EPA
TECHNOLOGY
TRANSFER
EFFICIENT PREPARED BY
TREATMENT OF U.S.
SMALL MUNICIPAL ENVIRONMENTAL
FLOWS AT PROTECTION
DAWSON.MINN AGENCY
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Increasingly rigid water pollution control stan-
dards require, in many cases, higher degrees of
sewage treatment than often provided by conven-
tional processes. The performance of many small
sewage treatment plants is frequently degraded by
the discharge of large quantities of solids in the
plant effluent. The solids discharges may be caused
by improper plant operation, surges of raw sewage
flow, or mechanical failures within the plant. These
problems are particularly common to small plants
which often receive limited operator attention and
which are frequently subjected to severe flow varia-
tions. A survey in 1960 of extended aeration plants
throughout the U.S. showed that they removed an
average of 86 percent of the BOD but only 62 percent
of the suspended solids found in the raw sewage.
There are also an increasing number of cases where
nitrification or nitrogen removal is required.
The Dawson, Minnesota, project was placed in
operation in October, 1974 and has demonstrated the
ability of a small plant (260,000 gpd capacity) to
provide highly reliable and efficient removal of
BOD suspended solids, and consistent nitrification.
Under controlled conditions, nitrogen removals of
up to 90% were also attained without the use of any
chemicals. These desirable goals were achieved at
reasonable costs and without any extraordinary
operator attention. The Dawson project has demon-
strated that small plants can meet very stringent
effluent goals (BOD and suspended solids less than
5 mg/l, ammonia concentrations of 0.1 mg/l) with-
out the use of expensive chemical treatment
processes.
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Figures 1 and 2 illustrate the process. Following
screening, the raw wastewater from Dawson is
pumped to an aeration channel or "oxidation ditch."
The process is a modification of the activated sludge
process. Aeration is provided by two floating aera-
tors which also keep the liquid moving around the
aeration channel at a velocity high enough to pre-
vent deposition of the solids. The aeration channel
at Dawson is designed so that the depth of mixed
liquor can be varied from 3 feet to 4.1 feet, provid-
ing 83,000 gallons of surge storage capacity within
the aeration channel. At design flows, the aeration
time in the channel is 17.7 to 25.4 hours, depending
upon the depth. Under the actual flows received,
aeration times averaged 35 hours during a 300-day
test period. The mixed liquor flows from the chan-
nel to a chamber which controls the flow to the
downstream clarifiers. By use of the storage volume
in the aeration channel, peak flows were reduced
about 31%, stabilizing the operation of the secondary
clarifiers. The mixed liquor flows to two down-
stream clarifiers in series. Design overflow rates
are 580 and 400 gpd/sq ft for clarifiers 1 and 2
respectively. Provisions were made to feed chemi-
cals and provide flocculation between the clarifiers
to assure the stringent effluent suspended solids
goal of 5 mg/l would be met. It has not proven neces-
sary to feed chemicals. Chlorination is provided
prior to discharge. Excess sludge is trucked to
nearby farmland where it is spread.
/deration Channel
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RAW SEWAGE
SUPERNATANT RETURN
WASTE
SLUDGE
INFLUENT
MANHOLE
TRUCK
LOADING
RETURN SLUDGE
CLARIFIER #7
FLOCCULATOR
CLARIFIER #2
CHLORINATION
EFFLUENT
Figure 7. Process Flow Sheet
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INDICATES SAMPLING
OR TESTING POINT
FLOATING
AERATOR NO. 2
FLOATING AERATOR NO. 1
SUBMERGENCE
ADJUSTMENT
MECHANISM
SLUDGE PUMP NO. 2
SLUDGE PUMP NO. 1
CLARIFIER NO. 1
CLARIFIER NO. 2
EFFLUENT
MANHOLE
CHLOKINt CONTACT TANK
CHEMICAL TRANSFCR
MANHOLC
CHEMICAL
STORAGE
Figure 2. Dawson Treatment Plant
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The performance of the Dawson plant over the
first 300 days of operation may be summarized as
follows:
BOD, mg/l
Influent
Effluent
Suspended Solids, mg/l
Influent
Effluent
Average
155
3
200
8
60-250
1-7
70-1621
1-30
If only 2 days of the suspended solids data are
eliminated, the average suspended solids content
of the final effluent becomes 5 mg/l. Throughout
the entire study, essentially complete nitrification
was achieved as ammonia concentrations in the
effluent were generally 0.1 mg/l or less. Nitrifica-
tion was achieved even during severely cold weather
when water temperatures in the aeration channel
were very near 0°C. Temperatures were below 6°C
for the first 100 of the 300-day study period. Sludge
ages were always 20 days or more. It was found that
denitrification could be achieved by careful control
of aeration so that dissolved oxygen levels went to
zero before the mixed liquor completed traveling
around the aeration channel to the aerator. Over
a 65-day period when such control was practiced,
nitrogen removals of 60-80% were typically achieved
with some values as high as 90%. The wastewater
itself provided the carbon source for the denitrify-
ing organisms so that no additions of methanol or
other carbon sources were required.
