S&A-TSB-22
TECHNICAL ASSISTANCE PROJECT
FORT COLLINS WASTEWATER
TREATMENT FACILITY
FORT COLLINS, COLORADO
JANUARY FEBRUARY, 1973
TECHNICAL SUPPORT BRANCH
SURVEILLANCE AND ANALYSIS DIVISION
ENVIRONMENTAL PROTECTION AGENCY
REGION VIII
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S&A/TSB - 22
TECHNICAL ASSISTANCE PROJECT
FORT COLLINS WASTEWATER TREATMENT FACILITY
FORT COLLINS, COLORADO
JANUARY - FEBRUARY, 1973
TECHNICAL SUPPORT BRANCH
SURVEILLANCE AND ANALYSIS DIVISION
U. S. ENVIRONMENTAL PROTECTION AGENCY
REGION VIII
MAY, 1973
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TABLE OF CONTENTS
Section Page No.
I. Introduction^ 1
II. Description of Plant 1
III. Summary of Assistance Project 4
A. Control Testing 4
B. Process Modifications 5
C. Performance Results 15
IV. Summary and Conclusions 21
V. Recommendations 24
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LIST OF FIGURES
Figure Page No.
1. Plant Flow Schematic 2
2. Effluent BOT)^ vs Time 17
3. Effluent TSS vs Time 18
4. Percent Reduction of BOD,, vs Time 20
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I. Introduction
On October 25, 1972, personnel from the Environmental Protection Agency,
Region VIII, and from the Colorado State Department of Health conducted an
operation and maintenance survey of the Fort Collins Wastewater Treatment
Plant #2 (activated sludge plant). The data collected during this study showed
• that the Fort Collins plant generally was producing a good quality effluent.
However, the data also indicated that cycles of good quality effluent and poorer
quality effluent were occurring. In an effort to produce a consistently high
quality effluent, a technical assistance project was initiated at the Fort
Collins facility on January 15, 1973.
The purpose of this report is to document the results from the technical
assistance project. The operational controls initiated during the project as
well as design limitations observed are outlined in the report. The results
achieved in producing a consistently high quality effluent are also discussed.
It is noted that Fort Collins also operates and maintains a trickling
filter wastewater treatment facility. No assistance was provided concerning
this plant.
II. Description of Plant
The Fort Collins Plant #2 is an activated sludge facility which treats
primarily domestic sewage from the south portion of the City. The daily average
flow in 1972 was about 20,817 cu. in/day (5.5 MGD) during the summer (high flow
due to infiltration) and 15,112 cu. m/day (3.9 MGD) during the winter.
Figure 1 depicts a schematic diagram of the various plant units. Flow
enters the plant by gravity and is pumped to the primary clarifiers. Raw
sewage may also be pumped, if required, directly to the distribution channel
between the two aeration basins. Effluent from the primary clarifiers enters
the distribution channel between the two aeration basins and is directed to
the basins by opening or closing the appropriate sluice gates. Sluice gates
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To Cache La Powdre River
Sludge and^/
Grease
mary V ' '
rifier ^West Aeration Basin
FIGURE 1
TECHNICAL ASSISTANCE PROJECT
PORT COLLINS WASTEWATER
TREATMENT FACILITY
FORT COLLINS, COLORADO
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are located at the head end and half way down each aeration basin. Flow from
the primary clarifiers can by-pass the aeration basins directly to the secondary
clarifiers or to the chlorine contact basin. This by-passing may be accomplished
by opening or closing a sluice gate located at the end of the aeration basin
distribution channel.
Mixed liquor from each aeration basin is directed to a collection box
located at the end of the two basins. Flow from the collection box is distributed
to the final clarifiers through mud valves.
Sludge is collected in the secondary clarifiers by suction type scraper
mechanisms and is returned through a common header to the aeration basins by
three variable speed centrifugal pumps. These pumps may be used and adjusted
independently. Return sludge may be added to the aeration basins through ports
at the head end of the basins or through ports located halfway down the basins.
Return sludge flow is measured by an inline venturi meter.
Excess activated sludge is wasted to the raw sewage wet well and then
pumped to the primary clarifiers. Prior to assistance, excess activated
sludge was wasted by lowering a telescoping valve located on the return sludge
line. During assistance this valve was plugged and sludge was wasted by
opening a two-inch gate valve located on a two-inch pipe which was welded
into the telescoping valve. The waste sludge flow rate varied with changes in
the return sludge flow. Therefore, it was necessary to determine the wasting
rate at various return sludge flow rates.
