S&A-TSB-23
TECHNICAL ASSISTANCE PROJECT
UPPER EAGLE VALLEY SANITATION DISTRICT
WASTEWATER TREATMENT FACILITY
AVON, COLORADO
MARCH - APRIL, 1973
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TECHNICAL SUPPORT BRANCH
SURVEILLANCE AND ANALYSIS DIVISION
S. ENVIRONMENTAL PROTECTION AGENCY
REGION VIII
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TECHNICAL ASSISTANCE PROJECT
UPPER EAGLE VALLEY SANITATION DISTRICT
WASTEWATER TREATMENT FACILITY
AVON, COLORADO
MARCH-APRIL, 1973
TECHNICAL SUPPORT BRANCH
SURVEILLANCE AND ANALYSIS DIVISION
U.S. ENVIRONMENTAL PROTECTION AGENCY
REGION VIII
JUNE, 1973
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TABLE OF CONTENTS
SECTION PAGE NUMBER
I. INTRODUCTION ]_
II. DESCRIPTION OF PLANT 1
A. BACKGROUND 1
B. PLANT FACILITIES 2
III. SUMMARY OF ASSISTANCE PROJECT 4
A. SUMMARY OF OPERATIONAL ASSISTANCE 4
B. PLANT PERFORMANCE EVALUATION 13
IV. SUMMARY AND CONCLUSION 14
V. RECOMMENDATIONS 15
VI. REFERENCES 17
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LIST OF FIGURES
FIGURES PAGE NUMBER
1. PLANT FLOW SCHEMATIC.
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I. INTRODUCTION
On September 14, 1972, Region VIII of the Environmental Protection Agency
(EPA), received a letter from the Colorado Department of Health requesting
operational assistance at the Upper Eagle Valley Sanitation District Waste-
water Treatment Facility. An on-site evaluation survey of the facility was
conducted by EPA on October 26 and 27, 1972 and a formal technical assistance
project was initiated on March 19, 1973. This report outlines the findings
and results of the technical assistance project and proposes several recommen-
dations for future consideration at the Upper Eagle treatment plant.
II. DESCRIPTION OF PLANT
A. BACKGROUND
Vail, Avon, and the surrounding area is provided wastewater treatment by
two separate sanitation districts. The Vail Sanitation District (VSD) serves
primarily the Town of Vail and discharges its effluent into Gore Creek. The
Upper Eagle VAlley Sanitation District (UEVSD) serves localities upstream
(the Big Horn Area) and downstream of the Town of Vail. The UEVSD discharges
its effluent to the Eagle River downstream of the confluence of the River with
Gore Creek.
Since a portion of the area served by the UEVSD is located upstream of the
VAil facility, an agreement between the two districts was reached providing
for a portion of Vail's collection lines to be used by the UEVSD to trans-
port sewage to the Upper Eagle treatment plant.
Flow from the Big Horn area is measured prior to entering the Vail inter-
ceptors. This flow is transmitted to the Vail plant along with sewage from
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Vail. At the Vail facility the flow is split and an amount approximately
equal to that flow measured from the Big Horn area is directed to the UEVSD
plant. Vail is required to pay the UEVSD for all wastewater sent to the UEVSD
plant in excess of that measured at the Big Horn metering station. It is
noted that the interelationship that exists between the plants provides a
means to adjust flows to a desired quanity between the two facilities and
therefore must be considered an important operational tool.
B. PLANT FACILITIES
Figure 1 shows a plant flow schematic of the Upper Eagle Wastewater
Treatment Facility. Flow enters the facility and passes through a bar screen,
grit channel, parshall flume, and comminutor before entering the aeration
basin. Flow continues through the aeration basin, final clarifier, and
chlorine basin before it is discharged to the Eagle River. Sludge removed in
the final clarifier is collected by a vacuum type sludge scraper mechanism and
is returned to the aeration basin by a constant speed centrifugal pump. Return
flow may be adjusted by opening or closing a plug valve located at the return
sludge discharge point in the aeration basin. The pretreatment facilities
and/or the aeration basin and secondary clarifier may be bypassed by opening
and closing the appropriate gates and valves.
