Water tools

Dissolved air flotation: saturator solubility, air released and separation area

Flotation works on the air that comes out of solution when pressurised recycle meets the flocculated water. Henry's law gives what the saturator can dissolve; a mass balance gives what is released.

Saturator pressure (gauge)kPa400 to 700 kPa gauge.

Water temperature°C

Recycle ratio%6 to 12%; more above 25 °C.

Saturator transfer efficiency

Air deficit of the influentmg/L

Mean bubble diameterµm10 to 100 µm.

Floc countper mL

Floc volumeppm

Influent flowm³/h

Separation-zone hydraulic loadingm/h10 to 20 conventional; 40 to 45 high rate.

Air

Air released in the contact zone

7.98g/m³ of influent

Air dissolved in the recycle

144.1mg/L

Air already in the flocculated water

24.3mg/L

Bubbles

Bubble volume

6,629ppm

Typical 3,500 to 8,000 ppm.

Bubble count

197,807per mL

Bubbles per floc

Bubble to floc volume ratio

2,651

Sizing

Separation-zone area

73.3m²

Recycle flow

100m³/h

How it works

Air solubility follows Henry's law, with the constant at temperature from the enthalpy of dissolution, and scales with absolute pressure:

H = K_{C} \exp (- \frac{Δ H}{R T}) C_{s a t} (1 atm) = \frac{ρ_{a i r}}{H} C_{s a t} (P) = C_{s a t} (1 atm) \frac{P_{a b s}}{101.325}

(1)

A balance over the recycle and the influent gives the air that comes out of solution, and from it the bubble volume and number:

C_{b} = \frac{e (C_{r} - C_{f l}) r - k}{1 + r} φ_{b} = \frac{C_{b}}{ρ_{a i r}} N_{b} = \frac{6 φ_{b}}{π d_{b}^{3}}

(2)

A_{s} = \frac{(1 + r) Q}{v_{h l}}

(3)

where

$H$: dimensionless Henry's constant of air
$K_{C}$: pre-exponential constant, 3,368
$Δ H$: enthalpy of dissolution, 10.28 kJ/mol
$R$: gas constant, 8.314 J/mol·K
$T$: absolute temperature, K
$C_{s a t}$: air solubility, mg/L
$ρ_{a i r}$: air density, mg/L
$P_{a b s}$: saturator absolute pressure, kPa
$C_{b}$: air released in the contact zone, mg/L of influent
$e$: saturator transfer efficiency
$C_{r}$: air in the recycle, mg/L
$C_{f l}$: air already in the flocculated water, mg/L
$r$: recycle ratio
$k$: air deficit of the influent, mg/L
$φ_{b}$: bubble volume fraction
$N_{b}$: bubble count, per mL
$d_{b}$: bubble diameter, µm
$A_{s}$: separation-zone area, m²
$Q$: influent flow, m³/h
$v_{h l}$: hydraulic loading, m/h

The separation zone is sized like a settling basin turned upside down: the rise velocity of the bubble-floc aggregate must exceed the hydraulic loading. Air released of 6 to 10 g/m³ usually gives the best clarification, with 1 to 2 × 10⁵ bubbles per mL at 10 to 100 µm. Warm water dissolves less air, so raise the recycle. These figures are a best case with no air lost to the contact tank.

Dissolved air flotation: saturator solubility, air released and separation area

How it works

Related reading