Presentado por:

Roberto Balda Ayala MsC. Ingeniera Sanitaria

DEFINITIONS GAS TRANSFER: gas molecules are exchanged between

a liquid and a gas at a gas liquid interface.

INCREASE OF THE GAS? DECREASE OF THE GAS?

Absorption desorption

DESORPTION OXYGEN RELEASE: algal activities

RELEASE OF TASTE AND ODOUR: volatile substances

RELEASE OF METHANE, H2S: anaerobic conditions

INDUCED GAS TRANSFER BY AERATION

O2 TO GROUNDWATERS

O2 TO SEWAGE

CO2 REMOVAL

NH4 REMOVAL

CH4 REMOVAL

VOLATILE OILS REMOVAL

ELEMENTS OF AERATION

towerTOWER Bandeja

Carbn coque

Soportes de la torre

TWO LAYERS

Liquid Gas

Interphase Gas - Liquid

Resistance of gas Moles to the liquid

For O2 (low solubility) limiting resistance is within the liquid film

SO, TO START:

Cs: D.O concentration (mg/L) of tab water Csw: D.O concentration (mg/L) of wastewater

Where: Betha = Csw/Cs : 0,92 0.98

(Cs)760 : Oxygen saturation values for water under standard conditions of T and Patm Donde Cs (20C

and 1 atm) = 9.2 mg/L

(Csw)760 = (475 0.00265 SD)/(33.5 + T)

At Patm due to T and Dissolved solids (DS)

Under standard pressure (760 mm Hg)

DS : mg/L

T : C

For wastewater, DO concentration at pressure P:

P = barometric pressure (mm Hg) Pv = Vapor pressure of water (mm Hg)

Csw = Cs = (Cs)760 * (P-Pv)/(760 Pv)

KLa = 2.303 * log (Csw C1)/(Csw-C2)

t2 t1

KLa : Oxygen transfer coefficient

KLa can be obtained with two (2) D.O. experimental measurements (aerators must not be running to set D.O. = 0, then they are re-started and plotted D.O. values vs. aeration time

dC/dt C

t

C : actual DO concentration at a certain time

Slopes are computed at time intervals t

C = g O2/m3

T = seconds

Draft force

Cs C

(the higher the O2 concentration the lower the absortion rate due to

transfer)

Slope: KLa 2.3

t

Log (Cs-Co)

(Cs-C)

0.1

1.0

Lab measurement of Oxygen Transfer Coefficient KLa

Not all the O2 really dissolved is available for m.o. :

KLa of wastewater

KLa of tab water

O2 available for m.o. under operation conditions

Absorbed O2 under standard conditions