form accumulation of ice. Consideration should be
given to temperature and detention time of the
liquid and freeboard in the tank in determining
temperatures that may be tolerated. Mechanical
aerator is not suitable for extended aeration type
where eventual conversion to step aeration is
contemplated.
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a.
Mechanical Aerator Oxygenation Capacity Tests: Oxygenation
capacity tests shall be performed in a test tank that is baffled
to the size and configuration of the aeration tank to be used.
Tests shall be performed as follows:
(1) The test tank shall be thoroughly cleaned and then filled
with fresh tap water.
(2) The temperature of the fresh water shall be as close to 20
degrees C 68 degrees F as possible.
(3) The aerator(s) shall be set at its maximum submergence with
the shroud in place.
(4) Submersible sample pumps shall be installed at three
operating depths, 2.4 m, 1.8 m, 1.2 m 8 ft., 6 ft. and 4 ft.
depths at two selected locations in the basin, on opposite sides
of the tank, one at 1/4 the distance from center of tank, and the
other at 3/4 the distance from the center.
(5) A catalyst, chloride or sulfate, shall be dissolved in the
tank contents, with the aerator running to insure mixing, to a
minimum concentration of 2 mg/l of cobalt ion.
(6) After the aerator has reached a steady state condition for 30
minutes, a sodium sulfite solution, which removes the D.O., shall
be released into the tank contents at the impeller. For a liquid
having an initial D.O. concentration of 10 parts per million (ppm)
approximately one pound of dry sodium sulfite will be required per
3780 L 1,000 gallons of liquid per test. This will provide an
excess to compensate for oxidation during mixing.
(7) As the D.O. rises in the tank, six samples, one for each
depth at each location, shall be drawn at one to five minute
intervals. Intervals should be selected to get at least six sets
of samples between 10 and 90 percent saturation. The D.O. shall
be run by the Azide Modification of the Iodometric Method as
described in AWWA EWW and an average obtained on the samples for
each sampling time.
(8) While the aerator is running, electrical measurements shall
be made on the input with a recording wattmeter.
(9) A semi-log plot of the data of the D.O. deficit against time
shall be constructed. Only points between 10 and 90 percent
saturation shall be considered. A line of best fit shall be drawn
through the plotted points.
(10)
The slope of the line shall be calculated.
SECTION 11375N
Page 84
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