2024 Volume 80 Issue 25 Article ID: 24-25007
In this study, we carried out a laboratory-scale investigation of the flocculation, concentration, and dewatering of the green alga Chlorella sorokiniana and the diatom Chaetoceros simplex var. calcitrans using polymer flocculants and slurries made with dried microalgae powder. The flocculation experiments were conducted using five different polymer flocculants, with variations in flocculant addition ratio, agitation conditions, and slurry pH. We then conducted gravity concentration and centrifugal dewatering experiments on the formed aggregates, varying the centrifugal strength. The results showed that fast and long agitation produced a higher recovery ratio of exceeding 98%. However, flocculants with larger molecular weights exhibited increased flocculation efficiency under conditions of slow agitation. pH fluctuations from 6 to 8 resulted in a maximum difference of 24.1% in recovery ratio when the flocculant addition ratio was 1%, but almost no change when the addition ratio was 3% or higher. The flocculant with the highest cationic degree in the range of this study showed high recovery rates of approximately 90% at a 2% addition ratio and over 95% at a 3% addition ratio for both microalgae species. The water content of aggregates decreased to about 65% for C. sorokiniana and about 72% for C. simplex var. calcitrans after centrifugal dewatering, suggesting that the dewaterability of algae varies depending on the substances constituting their surface. As the centrifugal intensity was increased, the water content converged to a constant value, which is about 65% for C. sorokiniana and about 70% for C. simplex var. calcitrans, indicating that efficient dewatering cannot be achieved with strong centrifugal separation. By flocculation and concentration, the volume of C. sorokiniana and C. simplex var. calcitrans were reduced to about 1/180 and 1/77, respectively, compared to those before flocculation. Centrifugal dewatering at 700 G or higher further reduced the volume of C. sorokiniana by about one-half and C. simplex var. calcitrans by one-third, respectively.
