How to size a DAF

The sizing of a Dissolved Air Flotation (DAF) is sort of a complicated process.

However, the’re precautions that need to be taken before starting the sizing process.

For one, a spreadsheet is a viable tool, finding the right formulas( for the length and the width), and location. DAF maybe an unknown to most.

They may not know of a plate settler, what a DAF does, or how important it may be.
For the record, a plate settler is very important.

So for those who don’t know, Dissolved Air Flotation Systems are created to remove three main categories of contaminants which consist of wastewater stream: suspended solids, biochemical oxygen demand, and oils and greases.

This system also assists with sludge dewatering and efficient solids removal.

In other words, it’s a treatment system.

The plate settler clarifier, for example, removes particles from the liquids.

A Plate settler is a primary tool in the creation of a DAF.

Here is a little information on how to size a DAF.


By using a spreadsheet, it will enable the engineer to input all of the design criteria, which will further solve for DAF sizing that is based on fluctuating solids loading.

Spreadsheets are operational on Microsoft Excel 2003 or later.

Spreadsheets help calculate, which is based on ppm of TSS and FOG combined, Flow rate and acceptable A/S ratio.

This helps define the inputs:

The combined TSS and FOG ppm content of wastewater.

The process flow rate (gpm)

Recycle System Pressure (in gage pressure, psi, that is typically 65 psig)

Recycle System Efficency ( which is 80.00%)

Temperature (in Farenheit)

The selected DAF model number based on minimum required DAF unit (below) and round up to the next larger DAF model

It also helps define the output:

Solids Loading ratio of TSS per min (lb)

Solids Loading ratio of TSS per hour (lb)

Solids Loading ratio of TSS per day (24 hr period)

Saturation Concentration (lb air/gal H2O) at atmospheric pressure

Theoretical Air Release (lb air/gal H2O)

Minimum required Sq. Ft. in DAF Unit (Separation Chamber):

gpm/sq.ft. hydraulic loading rate for Selected DAF Alone

Recirculation (RAD) Flow Rate Range

Theoretical Low-End Recycle (RAD) Flow Rate (gpm)

Low end gpm/sq.ft. hydraulic loading rate for selected DAFand theoretical RAD combined (gpm/sq.ft.)

Theoretical High-End Recycle (RAD) Flow Rate (gpm)

High end gpm/sq.ft. hydraulic loading rate for the selected DAFand theoretical RAD combined (gpm/sq.ft.

This is just a little example of what will be in the spreadsheet.

The Criteria

Here is the Standard Solids Loading Rate: 1.5 lb/sq. ft./hr

It’s preferred to never use more than 4 gpm/sq.ft hydraulic loading rate for the DAF and RAD combined

Lbs Air/Lbs Solids range = 0.015 – 0.040 is acceptable

Make sure apply 100% of the low end RAD flow rate when BELOW – 3000 ppm

Apply 75% of the high end RAD flow rate when BETWEEN 3000 – 5000 ppm

Lastly, typically apply 100% of the high end RAD flow rate when ABOVE – 5000

ppmcriteria when sizing the a DAF

Here are also a few formulas that is needed:

Recirculation Flow Rate:
(Lbs Air/Lbs Solids) x (Solids Loading Rate/min) x Theoretical Air Release

Dimensions for (Solids Loading Rate/min): Lbs/min

Dimensions for Theoretical Air Release: 1000 gal/0.62 lbs. @ 75 psig and 68 degrees F

Low end (Lbs Air/Lbs Solids) = 0.015
High end (Lbs Air/Lbs Solids) = 0.040

Conversion Factors:
1,000 mg = 1 gr
453.59 gr = 1 lb.
3.785 l = 1 gal.
Again, this is a long process, but its part of the job.

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