TECHNICAL ASSISTANCE PROJECT
GREELEY WASTEWATER TREATMENT FACILITY
GREELEY, COLORADO
JUNE - JULY, 1972
U. S. ENVIRONMENTAL PROTECTION AGENCY
SURVEILLANCE AND ANALYSIS DIVISION
TECHNICAL SUPPORT BRANCH
REGION VIII
AUGUST 1972
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S&A/TSB-4
TECHNICAL ASSISTANCE PROJECT
GREELEY WASTEWATER TREATMENT FACILITY
GREELEY, COLORADO
June - July, 1972
TECHNICAL SUPPORT BRANCH
SURVEILLANCE AND ANALYSIS DIVISION
U. S. ENVIRONMENTAL PROTECTION AGENCY
REGION VIII
August 1972
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TABLE OF CONTENTS
Section Page No.
I. Introduction 1
II. Purpose and Scope 1
III. Description of Plant 1
IV. Summary of Assistance Project 4
A. Control Testing 4
B. Process Modifications 5
C. Performance Results 8
V. Summary and Conclusions 13
VI. Recommendations 14
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LIST OF FIGURES
Figure Page No
1. Plant Flow Schematic 3
2. North Plant Performance (% Reduction of 6005 vs Time). . 9
3. Total Plant Performance (% Reduction of BOD5 vs Time). . 10
4. Total Plant Performance (Effluent BOD5 - mg/1 vs Time) . 11
5. Total Plant Performance (Effluent 6005 - Ibs. vs Time) . 12
ii
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I. Introduction
The Greeley Wastewater Treatment Facility has continually
experienced difficulties in achieving the desired degree of treat-
ment to meet the State's requirement of 80 percent reduction of
biochemical oxygen demand - 5 day (BODc). The plant's main problem
is organic overload. Despite the overload, modifications in the
plant's facilities and in operational modes have allowed an increase
in the percentage removal of 6005 from 40 percent in April of 1971
to the 70 percent range in April of 1972. Engineers from the Colo-
rado State Department of Health and the Superintendent of the Greeley
plant were responsible for developing the necessary modifications to
improve the plant's performance.
In an effort to further improve the plant's performance, the
Superintendent of the Greeley facility requested assistance from
the U. S. Environmental Protection Agency - Region VIII. An assis-
tance project was initiated at the Greeley plant on June 5, 1972.
This report summarizes the findings of that project.
II. Purpose and Scope
Plans have been developed to provide for new treatment facilities
at Greeley to alleviate the organic overload that exists at the present
plant. These facilities are scheduled for completion in March 1973.
Since future improvements have already been planned, the emphasis of
this report does not deal with present plant limitations and future
expansions. Only those portions of the existing facility that will
be used in the future are discussed and modifications that could aid
plant performance outlined. The main purpose of this report is to
document the results achieved in improving plant performance by im-
proved operation during the Federal Technical Assistance Project.
III. Description of Plant
Presently the Greeley wastewater treatment plant treats the waste
from the City of Greeley plus the wastes from a large packinghouse
(Monfort Packing Company). The present facilities consist of a trick-
ling filter plant (South Plant) constructed in 1955 and an activated
sludge plant (North Plant) constructed in 1965. In the past the
trickling filter plant was used to treat the industrial waste and the
activated sludge system was used to treat the domestic waste. Odor
problems and mechanical problems forced the shut down of the trick-
ling filter facility. During this time an attempt was made to treat
all the wastes in the activated sludge plant. The gross overload on
the activated sludge system reduced the plant's efficiency, resulting
in a very poor quality effluent. New facilities were planned to de-
crease the load on the activated sludge system. Since these new
facilities were not scheduled for completion until March 1973, efforts
to improve the effluent quality from the Greeley plant became necessary.
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The old trickling filter plant was modified and made operable to
serve as a pre-treatment system for the packinghouse wastes. The
filters were placed back in operation in December 1971. The pre-
treated waste from the trickling filter plant was then mixed with
the incoming domestic waste and treated in the activated sludge
system. Pre-treatment of the packinghouse waste on the trickling
filters greatly reduced the organic load on the activated sludge
plant.
Figure 1 shows the various units at the Greeley facility. The
activated sludge plant is located on the north side of the receiving
stream (the Cache La Poudre River) and the trickling filter plant is
located on the south side of the river. Sludge handling facilities
(anaerobic digesters) are located on the south side of the river.
All waste initially enters the plant on the south. Waste that has
been pumped to the north side of the river for treatment in the
activated sludge plant flows under the river to the south side for
disinfection in the chlorine contact tanks prior to discharge.
