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The Venturi Aeration Process

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					The Venturi Aeration Process:
   Understanding Oxygen Transfer
    and Wastewater Conditioning

         Venturi Aeration, Inc.
            41 Tallant Road
        Pelham, NH 03076-2236
             603-635-8239

                                   1
           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.


                                             2
         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)
                                                3
         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.
                                             4
         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.
                                              5
         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.”

                                                6
         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.
                                                 7
         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.

                                               8
      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
                                                9
         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).
                                            10
      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.

                                                11
      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.

                                                12
          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)



                                               13
      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.

                                              14
                    Summary of BOD5 Reduction Rates in Septage

             6000



             5000



             4000


B
O
             3000
D5



             2000



             1000



               0

     Hours           0   1   2   3   4   5   6   7   8   9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

                                                 Oxygen            Diff. Air       Hydro-Vac

                                                                                                          15
     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.
                                              16
     HYDROGEN ACCEPTOR HIERACRCHY
     Hydrogen      Hydrogen      Reduced
     Acceptor      Atom added    Product

A.   O2            + 4 H+        2 H2O

B.   2 NO3         + 12 H+       N2 + 6 H2O

C.   SO4=          + 10 H+       H2S + 4 H2O

D.   Oxidized      + xH+         Reduced
     Organics                    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
                                                      17
            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.
                                                   18
   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.
                                              19
   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.)
                                               20
   Applications of the Venturi Aerator - 4


• Aerobic Sludge Digesters

• Augment or enhance DAF systems during
  summer months.




                                             21
           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.

                                            22
                  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.
                                             23

				
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