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Design and Sizing of a Dissolved Air Flotation Separator

Dissolved Air Flotation Theory of Operation

If you are unfamiliar with basic DAF separator design principles, a review of the "Dissolved Air Flotation Theory of Operation" might prove helpful.

This basic document covers Stokes' law and other basic separation concepts.

General Considerations

DAF separators are typically considered very simple devices. However, several factors that could potentially affect safety, efficiency, and proper management must be given careful consideration prior to the installation or modification of any DAF separator or separation system:

  1. Flow Rate
    In general, DAF separators are sized by the flow rate verses the separation chamber's effective surface area (or projected surface area in the case of a DAF with enhanced "coalescing" or parallel plate media). Therefore, the effectiveness of any DAF separator is affected by the flow rate. The slower the flow, the better the results.

  2. Design Capacity
    A DAF separator has upper limits to the amounts of FOG (fats, oils and grease) and TSS (total suspended solids) that can effectively accumulate while it is in operation. If too much product accumulates in the receiving and middle chambers, it may flow into the wastewater outlet chamber and end up being discharged to the environment. Proper DAF design will allow for the removal and storage of accumulated products from the separator to ensure that the accumulated products do not effect the operation of the separator.

  3. Emulsifying Agents
    Detergents and soaps designed to remove solids and oily grime from equipment, vehicles, or other components can adversely affect the operation of a DAF separator. These types of surfactants, or "emulsifying" agents, are specifically formulated to increase the dispersal of solids, oils, greases, etc. into water, which is why they are such good cleaners. When these soapy wastewaters enter a DAF, it takes significantly longer for the products to separate, if they can, from the water. Excessive use of detergents can render an oil water separator inefficient by completely emulsifying oils into the wastewater stream and allowing them to pass through the system. Low-emulsifying soaps are available that allow oil separation to occur more quickly after the soapy water enters the oil water separator.

  4. Maintenance Practices
    The ability of DAF separators to function properly depends upon the timely performance of required service and maintenance. DAF separators must be monitored and maintained by competent personnel who understand how the systems operate. DAF separators should be given the same close attention given to any other important piece of equipment. The operators, users, and maintainers of the DAF separator must clarify who will be responsible for monitoring, inspecting, maintaining, and servicing the system. Frequent inspections should be made of the system and all associated piping, valves, etc. to prevent operational and mechanical failures or inefficiencies. Sludge and oil need to be periodically removed from the DAF separator to keep it operating properly. Additionally, leaks from DAF separators can result in environmental pollution, which can trigger costly investigative studies and cleanups. Rigorous implementation of a DAF inspection and maintenance plan can prevent discharges from the DAF separator that may contaminate the environment.

  5. Suitability of Dissolved Air Flotation Separation System to Process
    A DAF separator designed and installed to meet a past process requirement may no longer be suitable when process requirements change, and/or the original maintenance plan is no longer followed. A DAF separator that is put to a use for which it was not originally designed may be damaged or may not function properly, and could become an environmental liability. For example, a DAF separator designed to receive the wastewater discharge from a small engine wash rack will not be able to properly treat larger wastewater volumes from washing the exterior of locomotives. Process changes can also result in changes to the physical/chemical makeup of the wastewater being treated by a DAF separator. Finally, process changes may also necessitate the modification of storm water and wastewater drainage systems. Such systems should remain separate from each other because excessive drainage of storm water to a DAF separator could significantly impair its operation and efficiency.

  6. Contaminants Contained in the Wastewater Stream
    Metallic particles in the wastewater will settle into the sludge at the bottom of the DAF separator. Solvents or fuel compounds may also be entrained in the DAF separators' sludge. This sludge could require management under the Resource Conservation and Recovery Act (RCRA) as a hazardous waste if it exhibits certain toxicity characteristics. Therefore, it is important to prohibit the discharge of certain types of potential contaminants into a DAF separator, and to regularly analyze sludge samples to determine toxicity prior to disposal. To reduce the accumulation of sludge, floors should be dry-swept before washing. General improvements in spill/drip control and containment of hazardous materials and oils will also reduce the amount of contamination in DAF separator discharges.

Dissolved Air Flotation Decision - Flow Diagram


Identify Wastewater Source
Buildings and areas, as well as ALL activities and processes within the buildings and areas, that generate wastewater



Institute Pollution Prevention and Source Elimination / Reduction Procedures
Can the processes that generate the wastewater be eliminated?  Can the process be converted to a dry process?





Process altered or eliminated. No further discharge
No Further Action Required


Source Diversion
Can the process be moved to an area that has existing wastewater treatment equipment in place?  Can the existing equipment handle the increased flow?  Is moving the process, diverting the flow, economically feasible?





Process relocated or discharge diverted
No Further Action Required


Wastewater Compliance Evaluation
Identify permit limits on ALL pollutants generated at the site.  Characterize raw wastewater prior to any treatment if appropriate.  Characterize treated wastewater if existing treatment equipment is in place.



Discharge & Pretreatment Requirements
Does the raw wastewater meet permit limits and environmental requirements?  Does the treated wastewater meet permit limits and environmental requirements?





Discharge Meets Requirements
No Further Action Required


New Dissolved Air Floatation Separator or Treatment System Upgrades Required.  Proceed to Dissolved Air Floatation Separator Design - Flow Diagram


Dissolved Air Flotation Separator Design - Flow Diagram


Conduct a Wastewater Characterization Study
The engineer / Designer may conduct wastewater characterization study to establish separator or separation system design parameters.

