Harvesting of Microalgae

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In commercial algae production harvesting is generally done by centrifugation. However, the costs and energy demands for harvesting the algal biomass by these methods are high. In the technical and economical analysis on microalgae for biofuels it was shown that the investment costs for the centrifuges contributed up to 34% of the total investment on equipment. The study also showed that the centrifuge used 48.8% of the total energy consumption.

In a feasibility study, it was found that the total costs for concentrating the microalgae from 0.3 g/L to 100 g/L (10% dry matter) can be reduced from 2.72 Euro/kg (for centrifugation) to about 0.7 Euro/kg when the algae are pre-concentrated to 5% dry matter. This can be achieved by flocculation combined with flotation or sedimentation prior to further concentration by centrifugation or filtration. In addition the energy demand decreased from 4.76 kWh/kg to 0.4-0.6 kWh/kg.

Flocculation can be achieved in different ways (induced flocculation, auto- and bioflocculation or electroflocculation), but in general flocculation of the algal biomass is still poorly understood. The optimal conditions of the algae and the culture medium needed for effective flocculation are often unpredictable, which makes it difficult to find ways to control the harvesting process. In addition, after harvesting oil needs to be extracted from the biomass and often the cell wall is a big barrier to facilitate extraction and the thickness of the cell wall is affected by the conditions of the cells at the time of harvesting.

The development of pre-concentration processes for different oleaginous algae based on induced flocculation, electroflocculation, bioflocculation and autoflocculation. The algae can be different in cell size, cell shape,cell wall thickness and show different algal surface properties, such as the zeta potential and the surface tension of the algal suspension. These factors are expected to be crucial parameters for the flocculation processes. The algal aggregate size and size distribution as well as the aggregate density are important parameters for further concentration via flotation or sedimentation.

We want to derive mechanistic as well as kinetic models for the flocculation of algae based on the experimental results from our tests and validate the models. The derived mechanistic models should predict the effectiveness of the flocculation method for a given algae and the size and density of the algal flocks obtained. The kinetic model should predict the speed at which algal cells will flocculate at given medium and cell conditions. These models will help us to develop effective integrated harvesting processes for the different algae studied.

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