UNIVERSITY OF WATERLOO
Department of Civil Engineering
EnvE 472 Wastewater Treatment
Aeration Experiment
Purpose:
To measure the rate of oxygen mass transfer in a diffused bubble system and to
examine variables that influence the rate of mass transfer.
Background:
Mass transfer between gas and liquid phases is commonly encountered in the
environmental sector.
Oxygen supply to biological wastewater treatment
reactors and stripping of trihalomethanes from drinking water are common
examples. This experiment focuses on oxygen mass transfer.
When air and water are contacted with each other for a long period of time and
there are no reactions consuming oxygen in the water the concentration of
oxygen in the water will approach an equilibrium concentration (saturation
concentration) that is defined by Henry’s Law:
Cg
KH 
Cs
Where: Cg = gas phase concentration of oxygen (0.21 atm for oxygen in air)
Cs = equilibrium concentration of oxygen in water
KH = Henry’s Law coefficient
If the concentration of oxygen in the water is less than that specified by Henry’s
law, this provides a driving force for oxygen to move from the air and into the
water. The movement of oxygen from the gas to the water is referred to as
“mass transfer” and the rate at which oxygen can be transferred is an important
design characteristic of an aerator. In this lab, aeration will be provided by
diffused bubble aerators.
The rate of mass transfer is proportional to the difference in concentration
between the saturation dissolved oxygen concentration Cs and the actual
dissolved oxygen concentration C, the interfacial surface area per unit volume
between the gas and the liquid (a), and the volume (V) of the liquid. The
proportionality constant is the overall mass transfer coefficient (KL) based on
liquid concentrations. For a batch system the rate of mass transfer is equal to
the rate of mass accumulation in the water phase:
dM
= K L aV( C s - C)
dt
Since C=M/V we can also write:
dC
= K L a( C s - C)
dt
M = mass of oxygen transferred, mg
t = time, hours
C = dissolved oxygen concentration, mg/L
Cs = dissolved oxygen concentration at saturation, mg/L
a = gas-liquid surface area per unit volume of liquid, cm 2/cm3
KL = mass transfer coefficient, cm/h
Integration of this equation between (0,C0) and (t,C) gives:
ln (
C s - C0
)= KL a t
Cs - C
If we monitor C as a function of time we can plot the LHS of the above equation
versus t, and obtain the slope equal to KLa. It is usually difficult to measure “a”
by independent methods so the KLa product is reported as the mass transfer
coefficient.
The presence of contaminants (i.e. salts, surfactants, etc) in a wastewater will
impact upon oxygen mass transfer in two ways. The solubility of oxygen (Cs) in
water will often be reduced and hence the driving force for mass transfer
decreases. This is addressed through a correction factor (β) that is calculated
as:
β=Cs,ww/Cs,tapwater
where :
Cs,ww = solubility of O2 in wastewater
Cs,tapwater = solubility of O2 in tapwater
These solubilities should be determined at the same temperature (typically 20 oC)
The presence of contaminants in the wastewater will also impact upon K La. This
is addressed through a correction factor (α) that is calculated as:
α = KLaww/KLatapwater
where :
KLaww = mass transfer coefficient of O2 in wastewater
KLatapwater = mass transfer coefficient of O2 in tapwater
Procedure:
Two aeration columns will be employed in this experiment. One column contains
tapwater while the other contains primary clarifier effluent from the Waterloo
wastewater treatment plant. Each aeration column is outfitted with a diffused
bubble aerator and a dissolved oxygen probe/meter.
In the experiment oxygen will initially be “stripped” out of solution by bubbling
pure nitrogen gas through the column. Once the dissolved oxygen has been
virtually eliminated from the water the gas supply will be converted to air and
oxygen will be transferred into the water samples.
1.) Record the temperature of the water columns at the beginning and ending
of the testing
2.) During both the deoxygenation and oxygenation tests measure the
dissolved oxygen in the columns on a regular basis. Concentrations will
change more rapidly at the beginning of each test and hence
measurements should be taken more often (i.e. every minute). Once the
changes in concentrations slow down then readings might be taken less
frequently (i.e. every 5 minutes)
3.) The deoxygenation and oxygenation tests should be run long enough
such that there are only small changes in dissolved concentration with
time (i.e. the concentrations should approach a constant value)
Results and Discussion:
1. Estimate the saturation concentration of oxygen in water (C s) for each
experiment by plotting the measured concentrations versus time and
extrapolating the results to a horizontal line. Compare the measured C s
values to tabulated values.
2. For both the deoxygenation and oxygenation cycles of each experiment,
calculate -ln ((Cs - C1)/( Cs - C0)) for each measured value. Pay special
attention to which values are used for Cs in deoxygenation calculations (a
spreadsheet is handy for this).
3. Plot -ln ((Cs - C1)/( Cs - C0)) versus t - t0 for the oxygenation and
deoxygenation cycles of each experiment. The slopes of these lines are
equivalent to the mass transfer coefficients (KLa). Compare the KLa values
obtained for the deoxygenation and oxygenation cycles of each experiment.
4. Calculate values of α and β for the wastewater sample. Note: the α values
can be calculated separately for the deoxygenation and oxygenation cycles.
Compare the values obtained