Effluent from the secondary clarifier is directed to a chlorination basin
and is discharged to the Cache-La Poudre River.
Sludge and grease from the primary clarifiers. are pumped to anaerobic
digesters which are being operated in series (i.e., primary, secondary
digesters).
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Ill. Summary of Assistance Project
It was felt initially that the project at Fort Collins would be
directed at initiating control testing procedures to enable the operators to
"fine tune" the facility and eliminate the fluctuations in effluent quality.
It was determined, however, that some modifications were required to the
existing facilities to achieve desired operational results and flexibility.
Some of the desired modifications could not be made during the assistance pro-
ject and these changes will be discussed in the recommendations.
A. Control Testing
A series of control tests were initiated at the Fort Collins facility
to enable the operators to closely monitor the processes and to indicate when
process controls needed to be changed (i.e., adjustment of return sludge flow
rates, adjustment of the quantity of activated sludge to be wasted, adjust-
ment of dissolved oxygen concentrations, etc.). Several of the control tests
initiated were used by plant personnel before the assistance project. These
tests were modified slightly and incorporated into a routine testing procedure.
The control tests initiated were dissolved oxygen determinations,
centrifuge tests, turbidity, settleability tests, and sludge blanket depth
determinations. These tests were conducted four times per day, seven days per
week.
Dissolved oxygen (D.O.) tests were used to monitor the availability
of D.O. in the aeration basins. The most critical points for the availability
of D.O. were determined by taking measurements throughout the length of the
basins.
Centrifuge tests were used to determine variations in solids concen-
trations throughout the day and from day to day. Tests were conducted on samples
of mixed liquor taken at the discharge end of each aeration basin and on samples
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of return sludge. The centrifuge test values are expressed in percent solids
by.volume. Although it is not necessary for control, a correlation between
percent solids by volume and solids by weight was made. The results of this
correlation indicated that during the project one percent solids by volume was
approximately equal to 670 mg/1 by weight. This correlation will vary as the
characteristics of the sludge varies. For this report all solids concentrations
by weight were determined by using the correlation of one percent by volume equal
to 670 mg/1 by weight.
Turbidity tests were performed on samples of the effluent from one of the
final clarifiers. Test results were used to minotor the performance of the
activated sludge process prior to obtaining ,a BODj result.
Settleability tests were conducted on samples of the mixed liquor collected
at the discharge end of the aeration basins. Settleability tests were used to
monitor and observe sludge settling characteristics.
Sludge blanket depth determinations were made on the final clarifiers.
Results were used to monitor changes in the depth of the blanket and to determine
the amount of sludge that was accumulating in the final clarifier. It is noted
that the blanket depth determinations and wasting control procedures were begun
after the initial EPA inspection in October, 1972. The effect of using this
portion of the control testing procedures will be shown when effluent quality
is analyzed.
Data obtained from the various control tests were used to perform calcula-
tions and develop various graphs. The results of the calculations and the
trends from the graphs were used to interpret plant performance and control
plant operations.
B. Process Modifications
During the assistance project, various problems were encountered that
inhibited plant operation and flexibility. In most cases, these problems were
corrected by plant personnel. In fact, plant personnel are to be commended
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for their efforts in utilizing available material to satisfactorily complete
the required physical modifications within a short period of time. Other
modifications that could improve plant performance could not be completed
during the assistance project. These modifications will be presented and dis-
cussed in the report for future considerations at the Fort Collins plant.
1. Physical Modifications
The first change made at the Fort Collins Plant #2 involved the
method of returning activated sludge and wasting excess activated sludge. Prior
to assistance the return sludge flow was divided as equally as possible between
the four return sludge ports located in the aeration basins (See Figure 1).
To eliminate any possibility of sludge "short circuiting" through the aeration
basins it was decided to return the activated sludge through the two ports
located at the head end of each aeration basin. When this was attempted, the
increased pressure required to force the return sludge to the head end of the
aeration basins was sufficient to force solids out of the telescoping valve used
for wasting excess activated sludge and thus solids were wasted unintentionally.