Future plans call for the construction of an anaerobic, aerobic,
facultative lagoon system to be built to treat the packinghouse waste
and the waste activated sludge. Domestic waste will continue to be
treated in the activated sludge plant. The trickling filter plant
and its appurtenances are scheduled to be abandoned.
Emphasis during the technical assistance project was placed on
the operation of the activated sludge facility. Therefore, units
and operational controls for this portion of the facility will be
described in more detail.
Incoming raw sewage and industrial waste which has been
pretreated on the trickling filters are pumped to a splitter box
on the north side of the river. Sewage flows by gravity from the
splitter box to the two primary clarifiers. Each primary clarifier
has a central sludge thickening compartment. Prior to assistance,
sludge at times became too thick to pump from the primary clari-
fiers. Two 75 gpm piston pumps remove the sludge from the primary
clarifiers and pump it to the anaerobic digesters on the south side
of the river. Effluent from the primary clarifiers flows to a
channel located between the two aeration basins. Gates along the
channel allow settled sewage to be applied at three locations along
each aeration basin. Prior to the assistance project all sewage was
applied through the gate at the head end of each aeration basin.
Mixed liquor from each basin flows to two final clarifiers. Clarified
effluent is transported under the river to the chlorine contact basin
on the south side and then is discharged to the Cache La Poudre River.
Return sludge is removed from the final clarifiers by suction
type scraper arms and pumped by two centrifugal pumps through a
common header back to the aeration basins. Return sludge can be
added to the aeration basins at three separate locations along each
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ACTIVATED SLUDGE PLANT
Secondary 1Clarifiers
Aeration
Tanks
Activated
Sludge Control
Building
SplitterA
Box «-
TRICKLING FILTER PLANT
Chlorine Contact Chamber
Trickling
Filters
FIGURE 1
FEDERAL ASSISTANCE RKXJECT
Bumpy VfeSTBWTER TREATm«T FACILITY
Jue 1372 TO JULY ]972
PLANT FLOW SOCWTIC
Sludge Drying
Beds
North
Domestic Waste
-. ,, ,,.». ( primary
\^J Clarifier V^./Clarifier
Grit Chamber .S
rCComminutor
Industrial Waste
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tank. Sludge is wasted from the system by partially closing the valves
along the return sludge header, and thus forcing the return sludge up
to the splitter box where the wasted sludge is diverted to the primary
clarifiers. Measurement of waste sludge flow is accomplished by the
use of v-notch weirs. Return sludge flow is measured by an in-line
propeller type meter, accurate to 100 gpm. Return sludge flow rates
are adjusted by opening or closing a butterfly valve. Gate valves are
used to control the distribution of return sludge to each aeration basin.
The activated sludge portion of the plant was designed to achieve
80% removal at aeration basin loadings of 78 Ibs. of 8005 per 1000
cubic feet (13,200 Ibs. 8005 /day). Present loadings on the aeration
portion of the activated sludge plant are approximately 110 Ibs. 8005
per 1000 cubic feet of aeration basin volume (18,500 Ibs. BOD5/day)
or about 140 percent above design. Present flows to the activated
sludge portion of the plant are less than the average design flow
rate of 7 mgd.
IV. Summary of Assistance Project
Little could be done to improve the performance of the pretreatment
trickling filters due to the limited recirculation capabilities. There-
fore, the major emphasis of the technical assistance project was the
operation and control of the activated sludge portion of the plant.
A. Control Testing
The superintendent of the Greeley facility had initiated a
program of control testing. These tests were modified slightly
and used to control the plant's performance. The control tests
used were dissolved oxygen tests, centrifuge tests, turbidity,
settleability tests, and sludge blanket depths. These tests
were conducted five times a day by plant personnel.
A dissolved oxygen meter was used to measure the oxygen
concentrations of the mixed liquor in the aeration basins. Since
the plant is organically overloaded, the determination of dissolved
oxygen was necessary to indicate those conditions which depleted
the availability of oxygen.
Centrifuge testing was used to determine rapidly the variations
in solids concentrations throughout the day, as well as day to day.
Tests were conducted on the mixed liquor, return sludge, and
primary sludge. The centrifuge tests resulted in a percent solids
determination. A correlation between percent solids by centrifuge
and solids by weight was determined. The results of this corre-
lation indicate that one percent solids from the centrifuge test
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on mixed liquor and return sludge was equivalent to 660 mg/1
solids by weight. This factor will change as the sludge
characteristics change.