Sludge is wasted from the bottom of the secondary clarifier through an
air lift pump to the aerobic digester. Digested sludge is drained to a sludge
drying bed and digester supernatant is directed back to the aeration basin.
Dissolved oxygen and mixing is provided in both the aeration basin and
aerobic digester by air blowers coupled with surface and subsurface mechanical
agitators.
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TECHNICAL ASSISTANCE PROJECT
UPPER EAGLE VALLEY SANITATION DISTRICT
WASTEWATER TREATMENT FACILITY
AVON, COLORADO
MARCH-APRIL, 1973
PLANT FLOW SCHEMATIC
'Final Clarifier Effluents
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III. SUMMARY OF ASSISTANCE PROJECT
A. SUMMARY OF OPERATIONAL ASSISTANCE
At Upper Eagle the primary emphasis of the assistance project was on
operational control of the facility. No formal assistance was given on
procedures used to conduct various monitoring tests such as biochemical
oxygen demand (BOD^) and total suspended solids (TSS). During the project
the BODg and TSS analysis were performed by an EPA chemist. After comple-
tion of the Upper Eagle assistance project a few BOD^ and TSS analysis
were conducted by plant operators at. the Vail facility.
Presently it is not feasible for plant operators at Upper Eagle to
routinely perform BOD^ and TSS analysis because of the limited manpower
available and lack of the necessary equipment at the facility. The necessary
laboratory equipment should be purchased to enable plant personnel to run
selected monitoring analysis.
Control over plant operation at the Upper Eagle Treatment Facility
involved conducting various "control" tests and interpreting the corres-
ponding results. The control tests used were dissolved oxygen, centrifuge
turbidity, settleability, and sludge blanket depth. The centrifuge turbid-
ity and sludge blanket depth tests were conducted four times per day, seven
days per week, the settleability test was conducted two times"per day, seven
days per week, and the dissolved oxygen test was conducted periodically
during the assistance project. After assistance the control tests were
conducted twice per day during the days when a plant operator was at the
facility.
The dissolved oxygen (D.O.) tests ware used to monitor the D.O. concentration
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in the aeration basin and aerobic digester. For both the aeration basin and
aerobic digester the D.O. test was used to insure that the D.O. concentration
was maintained greater than 1 mg/1.
The centrifuge tests were used to determine the variations in solids
concentrations from day to day. Tests were conducted on samples of mixed
liquor taken at the discharge end of the aeration basin, on samples of return
sludge, on samples of sludge in the aerobic digester, and on samples of sludge
wasted to the aerobic digester. 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 indicate that during the project one
percent by volume was approximately equal to 1,000 mg/1 by weight. This
correlation will vary as the characteristics of the sludge vary. For this
report all solids concentrations by weight were determined using the correlation
of one percent by volume equal to 1,000 mg/1 by weight.
The turbidity tests were performed on samples of clarified supernatant
from the final clarifier. Test results were used to monitor the performance
of the activated sludge process prior to obtaining a BOD^ result.
The settleability tests were conducted on samples of mixed liquor
collected at the discharge end of the aeration basin. The tests were used
to monitor and observe sludge settling characteristics.
Sludge blanket depth determinations were made on the final clarifier.
Results were used to monitor the changes in the depth of the sludge blanket
and to determine the amount of sludge that was accumulating in the final
clarifier.
Data obtained from the control tests were used to perform calculations
and develop gfaphs which were used to interpret plant performance and control
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plant operations. After completion of the assistance project Upper Eagle
purchased all the necessary equipment to conduct the "control" tests and
are using the tests, calculations, and graphs to control plant operation.
Initiation of process control pointed out various procedures that must
be used to control plant operations and in addition pointed out various
limitations in plant design which hindered plant performance.