Therefore, the telescoping valve was plugged, and an alternative method of
wasting was accomplished by welding a valved two inch pipe into the telescoping
valve. This method of wasting allowed for smaller amounts of excess activated
sludge to be removed from the system over longer periods of time.
Operational problems were encountered in trying to adjust return
sludge flow rates to achieve optimum solids removal with a minimum amount of
water contained in the return sludge. The control tests indicated that dis-
crepancies existed in the laboratory data and the performance of the facility.
An investigation led to the discovery of leaking seals located around the
return sludge center well of each of the suction type secondary clarifiers.
The effect of the leaky seals was to allow mixed liquor to dilute the return
sludge flow. In fact, a solids balance conducted around the final clarifier
indicated that approximately 40 percent of the return sludge flow was comprised
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of mixed liquor. Excessively high return rates were required to compensate
for the dilute return sludge and to maintain a desired solids concentration in
the aeration basins. It is also felt that a portion of the cycling problem
that Fort Collins experienced with effluent quality can be attributed to the
leaking seals. The leak inhibited proper adjustment of return sludge flow
rates and thus allowed the plant to operate somewhat at its own will.
In an effort to conserve time, the seals on both clarifiers were
replaced in a one-day period. Although this virtually eliminated the leaking
problem, it also served to remove all the solids contained in both final
clarifiers from the system. (They were wasted to the primary clarifiers.)
This solids loss had a terrific impact on the assistance project as was deter-
mined through operational difficulties encountered after the seals were fixed.
This aspect of the assistance project will be discussed later in the report.
It is recommended that the seals be routinely checked at least once a year
and replaced when necessary. It is also recommended that the -replacement
process be designed to eliminate excessive losses of sludge from the activated
sludge system.
Prior to assistance, personnel at the Fort Collins wastewater
treatment plant were operating the mud valves used to direct the flow to the
final clarifiers so that the flow or mixed liquor would "back up" in the
aeration basins. This operation served a dual purpose of equalizing a small
amount of the incoming sewage flow and also increased the liquid depth in the
aeration tanks allowing the fixed surface mechanical aerators to supply
additional oxygen to the mixed liquor. Two problems were encountered with
this kind of operation.
The first problem was one of providing a consistently adequate
supply of dissolved oxygen. After several other methods had been tried, a
solution to controlling the dissolved oxygen concentration was achieved by
raising the adjustable weir on the effluent from the aeration basins. These
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weirs had not been adjusted for several years and had rusted tight in the
lowered positions. Raising the weirs allowed the water level in the basins
to increase and thus enabled the fixed mechanical aerators to supply additional
oxygen. The advantage of this arrangement was that water did not have to be
"backed up" in the aeration tanks daily to ensure an adequate supply of oxygen.
During the process of raising the effluent weirs on the aeration basins it
was noticed that the rubber seals along the bottom of the weirs was leaking.
In fact, the leak in the west aeration basin was so severe that a hydraulic
imbalance was created between the aeration basins and it became difficult to
achieve equal mixed liquor concentrations between the two basins. To compen-
sate for this leak, a portion of the effluent weir on the west basin was
blocked off and more equal mixed liquor concentrations were obtained. It is
recommended that the more permanent solution to the problem would be replacing
the rubber seals along the adjustable weir. It is also recommended that these
seals be routinely checked at least once a year and that care be taken when
replacing the seals to avoid excessive losses of sludge from the activated
sludge system.
The second problem encountered by daily adjusting the mud valves
to the final clarifiers was one of flow distribution. Many times the quantity
of sludge in one clarifier would greatly exceed the quantity in the other.
Since mud valves are not good valves for flow control, it was felt that if both
valves could be left wide open that equal flow and thus sludge distribution
would result. This proved not to be the case; however, since the collection
box where the mixed liquor is directed to the final clarifiers was not symmetri-
cal. The turbulence and non-symmetrical arrangement of this collection box
caused flow variations between the final clarifiers to occur. A wooden baffle
was placed in the collection box to eliminate a portion of the turbulence.
This resulted in a more uniform flow distribution to the final clarifiers.
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The final clarifiers at Fort Collins are equipped with a
suction-type scraper mechanism. An operational problem was encountered in
trying to "draw" a uniformly thick concentration from each of the eight re-
moval ports. Adjustments were made to try to balance the return sludge con-
centration from each port. It is recommended that these ports be monitored
routinely to insure a uniform sludge concentration in the return sludge flow.