The first step wastewater characterization study is to conduct a series of bench or "jar" tests.  These bench tests will help determine the systems "TARGET" effluent quality.



Determine the Type of Dissolved Air Flotation Separator or Separation System Necessary for Your Process
Evaluate the results of the characterization study and determine the category of oil to be removed.  Decide what type of separator or separation system to use.

The type of oil wastewater separator you need can vary greatly depending on the results of the wastewater characterization study. API and enhanced gravity separators (parallel plate or "coalescing") are suitable for a great variety of applications.  Emulsion breaking or dissolved air flotation systems may be required for processes with chemically or mechanically stable emulsions.



Do You Have an Existing Dissolved Air Flotation Separator at Your Facility
If there is an existing DAF separator on site, evaluate if it may be upgraded to accept the total flow or partial flow from the proposed source to reduce loadings.




Will Upgrading the Existing Dissolved Air Flotation Separator Bring the Discharge into Compliance
Can the existing system be upgraded by the installation of media (plate packs, coalescing packs, etc.) or adding emulsion breaking capabilities, etc.



Review Concerns and Requirements of the New Dissolved Air Flotation Separator or Separation System With the Engineer / Designer
List all provisions that need to be considered to ensure the new system will be accessible for maintenance, will meet site specific area classifications (ie: seismic and explosion proof area classifications, etc.) and will meet all regulatory and effluent discharge requirements.


Can the Existing Dissolved Air Flotation Separator be Used to Pre-treat the Wastewater Prior to the New Separator
Even if the existing DAF is not capable of handling the proposed load, it may be useable as a form of pre-treatment or used in conjunction with the proposed upgrades.




Write the Specifications for the New or Upgraded Dissolved Air Flotation Separator or Separation Systems
Write specifications for the new DAF separator, the DAF upgrade or the new DAF separation system required to meet all the above listed concerns and requirements.


Incorporate existing DAF into design


If determined that the existing DAF is no longer suitable for it's current use or unusable in the new system, incorporate the closure of the existing DAF into the specifications


Design the upgrade of the existing DAF

The Impact of TSS & FOG Ratios on DAF Design

There are many factors that impact the overall design of a dissolved air flotation system, but the most important by far is the combined TSS & FOG (in parts per million) content of the wastewater. For example, look at the following spreadsheets.

This spreadsheet will calculate a DAF unit based on ppm of TSS & FOG combined, Flow rate and acceptable A/S ratio:  
What is the combined TSS & FOG ppm content of wastewater: 3000
What is the process flow rate (gpm): 250
Lbs/Sq. Ft./Hr  (1.5 Typical) 1.5
Recycle System Pressure (in gage pressure, psi, typically 65 psig) 80
Recycle System Efficency (Typically 80.00%) 80%
Temperature (in Farenheit) 60
Call 630-762-0380 to talk to a Hydro-Flo application engineer
lb/gal of TSS = 0.03
Solids Loading ratio of TSS per min (lb); 6.26
Solids Loading ratio of TSS per hour (lb): 375.50
Solids Loading ratio of TSS per day (24 hr period): 9012.10
Saturation Concentration (lb air/gal H2O) at atmospheric pressure 0.0002190
Theoretical Air Release (lb air/gal H2O) 0.0009098
Minimum required Sq. Ft. in DAF Unit (Separation Chamber): 250.34
gpm/sq.ft. hydraulic loading rate for Selected DAF Alone 0.95
Recirculation (RAD) Flow Rate Range  
Theoretical Low End Recycle (RAD) Flow Rate (gpm): 125.00
Low end gpm/sq.ft. hydraulic loading rate for selected DAFand theoretical RAD combined (gpm/sq.ft.): 1.4259
Theoretical High End Recycle (RAD) Flow Rate (gpm): 275.17
High end gpm/sq.ft. hydraulic loading rate for the selected DAFand theoretical RAD combined (gpm/sq.ft.): 1.9968

Sizing the Dissolved Air Flotation Separator or Separation System

The final step is to insert the data into the "dissolved air flotation separator sizing spreadsheet".

Comparing and Evaluating Dissolved Air Flotation Separators and Separation Systems From Different Suppliers

Comparing DAF proposals is basically straight forward as long as the engineering is correct. How many square feet of surface area is the defining criteria. Typically if everyone is bidding a 50 square foot DAF system, the units are all going to be comparable.

If they are bidding a DAF with parallel plates installed (to improve the units overall efficiency) then you must be carful to make sure that everyone is on the same page


This is VERY important. Many separator manufacturers use the smallest media plate spacing possible in their DAF system. This allows them to post the largest projected surface area numbers possible, giving the customer the impression that they are purchasing the most efficient DAF available.

If using the smallest possible plate spacing were the answer to building the most efficient DAF, we would be building dissolved air flotation systems with 1/8" plate spacing, or smaller. Unfortunately, this is not the answer. Other considerations come into play. In actual practice, a plate spacing of 2" is the smallest plate spacing recommended. Many other issues come into play, such as excessively high cross sectional velocities and Reynolds numbers, plate pack distribution and short circuiting issues, as well as oil and sludge re-entrainment. All are common.