One of the major factors limiting performance was inadequate control
over the return sludge flow rate. Return sludge flow control is necessary
to provide control of the MLSS concentration throughout the day as the sewage
flow rate varies. This fact is increasingly important at Upper Eagle because
of the relatively short detention time in the aerator (4.5 hours at the design
flow of 4,163 cu m/day (1.1MGD)) for a plant of this type (i.e. no primary
clarifiers).
Two major reasons for the lack of adequate return sludge flow control
are insufficient manpower provided at the facility and inadequate physical
control over the return flow rate. Two plant operators are available to
cover plant operation eight hours per day, six days per week. These same
operators; however, are responsible for the collection system and at times
cannot be at the treatment plant for even the limited time of eight hours
per day, six days per week. To provide for adequate return sludge flow control
an operator must be at the plant to adjust the return flow rate during the
times when the sewage flow increases and decreases. This normally represents
a minimum time of sixteen hours per day seven days per week for a plant of this
size.
Sludge is returned to the aeration basin at Upper Eagle through one
constant speed centrifugal pump at a constant rate of approximately 3,785
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cu rn/day (1 MGD).
This constant return rate represented a fluctuation in the return flow
percentage (i.e. ratio of return sludge flow rate to incoming sewage flow
rate) of about 85% to about 200%. Before plant operators can control the
return flow rate there had to be a physical method developed to accomplish
the control. During assistance two methods were developed to try and control
the return flow rate. The first method involved using a pressure switch
activated by the liquid level in the return sludge wet well. The second
method involved installing a plug valve on the return sludge discharge line
to the aeration basin. This method of control was partially successful and
allowed the flow rate to be adjusted lower than the maximum output of the
return sludge pump. Although the rate of flow of return sludge could be
adjusted the flow rate could not be measured. No return sludge flow measuring
devices are provided at the Upper Eagle facility. Attempts were made to
estimate the flow rate, but a good estimation could not be made. It is
recommended that a flow measuring device be provided.
Additional problems with return sludge flow adjustments were also
encountered after the plug valve had been installed. The sludge collection
mechanism in the final clarifier at Upper Eagle is a vacuum type scraper
mechanism that has a center wet well connected to the sludge pick up ports.
The center wet well revolves around the fixed center support column. The
joint between the support column and the wet well is sealed by a flexible
rubber gasket which is provided to prevent the mixed liquor that enters the
clarifier from leaking into the return sludge wet well. During assistance
this seal was leaking and mixed liquor was allowed to dilute the return sludge.
The effects of this dilution became more and more pronounced as the return
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sludge flow was decreased. For example, when the return rate was decreased
a greater percentage of the return flow consisted of mixed liquor and a lower
percentage consisted of settled sludge from the bottom of the clarifier.
Eventually the settled sludge in the clarifier became thicker and thicker
and plugged the sludge pick up ports. At that point the return flow consisted
entirely of mixed liquor leading through the seal and no settled sludge in
the clarifier could be returned to the aeration basin. Because of the limitation
caused by the leaky seal, the return sludge flow rate adjustments were very
ineffective in controlling the MLSS concentration in the aeration basin.
It was pointed out by the plant operators that the leaky seal had been
repaired approximately one year prior to the assistance project. It is
recommended that the seal be repaired once again and that in the future the
seal be checked at least twice per year and replaced when necessary.
Another major plant deficiency that limits plant performance at Upper
Eagle is the size of the aeration basin. The plant flow pattern is designed
in a similiar fashion to an extended aeration facility, (i.e. no primary
clarifier, one aeration basin). However, at the average daily design flow
(4,163 cu m/day(l.l MGD)), the aeration basin detention time is 4.5 hours
rather than the 24 hours normally associated with extended aeration facilities.
The detention time at average flow during assistance (2,649 cu m/day(0.7
MGD)) was 7.1 hours which is still significantly less than a typical 24 hours.