Turbidity tests were conducted on the effluent from the
final clarifiers to monitor improvements in performance prior
to obtaining a BOD5 test result.
Settleability tests were conducted on the mixed liquor to
monitor and observe sludge settling characteristics.
Sludge blanket depth determinations were made on the final
clarifiers to monitor changes in the depth of the blanket.
Depth determinations were also made on the sludge in the pri-
mary clarifiers.
Results of these tests were used to perform daily calculations
and to develop graphs that are used to control the activated sludge
process.
B. Process Modifications
Various changes were made in the operational mode of the
activated sludge portion of the Greeley facility.
Conditions as they existed prior to Federal assistance are
outlined below:
All sewage was loaded at the head of each aeration basin.
Return sludge was pumped to the head end port and the center
port along each aeration basin. Rates for returning sludge
were set at twenty-five percent of the incoming flow. A
minimum of 500 gpm per pump or 1000 gpm return sludge rate had
been established.
Mixed liquor concentrations were built from 300 to 500 mg/1
on Monday to 900-1000 mg/1 on Friday. Mixed liquor concentra-
tions were dropping dramatically over the weekend when the waste
from the packinghouse decreased.
Activated sludge was being wasted to the primary clarifiers
only on Wednesday, Thursday and Friday. Mixed liquor was re-
turned to the splitter box which directs the flow to the pri-
maries to effect the removal of solids. No controlled wasting
was performed on the weekend or on Monday or Tuesday.
Modifications that were made to the above operational mode
are outlined below:
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Incoming sewage was "step" loaded to the aeration basins,
i.e., the sewage was loaded through each of the three gates
along the aeration basin. This modification was made because
of the excessive organic load on the plant. The organic over-
load requires an amount of oxygen greater than the plant was
designed to provide. Step loading equalizes the demand for
oxygen throughout the aeration basin, thus optimizing the use
of oxygen. Oxygen profiles were conducted along the side of
the aeration basin to determine the optimum gate settings.
For example, higher dissolved oxygen values in a portion of
the aeration basins indicated that more of the incoming sewage
load should be applied to that section.
At the start of the project, it was decided to try and
build the mixed liquor solids concentration to a maximum
value. It was felt that increased solids could better handle
the organic overload. As the solids concentration was increased,
the effluent clarity from the final clarifiers increased. How-
ever the effluent quality as measured by the suspended solids
concentration from the chlorine contact tank decreased. It was
determined that a cross-connection existed between the plant
effluent and mixed liquor from the aeration basins. A partially
closed valve was discovered to be the source of the cross-
connection. This leaky valve explains why the plant personnel
had been unable to maintain a solids concentration over the
weekends when the packinghouse waste was eliminated. After this
valve was shut completely off, solids built rapidly in the system.
It was felt that dissolved oxygen would be the critical factor
controlling the amount of solids that could be maintained. How-
ever, bulking of solids from the final clarifiers proved to be
the most critical factor.
Solids were wasted from the system by partially closing the
valves which controlled the distribution of return sludge to the
aeration basins. Flow adjustment of the desired quantity of
waste sludge was relatively easy to accomplish by measuring the
head on V-notch weirs. But, adjustment of the partially closed
return sludge valves to obtain an even distribution of solids
to each aeration basin was difficult to accomplish. Flow dis-
tribution of the return sludge to the aeration basins had to be
"eye-balled" and then one-half hour after a setting was made,
samples had to be collected from each aeration basin and percent
concentration by centrifuge had to be determined to find if a
solids imbalance due to return sludge flow differences had
occurred. Each time the return flow rates were readjusted, the
waste sludge flow rate changed due to the common header. Also,
each time return flow rates were increased or decreased, the
head on the gate valves changed, thus requiring a readjustment
of the flow distribution between the two aeration basins. To
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correct these difficulties would require flow measuring devices
(i.e., weir boxes) on the discharge return sludge ports. It would
also be more desireable to have a separate line for sludge wasting
which would eliminate the difficulties with a common header.
Return sludge rates were increased during the assistance project
from the rate of twenty-five percent of the incoming flow to selected
arbitrary rates. Rates were adjusted to the maximum return flows
possible throughout most of the day. These rates were reduced at
night and increased in increments in the morning as the flow increased.
During the initial portions of the assistance project and after
the cross-connection was discovered, sludge was wasted to keep the
final clarifiers from bulking solids. This required wasting rates
of 500 to 700 gpm during the week when the packinghouse was in oper-
ation. Wasting at these rates pinpointed the bottleneck in the
plant's capabilities. Waste activated sludge was pumped to the
primary clarifiers which are equipped with central sludge thickening
compartments. The sludge is then pumped, at a maximum rate of 150
gpm, to the anaerobic digesters on the south side of the river.
