Techno-economic comparison of flocculation combined with dissolved air flotation versus sedimentation for microalgae harvesting

Abstract

Microalgae are a promising new source of biomass but the harvesting method needs to optimised to reduce the production cost. In a two-step harvesting process, the bulk water is removed via flocculation (first step) followed by complete dewatering using centrifugation (second step). Flocs can be separated from the bulk water using either gravity sedimentation or dissolved air flotation (DAF). DAF requires a more complex setup and demands more energy than sedimentation, but it is faster and generates a sludge with a higher dry matter content. This study aims to compare the performance of DAF and sedimentation in laboratory experiments and then estimates the cost for a 100-hectare farm using sedimentation versus DAF for harvesting (step-1) and using a decanter centrifuge (step-2) for dewatering. Chlorella vulgaris was the model species and poly(dimethylaminoethyl methacrylate) (pDMAEMA) was synthesised and used as the flocculant. In bench-scale testing, sedimentation and DAF achieved maximum harvesting efficiencies of 87 % and 98 %, respectively. Sedimentation was slow and generated a sludge with low solids content (1.1 %). DAF achieved a fast separation and generated a sludge with 3× higher solids content of 3.4 %. When estimating the costs of biomass harvesting at 100-hectare scale, DAF was found to have higher capital and operating expenditure than sedimentation. However, when combining harvesting with the dewatering step in a two-stage harvesting-dewatering system, the total capital and operating expenditure was projected to be 20–38 % and 10–18 % cheaper, respectively, than sedimentation. This is due to the faster separation, higher sludge solids content, and greater separation efficiencies obtained via DAF than sedimentation. Hence, DAF is recommended over sedimentation for harvesting microalgae. Importantly, this study showcases that contrary to conventional belief, flocculation–DAF is cheaper than flocculation–sedimentation for large-scale algal harvesting.