2. Operational Modifications
Initially, it was felt that the lack of fine tuning was causing
the cycling effect in the effluent quality at the Fort Collins #2 plant.
However, it was determined that this was not the problem. The major operational
problem at the Fort Collins plant is one of developing and maintaining a
mature good settling sludge. After discussing past difficulties encountered
by present and previous operators, it became apparent that this problem has
plagued the plant since it began operation.
Developing and maintaining a mature good settling sludge at
Fort Collins requires that a sufficient mass of sludge be held in the system
to insure favorable food to micro-organism ratios consistent with the
physical facilities provided. When both seals in the final clarifiers were
replaced, a large portion of the solids that the operators had accumulated
in the system were removed. The result of this loss of solids was that a
poor settling sludge developed in the system. The first attempt at improving
sludge settleability was to simply stop wasting and let the sludge mass
increase to a desired level. This proved to be difficult since solids would
readily "bulk" from the final clarifiers and not allow a substantial increase
in sludge mass in the system. This has been a continual problem at the
Fort Collins plant in the past. The relationship of the size of the clarifiers
and aerators at the facility is such that increasing solids to a desired level
is hindered because bulking in the final clarifiers limits an increase in sludge
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mass by "automatically" wasting solids from the system. First glance at this
problem indicates that additional clarifiers are needed to eliminate the bulk-
ing. Although this may provide a solution, it is not felt that this is the
most desirable approach.
Aeration basin detention time is a limiting factor in allowing
the development of a good settling sludge at the Fort Collins facility. Normal
aeration basin detention times are from 6 to 8 hours for conventional activated
sludge systems. Based on daily average flows aeration basin detention times
at Fort Collins range from a high of 5.7 hours during winter months (Flow =
14,383 cu. m/day (3.8 MGD))to a low of 3.9 hours during summer months when
infiltration occurs (Flow = 20,817 cu. m/day (5.5 MGD)). Although the Fort
Collins plant is designed to operate as a conventional type of facility the
aeration basin detention times are minimal. This problem is accentuated when
a poor settling sludge exists in the system. Poor settling sludges require
high return sludge flow rates to maintain a desirable solids concentration in
aeration basins. The effect of the high return rates is that it cycles the
sludge quickly through the aeration basin. For example, the effective sewage
detention time is equivalent to the volume of the aeration basins divid.ed by
the incoming flow while the effective sludge detention time in the aerator is
equivalent to the volume of the aeration basins divided by incoming flow plus
the return sludge flow. This factor becomes increasingly significant when
trying to develop a mature good settling sludge.
To develop a good settling sludge requires that the sludge be
in the environment of the aerator in sufficient quantity so that desirable
food to micro-organisms ratios are achieved. Therefore, it was decided to
try to operate the Fort Collins facility in the contact stabilization mode.
All of the return sludge flow was returned to the head end of the aeration
basins while the incoming sewage was introduced at the inlet gates located
half way down the aeration basins. Supposedly, the head end of the aeration
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basins should have contained sludge at a concentration approximately that of
the return sludge flow and thereby should have increased the mass of sludge in
the aeration basins. However, due to the effective mixing provided by the
surface mechanical aerators, no differentation of solids concentration could
be detected in the aeration basins. The method of using contact stabilization
to provide adequate sludge detention time in the aeration basins would be
desirable especially during the summer months when high infiltration occurs
and aeration basin detention times are at a minimum. A suggested arrangement
would be to use one aeration basin as a reaeration tank and one as a contact
tank. A few minor modifications would give the operators the contact stabili-
zation flexibility.
Along these same lines an approach to increase the mass of
sludge at the Fort Collins plant was devised. Since it was difficult to
increase solids without severe bulking, it was decided to use one of the
aeration basins to store solids until a desired mass was accumulated. This
was accomplished by loading only one aeration basin with incoming sewage and
putting only a portion of the return sludge flow in the other basin. This
allowed an accumulation of solids at a higher concentration in the aeration
basin not receiving any sewage flow. When the desired mass of solids had
been accumulated in the system, the plant was switched back to the conventional
mode of operation (i.e., all sewage and return sludge introduced at the head
end of the aeration basins). High mixed liquor concentrations (approximately
equal to 3000 mg/1) were achieved in both aeration basins. The poor settling
solids were kept in the system by minimizing bulking. The procedure used to
minimize bulking was to by-pass primary effluent directly to the final clari-
fiers and thus decrease the solids loading on the secondary clarifiers. This
method of operation was carried on about a week to give the sludge a chance to
develop better settling characteristics. As the sludge began settling better,
less and less primary effluent had to be by-passed to the secondaries and
finally all the flow was handled through the aeration basins.
