Dissolved Air Flotation Design Calculation

DAF Dissolved Air Flotation is a much commonly used wastewater pretreatment equipment to purity wastewater by removing the insoluble suspended solids, colloids and oil & grease. According to Technical Specifications for Floatation Process in Wastewater Treatment HJ2007-2010, the main points for dissolved air flotation design calculation are as below:

1.1
Effective water depth is generally <2.5m.
For horizontal-flow dissolved air flotation, length-width ratio is generally 2:1 – 3:1.
For vertical-flow dissolved air flotation, length-width ratio is generally 1:1.
Width is better to be<6m.
Length is better to be<15m.

1.2
To the water flow ascending velocity in Contacting Zone:
Flow in bottom takes around 20mm/s, and flow in top takes 5-10mm/s.

HRT(hydraulic retention time) > 1 minute.
Tilt angle of the baffle/wall of Contacting Zone is generally 70°.

1.3
Surface load of Separation Zone is better to be 4-6m³/(m²·h).
HRT 10-20 minutes.

1.4
Water Recirculation Ratio for air-saturated water should be determined by calculation.
Generally, 15%-30%.

Dissolved Air Flotation Design Calculation:
2.1
Air Required for Air Flotation Tank Q_g:
Q_g=(γ*Q*R*A*ψ)/1000
γ: Volume-weight of Air, in g/L
Q: Designed Capacity for Dissolved Air Flotation, in m³/h
R: Water Recirculation Ratio for Air-saturated Water, determined through test
A: Air Releasing Capacity/Quantity, in L/m³
ψ: Correction Coefficient for Water Temperature, generally, 1.1-1.3

If you don’t have data from test, then you can calculate as:
Q_g=[γ*C*(f*P-1)*R*Q]/1000

C: Air Solubility at One Atmospheric Pressure at Certain Temperature, in ml/L·atm
p: Air-dissolving Pressure, in atm
f: Air-dissolving Efficiency, f=0.8-0.9

2.2
Air-solid Ratio a:
a is related to the hydrophobicity of suspended particles, generally, a=0.005-0.006. determined by test.
If lab test is not available, then it can be calculated as:
a=Q_g/(Q*S)=[γ*C*(f*P-1)*R]/(1000*S)
S: Mass Concentration of Suspended Solids in Wastewater, in kg/m³

2.3
Water Recirculation Ratio R:
R=Q_r/Q=(1000*a*S)/[γ*C*(f*P-1)]
Q_r: Quantity/Volume of Air-saturated Water, in m³/h

2.4
Rated Air Capacity of Air Compressor Q_g’:
Q_g’=ψ’*Q_g/(60*γ)
Q_g’: Rated Air Capacity of Air Compressor, in m³/min
ψ’: Security Coefficient, 1.2-1.5

2.5
Quantity/Volume of Air-saturated Water Qr:
Q_r=Q_g/(736*f*P*K_t)
f: Air-dissolving Efficiency
P: Air-dissolving Pressure, in atm
K_t: Solubility Coefficient