Harvesting Microalgae Grown on Wastewater

This project investigated the lifecycle costs of coagulants used for harvesting microalgae for biofuel production. Algae were grown in semi-continuous culture in pilot-scale photobioreactors (PBRs) under natural light conditions with anaerobic digester centrate as the feed source. Algae suspensions were collected from the PBRs, and the optimal coagulant dosages for metal salts (alum, ferric chloride), cationic polymer (Zetag 8819), anionic polymer (E-38) and natural coagulants (Moringa Oleifera and Opuntia ficus-indica cactus) were determined using jar tests. The relative dewaterability of the algae cake was estimated by centrifugation. Alum, ferric chloride and cationic polymer were all capable of > 91% algae recovery at optimal dosages of 170 (alum), 185 (ferric) and 44 (polymer) kg coagulant/MT of algae. Economic analysis and life cycle assessment studies revealed the polymer had the lowest cost but the highest greenhouse gas emissions and energy usage, while ferric chloride had the highest cost but the lowest greenhouse gas emissions and energy usage.