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In summary, the aeration basins and final clarifiers at Fort
Collins are sized so that difficulties are encountered in trying to develop
and maintain a good settling sludge. The major factor influencing these
difficulties is the minimal size of the aeration basins. When low solids
concentrations occur, such as during start-up or as a result of breakdowns
or maintenance, it is recommended that the procedure used during assistance
(i.e., building solids in one aeration basin and storing them in the other) be
used to aid in rapid development of a good settling sludge.
Another major operational problem at the Fort Collins plant is
the limited time during the day the plant is operated. Due to the relatively
small aeration basins it becomes necessary to continually readjust return
sludge flow rates as the flow changes. For example, in extended aeration
activated sludge plants (aeration basin detention time = 24 hours) the incoming
flow has little impact on the contents of the large aeration tank and as a
result, return adjustments, although necessary, are not as critical. The
smaller the aeration basin becomes (i.e., shorter detention times) the more
sensitive the mixed liquor concentration is to the incoming flow. Since the
Fort Collins plant has only eight hours a day operation all adjustments to
return rates must be accomplished in an eight-hour period. This proved to be
unsatisfactory during the assistance project ,and plant personnel were requested
to come in "after hours" to make adjustments in return sludge flow rates.
However, the flow does not significantly change until 2:00 a.m. in the morning,
and this was too late to expect the operators to stay on the job. The result
of not adjusting return sludge flow rates especially in a plant with short
aeration basin detention times is that a lower inventory of solids must be
carried to avoid bulking problems. The lower solids inventory and the lack
of return adjustments to maintain the highest mixed liquor concentration
possible, results in a poorer quality effluent than could be achieved if
return adjustments were made. It is recommended that twenty-four-hour
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operation be provided at the Fort Collins facility so that optimum performance
can be achieved.
3. Future Modifications
Although numerous operational and physical modifications were
made at the Fort Collins plant several areas that pose operational problems
still exist. Perhaps the most significant area of difficulty encountered at
the Fort Collins plant was the relationship between the volume of the aerators
and the clarifiers. The effect of this imbalance in volumes can be pointed
out by first comparing two variations of the activated sludge process (i.e.,
high rate processes versus conventional processes). High rate activated sludge
processes are characterized by relatively small aeration basins and large final
clarifiers. Settling characteristics associated with the process are generally
considered to be relatively poor. Conventional activated sludge processes
generally exhibit better settling characteristics and are characterized by
larger aeration basins and smaller clarifiers in relation to a high rate
process. At Fort Collins the relatively short detention times in the aeration
basins result in the process tending toward the high rate activated sludge
variation. As a result, the clarifiers appear to be unsatisfactory any time
the system gets out of balance. Although an additional clarifier may provide
satisfactory performance, a better approach would be to provide additional
aeration capacity. This would enable the process to tend more toward the con-
ventional activated sludge process and its associated better settling character-
istics. The result would be to provide a more easily controllable plant
capable of producing a higher effluent quality.
Another modification that would aid the operators in control-
ling the present facility would be to provide them with the flexibility to
operate one aeration basin as a reaeration tank and the other as a contact
tank. This modification would allow plant personnel to maintain a high
inventory of sludge during the summer months when infiltration rates increase
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significantly. During the summer months in the past, difficulties have been
encountered in maintaining an adequate amount of sludge due to solids losses
in the effluent from the final clarifiers.
Another area of operational difficulty was encountered in trying to
adjust and maintain low return sludge flow rates. Low rates were accomplished
by partially closing the plug valves provided on the discharge side of the
centrifugal return sludge pumps. This proved unsatisfactory because during
the night the valves would become plugged with debris and significantly reduce
the desired return flow rates. This problem could be reduced if twenty-four
hour operation were provided. Also, at low return sludge rates precise flow
measurement was difficult. It is suggested that additional flow meters that
provide more precise flow readings at low flows (i.e., less than 63.1 I/sec
(1000 gpm)) be installed. Consideration should also be given to providing
a smaller return sludge pump so that valving with the associated plugging
problems could be minimized.
