AERATION PROCESS IN SUBMERGED MEMBRANE BIOREACTORS FOR WASTE - - PowerPoint PPT Presentation

AERATION PROCESS

IN SUBMERGED MEMBRANE BIOREACTORS FOR WASTE WATER TREATMENT

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SEM Photomicrograph of a Biofilm Formed on a Cellulose Acetate RO Membrane fed with Pretreated Municipal Wastewater (at Water Factory 21, California). Biofilm is Displayed in Edge View to Illustrate the Layering of Cells/EPS, Layering Effect may result from Changes in Compaction as influenced by the System Operating Pressure. T = top (feed-water) Surface of biofilm; B = bottom of biofilm nearest to membrane; flux direction indicated by arrow. Bar = 1m.

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PERMEATE FLUX

Poiseuille — Hagen

• f

law the to according

h . 2 m 1 ) R R ( . p

CL M p

TM

p

J

         

  



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EFFECT OF SUPERFICIAL GAS VELOCITY ON CAKE THICKNESS

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Va[m3 . m..2 . H.1]

Effect of Bubbling Strength , V

a, on Steady-State flux, Jss, of Intermittently Aerated

Activated Sludge: Dp = 0.5 m, P = 30 kPa, MLSS = 8 kg. M1, T = 20 OC

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Effect of Air-Liquid Two-Phase Flow Velocity, u*, on Steady-State Flux, Jss, of Intermittently Aerated Activated Sludge: Dp = 0.5 m, P = 30 kPa, MLSS = 8 kg / m3 , T = 20 OC

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Cross-Flow Microfiltration Model for Suspension with Particles of Different Sizes

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Principle of Operation of the Plant SH

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OPTIMISATION OF AERATION PROCESS IN SUBMERGED MEMBRANE BIOREACTORS FOR WASTEWATER TREATMENT

OBJTECTIVE

• Aeration is required for

(a)

Oxygenation of the MLSS, and (b) Creation

• f

airlift to produce adequate crossflow velocity over the submerged membrane to minimize its fouling and enhance its microfiltration performance

• Fine air bubbles are considered to be efficient for oxygenation

and coarse air bubbles for airlift

• In order to achieve minimum energy consumed for generation

the bubbles, the aeration process need to be optimised by determining appropriate mix of the fine and coarse bubbles in terms of their size and population

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OXYGENATION

• Under process conditions, the oxygen transfer rate must

meet the demand of the biomass in the aeration tank

• The oxygen demand of the biomass can be determined on

the basis of bio-kinetic theories ( e.g with the software GPS-X, Version 5.0, based on ASM 3)

• The dissolved oxygen concentration (CL) is to be

maintained by the oxygen transfer rate through the air bubbles

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ACTIVATED SLUDGE MODEL NO.3 (ASM 3)

ORGANIZATION International Association of Water Quality (IAWQ). TASK GROUP Task Group of Mathematical Modeling for Design and Operation of Biological Wastewater Treatment Willi Gujer*, Mogens Henze**, Takahashi Mino*** and Mark van Loosdrecht

*Swiss Federal Institute for Aquatic Science and Technology and Swiss Federal Institute of Federal Technology, EAWAG/ETH, 8600 Duberdorf, Switzerland **Technical University of Denmark, Building 115, 2800 Lyngby, Denmark *** The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113, Japan Delft University of Technology, Julianalaan 67, 2628 BC Delft, The Netherlands.

CAPABILITY

• The activated Sludge Model No.3 (ASM3) can predict oxygen consumption,

sludge production, nitrification and denitrification of activated sludge systems.

• Typical kinetic and stoichiometric parameters are provided for 10 oC and 20 oC

together with the composition of a typical primary effluent in terms of the model components.

