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The influence of floc size and hydraulic detention time on the performance of a dissolved air flotation (DAF) pilot unit in the light of a mathematical model

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The influence of floc size and hydraulic detention time on the performance of a dissolved air flotation (DAF) pilot unit was investigated in the light of a known mathematical model. The following design and operational parameters were considered: the hydraulic detention time (tdcz) and hydraulic loading rate in the contact zone, the down-flow loading rate in the clarification zone, the particle size distribution (d F), and the recirculation rate (p). As a reference for DAF performance analysis, the proposed β.td parameter from the above mentioned mathematical model was employed. The results indicated that tdcz is an important factor in DAF performance and that d F and floc size are also determinants of DAF efficiency. Further, β.td was sensitive to both design and operational parameters, which were varied in the DAF pilot plant. The performance of the DAF unit decreases with increasing β.td values because a higher td (considering a fixed β) or a higher β (e.g., higher hydrophobicity of the flocs for a fixed td) would be necessary in the reaction zone to reach desired flotation efficiency.

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Ø :

Floc volume concentration in the inlet of the flotation unit

P :

Recirculation fraction (Q R /Q ab), where Q R is the recirculation flow and Q ab is the inlet flow (L3 T−1)

α :

Flux of particles

S v :

Expressed as the dissolved (precipitable) air volume concentration in the recirculated water flow (volume of dissolved air/volume of recirculated water)

n :

Number of required microbubbles to be attached to each floc to promote its removal after leaving the contact zone

V bf :

Rise velocity of aggregate (L T−1)

d B :

Air bubbles diameter, uniformly sized (L)

d F :

Floc diameter, uniformly sized (L)

η :

Kolmogorov microscale of turbulence

p :

Recirculation rate (−)


Hydraulic loading rate in the separation zone (L T−1)


Hydraulic detention time in the contact zone (T−1)

ε :

Energy dissipation per mass unit (J kg−1 s−1)

\(\rho\) F and \(\rho\) l :

Specific mass for floc and liquid, respectively

ν :

Cinematic viscosity of the water (L2 T−1)

L and T :

Refer to general length and time units, respectively


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The authors are grateful to FAPESP for providing financial support of this research. This paper was presented in part at the 6th International Conference on Flotation in Water and Wastewater in New York (EUA) in November 2012.

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Correspondence to R. B. Moruzzi.

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Moruzzi, R.B., Reali, M.A.P. The influence of floc size and hydraulic detention time on the performance of a dissolved air flotation (DAF) pilot unit in the light of a mathematical model. Bioprocess Biosyst Eng 37, 2445–2452 (2014). https://doi.org/10.1007/s00449-014-1221-6

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  • Particle
  • Image analysis
  • Mathematic model
  • Dissolved air flotation