Waste sludge flow control and measurement at the Fort Collins plant lacks
the flexibility that the operator needs to maintain close control over his
process. Waste sludge flow rates are difficult to determine and change
constantly as the return sludge flow, rate is adjusted. A more desirable and
controllable approach would be to provide a wasting system that is separate
from the return sludge system and one where wasting rates could be adjusted
to a desired level and measured.
Two other modifications that would assist plant personnel in providing a
consistently high quality effluent would be to provide the flexibility to use
one or both of the primary clarifiers as flow equalization basins and to provide
the capability to add polymers or chemicals to the aeration basin effluent to
aid settling in the final clarifiers.
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Flow equalization could be accomplished by providing a method
for the primary clarifiers to drain to the influent wet well during the night.
The result of this modification would be to lessen the impact of peak daily
flows on the mixed liquor solids concentration in the present aeration basins.
Also, the solids loading rate on the final clarifiers would be lessened by
decreasing the magnitude of peak flows.
The ability to add polymers and chemicals to the mixed liquor
to aid settling would allow the operators to minimize solids losses in the
effluent during period of imbalance (i.e., breakdowns, shutdowns for main-
tenance, periods when poor settling sludge exists, etc.)
C. Performance Results
The primary objective of the technical assistance project at Fort
Collins was to produce a consistently high quality effluent. To demonstrate
the "cycling" effect on plant effluent quality, plant data was analyzed for
4-1/2 months prior to the initiation of the assistance project. A period of
4-1/2 months after initiation of the assistance project was also analyzed to
show the results of assistance. Data analyzed included the biochemical
oxygen demand (BOD5) of the plant influent, final clarifier effluent, and
chlorine contact tank effluent (i.e., plant effluent) and the total suspended
solids (TSS) of the chlorine contact tank effluent. The samples for these
analyses were collected three times each day and were composited. No attempt
was made to composite samples proportional to flow. The laboratory analyses
were performed the following day.
During the assistance project, plant operators spent a considerable
amount of time repairing plant deficiencies noted during assistance and
learning and using the techniques initiated to control plant operation. Due
to this increased demand on their time, the TSS results are incomplete during
the course of the assistance project.
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Figure 2 shows the 7-day moving average BOD- concentration of the
final clarifier effluent and chlorine contact tank effluent. Figure 3 shows the
7-day moving average TSS concentration of the chlorine contact tank effluent.
Data is shown for the period of September, 1972, through May, 1973. These
figures show the cycling of effluent quality that occurred prior to the
assistance project (before January 15, 1973). The BODg concentration of the
final clarifier effluent varied from about 55 mg/1 to about 13 mg/1 and the
BOD,- concentration of the chlorine contact tank effluent (plant effluent) varied
from about 45 mg/1 to about 10 mg/1. The plant effluent TSS concentration
varied from about 63 mg/1 to about 15 mg/1. The fluctuations in effluent quality
are in part a result of many of the deficiencies that existed in the facility
prior to technical assistance (i.e., leaky seals, inadequate flow splitting,
small aerators, return sludge flow control, etc.)
It should be noted that a portion of the operational controls used
during the formal assistance project were initiated approximately one month
prior to beginning the project. (Shown on Figures 2 and 3). After initiation
of these controls plant effluent BOD^ concentrations were achieved below 20 mg/1
and are comparable to BOD5 concentration values achieved after completion of
the assistance project. Plant effluent TSS, however, was between 20 and 30 mg/1
during this one month period, much higher than TSS values achieved after
completion of the assistance project. Apparently, changes in plant operation
as a result of the assistance project provided conditions for better solids
capture and lower TSS values.
During assistance plant effluent quality was not satisfactory due to
operational problems encountered as a result of the many changes. After these
operational problems were solved, however, plant effluent BOD5 and TSS values
were never greater than 20 mg/1 (i.e., the 1975 Colorado Effluent Standard).
The final clarifier effluent BOD value did exceed 20 mg/1 for a short period
of time during the first of May. These higher BOD- values can be attributed
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50,
40
30
20
10
TECHNICAL ASSISTANCE PROJECT
FORT COLLINS WASTE WATER
TREATMENT FACILITY
FORT COLLINS, COLORADO
EFFLUENT BOD
VS.
