Design & Modelling Platform
The platform, developed by NTUA in collaboration with the D-Factory consortium, implements process modelling, synthesis and design, optimisation, flow-sheeting and techno-economic analysis.
Design & Modellingt platform
D-Factory partners have worked together to prepare a range of products based on biomass from Dunaliella salina.
For full details, consult the products sheet and the application note.
Process followed by Dunaliella salina at Monzón Biotech
The D-Factory partner Monzón Biotech has released a video on the process followed by Dunaliella salina algae in their facilities in Monzón, Huesca.
Watch the video
Biomass processing and products extraction
The key point in biorefineries is the processing of the sustainably-produced biomass into a spectrum of marketable products and bioenergy. The extraction process from the algal biomass in a biorefinery should allow recovery of a range of fats and oils, carbohydrates, and proteins which can be valorized as food, feed, bio-based products (chemicals and materials) and biofuels. Dunaliella biomass is a rich source of beta-carotene and could provide further products for applications in human nutrition, pharmaceuticals and cosmetics. An integrated algal biorefinery reduces the cost of production by optimizing processing systems in order to recover many valuable products from the same algal biomass. Therefore, in the D-Factory several extraction methods are used (supercritical CO2, high-performance counter-current chromatography, membrane technology, hydrophobic interaction resins and ion exchange resins). Each of these technological pathways is suitable for efficiently recovery of a specific class of products. The aim of the D-Factory investigation is to show the potential benefit of process integration, with a special emphasis on demonstrating minimal waste. By-products can thus be further collected, to provide valuable added value to factory operations.
Drying the cell biomass
After harvesting, the collected algal biomass, usually a liquid suspension, should be processed immediately. If it has to be shipped for processing it should be dried to avoid costs of transporting water. In the D-factory we use two methods:
1) Freeze-drying or lyophilisation, a dehydration process which involves freezing the material and then reducing the surrounding pressure to allow the frozen water to sublimate (from solid to gas). Rapid freezing is critical to avoid the formation of large water crystals. Water is then removed in two stages. The first stage involves sublimation at a temperature between -20° and 0° C. The second stage uses a slightly higher temperature to remove any bound water molecules. Dunaliella cells can be problematic to freeze due to the presence of glycerol which acts as a cryoprotectant (prevents freezing). Freeze drying tends to cause less damage to organic materials than spray drying, but is more expensive and is typically used for premium products such as high-quality instant coffee to give a better flavour than spray dried coffee.
2) Spray-drying. Here, the algal suspension is rapidly dried with a hot gas. The liquid input, usually a suspension, is transformed by spraying it through a nozzle into a hot vapour stream. During the vaporisation, small droplets are created (20-180 micron), the water quickly evaporates from the droplet and the dried solid powder is collected. This method, suitable for many thermally-sensitive materials such as foods and pharmaceuticals, has the advantage of acting very quickly and in a single step which helps to maximize the profit and simplify the process. During the spray-drying procedure, Dunaliella cells require careful study of the inlet and outlet temperatures and the residence time to avoid oxidation and degradation of any beta-carotene.
Microalgae biorefineries spend almost one third of their budgets on algal harvesting. Microalgal harvesting uses bioprocessing procedures such as centrifugation, filtration, and electroflocculation. Algal harvesting is usually performed in two steps: an initial pre-concentration (thickening) procedure that aims to increase the concentration of algal slurry to between 0.5 -1% of the suspended material, followed by a second dewatering step to reach a final concentration of 15–30%. The goal of D-Factory is to harvest Dunaliella cells efficiently, maximising recovery of biomass while minimizing residual waste. Harvesting Dunaliella cells is challenging due to some of their physical features:
- Their relatively small size (~10 μm diameter) and their ability to float in a high specific gravity, high viscosity brine;
- Their growth at low density in large-scale cultures (usually below 1 g/l), requiring very large culture volumes to obtain a significant amounts of material;
- The lack of a rigid cell wall. Instead Dunaliella have a thin elastic plasma membrane, enriched in free sterols and able to rapidly adapt to changes in external salt concentration, and covered with sticky glycoproteins and polysaccharides.
To cope with these harvesting issues, the D-Factory utilizes three harvesting systems, depending on the end-products required.
- Stacked disk clarifier centrifuges. These are the most common industrial centrifuges and ideally suited for separating particles of the size of algae and concentrating them from Dunaliella cultures. A disc-stack centrifuge consists of a relatively shallow cylindrical bowl containing a number (stack) of closely spaced metal cones (discs) that rotate with the bowl. The mixture to be separated is fed to the centre of the stacked discs where it is subjected to centrifugal forces. The dense phase travels outwards on the underside of the discs while the lighter phase is displaced to the centre. These centrifuges offer continuous flow with automatic discharge, well suited to harvesting Dunaliella cell biomass. The Dunaliella cells are enriched in β-carotene that accumulates as droplets in the periphery of chloroplasts and remains within the algal biomass during separation. However, cell rupture causes the leak of glycerol and other water soluble compounds to the surrounding water, hindering their recovery from the dilute solution in the brine. Here an introduction to the Westfalia technology that the D-Factory uses. The first photograph on the left shows the Westfalia machine installed at Monzon as part of the D-Factory programme.
