Microalgae dewatering is a major obstruction to industrial-scale processing of microalgae for biofuel prodn. The dil. nature of harvested microalgal cultures creates a huge operational cost during dewatering, thereby, rendering algae-based fuels less economically attractive. Currently there is no superior method of dewatering microalgae. A technique that may result in a greater algal biomass may have drawbacks such as a high capital cost or high energy consumption. The choice of which harvesting technique to apply will depend on the species of microalgae and the final product desired. Algal properties such as a large cell size and the capability of the microalgae to autoflocculate can simplify the dewatering process. This article reviews and addresses the various technologies currently used for dewatering microalgal cultures along with a comparative study of the performances of the different technologies.

Dewatering of microalgal cultures : a major bottleneck to algae-based fuels

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

Research studies on microalgae have increased in the last decades due to the wide range of applications associated to these photosynthetic microorganisms. Microalgae are an important source of oils and other biomolecules that can be used in the production of biofuels and high-valued products. However, the use of microalgae in these green processes is still not economically viable. One of the main costs associated to microalgal production is related to the harvesting process, as it usually accounts for about 20–30% of total cost. Therefore, this review focuses on the main harvesting processes applied to microalgae, presenting the main advantages and disadvantages of each method, to allow the selection of an appropriate procedure to effectively separate microalgal biomass from the culture medium. To reduce the associated costs, it is common to harvest microalgae in a two-step separation: (i) thickening procedures, in which microalgal slurry is concentrated to about 2–7% of total suspended solids; and (ii) dewatering procedures, which result in the concentration of microalgal slurry to 15–25% of total suspended solids. Selection of the adequate harvesting methods depends on the characteristics of the target microorganism and also on the type and value of the end product.

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Harvesting techniques applied to microalgae: A review

Constraints to commercialization of algal fuels.

Methods of downstream processing for the production of biodiesel from microalgae

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Harvesting fresh water and marine algae by magnetic separation: Screening of separation parameters and high gradient magnetic filtration

First test flights using blends with algae oil are already carried out and expectations by the aviation and other industries are high. On the other hand technical data about performance of cultivation systems, downstream processing, and suitability of algae oil as fuel are still limited. The existing microalgae growing industry mainly produces for the food and feed market. Energy efficiency is so far out of scope but needs to be taken into account if the product changes to biofuel. Energy and CO2 balances are used to estimate the potential of algae oil to fulfil the EU sustainability criteria for biofuels. The analysis is supported by lab tests as well as data gained by a pilot scale demonstrator combined with published data for well-known established processes. The algae oil composition is indicator of suitability as fuel as well as for economic viability. Approaches attaining high value fractions are therefore of great importance and will be discussed in order to determine the most intended market.

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Composition of Algal Oil and Its Potential as Biofuel

Biofuels from microalgae: photoconversion efficiency during lipid accumulation.

A variety of high value products have so far been produced with algae and the transition to algae mass cultures for the energy market currently arouses the interest of research and industry. The key to efficient cultivation of microalgae is the optimization of photobioreactors that does not only allow for efficient light capture but also takes account of the specific physiological requirements of microalgae. Three fundamental reactor designs (bubble columns, flat plate reactors, and tubular reactors) are common and are discussed together with some elaborate derivatives in the following. Every concept excels with specific advantages in terms of light distribution, fluid dynamics, avoidance of gradients, and utilization of the intermittent light effect. However, the integration of all beneficial characteristics and simultaneously the compliance with energetic and economic constraints still imposes demanding challenges on engineering.

Closed Bioreactors as Tools for Microalgae Production

The unicellular microalga Phaeodactylum tricornutum exhibits the ability to accumulate triacylglycerols to a high specific content when nutrients are limited in the culture medium. Therefore, the organism is a promising candidate for biodiesel production. Mathematical modeling can substantially contribute to process development and optimization of algae cultivation on different levels. In our work we describe a linear programming approach to model and simulate the growth and storage molecule accumulation of P. tricornutum. The model is based on mass and energy balances and shows that the organism realizes the inherent drive for maximization of energy to biomass conversion and growth. The model predicts that under nutrient limiting conditions both storage carbohydrates and lipids are synthesized simultaneously but at different rates. The model was validated with data gained from batch growth experiments.

Robert Dillschneider

Papers

Harvesting fresh water and marine algae by magnetic separation: Screening of separation parameters and high gradient magnetic filtration

Composition of Algal Oil and Its Potential as Biofuel

Biofuels from microalgae: photoconversion efficiency during lipid accumulation.

Closed Bioreactors as Tools for Microalgae Production

A linear programming approach for modeling and simulation of growth and lipid accumulation of Phaeodactylum tricornutum.