Algae have been recognized as a sustainable source of biomass for the production of biofuels, bioproducts, and other valuable chemicals. To ensure the economic viability and environmental sustainability of algae-based processes, it is essential to develop efficient and cost-effective harvesting and dewatering techniques. This article provides an overview of different harvesting methods for algae biomass and discusses recent advances in developing sustainable algae biomass solutions.
Gravity Sedimentation
Gravity sedimentation is a simple and passive method that allows algae to settle at the bottom of the cultivation pond or tank due to their higher density compared to water. The efficiency of gravity sedimentation depends on several factors, such as algal species, size, shape, density, concentration, and water temperature. Although this method has low energy requirements, it has some limitations, including slow settling rates and low recovery efficiencies for small-sized or buoyant algae cells.
Centrifugation
Centrifugation is a widely used method for harvesting algae biomass due to its high efficiency and ability to handle large volumes of culture. It works by applying a centrifugal force to separate algae cells from the liquid medium based on their differences in density. While centrifugation can achieve high recovery rates (up to 99%) and produce concentrated biomass with low moisture content, it has high energy requirements and capital costs, which may not be suitable for large-scale applications.
Filtration
Filtration is another common technique used for harvesting algae biomass. It involves passing the algal suspension through a porous medium (e.g., filter paper or membrane) that retains the algae cells while allowing water to pass through. The efficiency of filtration depends on factors such as pore size, filter material, pressure applied, and algal cell characteristics. Filtration can be an effective method for concentrating algae biomass with relatively low energy requirements. However, fouling and clogging of the filter medium can be a major challenge, especially for small-sized or sticky algae cells.
Flotation
Flotation is a separation technique that exploits the differences in surface properties (e.g., hydrophobicity) between algae cells and water. It can be achieved by introducing gas bubbles into the algal suspension, which attach to the algae cells and cause them to float to the surface, where they can be easily removed. Flotation can be an effective method for harvesting buoyant algae species with low energy requirements. However, it may require chemical additives (e.g., flocculants or surfactants) to enhance the flotation efficiency, which may increase the cost and environmental impact of the process.
Flocculation
Flocculation is a process that promotes the aggregation of algae cells into larger flocs or clumps that can be easily separated from the liquid medium by sedimentation, flotation, or filtration. Flocculation can be induced by various mechanisms, such as pH adjustment, addition of chemical flocculants (e.g., aluminum sulfate or chitosan), or bioflocculation using microorganisms or extracellular polymeric substances. Flocculation can significantly improve the efficiency of downstream harvesting and dewatering processes with relatively low energy requirements. However, the use of chemical flocculants may have potential environmental concerns due to their toxicity and residual effects on water quality.
In conclusion, several methods are available for harvesting and dewatering algae biomass, each with its advantages and limitations. The choice of an appropriate method depends on various factors such as algal species, cultivation scale, desired product quality, and economic considerations. Developing sustainable algae biomass solutions requires a holistic approach that considers not only the efficiency and cost-effectiveness of individual harvesting techniques but also their integration within the overall biorefinery concept and their environmental impact.
