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:

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:

1

Since C=M/V we can also write:

2

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, cm2/cm3

KL = mass transfer coefficient, cm/h

Integration of this equation between (0,C0) and (t,C) gives:

3

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 20oC)

The presence of contaminants in the wastewater will also impact upon KLa. 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 (Cs) for each experiment by plotting the measured concentrations versus time and extrapolating the results to a horizontal line. Compare the measured Cs 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