It is relevant that without flocculant addition the velocities obtained
along the column were fairly constant (0.05–0.08 mm/s) and much
lower than those obtained with flocculated samples (0.21–0.56 mm/s
with Ecotan and 0.16–0.35 mm/s with Tanfloc). Comparing flocculants,
higher velocities along the column (with a maximum of 0.56 mm/s at
40 cm), were obtained with Ecotan (10 mg/L). In contrast, higher
doses of Tanfloc (50 mg/L) yielded slower settling velocities of about
0.35 mm/s. A conventional gravity settler should be designed in order
to retain 90% of the biomass with a surface loading rate of 0.3 mm/s
[23]. Considering the settling velocities obtained in this study
(Table 4), a velocity of 0.3 mm/s was obtained at column depths of 20
and 30 cmfor Ecotan and Tanfloc, respectively. Therefore, settler surface
area with Ecotan would be 2-times smaller than Tanfloc and 8-times
without flocculant.