TIME
7-DAY MOVING AVERAGE
BEFORE CHLORINATION
7-DAY MOVING
AVERAGE AFTER
CHLORINATION
10 20 30
SEPTEMBER
OCTOBER
30
10 20
NOVEMBER
30
DECEMBER
10 20
JANUARY
10 20
FEBRUARY
10 20
APRIL
30
10
MAY
20
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60
FIGURE 3
TECHNICAL ASSISTANCE PROJECT
FORT COLLINS WASTE WATER
TREATMENT FACILITY
FORT COLLINS, COLORADO
EFFLUENT SUSPENDED SOLIDS
VS.
TIME
TIME
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to higher plant flows (due to infiltration) and the associated problems with
the relatively short aeration basin detention times and inadequate return
sludge flow control (due to limited plant staffing). These problems were
partially overcome by the end of the month of May. The fluctuations in plant
effluent quality (i.e., periodic high BOD- and TSS values) can be expected to
continue as long as the problems with the short aerator basin detention time
and inadequate plant staffing continue. The length of time and number of
times the plant experiences these high BOD5 and TSS values should be reduced
due to the operational controls provided at the plant as a result of the
assistance project.
Figure 4 shows the percent removal of BOD^ through the entire plant
(i.e., plant influent through chlorinated effluent) and through the final
clarifier effluent (i.e., plant influent through final clarifier effluent).
The total plant BOD removal percentage was always greater than 90% after
assistance. The difference in the BOD- removal .percentages and BOD,, concen-
trations in the chlorinated effluent from those shown for the final clarifier
effluent may be attributed to reductions in the BOD5 due to chlorination
and possible chlorine interference with the BOD_ test (chlorinated effluent
samples are not dechlorinated). The differences between these results are
not always consistent and can be expected to vary as the plant performance
varies. It is felt that the most accurate measure of the activated sludge
performance is provided by final clarifier effluent sample analysis. To
accurately measure the activated sludge performance it is suggested that final
clarifier effluent BOD,- analysis be continued.
Results of the data analysis shown in this portion of the report
indicate that a consistently high quality effluent was developed and main-
tained for approximately 2-1/2 months. However, the more important results
are that the present plant operators were provided the means to recover from
poor effluent quality in a minimum amount of time.
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100
AiA
I AVERAGE AFTER
4 CHLORINATION
7-DAY MOVING
AVERAGE BEFORE
CHLORINATION
TECHNICAL ASSISTANCE PROJECT
FORT COLLINS WASTE WATER
TREATMENT FACILITY
FORT COLLINS, COLORADO
60
10 20 30
SEPTEMBER
10 20 30
OCTOBER
10 20 30
NOVEMBER
10 20 30
DECEMBER
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IV. Summary and Conclusions
Data collected during an operation and maintenance survey of the Fort
Collins Wastewater Treatment Plant #2 indicated that cycles of good quality
effluent and poorer quality effluent were occurring. A technical assistance
project was initiated by personnel from Region VIII of the U. S. Environmental
Protection Agency on January 15, 1973, in an effort to produce a consistently
high quality effluent from the Fort Collins facility.
A series of control tests and daily calculations were initiated at the
Fort Collins facility to enable the operators to closely monitor the processes
and to indicate when process controls needed to be changed. It is recommended
that these control tests and calculations continue to be used to guide the
operators in maintaining good operational control.
Initially, it was felt that the assistance project would be directed at
"fine tuning" the facility. However, both physical and operational modifi-
cations were required to achieve desired results and flexibility.
The series of important physical modifications made to the Fort
Collins facility are summarized below:
1. A modification was made to the location that the return sludge
was introduced to the aeration basins (i.e., all return sludge was introduced
at the head end of each basin).
2. The method of wasting excess activated sludge was changed to
allow smaller amounts of sludge to be wasted over longer periods of time.
3. Leaky seals were discovered and replaced in the center well of
the final clarifiers. Routine maintenance inspections and replacement of
these seals is required in the future.
4. The adjustable effluent weirs on the aeration basin were placed
into service to provide a means of flexible dissolved oxygen control. The
seals adjacent to these weirs were observed to be leaking and replacement
is recommended in the near future.
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5. A wooden baffle was placed in the collection box at the end of
the aeration basins to eliminate excessive turbulence and allow a more equal
distribution of flow to the final clarifiers.
