Imogen Foubert

Flocculation as a low-cost method for harvesting microalgae for bulk biomass production

The global demand for biomass for food, feed, biofuels, and chemical production is expected to increase in the coming decades. Microalgae are a promising new source of biomass that may complement agricultural crops. Production of microalgae has so far been limited to high-value applications. In order to realize large-scale production of microalgae biomass for low-value applications, new low-cost technologies are needed to produce and process microalgae. A major challenge lies in the harvesting of the microalgae, which requires the separation of a low amount of biomass consisting of small individual cells from a large volume of culture medium. Flocculation is seen as a promising low-cost harvesting method. Here, we overview the challenges and possible solutions for flocculating microalgae.

Flocculation of Chlorella vulgaris induced by high pH: Role of magnesium and calcium and practical implications

Microalgae hold great potential as a feedstock for biofuels or bulk protein or treatment of wastewater or flue gas. Realising these applications will require the development of a cost-efficient harvesting technology. Here, we explore the potential of flocculation induced by high pH for harvesting Chlorella vulgaris. Our results demonstrate that flocculation can be induced by increasing medium pH to 11. Although both calcium and magnesium precipitated when pH was increased, only magnesium (≥0.15 mM) proved to be essential to induce flocculation. The costs of four different bases (sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide and sodium carbonate) were calculated and evaluated and the use of lime appeared to be the most cost-efficient. Flocculation induced by high pH is therefore a potentially useful method to preconcentrate freshwater microalgal biomass during harvesting.

Evaluation of electro-coagulation-flocculation for harvesting marine and freshwater microalgae.

Although microalgae are considered as a promising feedstock for biofuels, the energy efficiency of the production process needs to be significantly improved. Due to their small size and low concentration in the culture medium, cost‐efficient harvesting of microalgae is a major challenge. In this study, the use of electro‐coagulation–flocculation (ECF) as a method for harvesting a freshwater (Chlorella vulgaris) and a marine (Phaeodactylum tricornutum) microalgal species is evaluated. ECF was shown to be more efficient using an aluminum anode than using an iron anode. Furthermore, it could be concluded that the efficiency of the ECF process can be substantially improved by reducing the initial pH and by increasing the turbulence in the microalgal suspension. Although higher current densities resulted in a more rapid flocculation of the microalgal suspension, power consumption, expressed per kg of microalgae harvested, and release of aluminum were lower when a lower current density was used. The aluminum content of the harvested microalgal biomass was less than 1% while the aluminum concentration in the process water was below 2 mg L−1. Under optimal conditions, power consumption of the ECF process was around 2 kWh kg−1 of microalgal biomass harvested for Chlorella vulgaris and ca. 0.3 kWh kg−1 for Phaeodactylum tricornutum. Compared to centrifugation, ECF is thus more energy efficient. Because of the lower power consumption of ECF in seawater, ECF is a particularly attractive method for harvesting marine microalgae. Biotechnol. Bioeng. 2011;108: 2320–2329.

Harvesting microalgal biomass using submerged microfiltration membranes.

This study was performed to investigate the applicability of submerged microfiltration as a first step of up-concentration for harvesting both a freshwater green algae species Chlorella vulgaris and a marine diatom Phaeodactylum tricornutum using three lab-made membranes with different porosity. The filtration performance was assessed by conducting the improved flux step method (IFM) and batch up-concentration filtrations. The fouling autopsy of the membranes was performed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and Fourier transform infrared spectroscopy (FTIR). The cost analysis was estimated based on the data of a related full-scale submerged membrane bioreactor (MBR). Overall results suggest that submerged microfiltration for algal harvesting is economically feasible. The IFM results indicate a low degree of fouling, comparable to the one obtained for a submerged MBR. By combining the submerged microfiltration with centrifugation to reach a final concentration of 22% w/v, the energy consumption to dewater C. vulgaris and P. tricornutum is 0.84 kW h/m(3) and 0.91 kW h/m(3), respectively.

Antioxidant potential of microalgae in relation to their phenolic and carotenoid content

In the past decades, food scientists have been searching for natural alternatives to replace synthetic antioxidants. In order to evaluate the potential of microalgae as new source of safe antioxidants, 32 microalgal biomass samples were screened for their antioxidant capacity using three antioxidant assays, and both total phenolic content and carotenoid content were measured. Microalgae were extracted using a one-step extraction with ethanol/water, and alternatively, a three-step fractionation procedure using successively hexane, ethyl acetate, and water. Antioxidant activity of the extracts varied strongly between species and further depended on growth conditions and the solvent used for extraction. It was found that industrially cultivated samples of Tetraselmis suecica, Botryococcus braunii, Neochloris oleoabundans, Isochrysis sp., Chlorella vulgaris, and Phaeodactylum tricornutum possessed the highest antioxidant capacities in this study and thus could be a potential new source of natural antioxidants. The results from the different types of extracts clearly indicated that next to the well-studied carotenoids, phenolic compounds also contribute significantly to the antioxidant capacity of microalgae.

