Transcript The Venturi Aeration Process

The Venturi Aeration Process:
Understanding Oxygen Transfer
and Wastewater Conditioning
Venturi Aeration, Inc.
41 Tallant Road
Pelham, NH 03076-2236
603-635-8239
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Oxygen Transfer Basics
• There are various wastewater characteristics
that affect the amount of Oxygen that gets
dissolved in wastewater:
• Soluble salts (TDS), particulate (TSS),
surface active substances (algae, O&G),
temperature, atm pressure, et. al.
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Oxygen Transfer Basics - 2
• Additionally, the solubility of oxygen is
also impacted by other mechanical and
kinetic properties:
• Tank geometry (circular, rectangular,
square, etc.; rounded corners, conical
bottom, height of tank, material of
construction, in-ground, above ground, etc.
• Type of aeration device,
• Intensity of mixing (Reynolds Number)
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Oxygen Transfer Basics - 3
• Two Predominant Oxygen Transfer Models:
• 1. Buoyancy Transfer Model - Diffusers
release an air bubble at the bottom of the
tank using the surface area of the bubble to
transfer oxygen, (design variables) and
• 2. Kinetic Transfer Model - using
mechanical energy to transfer oxygen
into a liquid.
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Oxygen Transfer Basics - 4
• Two Film Theory of Oxygen Transfer: the
rate of oxygen (gas) transfer is proportional
to the difference between the existing
concentration of the gas and the equilibrium
of the gas in solution.
• This means that as a gas approaches
equilibrium it encounters greater resistance
at the interface of the liquid and the gas,
thus reducing the solubility of the gas.
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Oxygen Transfer Basics - 5
• The Standard Oxygen Transfer Rating
(SOTR) is a unit of measurement that
quantifies the “oxygen” transfer efficiency
of a specific type of aeration device using
either a “Buoyancy” or “Kinetic” oxygen
transfer model.
• The Standard is maintained by the ASCE
and all testing is done in “Clear Water.”
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Oxygen Transfer Basics - 6
• Because the ASCE testing is done in clear
water (low TDS, TSS, etc.) there are three
(3) values that are used to correct for the
wastewater characteristics to develop an
AOTR (Actual O2 Transfer Rating):
• The alpha () value corrects for the type of
aeration device, tank geometry, intensity of
mixing.
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Oxygen Transfer Basics - 7
• The beta () value corrects for soluble salts
(TDS), particulate (TSS), surface active
substances, etc.
• The theta () value adjusts for the solubility
of oxygen at specific temperatures.
• All aeration devices have a standard SOTE
however, their values must be adjusted for
the above three values for an AOTR.
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Various Kinetic Transfer Model
Mechanical Aeration Devices’SOTE
• Following are various AOTR’s for mechanical
aeration devices:
lbs. O2/hp/hr
• Venturi Aerator
2.73 -3.06
• Surface aerator w/draft tube 1.2 - 2.1
• Surface high speed
1.2 - 2.0
• Submerged turbine
1.0 - 2.0
• Submerged turbine/sparger 1.2 - 1.8
• Surface brush and blade
0.8 - 1.8
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The Venturi Aeration Process
• The venturi aeration process has a high
oxygen transfer efficiency because of the
following factors:
• High Gas / Liquid ratio: 2.2 : 1.0
• Intensity of mixing is internal in the
device’s mixing & oxidizing zone.
• Constant supply of an oxygen-deficient film
to overcome the resistance of a partially
oxygenated liquid (Two film theory).
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The Venturi Aeration Process - 2
• Kinetic force of the discharge is used for
mixing and equalization (2 fps). The best
use of this feature is in a circular tank.
• Stripping substances with weak Henry’s
constants, e.g. CO2 and VOCs.
• Oxidizing sulfur-containing molecules, e.g.
hydrogen sulfide and -mercaptans for
effective odor and corrosion control.
