Evaluation of flocculation, sedimentation, and filtration for dewatering of Scenedesmus algae

Algae can be used as a feedstock for agricultural fertilizers, livestock and poultry feeds, anaerobic digestion, and biofuel production. For each end product, the requirements for moisture content (or solids content) vary, such that a desirable water removal strategy needs to be adaptable to varying levels of water removal. Flocculation, sedimentation, and filtration were evaluated as possible strategies for thickening and dewatering of algae. The goal of this study was to validate that algae cells treated by such means could be processed by vacuum belt filters and to determine the conditions under which the solids content could be increased to 5 to 25 wt%. The flocculation and sedimentation studies focused on conditions needed to thicken algae from a culture concentration range of 0.4 to 1 g L-1 to an end-product concentration range of 15 to 50 g L-1. Sedimentation rates of Scenedesmus actus were measured with varying flocculant dosages (0 to 25 ppm) for various flocculants. The highest level of compaction was achieved with a synthetic cationic polymeric flocculant with higher molecular weight at a dosage of 15 ppm, which provided 16.3 mL of compacted solids (3.3 wt% solids). Subsequently, solids were successfully separated as a cake via gravity and vacuum filtration. The filtration studies focused on the conditions needed to filter flocculated algae slurry from a concentration range of 15 to 50 g L-1 to a product at a concentration range of 50 to 250 g L-1. Filtration rates of Scenedesmus actus were measured on algae slurry treated with 10 to 15 ppm of a synthetic cationic polymeric flocculant. Processing parameters such as cake formation time, filtration rate, and mass throughput were evaluated against variables such as cake thickness, feed concentration, and processing time.