One of the disadvantages of designing a plant with conventional detention
times (i.e. 4-6 hours) and no primary clarifier is that settleable organic
material is recycled with activated sludge (i.e. it settles in the final
clarifier and is returned with the activated sludge to the aeration basin).
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This settleable organic material is in most cases more difficult to break
down by bacteriological activity and as a result must be recycled many times
in a system that has a short aeration time in order to achieve a stable end
product. The effect is to overload the system and reduce the quantity of
settleable organic material converted to a stable end product and quantity
of colloidal and dissolved organic material converted to settleable sludge
solids. An approach to increase these conversions is to carry a higher MLSS
concentration. The higher MLSS concentration in part compensates for the
shorter aeration basin detention time. The problem encountered in this
approach is that the size of the aeration basin (small) and final clarifier
(large) establishes an interrelationship that directly affects the systems
ability to maintain a selected "activated" MLSS concentration. Generally a
small aerator is associated with systems that have lower MLSS concentrations
(high rate systems).
Another limitation in plant design at Upper Eagle that indirectly affects
plant performance is the aerobic digester capability at the facility. During
assistance the digester was not performing satisfactorily and as a result
removal of excess sludge from the activated sludge portion of the plant was
severely hindered. The poor digester performance was characterized by a
sludge that would not develop a clear supernatant layer even after extended
periods of quiescense (i.e. 24 hours). To allow room for waste sludge in
the digester, sludge had to be drawn to the snow covered drying beds.
Several factors which contributed to the inadequate digester performance
during the assistance project were excessive solids loading to the digester,
low temperature of the digester contents, and inadequate detention time in the
digester.
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Approximately one week before and during the assistance project, large
quantities of sludge were wasted to the aerobic digester within a short time
period. The purpose of wasting a large quantity of sludge within a short
time period was to aid in attaining the optimum MLSS concentration. The
effect of the heavy solids loading to the digester was that normal digestion
appeared to be upset and adequate supernating in the digester was prevented.
Future plant operations should strive to achieve a balance in solids
distribution in the plant so that heavy solids loadings to the digester are
avoided.
A second factor, low temperature in the digester, was felt to be the
leading cause of inadequate digester performance during assistance. Frequently
the temperature of the sludge in the digester was so low that ice formations
were observed in and around the edges of the digester. The low temperature
of the sludge caused the microorganisms to be less active and therefore
decreased the rate of sludge digestion. A cover over the existing digester
would help this situation to a degree but other considerations such as an
external heat source or alternate means of ultimate sludge disposal must be
considered to compensate for the decreased sludge digestion rate caused by
lower temperatures.
A thorough analysis of the detention time in the digester was not made
because sufficient data was not obtained during the project. The design
detention time; however, is 5 days (2). This design detention time is not
sufficient for adequate digestion. It is recommended that a thorough analysis
be made of the detention time in the digester, with respect to adequate
digester performance, prior to initiating major modifications apparently
necessary at the Upper Eagle facility. A suggested minimum detention time for
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an aerobic digester associated with a plant like Upper Eagle's is 20 days (3).
Another limitation in plant design that degraded plant effluent quality
was the absence of a skimmer in the final clarifier. No method of removing
floating material is available at the Upper Eagle facility and as a result
the plant effluent quality is degraded. It is recommended that a skimming
device be provided.
The plant limitations that have been outlined above inhibited the ability
to provide adequate operational control at the Upper Eagle facility.
Therefore, operational assistance turned into developing methods to make the
best of the required mode of operation.
During the assistance project the sewage flow rate to the Upper Eagle
facility was increased because of the sever problems that were being
encountered with plant operation at the Vail Wastewater Treatment Facility.
Future operation of the Upper Eagle facility should include the capability
to reduce flow by letting Vail take on an additional volume of sewage. This
interrelationship is a useful tool that must be fully developed between both
facilities. It is noted that the increased flow at Upper Eagle during
assistance did cause a slight deterioration of plant effluent quality.
