MEASURING OXYGEN TRANSFER IN AN ACTIVATED SLUDGE TANK IN OPERATION

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MEASURING OXYGEN TRANSFERIN AN ACTIVATED SLUDGE
TANK IN OPERATION
UNIVERSITY OF AALBORG, SEPTEMBER 2009 ERASMUS
TEACHING STAFF VISIT - SEMINAR B

• Salvatore Nicosia, Assoc. Prof. in Environmental
  Engineering
• DIIAA, Dipartimento di Ingegneria Idraulicaed
  Applicazioni Ambientali,
• Università di Palermo Viale delle Scienze, 90128
  Palermo, ITALY(nicosia_at_idra.unipa.it)

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THE PHENOMENA AND REACTIONS GENERALLY MAKING UP
THE UNIT OPERATION OF OXYGEN TRANSFER (PICTORIAL)
O2 from atmosphere
Mixed liquor out with the same DO as in the tank
Waste water in with its DO
Non-utilized O2
Recycle sludge with its DO
O2 transferred to mixed liquor and used up
VML
ACTIVATED SLUDGE TANK
O2 supplied
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THE FLUXES OF OXYGEN MASS TRANSFER AND USE
(QUANTITATIVE WO as kgO2/h)
Win,atm kLaatm? (Ceq CML) ? VML
Wout,atm
Wout,ML (Q Qrec) ? CML
Win,ww Q ? Cin
Wtransf kLaML? (Ceq CML) ? VML
VML
Win,rec Qrec ? Crec
ACTIVATED SLUDGE TANK
Win,aer
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THE BALANCE EQUATION FOR O2 (kgO2/h) - 1

• O2 intake from atmosphere is usually neglected
  this simplification loses significance for
  surface aeration systems, ad is relevant only if
  bubble aeration is made.

Mass of oxygen entering
Mass of oxygen exiting used

• Now we must state the system conditions during
  the oxygen transfer measure (?)

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THE BALANCE EQUATION FOR O2 - 2

• Possible and practical conditions
• during an oxygen transfer measure

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ADVANTAGES AND DRAWBACKS OF THE OPERATIONAL
CONDITIONS
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HOW CAN WE SOLVE THE EQUATION? (1)

• A- The simplest system physical batch re-aeration

Wout,atm
Win,atm kLaatm? (Ceq CML) ? VML
Wtransf kLaML? (Ceq CML) ? VML
VML
ACTIVATED SLUDGE TANK
Win,aer
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HOW CAN WE SOLVE THE EQUATION? (2)

• Physical batch re-aeration (cont)

By changing the variable CML into (Csat CML),
where Csat is the actual saturation concentration
in that basin, not foreseeable theoretically we
get
a 1st order differential equation, linear,
homogeneous, which yields
or , as well
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VIEWING IT GRAPHICALLY (1)

• Graphically, kLa is the slope of the straight
  line that interpolates the DO concentrations.
• In order to draw the line we must assign Csat a
  value, which is an asymptotic one ? we can
  estimate but not measure it.
• Ceq is taken as the value that makes best adhere
  the experimental points to the interpolating
  line.
• It is advisable to make Ceq range between the
  highest DO value measured during the experiment
  and the concentration in clean water.

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VIEWING IT GRAPHICALLY (2)
The value of kLa just calculated enables us to
draw the curve of DO concentrations versus time
as they would have been in a perfect run.
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HOW CAN WE SOLVE THE EQUATION? (3)

• B- Complete system cyclic aeration mode
  differential equation

We can skilfully group together the variables
that contain CML in order to finally get at the
differential equation
1st order, linear, non homogeneous which can be
solved in a well known way. The oxygen transfer
coefficient, kLa, is calculated at last.
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HOW CAN WE SOLVE THE EQUATION? (4)

• C- Complete system cyclic aeration mode
  mathematical artifice
• If there were equilibrium between oxygen transfer
  and use, the derivative term would go to zero and
  we should get

• By substituting this in the general equation and
  cancelling out common terms, we can finally write

which is easily integrated, linearized etc. as in
(A).
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THE EQUATION SOLVED
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THE OPERATION PLAN, 1- DO MEASURE POINTS
M.P. A ?
Bottom turbines, 2 x 7,45 kW
1 Surface turbine, 9 kW
M.P. B ?
V 410 m3 (F/M) ratio 0.04 unit power input
0,058 kW/m3
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THE OPERATION PLAN, 2- SCHEDULED RUNS
Quite ideally
S.S.
Steady State
S.S.
DO
t
Aerators off
Aerators off
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THE FIGURES OF OUR CASE-STUDY

• The slope of the straight interpolar line is
• 0,0026 s-1 9,36 h-1
• The ratio (Qi Qr)/V (8 8) / 410 0,04 h-1
• Therefore, the coefficient of oxygen transfer of
  the whole of those aeration machines in that
  basin is 9,32 (gO2transf/m3) / ((gO2deficit/m3)?h)
  .
• As the rated coefficient in clean water of the 2
  bottom turbines together is 20
• even neglecting as a first approximation the
  effect of the small surface turbine
• the efficiency of the aeration system is lower
  than 50.

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A POSSIBLE EXPLANATION FOR THIS RESULT
Rated influence circle for the bottom aerators

• The bottom aerators are too close to the walls
• SS concentration in ML is a bit high (3,87 kg
  SST/ m3)

Actual influence circle
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ACCESSORY MEASURE BIOMASS OUR

• A sample of MLSS is put into a stirred bottle,
  fitted with a DO probe
• Assumed respiration kinetics 0 order
• d DO / dt const ?
• ? (DO DO0) kres (t t0)

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APPENDIX
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LÉVOLUTION THÉORIQUE DES CONCENTRATIONS DO.D.
AU COURS DU TEMPS
De léquation intégrée on peut calculer  
On voit que la concentration en oxygène à
saturation dans les conditions de travail, C
nest rattrapée que après un temps théoriquement
infini. On obtient aussi la vitesse instantanée
de réoxygénation de leau 
et le flux unitaire doxygène transféré qui est 
où VR est le volume du bassin daération. On
obtient ainsi le diagramme suivant qui
reconstruit lessai expérimental.