The sludge produced by a high rate activated sludge process (i.e.,
110 Ibs. BODc per 1000 cubic ft.) generally does not settle or
concentrate well. It was quickly determined that the sludge
thickeners could not cause thickening of the 500 to 700 gpm of
waste sludge to a degree that all solids could be removed by the
available 150 gpm sludge pumps. Therefore, the primary clarifiers
filled with solids and effluent suspended solids from the primary
clarifiers increased from less than 100 mg/1 to greater than 300
mg/1. These septic solids placed an extreme overload on the aera-
tion portion of the activated sludge system. It is doubtful that
the anaerobic digesters which are already overloaded could have
handled the solids that were wasted even if they could have been
removed from the primary clarifiers.
After the limitation on sludge pumping and thus wasting of
activated sludge was determined, an effort was made to develop
a satisfactory operational mode. It was decided to continue
to operate the aeration portion of the system to get optimum
solids production, since a large portion of the BODs in the
waste is used in assimulative and endogenous respiration during
cell growth. Wasting to the primaries was then optimized by
monitoring the sludge blanket in the primary clarifiers. Wasting
was continued until the primary clarifier blankets began to
increase to the point that "bulking" of solids from the primaries
would occur. In this manner, all the solids that could be
handled by the system were removed. Also, all the 8005 that
could be utilized in solids production was removed. Those solids
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that could not be removed by the system were lost in the effluent.
This mode of operation provided the maximum degree of treatment
from a grossly overloaded plant.
C. Performance Results
The performance of the North (activated sludge) plant is
depicted in Figure 2 by the plot of percentage reduction of
BOD5 versus time. Prior to assistance the 7-day moving average
indicates that the percent reduction of 8005 was approximately
65 percent. After assistance the percent reduction of BODg was
between 80 and 90 percent. The fluctuation in the percentage
reduction of BOD^ after assistance reflects the somewhat spor-
adic loss of solids that occurs. As outlined previously, the
system cannot handle all of the solids and therefore some
solids must be lost in the effluent. It is important to note
that the efficiency of removal from the North plant is calculated
on the load to that facility. In-plant loads (i.e., digester
supernatant) are discharged to the North Plant. Therefore, it
is possible for the North plant to have an efficiency equal to
or greater than the total plant efficiency.
Figure 3 shows the total plant performance for percent
reduction of BODg. A gradual increase in percent reduction
was occuring prior to the start of the Federal Assistance
project. This increase reflects the improvement in performance
of the trickling filter with the warmer summer temperatures.
After the start of the Federal Assistance project, percentages
of BODg reduction, as indicated by the seven day moving average,
increased from the 70 percent range to the 80 and 90 percent
range. Again the sporadic loss of solids from the activated
sludge plant prevents the development of a consistent removal
pattern.
Figure 4 depicts the improvement in plant performance as
measured by effluent BODg concentration. The decline in
effluent concentration prior to assistance is due to the increased
performance of the trickling filters. The effluent BODc, as
indicated by the seven day moving average, had decreased to
approximately 105 to 110 mg/1 prior to assistance. After assistance
the 7-day average effluent BODs concentration ranged from 28 to
90 mg/1 with an average of about 60 mg/1. Thus, after Federal
Assistance was initiated, the effluent 6005 concentration was
reduced by at least 15 mg/1 and as much as 82 mg/1 with an
average reduction of about 45 mg/1.
Figure 5 shows the pounds of BODg in the effluent versus time.
Again the decline of BODg in the effluent prior to assistance is
due to the increased performance of the trickling filters. The
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7-Day Moving Avg.