The method of wasting activated sludge was modified to conserve digester
capacity and to account for the quanity of solids wasted. As thick a sludge
as possible was wasted to the digester by shutting off the return sludge pump
and allowing the sludge to accumulate and concentrate in the secondary clarifier.
This sludge was then wasted to the digester at as slow a rate as possible. It
is recommended that sludge continue to be wasted as thick as possible until
adequate digester performance is attained. At that time a thick sludge should
continue to be wasted but the return sludge flow rate should not be stopped
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prior to wasting. Each time sludge was wasted the concentration was
determined using the centrifuge. The volume of sludge wasted was determined
by measuring the volume occupied by the wasted sludge in the digester. Using
both the volume and concentration of sludge wasted the quantity of sludge
removed from the activated sludge system was determined. It is recommended
that this method of determining the amount of sludge wasted be used in future
plant operations.
At the beginning of the federal assistance project, the Upper Eagle
plant was glutted with sludge solids which were being rapidly recycled
throughout the system by the constant high return sludge flow rate. By
interpreting the control tests and by analyzing unit sizes various operational
changes were concluded. In summary, it was decided to increase the MLSS to
as high a concentration as possible to obtain the best treatment using the
minimum aeration basin detention time provided. The approach to increasing
MLSS is outlined as follows: Excess sludge was in the system and was hindering
the activated sludge's ability to dewater. This excess sludge was wasted. As
the excess sludge was wasted the settleability tests indicated that the
remaining sludge in the system dewatered much better to almost twice the
concentration as with prior tests in a given time period (i.e. 60 minutes).
A plug valve was installed so that return sludge rates could be decreased to
take full advantage of the sludge's ability to dewater and to limit the
quanity of excess water that was being recycled to the aeration basin. This
mode of operation, if it had been successful, should have produced the highest
effluent quality. However, wasting limitations, inadequate return sludge
control, leaky seals in the clarifier, and other problems forced this method
of operation to be abandoned.
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The return sludge flow rate, by necessity (i.e. to prevent plugged
sludge pick up ports in the clarifier because of leaky seals), was returned
to its original high rate and the maximum MLSS concentration was achieved
by allowing the solids in the system to build and still avoid bulking in
the final clarifier. This method of operation is highly undesirable in
light of the control the operators could have if adequate facilities and
controls were available.
B. Plant Performance Evaluation
During assistance plant performance at Upper Eagle was satisfactory,
(effluent BOD^ and TSS was approximately 25 mg/1 except during the higher
flow period as explained above when effluent BODg and TSS was about 40 mg/1).
However, it is felt that a consistent, high'-quality effluent would be difficult
to maintain due to inadequate control over the treatment plant operation and
limitations in plant design. Inadequate control over plant operation was
due primarily to inadequate control over the return sludge flow rate. Control
over the return flow rate was not provided because of limited manpower
available at the plant, leaky seals in the final clarifier, and inadequate
physical control .over the return rate itself. Physical control over the
return rate was provided during the assistance project by placing a plug
valve on the return sludge line. Using the plug valve the return sludge flow
rate can be adjusted. The adjustments; however, cannot be used to adequately
control plant operation until the leaky seals in the clarifier are repaired
and adequate manpower at the plant is provided. It should also be noted that
only one return sludge pump is provided at Upper Eagle. It is apparent that
if repairs are required on that pump the entire return sludge flow capability
is eliminated and plant effluent quality would deteriorate. To prevent complete
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elimination of the return sludge capability during periods of pump maintenance
it is recommended that a separate return sludge pump be provided at Upper
Eagle. It is also recommended that the leaky seals in the clarifier be
repaired and that adequate plant staffing be provided. At that time the plug
valve or some other method of controlling the return flow rate should be used
and plant operation should be controlled in order that the most consistent,
highest quality effluent be attained with the facilities available.
