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BIOCHEMICAL OXYGEN DEMAND.
DISSOLVED OXYGEN AND VELOCITY

CHECKS ON AIRATORS AT TH!
EAST LANSING. MICHIGAN
”WAG! TREATMENT PM

M for It. DOW of B. 5.
MICHIGAN STATE COLLIGI
Robert C. Rica
1949

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Biochemical Oxygen Demand, Dissolved Oxygen and
Velocity Checks on Aerators at the East
Lansing, Michigan Sewage Treatment
Plant
A Thesis Submitted to
The Faculty of
MICHIGAN STATE COLLEGE

of
AGRICULTURE AND APPLIED SCIENCE

by

Robert C. Riess
-...---""

Candidate for the Degree of

Bachelor of Science

June 19L9

Acknowledgements

Professor Frank R. Theroux - For his suggestion of the original
idea and his help in getting the

apparatus and material together.

Mr. Maurice Richman and

Er. Henry King - For their help at the East

Lansing plant in running the
tests, use of materials, and

general knowledge of the plant.
Patricia W. Riess - My wife, who has helped me

immensely on this paper and all

others during my college years.

217908

In 1938, the City of East Lansing, Michigan, and
Michigan State College at East Lansing deemed it necessary to
construct a sewage treatment plant. At that time, the com-
bined population totaled 10,000, of which h,000 were students
at the College. Up until the time of construction of the
sewage plant: the sewage from the College was passed into an
Imhoff tank which was, and still is, on the site of the
present sewage treatment plant. The sewage from East hinsing

was untreated.

An activated sludge type plant was decided upon and
construction began on the sewage treatment plant in 1938; on
August 21, 1939, the plant was in Operation. The plant con-
sisted of two primary settling tanks, nine aerators of the
simplex downdraft mechanical type, two secondary or final
settling tanks of the Dorr Squarex type, a sludge digestor,

and ten sludge drying beds.

The plant operated successfully for a number of
years treating an average of 1.7 million gallons of sewage

per day.

By 19L6, the combined populations of East Lansing
and Michigan State College had increased to an estimated
30,000, and an addition to the existing sewage plant became a
necessity. In 1946, enlargement began and by l9h8, construc-

tion was completed. The new additions enlarged the plant to

   

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four primary settling tanks, seventeen aerators of the simplac
downdraft mechanical type, four secondary settling tanks, two
Dorr squarex types, three sludge digestors, and seventeen

sludge drying beds. One of the sludge digestors in this case

is the old Imhoff tank converted to use.

The aerators of the original plant were twenty-three
feet, three inches square, and had an Operating depth of
thirteen feet, four inches. They were placed in three rows
of three each, and flow could be regulated between aerators

by means of a system of baffle gates in the concrete partitions.

The aerators were equipped with a 5 Hp motor that
turned the impellers at 900 RPM, forcing the air down a
twelve inch drawdown tube. The total capacity of these nine

aerators is h88,000 gallons.

The new aerators are without baffles and measure
twenty-seven feet wide and one hundred one feet long, with an
Operating depth of fourteen feet, seven inches. They were
originally equipped with a twelve inch diameter drawdown tube,
but they proved ineffective and, at the start of this test,

were being replaced by fourteen inch diameter drawdown tubes.

These aerators are placed four in a row in two rows.
They are aerated by a 5 Hp motor operating at 900 RPM, and
carrying a total capacity of 528,760 gallons. This makes the

combined capacity of the aerators 1,016,760 gallons.

During the aeration period, the pattern of the flow

is outward from the drawdown tube or toward the edges of the

-3...

aeration tank. At some outer points in the new aerators
where the flow of the two aerators meet, whirlpools and
eccentric currents are plentiful. In the old aerators, no
such condition exists due to the boundaries of the concrete

walls.

Several of the old aerators were equipped with four
steel flutes, fourteen inches wide by ten inches deep and
seven feet four inches long. These were placed so the bottoms
of the flutes were seven inches below the surface of the
sewage and arranged diagonally out from the drewdown tube.
The flow pattern on those aerators equipped with the flutes
was outward in the areas between the flutes and inward inside
the flutes. Thus it may be seen that at the time the tests
were run, four distinct arrangements of aerators were in
use, i. e., aerators with baffles, aerators with baffles and
flutes, aerators without baffles and twelve inch drawdown

tubes, and aerators without baffles and fourteen inch aerators.

The tests were run almost exclusively on the latter
two aerator types as the primary object of the tests was to
determine if the new aerators were Operating to capacity and
as desired. However, at some times during the research, the
new aerators were not operating and so several tests were run
on the Old aerators for a comparison and the results from

those tests are included in this report for added information.

During the course of these tests, the usual plant

operating procedures were not always followed due to an

unusual amount of breakdown of equipment and misfortunes.

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-4-

Consequently, the results obtained may present an untrue

picture of the plant when it is Operating at full capacity and

under design conditions.

At the beginning of the tests run, the twelve inch
drawdown tubes in aerators 10, 12, 1A, and 16 were being re-
placed with the fourteen inch drawdown tubes, and so the tests
were run on aerators ll, 13, 15, and 17, with their twelve
inch drawdown tubes. It was honed to thus be able to run a
set of tests on the latter aerators with their twelve inch
drawdown tubes and to run the same tests on them after being
replaced with fourteen inch drawdown tubes. Unfortunataly,
the fourteen inch tubes were not in place when the tests were

terminated.