6. Modifications and adjustments were made to the return sludge
ports in the final clarifiers to achieve removal of a uniformly thick return
sludge.
Operational modifications were developed at the Fort Collins plant to
minimize the time required to grow and maintain a good activated sludge. A
large amount of sludge was initially wasted from the Fort Collins plant when
the leaky seals in the final clarifiers were replaced. The poor settling
sludge that developed after this solids loss and the problems encountered in
trying to increase the sludge mass indicated that one of the main operational
problems at the Fort Collins facility was developing and maintaining a mature
good settling sludge. This fact was supported by prior difficulties that
plant personnel had experienced in 'maintaining consistent control over the
facility. A method to increase the sludge mass to a quantity that would
encourage the development of a good settling sludge was achieved after several
different attempts. The method involved using one of the aeration basins to
"store" activated sludge solids while the other aerator was used to treat the
incoming sewage and grow solids. After a desired mass of solids were
obtained, they were dispersed throughout the system and held until a good
sludge had a chance to develop. This method can be used in the future to
rapidly develop a desired sludge.
Other operational modifications used during the project were to initiate
"after hours" adjustment of process controls. Future operation of the facility
should include greater than eight hour a day operation or effluent quality will,
of necessity, be less than optimum. Twenty-four hours a day operation is
recommended.
Other modifications that would improve plant performance at Fort Collins
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could not be completed during the assistance project. These Modifications are
summarized below:
1. It is suggested that consideration be given to adding additional
aeration capacity at the Fort Collins plant to provide a more easily controll-
able facility capable of producing a higher quality effluent.
2. The capability of operating one aeration basin as a reaeration
tank and one basin as a contact tank (i.e., similar to the contact stabiliza-
tion variation of the activated sludge process) should be provided at the
Fort Collins facility.
3. Difficulty was encountered in adjusting and maintaining low
return sludge flow rates. Consideration should be given to providing a
smaller pump and metering equipment that would provide precise measurement
and control at low return rates.
4. Inadequate flow control and measurement still exist on the
waste sludge flow system despite modifications that were made. It is
suggested that an independent wasting system be developed and variable flow
control and metering be provided.
5. The capability of using one or both of the primary clarifiers
as flow equalization basins would provide the operator with flexibility to
maintain a higher solids mass in his system especially during summer months
when infiltration occurs.
6. The capability to add polymers and chemicals to aid settling
would allow the operators to minimize solids loss during periods of im-
balance.
The primary objective of the assistance project at Fort Collins was
to produce a consistently high quality plant effluent. As a result of the
project the cycling effect was reduced substantially. Periodic effluent
quality fluctuation can be expected, however, due to problems with the
short aeration basin detention time and inadequate plant staffing.
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Plant effluent BOD concentrations had been reduced during the month
prior to assistance to below 20 mg/1. However, effluent TSS concentrations
were between 20 to 30 mg/1. A portion of the operational controls used
during assistance had been initiated during this time. After assistance,
the seven-day moving average values of BOD,, and TSS concentrations in the
plant effluent were below 20 mg/1. Apparently, changes in plant operation
as a result of the assistance project provided conditions for better solids
capture and lower TSS values. Seven-day moving averages of BOD,, removal
percentages for the plant were consistently greater than 90% after completion
of the assistance project.
V. Recommendations
The following recommendations are made:
1. Control tests and calculations initiated during the assistance
project should be continued.
2. The rubber seals in the center well of the final clarifiers and
adjacent to the effluent weirs on the aeration basins should be inspected
routinely and replaced when necessary.
3. The operational mode used during assistance to develop a good
settling sludge should be used when required in the future to minimize
periods of poor effluent quality.
4. Twenty-four hour a day operation should be provided at the
treatment facility.
5. Additional aeration capacity should be considered.
6. Flexibility to operate in the "contact stabilization" mode
should be provided.
7. Modifications to provide better return sludge flow control and
measurement at low rates should be provided.
8. Modifications to provide better waste sludge flow control and
measurement should be provided. Consideration should also be given to separa-
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tion of the wasting and return sludge flow systems.
9. Flexibility to use the primary clarifiers as flow equalization
basins should be provided.
10. Flexibility to add polymers and chemicals to the mixed liquor
to aid settling should be provided.
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