal potential, while tea saponin converted algal surface from hydrophilic into hydrophobic. Overall, two surfactants used here could facilitate attachment of C. vulgaris onto bubbles, making the algae easier to be harvested, thereby increasing the flotation recovery.

Development of heat-aided flocculation for flotation harvesting of microalgae

Biofuel from microalgae has significant potential, but the economics of production are still not on par with fossil fuels. One stage of production commonly cited as requiring improvement is harvesting the microalgae from dilute culture solutions. Flotation has emerged as a promising harvesting method that avoids the high energy costs of methods such as centrifugation, but requires the addition of chemical flocculating agents that add to the cost and can contaminate the biomass. To address this, a novel derivative, heat-aided flocculation, is examined. By potentially using waste heat from industry, flotation can be achieved without addition of chemicals.

Heat-aided flocculation for flotation harvesting of microalgae

Microalgae biofuels are a promising renewable fuel that can be produced whilst mitigating industrial CO2. However, their production is still not economically comparable to fossil fuels, with microalgae harvesting recognized as an area requiring development. Flotation is emerging as a promising harvesting method, but typically requires addition of flocculating chemicals to allow a foam concentrate to be formed and collected. In addition to adding to process costs, these additives contaminate the biomass and limit applications, such as for animal feed, for the non-biofuel component. An alternative method to achieve efficient harvesting is proposed that could use waste industrial heat to aid the flotation process and avoids the need for chemical additives. The best separation was achieved at 85°C, when the Scenedesmus sp. culture was concentrated to 2.78g/L from an initial density of 0.13g/L. This result gives a concentration factor of 25.8 and a recovery of 83%.

Harvesting of Spirulina platensis by cellular flotation and growth stage determination

To investigate an effective harvesting method for Spirulina platensis. Eighty per cent of S. platensis cells in the logarithmic growth phase were harvested by flotation when the cells were set in a static condition for 2 h. The optimum harvesting time was about day 6 of cultivation. The flotation activity of S. platensis cells was enhanced by the addition of NaCl. The harvesting of S. platensis by flotation is a cost-effective and straightforward method that can retain the algal quality. The optimum harvesting time of S. platensis can be predicted by the cellular protein to carbon ratio. Flotation harvesting is also applicable to other cyanobacteria with gas vesicles.