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SUBMERGED MEMBRANE BIO-REACTOR

(for Wastewater Treatment)

AERATION ENERGY OPTIMISED

ASSUMPTION FOR ANALYSIS

• Pre-screened and pre-settled municipal wastewater
• Interloop reactor
• Steady state conditions
• Confined and coalesence – free bubbles
• No impediments to the oxygen mass transfer
• Planar aerator
• Membrane to operate below the critical flux

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AIR BUBBLES IN QUIESCENT LIQUID

Pat L CL L H  UB

D

dB  de, d VB = /6 d3

B B

A =  d2

B

dB B = (L - G)VB

AIR

O2 = 23% G , CG

Total Pressure inside the Bubble: PT = Pat + L gH + 4 / dB Shape Regimes = f(EO, M, Re)

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AIR BUBBLES DIAMETER

diameter Sauter Mean Bs d : Where 2 Bi d . a 1 i 3 Bi d a 1 i Bs d 1/3 b) . 2 (a Bi d         

b a

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OXYGEN CONTENT IN AIR BUBBLES

• Oxygen content in air bubbles is 20.95 % by volume and

23.15 % by weight of dry air.

• With

air = 1.293 [kg/ m3] O2 ~ 300 [g/m3of air] —

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AIR BUBBLES

• The

fluid mechanics

• f

the submerged membrane models is dominated by the expansion and rise of air bubbles introduced through the crossflow aeration under the modules.

• Consequently, the

bubble formation and bubble rise velocity decisively influence the hydraulic environment

• f

the membrane surface in the case of a given module geometry.

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AIR BUBBLES

(Contd.)

• Oxygenation is best served by fine bubbles, as they have :
• 1. Higher interfacial area (contact area / volume) for
• xygen transfer to the liquid compared to the coarse

bubbles at equal flow rates.

• 2. Fine bubbles have less rise-velocity in the liquid

compared to coarse bubbles and therefore more residence time is available for O2 - mass transfer.

• Minimization of fouling of membrane is best effected by coarse

bubbles, as they generate higher cross-flow velocity over the membrane by air-lift effect compared to fine-bubbles and produce flux through the membrane (permeability).

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AIR BUBBLES

(Contd.) VELOCITY OF BUBBLES

• TERMINAL VELOCITY OF ISOLATED BUBBLES

(Stationary Fluid)

— NOT APPLICABLE FOR THE BUBBLE SIZE RANGE IMPORTANT FOR BIOLOGICAL WASTEWATER TREATMENT

• For bubbles > 5.7 mm dia. The rise velocity in non-Newtonian liquids:

Where :

UB

—

Velocity of air bubble dBe

—

Equivalent bubble dia L

—

Density of liquid G

—

Density of gas L

—

Viscosity of liquid

 

dia mm 20 for m/s 0.3 U . g . e bubbles cap spherical for ) d . (g 0.71 U 2. dia mm 8 . for s / m 1 . U . g . e mm 1 d bubbles spherical for 1) e R Law (Stokes μ 18 ρ ρ . g . d U 1.

B 0.5 Be B B Be L G L Be B

          

2 / 1

Be B

d 23 . 2 U 

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VOLUME FLOW OF AIR

) (Frequency ] s [ second per produced bubbles

• f

Number — . N ] s / [m flow

• f

rate volume Air — . V : Where ) generation bubble free e coalescenc (for — d . /6 . . N . V

• 1

B 3 G 3 B B G

 

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POWER DELIVERED TO THE AIR BUBBLE

• Bubbling, like other polarization control techniques, has energy demand.
• Future efforts are likely to be aimed at minimization of energy.
• There is need to identify the optimal bubble size and frequency for a

given application.

• VB

= f(Q, dor, , , , p, K, Vch, g, H) Where: dor

• dia of orifice

 p

• density of dispersed phase

K

• Orifice constant

Vch

• Chamber Volume

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ENERGY REQUIRED FOR OXYGENATION

• The Oxygen Quantity supplied ~ several parameters:

Bubble size, O2 – content in water, content

• f other gases (e.g. CO2), water depth and

temperature, contamination with surfactants.

• The range of bubble sizes generated by the aeration

system and the energy used for bubble generation are of interest.

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ENERGY REQUIREMENT

• The decrease in activated sludge volume through higher MLSS

concentrations inevitably leads to higher aeration requirements or to HIGHER POWER CONSUMPTION for aeration purposes.

• If  - factor of 0.75 is determined for a conventional activated

sludge process with MLSS of 4 g/L, an increase of the MLSS to 20 g/L with the MCASP will result in an increase in energy requirements for aeration of almost 400%.