Daily Values
FEDERAL ASSISTANCE REJECT
GREELEY WASTEWTER TREATMENT FACILITY
JUNE 1972 TO JULY 1972
NORTH Piwr PERFORWNCE
PERCENT REDUCTION OF BOifc
50
TIME
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Daily Values
»• 7-Day
% Moving Average
FEDERAL ASSISTANCE PROJECT
GREELEY WASTEWATER TREATMENT FACILITY
JUNE 1372 TO JULY 1972
TOTAL PLANT PERFORMANCE
PERCENT REDUCTION OF BOD5
vs
Tin-
50
5/15
20
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200
160
CH20
UN
Q
O
CD
80
FIGURED
FEDERAL ASSISTANCE PROJECT
GREELEY WASTEWATER TREAirtNT FACILITY
JUNE 1972 TO JULY 1972
TOTAL PLANT PERFORMANCE
EFFLUENT BODc (MG/L)
7 Day Moving Average
Daily value
5/15 20
25
30
6/5
10
15
TIME
20
25
30
7/5
10
15
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10
CO
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BOD5 in the effluent, as indicated by the seven day moving
average, had decreased to between 5000 and 6000 Ibs./day
prior to assistance. After assistance the seven day moving
average indicates a BODg load in the effluent that is quite
variable, but less than the effluent BOD5 load before assis-
tance. Data for the time period represented in Figure 5 was
grouped and averaged. Prior to assistance, the average BOD5
load discharged to the river was 6500 Ibs./day. After assis-
tance the average BODs load discharged to the river was 3500
Ibs./day which represents a decrease of 3000 Ibs. of BODg per
day (population equivalent of about 15,000 people) discharged
to the Cache La Poudre River.
The results outlined above indicate a definite improvement
in performance of the Greeley facility. The majority of this
improvement is due to the discovery of a partially opened
valve which provided a cross-connection between the mixed
liquor and the final effluent. Also, change of the operational
mode caused an improvement in effluent quality as measured by
BODs. Consistent effluent quality will be difficult to achieve
at the Greeley plant due to the organic overload received at
the plant and the plant's inability to handle sludge solids
which causes sludge bulking.
V. Summary and Conclusions
Modifications to the Greeley Facility have allowed an increase
in the percentage removals of BOD5 from 40 percent in April of 1971
to the 70 percent range in April of 1972. Engineers from the Colo-
rado State Department of Health and the Superintendent of the Greeley
plant were responsible for developing the necessary modifications to
improve the plant's performance. At the request of the plant Super-
intendent, assistance was provided by the U. S. Environmental Protection
Agency. Percentage removals were increased to the 80 percent range
during the course of this assistance. During certain periods, 7-day
average removal efficiencies were in the 90 percent range. As a
result of the improved efficiencies a reduction about 3000 Ibs. of
BOD per day has been removed from Greeley's effluent.
The majority of the improvement at the Greeley plant was due to
the discovery of a partially opened valve which provided a cross-
connection between the mixed liquor from the activated sludge plant
and the final effluent. An improved operational mode for the
activated sludge portion of the plant also resulted in improved
effluent quality. Consistent effluent quality will be difficult
to achieve at the Greeley plant due to the organic overload
received at the plant and the plant's inability to handle sludge
solids which causes sludge bulking or loss of solids.
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Achieving optimum reduction of BODs at the Greeley plant will
require close control over the activated sludge portion of the
facility. The plant will have to be controlled to obtain the maximum
reduction of 6005 by assimulative and endogenous respiration and the
maximum removal of solids with the present sludge handling facilities.
The majority of difficulties that presently exist at the Greeley
plant should be alleviated by the addition of a new facility that is
under construction. However, several areas within the plant that will
continue to be used in the future should be modified or evaluated.
These are:
1. Weir boxes should be placed on the return sludge ports
to each aeration basin so that equal flow distribution
and the resulting solids distribution can be achieved.
2. Flow control and flow measuring devices should be modified
on the return sludge system so that increments of flow
less than ±100 gpm can be set and maintained.
3. The sludge wasting system should be a separate system to
avoid the balancing problems that occur in a combined
system.
4. The primary sludge pumping capabilities should be evaluated
to see if they will be adequate for future needs.
VI. Recommendations
The following recommendations are made:
1. Control testing should be continued at the Greeley facility
to aid in the operation of the activated sludge portion of
the plant.
2. The activated sludge portion of the plant should be operated
to obtain maximum solids production from the reduction of
BODc, thus optimizing the reduction of BOD5 as a result of
cell assimulation and endogenous respiration.
3. The maximum amount of solids that can be handled by the
present sludge handling system should be removed daily.
This will increase solids and associated 6005 removals
and will minimize the amount of solids that will be lost
in the effluent.
4. The operational mode initiated during Federal Assistance
should be continued (i.e., step loading, maximize mixed
liquor concentrations, etc.).
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5. Weir boxes should be placed on the return sludge ports to each
aeration basin to allow for adjustment of flow distribution.
6. Return sludge flow control and measuring control should be
modified to allow for the adjustments of flow less than
±100 gpm.
7. Sludge wasting (both primary and activated) should be evaluated
to determine if these systems will be adequate for future needs.
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GPO 844 • 880
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