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The Venturi Aeration Process - 3
• Degassing of gases imbedded in organic
materials enhances settling of solids in
clarifiers.
• Large amounts of induced DO cause fats,
oils, and grease to hydrolyze and float for
skimming.
• By stripping CO2 pH is raised allowing for
nitrification (pH >6.8) to begin.
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Interdependence of
pH, Alkalinity and CO2
• A formula for pH showing the
interdependence of pH, alkalinity and
carbon dioxide:
• pH = 6.35 + log(alkalinity/carbon dioxide)
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The Venturi Aeration Process - 4
• BOD Reduction, the large amounts of DO
immediately allow BOD reduction begin.
• The venturi nozzle has a pressure
differential that causes organic materials to
break apart (implosion) which increases
their surface area making them more readily
available for microbial digestion.
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Summary of BOD5 Reduction Rates in Septage
6000
5000
4000
B
O
D5
3000
2000
1000
0
Hours
0
1
2
3
4
5
6
7
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9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Oxygen
Diff. Air
Hydro-Vac
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Applications of the Venturi Aerator
• Collection system (lift stations for odor and
corrosion control, and BOD reduction).
• Headworks for sludge separation - enhances
settling and performance of the primary
clarifier.
• Septage Receiving - for odor control, BOD
reduction, degassing H2S and “shearing”
organic materials.
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HYDROGEN ACCEPTOR HIERACRCHY
Hydrogen
Acceptor
Hydrogen
Atom added
Reduced
Product
A.
O2
+ 4 H+
2 H2O
B.
2 NO3
+ 12 H+
N2 + 6 H2O
C.
SO4=
+ 10 H+
H2S + 4 H2O
D.
Oxidized
Organics
+ xH+
Reduced
Organics
E.
CO2
+ 8 H+
CH4 + 2 H2O
(A)(B)(E) result in odorless products
(C) Malodorous compound H2S
(D) Odorous products e.g. -mercaptans
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Explanation of BOD5
Reduction Rates In Septage
• 1. Reduction in BOD5 for fine bubble ceramic
diffusers in septage with air from blower (EPA
data).
• 2. Reductionof BOD5 for pure Oxygen through
fine bubble ceramic diffuser in 1. Above (EPA
data).
• 3. Reduction in BOD5 for venturi aerator using
ambient air. (GLSD data 1993) Note: the same
reduction curve as Pure Oxygen thru diffusers.
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Applications of the Venturi Aerator - 2
• Mixing and Equalization
• Supernatant aeration from digesters
• Landfill leachate aeration prior to
headworks.
• Oil & Grease Recovery.
• Effluent aeration to streams or wetlands.
• Stripping PCE, TCE, etc. from industrial
wastestreams and groundwater.
• Lagoon aeration with two zones.
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Applications of the Venturi Aerator - 3
• Pre-aeration in front of SBRs, RBCs,
Bioclere/FAST trickling filters to protect
zoogloeal mass from toxic shock, reduce
temperature (+DO), adjust pH and achieve
BOD reduction.
• Used as a venturi DAF application by
discharging into stilling well (DO >12.5).
• Bioreactors for degradation of difficult
substances (e.g. HCHO, acetone, etc.)
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Applications of the Venturi Aerator - 4
• Aerobic Sludge Digesters
• Augment or enhance DAF systems during
summer months.
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Design Considerations
• Start with total oxygen demand required for
desired reductions and sizing unit(s).
• Determine what features need to be factored
into the design.
• Flooded or lift suction.
• Location of equipment.
• Use of Gorman-Rupp self-priming pumps.
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Summary
• The venturi aeration process is simple, easy
to install into existing systems, has no
moving parts.
• The mechanical energy is derived from the
pump accelerating the liquid through the
venturi nozzle creating the vacuum to
aspirate ambient air into process liquid.
• The venturi aeration process provides many
enhancements to existing wastewater
treatment systems and their performance.
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