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STANDARD OXYGEN TRANSFER RATE (SOTR)

• SOTR (ASCE)

Mass of oxygen transferred per unit time into a given volume of water and reported at standard conditions OC (in Europe)  Oxygenation Capacity SOTR = = KL a20 C*20 V

Where :

V = Tank Volume CL = Bulk liquid phase oxygen concentration t = time KL = Overall liquid film coefficient a20 = Interfacial area / unit liquid volume C*20 = Clean water oxygen concentration at diffuser depth and 20OC

STD L

dt dC V      

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STANDARD OXYGEN TRANSFER EFFICIENCY (SOTE)

] h / m [ conditions dard tan s at rate flow Air G ] h / kg [ Oxygen

• f

rate Flow Mass W Rate Transfer Oxygen dard tan S SOTR : Where G 30 . SOTR W SOTR SOTE

• 3

N S O S O

    

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The Initial Bubble Size Required to Aerate Water, to Achieve 95% Transfer of Available Oxygen. Bubble Initially Contains Air

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The BOD Curve. (a) Normal Curve for Oxidation of Organic Matter (b) The Influence of Nitrification

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Translation of Clean to Dirty Water Oxygen Transfer Rates

) 20 ( La ) T ( La 20 T 20 20 20 T La La

K K and v P e d w s P v P e d w b P * s C * s C ) water clean ( * C ) field ( * C ) cleanwater ( K ) field ( K Where

* C ) 20 T ( ) C * C ( ) )( SOTR ( f OTR

20 L 20

               



         

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APPLICATION TO PROCESS CONDITIONS

 

* 20 L * 20 20 T f * 20 20 L L * f f L f L * f f L

• cess

Pr L f

C ) C C ( SOTE OTE C . a . K ) C C ( a . K SOTR OTR V C C a . K dt dC V OTR

• 

   

                      



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NOMENCLATURE

OTRf Oxygen transfer rate under process (field) conditions [kg / h] SOTR Standard Oxygen Transfer rate at 20OC, 1 atm and 100% relative humidity [kg/h]  Wastewater Correction factor for oxygen transfer coefficient [-]  Wastewater correction factor for oxygen saturation [-]



Temperature correction factor for oxygen saturation [-]  Pressure correction factor for oxygen saturation [-]  Temperature correction factor for oxygen transfer coefficient [-] CL Bulk liquid phase oxygen concentration [mg / L] Clean water oxygen saturation at diffuser depth submergence and 20OC [mg/L]

* 20

C

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C* Dissolved oxygen saturation concentration [mg/L] C*

ST

Surface saturation concentration at temperature T [mg/L] CS

* 20

Surface saturation concentration at temperature 20oC [9.09 mg / L] T Absolute temperature [OK] KLa Oxygen transfer coefficient [h-1] KLaT Oxygen transfer coefficient at temperature T [h-1] KLa20 Oxygen transfer coefficient at temperature 20OC [h-1] Pb Barometric Pressure [k Pa] PV20 Vapor pressure at temperature 20OC [k Pa] PS Standard barometric pressure [101.325 k Pa] de Effective saturation depth at infinite time [m] w Specific weight of water [N/m3]

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RHEOLOGICAL QUALITIES OF ACTIVATED SLUDGE FROM MCASP

• The parameter viscosity is unsuitable for comprehensively

describing the flow qualities of activated sludge

• The representative viscosity at a shear rate of 23 s-1 is

approximately 10 times greater than the representative viscosity at a shear rate of 1,200 s-1

• The activated sludge from MCASP plants show a

pseudo plastic behavior

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OXYGEN INPUT

t K. , C C ln t . K ) C (C C d K.dt dt . K . K C (C C d C d C d . w M . w ρ1 . M ) C (C U . d .H k . . V . 6 ΔM . . N . M U H . ) C (C k . d . π ΔM

L L

C O C L S C C L S L W L, B ) L s L 2 S B B G 2 B 2

O

t O O 2 O 2 O

L L

L B L s L B

              

     

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OXYGEN INPUT

(Contd.)

tank water

• f

area

• verall

the into supply air

• f

velocity Mean w V . V H K size Bubble U . d k . 6 K : Where ] e 1 [ C C tank free Oxygen O C for

~ ~

G W , L B B L B kt S L O , L

                     

     



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OXYGEN MASS TRANSFER EFFICIENCY

• Aeration

systems using fine bubbles should be considered advantageous (for water).