- Evodos dynamic settling machines. These are semi continuous, fully automated systems that harvest cells as a paste with a smooth discharge, in contrast to traditional centrifuges which deploy a continuous discharge system that favour high shearing forces. The Evodos machine is constructed with different hydrodynamics, enabling better separation sharpness at only 3,000 – 4500 xg. Although similar to bowl centrifuges, the new Evodos 50 machines allow fully automatic discharge of the algae paste, and are designed at industrial scale, without operator assistance. The D-Factory has tested two models of Evodos dynamic settling machines – the Evodos 10 and the Evodos 50. Both perform well using Dunaliella strains, the larger machine achieving very high recoveries of β-carotene within predominantly intact Dunaliella cells. Further information on Evodos spiral plate technology and the performance of their machines can be found at their website. The second picture in this post shows the Evodos 50 machine installed at NBT in Eilat as part of the D-Factory.
- Membrane filtration. Membrane filtration approaches can be tailored to the size of organism being separated. For example, ultrafiltration uses membranes with a very small pore size that are capable of retaining small es. Larger organisms such as microalgae can be separated from aqueous media using microfiltration, using membranes with slightly larger pore sizes of around 0.1 μm. These membranes retain microalgae, while allowing solutes as well as bacteria and viruses, to pass through. Further scientific discussion of membrane filtration are included at the link. The small pore sizes and surface characteristics of filtration membranes present a major problem in microalgal separation since all membranes are susceptible to surface fouling, leading to a significant flux declines and contamination. Clogging and adhesion of the elastic, glutinous Dunaliella cells at the membrane surface is a major problem during filtration.
A recent, detailed review of microalgal harvesting
has recently been published in the scientific literature at the link.
Algae as food source
In November 2016, Prof Patricia Harvey, D-Factory coordinator, was invited to give a talk to FOOD2030 - a workshop on aquatic food products and new marine value chains hosted by the European Commission. Here you can find the slides of her lecture “Potential of New Algae Value Chains for Food” where some interesting facts and figures on commercial algae production for the food sector are reported.
Intensive microalgae cultivation systems
Intensive microalgae cultivation systems include open raceways, and closed photobioreactors.
Raceways are shallow artificial ponds (ca. 25 cm depth) which can vary in size up to a production surface area of approximately 3000 m². In the raceways, long arm, slow revolution paddle wheels make the water flow continuously around the circuit. Vertical mixing is an important factor for the algal growth because it influences the frequency by which cell will travel from bottom (dark zone – no photosynthesis) to surface (light zone - photosynthesis) of the open pond. In large outdoor open raceways such those at D-Factory partner NBT in Eilat, natural environmental conditions enhance algal growth, while advances in biotechnology and engineering control factors affecting cell development and water chemistry. Variation in environmental conditions during cultivation allow the production of different compounds (e.g. carotenoids, glycerol, phytoene), with the algal cells acting as “cell factories”.
At A4F, another D-Factory partner located in Portugal, highly intensive cultivation takes place in enclosed photobioreactors (PBRs). Unlike raceways, PBRs are contained systems, isolated from their environment, and providing specific growth conditions (e.g. light, culture medium, cooling, etc.) through which to regulate algal growth rates and purity. The designs of PBRs range from long, narrow plastic tubes, to plastic bags and rotating trays. They are particularly suitable where contained growth is required (e.g. for the culture of genetically manipulated algae).
A combination of PBR and raceway cultivation is often used for large-scale algal cultivation, with the direct transfer of fast growing, dense algal inocula from PBRs into open raceway ponds.
Dunaliella Science Digest
Here you can find the latest Dunaliella Science Digest summary produced by the D-Factory team with the most updated papers on Dunaliella and related products. Publications are indexed by Authors with links to the Internet
What is Dunaliella salina?
is a type of halophile green micro-algae. Halophiles are organisms living and thriving in environments of high salinity. D. salina
is well known for its high concentration of carotenoids that provide protection against the intense light normally measured in salt evaporation ponds. Due to its carotenoid accumulation, D. salina
has various applications in health and nutrition.
1. The biology of Dunaliella
is extremely well understood.
2. It is halotolerant, occupying a somewhat restricted ecosystem based on high salinity. This means it is likely to be cultivated more consistently and reliably than would be the case for many other microalgae.
3. It is one of only a few microalgal genera that are currently cultivated commercially and sold as a lyophilised or spray-dried algal food additive.
4. During the last 14 years we have accrued a bank of experience to contain contamination or the effect of predators on Dunaliella
5. It produces a range of compounds of great interest in pharmaceutical, cosmetic, nutraceutical and other applications.
6. The latest CO2
-enriching technologies are easily adapted to enhance the level of CO2
captured by these algae in photosynthesis whether cultivated in raceways or PBRs.
If you wish to have more detailed information on Dunaliella salina
, click here