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Procedure for the Design and Sizing of a Dissolved Air Flotation
Separator or Separation System
Table of Contents
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Dissolved
Air Flotation
Separator Theory of Operation
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General Considerations
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Dissolved Air Flotation Separator Decision - Flow Diagram
Determining if your process require an oil water separator
or separation system
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Dissolved Air Flotation Separator Design - Flow Diagram
Basic process in the design of an oil water separator or
separation system
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The Impact of
TSS & FOG Ratios on DAF
Design
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Sizing the
Dissolved Air Flotation Separator or Separation System
Inputting the accumulated data to the Hydro-Flo Dissolved Air
Flotation
Separator Sizing Spreadsheets to accurately size an oil water
separator or separation system for your application
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Comparing and Evaluating
Dissolved Air Flotation Separators and Separation
Systems from Different Suppliers
Reviewing and filling out the Hydro-Flo Technologies "Application
Questionnaire" will also help you gather pertinent information
required for the proper design and application of DAF
separators and separation systems.
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.
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:
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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.
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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.
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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.
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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.
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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.
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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.
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STEP 1:
Identify Wastewater
Source
Buildings and areas, as well as ALL activities and processes
within the buildings and areas, that generate wastewater
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STEP 2:
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?
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STEP 2-A:
Process altered or
eliminated. No further discharge
STOP
No Further Action Required
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STEP 3:
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?
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STEP 3-A:
Process relocated or discharge
diverted
STOP
No Further Action Required
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STEP 4:
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.
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STEP 5:
Discharge & Pretreatment
Requirements
Does the raw wastewater meet permit limits
and environmental requirements? Does the treated
wastewater meet permit limits and environmental requirements?
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STEP 5-A:
Discharge Meets Requirements
STOP
No Further Action Required
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STEP 6:
New Dissolved Air Floatation Separator or
Treatment System
Upgrades Required. Proceed to Dissolved Air Floatation Separator Design -
Flow Diagram
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STEP 1:
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.
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STEP 2:
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.
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STEP 3:
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.
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STEP 3-A:
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.
 
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STEP 4:
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.
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STEP 3-B:
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.
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STEP 5:
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.
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 STEP
3-C:
Incorporate existing DAF into design |
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3-D:
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 |
| STEP
3-E:
Design the upgrade of the existing DAF |
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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.
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This spreadsheet will calculate a DAF unit based on ppm of
TSS & FOG combined, Flow rate and acceptable A/S ratio: |
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Input |
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What is the combined TSS & FOG ppm content of wastewater: |
3000 |
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What is the process flow rate (gpm): |
250 |
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Lbs/Sq. Ft./Hr (1.5 Typical) |
1.5 |
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Recycle System Pressure (in gage pressure, psi, typically 65
psig) |
80 |
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Recycle System Efficency (Typically 80.00%) |
80% |
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Temperature (in Farenheit) |
60 |
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Call 630-762-0380 to talk to a Hydro-Flo application
engineer |
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Output |
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lb/gal of TSS = |
0.03 |
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Solids Loading ratio of TSS per min (lb); |
6.26 |
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Solids Loading ratio of TSS per hour (lb): |
375.50 |
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Solids Loading ratio of TSS per day (24 hr period): |
9012.10 |
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Saturation Concentration (lb air/gal H2O) at atmospheric
pressure |
0.0002190 |
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Theoretical Air Release (lb air/gal H2O) |
0.0009098 |
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Minimum required Sq. Ft. in DAF Unit (Separation Chamber): |
250.34 |
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gpm/sq.ft. hydraulic loading rate for Selected DAF Alone |
0.95 |
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Recirculation (RAD) Flow Rate Range |
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Theoretical Low End Recycle (RAD) Flow Rate (gpm): |
125.00 |
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Low end gpm/sq.ft. hydraulic loading rate for selected
DAFand theoretical RAD combined (gpm/sq.ft.): |
1.4259 |
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Theoretical High End Recycle (RAD) Flow Rate (gpm): |
275.17 |
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High end gpm/sq.ft. hydraulic loading rate for the selected
DAFand theoretical RAD combined (gpm/sq.ft.): |
1.9968 |
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The final step is to insert the data into the "dissolved
air flotation
separator sizing spreadsheet".
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
NOTE:
If plates are used to
increase the separators efficiency, DO NOT COMPARE DISSOLVED AIR
FLOTATION SYSTEMS
BASED SOLELY ON THE AMOUNT OF PROJECTED SURFACE
AREA!
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.
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