• Systems for the aeration of wastewater must be considered insufficient if they

cannot produce a minimum oxygen input per unit time required for wastewater biology, regardless of their respective transfer efficiency.

• CONCLUSION

The aeration systems used today have a considerable potential for further development from the point of view of increase in the oxygen input in relation to the energy required.

C) 20 at water (for D 3.236 d distance) any without

• ther

the follows bubble (one d U f K . g . w s C max ) ( ) C s (C . , K . K E w V kWh 2 O g k E . M

O

1/3 N B B B max B L W L B 2 O

2 2

O O

η η

            

• 

      

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OXYGEN UTILIZATION

• The results show clearly that in order to achieve high
• xygen utilization efficiencies, it is necessary to use

bubbles which are smaller than made in established technology.

• Using air for example, the calculations point to the need

to be able to generate bubbles in the range 300 – 1000 m in diameter for aeration of depths below 6 m.

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Principle Principle Principle

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AIR LIFT

• Superficial liquid velocity

Where:

 Superficial liquid velocity in riser [ms-1] hD  Height of gas-liquid dispersion [m] hL  Height of ungassed liquid [m] r  Fractional gas holdup in the riser [ - ] d  Fraction gas holdup in the downcomer [ - ]   Overall fractional holdup [ - ] KT  Fractional loss coefficient for the top zone of the reactor [ - ] KB  Fractional loss coefficient for the bottom zone of the reactor [ - ] Ar  Cross-sectional area of riser [m2] Ad  Cross-sectional area of downcomer [m2]

0.5

2 2 2 D

) ε (1 1 ) A r A ( K ) ε (1 K ) ε r (ε h g 2 U

d d d Lr

B r T

             

    

Lr U

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• Internal Loop Reactor

separation liquid gas without 0.9) (k r ε k m/s 1.35 U U ) U (U 1.35 0.24 U (Emp.) b A d A 11.402 K 0.5 ) d ε (1 1 . d A r A K d ε r ε h 2g Lr U B K T K

d ε r ε

Gr Lr 0.93 Lr Gr Gr 0.789 B B D

2 2

               

                                         

REARRANGED

1 ) ε )(1 ε (ε h 2g ) A r A ( . K . U

2 D 2 B 2 Lr

d d r d

  

 accounts for reactor geometry

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• Gas holdup

D 2 B 2 Lr r d

h 2g ) d A r A .( K . U For h h h

ε ε ε

D L D

   

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• Gas Liquid Interfacial Area

) ε (1 d ε 6 a and phase) liquid

• n

(based ) ε (1 d ε 6 a

d r r

Bd Ld Br Lr

d

    

• Residence Time

Ld Ld Lr Lr

U ) ε (1 h t U ) ε (1 h t

d d r r

   

] s [ downcomer in time residence Liquid t ] s [ riser in time residence Liquid t [m] downcomer

• f

height h ] m [ riser

• f

height h ] 1 [m down in phase) liquid (basedon area l interfacia liquid gas Specific a ] 1 [m riser in phase) liquid

• n

(based area terfacial in liquid gas specific a : Where

Ld Lr d r Ld Lr

         

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AIRFLOW RATE EFEECT ON FLUX IN SUBMERGED SYSTEMS

Airflow rate (m3 h-1) Gas holdup (h-1) Aeration Intensity (m h-1) Flux (L m-2 h-1) Config Reference

0.55 0.8 0.05 — HF Bouhabila et al (1996) 42 2 10 12 HF Ueda et al.. (1996) 3.2 – 4.3 0.6 – 0.8 1.9 – 2.5 1.1 – 2.2 HF Dufresne et al. (1998) 0.8 0.8 8 12 FP Dufresne et al. (1997) 100 11 2.2 31 FP Gunder et al. (1999) 80 13 1 20 FP Gunder et al. (1998) 78 35 0.9 18 FP Gunder et al. (1998) 0.72 0.009 0.7 8.3 HF Visvanathan et al (1997)

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START

UGr, Ab, Ad, Ar, and hL

Assume ULr (0.01 – 2 m-1s)

Calculate r Calculate  d or  d = 0 Calculate  Calculate hD Calculate KB Calculate ULr

