Harvesting microalgae for biofuel : processes and mechanisms
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The application of microalgae for biofuel production is a subject of increasing interest as fuel prices continue to fluctuate and the United States aims to secure a reliable, domestic fuel source. Though microalgae have proven to be very efficient at producing oil-rich lipids, the optimum conditions for algae cultivation and methods for harvesting and oil extraction have not been determined. In particular, the harvesting component is especially important to the effectiveness of the overall process because of the large volumes of algae-rich water that must be processed, the strict requirements for downstream lysing, oil extraction and fuel production and the necessity to generate algae biomass with significant post-extraction byproduct value. A number of solid-liquid separation technologies have shown some potential for achieving microalgae/water separation; however, application of these processes to biofuel production requires an evaluation of treatment effectiveness as a function of water quality, algae particle characteristics, and process chemistry. The goal of this research was to identify and evaluate several potentially viable harvesting methods that could be incorporated into end-to-end algae to biofuel production. To achieve this goal, a literature review was conducted to identify the most promising harvesting methods for biofuel applications, and bench scale tests were performed for several harvesting processes. A number of significant findings were identified. Batch algae coagulation experiments with ferric chloride, chitosan, and pH-induced autoflocculation suggest that coagulants can provide effective treatment, but the effectiveness is dependent on water composition and pH. Electrocoagulation experiments indicated that dissolution of the sacrificial electrode led to high metal concentrations in the algae. Pre-oxidation with ozone increased the removal of freshwater Neochloris oleobundans by 20-80% after subsequent flocculation and sedimentation compared with non-ozonated samples. Most notably, this research identified the importance of optimizing water quality and algae particle characteristics for a particular algae harvesting process. Implementing a harvesting process that takes advantage of the natural constituents of a water and the surface characteristics of an algae culture, minimize treatment requirements and enables smoother integration with subsequent processing steps.