It was found that the plant could be operated for
as long as 90 days without the need for sludge wast-
ing by allowing the mixed liquor solids to build up
to 10,000 mg/l before wasting sludge. The mixed
liquor solids readily settled to solids concentrations
of 2.5 to 3.5%.
The high degree of treatment noted above has
been consistently achieved without any extra-
ordinary operator skills or attention. About 4 to 6
manhours per day are spent at the plant. A typical
day in the life of a plant operator at Dawson includes
the following duties:
1. Read influent flow meter.
2. Clean bar screen.
3. Skim settling tanks.
4. Make a visual check of all equipment to make
sure it is operating.
5. Measure temperature of air, channel and
settling tank contents.
6. Collect necessary samples for analysis (pri-
marily raw sewage, MLSS and effluent).
7. Check DO of channel.
8. Check chlorine residual and set feed rate.
9. Wash down the walkways and building walls.
10. Sludge hauling (when required).
11. Cutting the grass or shoveling the snow as
necessary.
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Floating Aerators
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Table 1 presents the construction costs of the
Dawson plant both at the time of the actual bid
(1971) and at an EPA sewage treatment plant index
reflecting second quarter, 1976 cost levels.
Operation and maintenance costs are given in
Table 2. Power costs, of course, are related pri-
marily to the electric motors that drive the various
items of process equipment. The total connected
horsepower is 53 hp, not all of which is in service
continuously. Heating costs relate to an oil fired
furnace and hot air blower system within the con-
trol building and laboratory. This is the only struc-
ture on the plant site that is equipped with a per-
manent heating system. Due to freezing problems
in the clarifier building, portable heater units are
used at certain times during the winter months.
This cost is included under miscellaneous supplies
and replacement parts. Miscellaneous supplies and
replacement parts included such items as a space
heater for providing temporary heat in the clarifier
building, three furnace repairs, replacement of a
hydraulic hose on the aerators, grease and oil, clean-
ing supplies, lawn mower parts and similar items.
There were no major repairs necessary to the process
equipment items. The transportation item reflects
the cost of disposing of sludge on nearby fields. The
sludge is disposed of in liquid form as weather
conditions permit.
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Table 1
DAWSO4N CONSTRUCTION COSTS
Approximate Unit
Process or Item Costs
Raw Sewage Pumps and
Related Items
Channel Aerators and
Related Items
Clarifiers and Housing
Return Sludge Pumping System
Control Building and Laboratory
Other (Chlorination, Chemical
Feed System)
^^
Bid Date
November 30, 1971 Estimated Costs
@EPA STP Index = 166 @ EPA STP Index = 255
$ 18,000
100,000
65,000
12,000
43,000
12,000
$250,000
Table 2
$ 28,000
153,000
100,000
18,000
66,000
18,000
$383,000
DAWSON OPERATION AND MAINTENANCE COSTS a
** ~"
ir*
Raw
Cost Sewage Channel &
Item Pumping Aerators
Power 0.5 3.6
Heating Costs
Chlorine
Misc. Supplies
& Replacement Parts
Labor 0.5 1.5
Transportation
Subtotals 1.0 5.1
Return Control
Clarifier Clarifier Sludge Bldg. &
No. 1 No. 2 Pumping Laboratory
0.1 0.1 1.2 0.5
0.7
2.0
1.0 1.0 1.2 6.0
1.1 1.1 2.4 9.2
Other Totals
0.2 6.
0.
1.0 1.
0.6 2.
2.3 13.
0.2 0.
4.3 24.
2
7
0
,6
5
2
2
a Average flow of 174,650 gallons per day
Cents/1,000 gallons treated
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The technology demonstrated at Dawson provides
a reliable, flexible, and highly efficient treatment
approach applicable to many small treatment
plants. This is particularly significant since about
70% of the municipal plants in the United States
are less than 1 mgd in capacity. The high degrees of
nitrification and of removal of suspended solids and
BOD achieved without the need for chemical treat-
ment techniques may make the approach cost-
effective in many water quality limited areas. The
simple, stable sludge handling aspects also con-
tribute to the attractiveness of the process for small
plants. The potential for nitrogen removal demon-
strated at Dawson, although not yet fully defined,
may make the process attractive for application to
instances where nitrogen removal is required.
For Further
Information:
Detailed information on this project is
available in the form of a Final Report. This
Report can be obtained by writing:
Benjamin Lykins, Jr.
U.S. EPA-MERL
26 W. St. Clair
Cincinnati, Ohio 45268
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