Compare Assumed ULr With Calculated ULr

Write, ULr, ,  r,  d and hD No

Flow Chart for the prediction of liquid velocity in airlift reactors

STOP

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Effect of Gas Velocity on Final Flux in Filtration with Submerged Membranes

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HYDRAULIC PERFORMANCE VERSUS CROSS-FLOW VELOCITY

Cross Velocity (ms-1) TMP (bar) Flux (L m-2 h-1) Permeability (L m-2 h-1 bar-1 ) References Submerged 0.5* 0.3 21 70 Ishida et al.(1993) 0.4* 0.15 12 80 Ueda et al. (1996) 0.3 – 0.5* 0.3 17 57 Shimizy et al. (1996a)

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Effect of Cross-Flow Velocity on Permeability

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ENERGY REQUIRED FOR GENERATING AIR BUBBLES

The energy supplied to the aeration system per unit time: Total pressure loss: Our interest: Volume of single bubble:

st tot tot k E

P p . . . 1 P

V

   

pex hyd pipe tot

p       

B L hyd

d 4 . g .

H

    

] d , d [ d 6 V

e B B

3



       

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Total aeration gas volume/time: Hydraulic energy per unit time:

        

3 B tot

d 6 n . V

] d 4 g [ ) d 6 ( n . .

B L B hyd tot EH

H V P

3

       

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STANDARD AERATION EFFICIENCY (SAE)

• SAE

 The rate of Oxygen transfer per unit power input (based on DP or WP)

•  Adiabatic Law

Where :

DP = Delivered Power [kW] AP = Air Power [kW] GS = Air flow rate at std. conditions [m3

N / h]

PD = Absolute pressure downstream of blower [kPa] Pa = Absolute pressure upstream of blower [kPa] k = Coefficient of adiabatic compression

                      1 k a P d P s G 1 . AP DP DP SOTR SAE

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Where:

PE  Power [N.m] K  Efficiency of the system [ - ] ptot  Total loss of pressure [a] in the system Pst  Energy per unit time required for stirring water [Nm] ppipe  Loss of pressure in the air supply piping [Pa] phyd  Pressure loss determined by the water depth in the tank [Pa] pex  Pressure loss component as excess pressure [Pa] E  Energy [kg m2/s2] VB  Volume of air bubble [m3]   Density of air [kg/m3] H  Water depth [m]   Surface tension [kg/s2] dB  Bubble dia [m] n  Number of bubbles [-]

] S 3 m [ flow air

• f

rate Total tot V. 

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Theoretical Calculation of the Specific Energy Requirement (Plants SP and SH)

SP SH

Average flux 17 13 Permeate Volume Flowrate m3 / h 1.4 1.0 Suction height H m 1.5 2.5 Power of Pumps kW 0.011 0.014 Air Volume Flow Nm3 / h 68 – 80 68 - 100 Blowing Depth (cross flow and biology) m 2.6 2.0 Power of Blowers kW 0.9 – 1.1 0.7 – 1.0 Specific Energy Requirement kWh/m3 0.7 – 0.8 0.7 – 1.0

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Specific Energy Requirement for Membrane Filtration and Oxygen Supply Dependent on the MLSS Concentration for MCASP (Plants)

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SMBR MODELLING

Register all influences on the hydrodynamics of aeration through a model

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COMPUTATIONAL MODEL

DEFINE TANK GEOMETRY

SPECIFY MEMBRANE CHARACTERISTICS & CONFIGURATION SPECIFY RAW INFLUENT RATE OF FLOW & COMPOSITION

S M B R

EFFLUENT OF SPECIFIED RATE OF FLOW AND COMPOSITION

M L S S AIRLIFT GPS-X-V.5.0

O2 FOR OXYGENATION X – FLOW TMP COARSE BUBBLES O2 IN FINE BUBBLES O2 IN COARSE BUBBLES AIR BUBBLES SIZE, POPULATION VELOCITY AIR BUBBLES SIZE, POPULATION VELOCITY ENERGY REQD. FOR FINE BUBBLES ENERGY REQD. FOR COARSE BUBBLES OPTIMIZATION PROCESS

• MIN. ENERGY FOR SMBR

SUBROUTINE SUBROUTINE

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