Before the results of the tests are examined, a word
or two must be said about them and how they were conducted.
The dissolved oxygen tests(D. 0.) were all run using Winkler's
method and two samples run of each position. The biochemical
oxygen demand tests (B. O. D.) were also run according to
Winkler's method after their five day incubation period. The
velocity tests were run by use of a stream gage at various
depths and the time in seconds noted when ten clicks or ten
revolutions had sounded in the earphones. This was changed
to velocity in feet per second by a chart in the carrying case

of the stream gage.

In all cases the results given are an average as the

results obtained were sometimes very erratic, especially in

the velocity tests. Sometimes on two successive trials at the

-5-

same depth in the same aerator area and position, the time
for ten revolutions varied by as much as thirty seconds.
This is probably due to a reversal of the gage such as might

be caused by whirlpool interference.

In examining the results, it will be useful to know
that aerator 16 is equipped with a fourteen inch drawdown tube
and no baffles; aerator 15 has a twelve inch drawdown tube
and no b ffles; aerator A has a twelve inch drawdown tube
and baffles, while aerator 5 has a twelve inch drawdown

tube, affles, and flutes.

Taking each aerator individually and analyzing its
performance, it will be noted that in aerator 15 (Table 1),
the area nearest the aerator drawdown tube has the lowest
B. O. D., but that the B. 0. D. is not the same on a locus
around the drawdown tube. This indicates that the amount of
oxygen rising along the outside of the shaft is not uniform
throughout the aerator. The D. 0. and velocity tests bear

this out.

The velocity tests at the various depths for this
aerator show that a diagonal line through positions 19, 30,
and A2 have their highest velocities at the three feet, two
inch level of the water, while a diagonal through 1, l7, and
32 shows that the highest velocities recorded are at the

seven feet, three inch depth.

In aerator A, (Table 2) the D. 0. test was run

several times with no results obtained while the B. 0. D.

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test again indicates that the lowest B. O. D. is nearest the

drawdown tube and not uniform in value at those positions.

The velocities again seem to follow a pattern like
that of aerator 15 with a diagonal through positions 17 and
32, having the highest velocities at the seven feet, three indi
depth and the highest velocities at the three feet, two inch

on a diagonal through 19 and 30.

In aerator 5 (Table 3), as with aerator A, no results
for D. 0. were obtained and, due to the flutes being present,
the usual testing positions were not available in all cases.
Due to that situation, the results are a little more difficult

to analvze.

The B. O. D. test seems to indicate, however, that
the greatest B. O. D. is at the extremities (positions 7 and
L2), while directly around the aerator the next highest B. O. D.
is obtained. This means that the lowest B. O. D. would be
in between these positions, and the results obtained bear this

out.

In aerator 16 (Table A), the velocities again form
a pattern of sorts as the highest velocities are on a.
diagonal line through 19 and 30 at a depth of ten feet, zero
inches. However, no pattern is found through positions 17
and 32, as position 17 has its highest velocity at the two
feet, three inch level, and position 32 has its highest

velocitv at the ten feet, zero inch level.

-7-

The B. O. D. tests show some uniformity near the
drawdown tube as the four positions (17, 19, 30 and 32) only

vary by 0.7 Ppm from the highest to lowest B. 0. D.

The D. 0. tests show results similar to the B. O. D.
as the most uniform and greatest D. 0. results were obtained
at positions nearest the drawdown tube. The D. O. at these
positions (17, 19, 30 and 32) varied by only O.h ppm from

the highest to lowest values.

Before the tests were run, it was assumed that the
velocity was greatest near the bottom of the aerator and at a
locus nearest the drawdown tube. The obtained results indi-
cate that the highest velocities are obtained near the drawdown
'tube, but not necessarily at a point near the bottom. In a
number of cases, the highest velocity was at the seven foot,
three inch level, and, in some cases, at the three feet, two

inch level.

It was also thought that the air released at th
bottom of the drawdown tube surged upward t00 vertically, and
thus did not aerate the entire surface area but only that
immediately around the drawdown tube. For the most part, the
results indicate this to be true, but also show that the next
highest D. 0. tests are obtained at the extremities of the
aerators, leaving a locus of low D. 0. between the drawdown

tube and aerator extremities.

The results taken as a whole seem to indicate that

~8-

the aerators are functioning as designed, but no definite

conclusions can be drawn from the tests run for several

reasons. One of the main reasons is that during the period
the tests were run, the plant was not functioning properly
and so the results are not indicative of a plant running at

full capacity or design capacity.

Another item against the results obtained is that
the time to run the tests was so limited that a complete
analysis could not be made. To be complete, more tests would
have to be run of each position, of more positions, and of

more aerators.

Another item that would make the results more
valuable would be to run the B. O. D., D. 0., and velocity
tests on the aerator positions at as nearly possible the
same time for an aerator. The results of this report for
the tests mentioned sometimes were run as far apart as a

week's time, due to necessity.