STEADY STATE MIXED LIQUOR COLUMN TESTING
Redmon Engineering Company makes significant use of the
steady state mixed liquor column test to obtain suitable data for predicting
both the oxygen uptake rate and alpha factor for various locations within
an aeration basin.
The test is conducted in a columnar tank, 2.5 feet (0.76 m) in diameter
by 11.0 feet (3.35 m) high, operated at a water depth of about 10.5 feet
(3.2 m). A schematic diagram of the test setup is illustrated in Figure
1. Mixed liquor from an operating activated sludge plant is pumped continuously
through the column to maintain a short hydraulic detention time (8 12 minutes).
Overflow from the column is allowed to return to the basin from which it
was removed by a discharge pipe near the top of the tank. Diffusers of known
clean water oxygen transfer efficiency are tested for gas phase oxygen transfer
efficiency, under existing process conditions, using the offgas method as
developed by Ewing Engineering Company and reported by Redmon, Boyle and
Ewing(1). The D.O. concentration is monitored at two depths corresponding
to approximately 1/4 and 3/4 of the side water depth, as well as in the
vicinity of the sewage pump. The applied airflow rate of the diffuser(s)
is measured with a variable area rotameter for which suitable observations
of air temperature and pressure are recorded. The ratio of the standard
oxygen transfer efficiency of the diffusers tested in wastewater (SOTEpw)
to those encountered under similar conditions in clean water (SOTEcw), is
defined as alpha.
A mass balance conducted around the test column provides an estimate of
the oxygen uptake rate at each sampling location and operating condition
tested. The mass balance calculations are presented as Figure 2.
The column tests conducted by Redmon Engineering Company are based on early
work by Doyle, et al.(2) and Stenstrom, et al.(3). Doyle demonstrated the
alpha was depth sensitive and that bench scale alpha testing did not properly
simulate the hydrodynamics and bubble residence times encountered in typical
aeration tanks of 15 feet of depth (4.57 m) and greater. Although the column
test is particularly well suited for simulating fine pore grid aeration
systems it has other applications as well.
One of the most promising applications of the method is the measurement
of oxygen uptake rate under DO conditions equivalent to those in the biological
system from which mixed liquor is being pumped. The test procedure carefully
measures the mass flow rate of air being supplied to the operating diffuser(s)
in the bottom of the column. Offgas analysis permits accurate appraisal
of the gas-phase oxygen transfer efficiency of the diffuser allowing for
accurate determination of the mass of oxygen transferred to the mixed liquor
in the column. Mass balance calculations permit the computation of OUR under
the DO conditions existent in the full scale system.
The above approach is quite different from BOD bottle uptake tests and respirometric
measures of OUR that occur at DO concentrations of 4 mg/l to 6 mg/l. Under
these fully aerobic conditions, measured oxygen uptake rates can be substantially
greater than the actual OUR in the full scale reactor at DO limiting values
of 0.2 to 2.0 mg/l. Since the steady state column procedure measures OUR
at DO conditions equivalent to those in the full scale basin, the observed
OUR values are considered reliable indicators of the actual OUR of the system
at the sampling location in question. This approach eliminates the sources
of error extensively discussed by Mueller and Stensel(4).
Column DO is controlled by the quantity of air supplied to the test diffusers
in the column. As the air flow rate is increased so is the DO concentration
in the column. Typical practice is to operate the column at several DO concentrations
(e.g. 0.5, 1.0, 2.0 and 3.0 mg/l) and to observe OUR as a function of DO
concentration.
When sampling mixed liquor from near the inlet end of a plug flow basin,
where the DO concentration is generally between about 0.1 and 1.0 mg/l,
it is generally observed that the OUR increases until a DO concentration
of 2.5 to 3.0 mg/l is obtained. At this concentration and above, OUR is
typically observed to be constant.
Actual OUR column data obtained from a single station at the inlet end of
an activated sludge plant over a two day period is illustrated as Figure
3. When the DO concentration in the column was controlled to be equivalent
to the concentration in the full scale basin, OUR values were measured to
be approximately 43 to 44 mg/l/hr. The OUR values were observed to increase
substantially as the DO concentration in the column was increased. Employing
BOD bottle uptake tests or respirometric measures of OUR at DO concentrations
of 4 to 6 mg/l in this specific evaluation would indicate OUR values roughly
25% to 65% greater than the actual OUR in the full scale reactor at the
DO limiting values of 0.55 to 0.80 mg/l.
References
1. Redmon, D.T., Boyle, W.C., and Ewing, L., Oxygen Transfer Efficiency
Measurements In Mixed Liquor Using Off-Gas Techniques, Journal WPCF, Volume
55, Number 11, November, 1983.
2. Doyle, M.L., Boyle, W.C., Rooney, T.,and Huibregtse, G.L., Pilot Plant
Determination Of Oxygen Transfer In Fine Bubble Aeration, Journal WPCF,
Volume 55, Number 12, December, 1983.
3. Hwang, H.J., Stenstrom, M., Evaluation Of Fine-Bubble Alpha Factors In
Near Full-Scale Equipment, Journal WPCF, Volume 57, Number 12, December,
1985.
4. Mueller, J.S., and Stensel, H.D., Biologically Enhanced Oxygen Transfer
In The Activated Sludge Process, Research Journal WPCF, Volume 62, Number
2, March/April,1990.