IV. SUMMARY AND CONCLUSIONS
Region VIII of the Environmental Protection Agency received a letter
from the Colorado State Department of Health requesting assistance concerning
the operation of the Upper Eagle Valley Wastewater Treatment Facility. An
initial evaluation of this plant was made on October 26, 1972 and a formal
technical assistance project was initiated on March 19, 1973.
During assistance, plant personnel were given instruction in conducting
and interpreting various process control tests. Results from these tests
pointed out various procedures that must be used to control plant operation
and in addition pointed out various limitations in plant design which hindered
plant performance.
One of the major factors limiting plant performance was inadequate control
over the return sludge flow rate. A plug valve was installed so that the
return rate could be physically adjusted; however, limited manpower at the
facility and a leaky seal in the clarifier prevented attempts to control this
rate. In addition to this lack of return control, the return flow rate could
not be measured.
Another plant deficiency that limits plant performance is the size of
the aeration basin. The basin is too small for the type of facility that
exists at Upper Eagle and a consistently high quality effluent would be
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difficult to achieve and maintain. In future plant modifications consideration
should be given to expanding the aeration basin facilities.
Inadequate digester performance indirectly affected plant performance
during the assistance project. The reasons for poor performance were cold
temperature of the digester contents, inadequate detention time in the digester,
and excessive solids loading to the digester. Present digester performance
can be improved by providing proper wasting control of excess activated
sludge and by modifying the digester to include placing a cover over the
digester and possibly providing an external heat source to heat the digester
contents. An alternate means of ultimate disposal should also be considered.
In addition expansion of the digester capacity to allow for a longer sludge
detention time appears to be necessary.
Another plant limitation at Upper Eagle is that no method of removing
floating material is available. The obvious effect of this, limitation is
a degraded plant effluent.
Although various operational changes were initiated during the Upper
Eagle assistance project, plant deficiencies inhibited proper operational
control. This lack of operational control limited the improvement of
effluent quality and required an operational mode to be continued that was
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highly undesirable.
V.. RECOMMENDATIONS
Based on the results of the technical assistance project the following
recommendations are made:
1. Purchase the necessary laboratory equipment to enable plant personnel
to run selected monitoring analysis.
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2. Provide a flow measuring device on the return sludge flow line so
that the flow rate can be accurately measured.
3. Repair the leaky seal in the final clarifier as soon as possible
and in the future check the seal at least twice per year and replace it
when necessary.
4. Maintain the relationship between the Vail and Upper Eagle Treatment
Facilities so that additional flow can be accepted at one facility when the
other is experiencing operational difficulties.
5. Provide an alternate return sludge pump that can be used when the
existing pump is out of service.
6. Future plant expansion at Upper Eagle should include consideration
of enlarging the existing aeration basin capacity. In the interim period
a lower flow capacity would be desirable.
7. To aid present digester performance a cover should be placed over
the digester. Also consideration should be given to providing an external
heat source or an alternate means of ultimate sludge disposal to insure
continued operation in winter months. Any plant expansion should investigate
the sludge detention time in the existing digester which is believed to be
inadequate.
8. Place a shimmer on the final clarifier to remove floating material
from the waste stream.
9. Provide adequate plant staffing to control plant operation during
periods of increasing or decreasing sewage flow or for a minumum of 16 hours
per day, 7 days per week.
10. Continue conducting the operational control tests and operational
practices that were initiated during the assistance project.
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VI. REFERENCES
1. "Operation and Maintenance Manual, Sewage Treatment Plant, Upper
Eagle Valley Sanitation District".
2. "Process Design Manual for Upgrading Existing Wastewater Treatment
Plants" prepared for the U. S. Environmental Protection Agency by Roy F.
Weston, Incorporated, Contract No. 14-12-933, U. S. Environmental Protection
Agency, Region VIII, Denver, Colorado 80203 (October, 1973).
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