Effect of shear rate on floc characteristics and concentration factors for the harvesting of Chlorella vulgaris using coagulation-flocculation-sedimentation.

Coagulation-based separation has been increasingly applied to microalgal harvesting because of its competitive cost and high scalability. The characteristics of flocs formed during coagulation/flocculation are critical for efficient harvesting. However, few studies have been devoted to systematically investigating the structural characteristics of microalgal flocs and their influences on subsequent settling performance. In this paper, the dynamic mean size and fractal dimension, strength, regrowth and settling performance of Al3+ coagulated Chlorella vulgaris flocs were characterized at various flocculation shear rates. The influence of shear rate on floc characteristics was revealed. An appropriate shear rate (9 s-1) produced more desirable microalgal flocs (in terms of size and compactness), with better settling performance and a higher concentration factor, than higher or lower shear rates, favoring their separation and subsequent harvesting. At this condition, the concentration factor reached 13.50, which was a 177.21% improvement over the 4.87 reached at a low shear rate.