Influence of organic matter generated by Chlorella vulgaris on five different modes of flocculation

Microalgae excrete relatively large amounts of algal organic matter (AOM) that may interfere with flocculation. The influence of AOM on flocculation of Chlorella vulgaris was studied using five different flocculation methods: aluminum sulfate, chitosan, cationic starch, pH-induced flocculation and electro-coagulation-flocculation (ECF). The presence of AOM was found to inhibit flocculation for all flocculation methods resulting in an increase of dosage demand. For pH-induced flocculation, the dosage required to achieve 85% flocculation increased only 2-fold when AOM was present, while for chitosan, this dosage increased 9-fold. For alum, ECF and cationic starch flocculation, the dosage increased 5-6-fold. Interference by AOM is an important parameter to consider in the assessment of flocculation-based harvesting of microalgae.

Dynamic mathematical model of the crystallization kinetics of fats

A new model able to describe the kinetics of isothermal crystallization is presented: it is a model written in the form of a differential equation allowing use under dynamic temperature variations. It describes the crystallization process as if it is a reversible reaction with a first order forward reaction and a reverse reaction of order n. The model has the advantage of having an analytical solution under isothermal conditions that facilitates parameter estimation. The quality of this model was compared with the more traditional Avrami (with and without induction time) and Gompertz models using different model selection criteria. To show the universality of the model, different fat samples, different crystallization temperatures and different measuring techniques were used for model evaluation. The new model was selected as the best for the majority of the samples and this independent of the model selection criterion used.

Harvesting microalgal biomass using a magnetically induced membrane vibration (MMV) system: filtration performance and energy consumption.

This study was performed to investigate the effectiveness of submerged microfiltration to harvest both a marine diatom Phaeodactylum tricornutum and a Chlorella vulgaris in a recently developed magnetically induced membrane vibrating (MMV) system. We assess the filtration performance by conducting the improved flux step method (IFM), fed-batch concentration filtrations and membrane fouling autopsy using two lab-made membranes with different porosity. The full-scale energy consumption was also estimated. Overall results suggest that the MMV offers a good fouling control and the process was proven to be economically attractive. By combining the membrane filtration (15× concentration) with centrifugation to reach a final concentration of 25% w/v, the energy consumption to harvest P. tricornutum and C. vulgaris was, respectively, as low as 0.84 and 0.77kWh/m(3), corresponding to 1.46 and 1.39 kWh/kg of the harvested biomass.

Modelling of the crystallization kinetics of fats

Abstract Experimental techniques used to obtain crystallization data are compared. Furthermore, the crystallization models used by different authors are presented in relation to their background. Compared to the mostly used Avrami model, the fit of the Gompertz model seems to be better. The fit of a new model developed at the authors‘ laboratory is even better for the majority of the samples. The applied parameter estimation methods are also reviewed. Special attention is given to the problems with linearizing the Avrami model. Finally, some future trends are highlighted.

Influence of Drying and Storage on Lipid and Carotenoid Stability of the Microalga Phaeodactylum tricornutum

The influence of short-term storage and spray- and freeze-drying of fresh microalgal paste on the stability of lipids and carotenoids of Phaeodactylum tricornutum was investigated. Furthermore, the effects of storage time (14 and 35 days) and condition (vacuum packed vs non vacuum packed, -20 °C vs 4 °C vs 20 °C) after spray- and freeze-drying were studied. Total lipid content, free fatty acid content, carotenoid content and degree of lipid oxidation were measured. No effects of spray- and freeze-drying and subsequent storage were found on total lipid content, except for short-term storage of the fresh microalgal paste, which led to pronounced lipolysis and therefore a lower total lipid content. Freeze-dried microalgae were found to be more susceptible to lipolysis upon storage than spray-dried microalgae. On the other hand, spray-dried microalgae were more susceptible to oxidation than freeze-dried microalgae, possibly due to breakdown of protecting carotenoids upon spray-drying. Hardly any effect of storage condition was observed for any of the parameters tested.