S&A/TSB-2
TECHNICAL ASSISTANCE PROJECT
LONGMONT WASTEWATER TREATMENT FACILITY
LONGMONT, COLORADO
MARCH - MAY, 1972
LONG
££ VRAI
JWY-2
U.S. ENVIRONMENTAL PROTECTION AGENCY
SURVEILLANCE AND ANALYSIS DIVISION
TECHNICAL SUPPORT BRANCH
REGION VIII
MAY 1972
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S&A/TSB-2
TECHNICAL ASSISTANCE PROJECT
LONGMONT WASTEWATER TREATMENT FACILITY
LONGMONT, COLORADO
March - May 1972
TECHNICAL SUPPORT BRANCH
SURVEILLANCE AND ANALYSIS DIVISION
U. S. ENVIRONMENTAL PROTECTION AGENCY
REGION VIII
May 1972
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TABLE OF CONTENTS
PAGE NO.
I. Introduction 1
11. Purpose and Scope 1
III. Description of Plant 1
A. Background 1
B. Plant Facilities 2
IV. Summary of Project 4
A. Summary of Laboratory Assistance 4
1. Review of Laboratory Procedures 4
2. Accompli shments 6
B. Summary of Operational Assistance 6
1. Control Testi ng 6
2. Data Interpretati on 7
C. Summary of PI ant Performance 8
1. Analysis of Rock Filter 8
2. Analysis of ABF Filter 11
3. Analysis of Other Plant Units 18
4. Analysis of Total Plant Performance 19
V. Summary and Conclusions 21
VI. Recommendations 23
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LIST OF FIGURES
TITLE PAGE NO.
Figure 1 - Plant Flow Schematic 3
Figure 2 - Flow to Filters vs. Time 9
Figure 3 - Load to Rock Filter vs. Time 10
Figure 4 - Effluent BOD5 - Rock Filter vs. Time 12
Figure 5 - Ratio Effluent BODs to Loading - Rock Filter vs. Time 13
Figure 6 - Load to ABF Filter vs. Time 15
Figure 7 - Effluent BOD5 - ABF Filter vs. Time 16
Figure 8 - Ratio Effluent BOD5 to Loading - ABF Filter vs. Time 17
Figure 9 - Effluent BOD5 vs. Time 20
Figure 10- Percent Reduction of 8005 vs. Time 22
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I. INTRODUCTION
Region VIII of the United States Environmental Protection Agency devel-
oped an Accomplishment Plan for the Metropolitan Denver-South Platte River
Basin areas. A portion of this plan called for an evaluation of the waste
water treatment facilities in the Metropolitan Denver area regarding their
operational and maintenance practices. (Initially only five plants were
selected for visits.) One of the purposes of the plant evaluations was to
determine those facilities where regional resources could be used to improve
existing effluent quality by improving operational controls.
The waste water treatment facility at Longmont, Colorado, was one of the
plants evaluated. The Longmont plant was producing a poor quality effluent
(in violation of Colorado's Water Quality Standards, i.e.. 80 percent removal
of BODs) and it was concluded to be a candidate for technical assistance. An
assistance project aimed at improving the plant's operations and upgrading
effluent quality was begun on March 20, 1972. Simultaneously a project to
provide assistance in conducting laboratory analyses was started.
II. PURPOSE AND SCOPE
The purpose of this report is to summarize the results and findings of
the technical assistance project that was conducted at the Longmont, Colorado,
waste water treatment facility.
The initial objective to improve the plant's operations and the effluent
quality was successful to a degree and is documented in this report. In
addition, those portions of the plant that apparently limited further improve-
ment of effluent quality are discussed. The laboratory assistance is outlined
and some of the problems encountered are also discussed.
III. DESCRIPTION OF PLANT
A. Background
The Longmont Wastewater Treatment Facility was recently expanded (1971).
Prior to plant expansion, the city had been provided secondary treatment by a
rock media trickling filter. The new plant expansion initially called for
replacement of the existing rock media with redwood slats and, in addition,
the construction of a new redwood media filter (the Del-Pak Media Corporation
provided the redwood slat media at Longmont). Actually, only the new redwood
media filter was constructed since it was decided to determine if the redwood
and the associated process designated as the Activated Biofilter (ABF)* could
handle the high BODs loading rates used in design of the redwood filters.
Redwood was purchased to replace the rock media in the old filter but the slats
have not been installed to date.
ABF and Activated Biofilter are trademarks of Del-Pak Corporation.
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The plant has continually experienced problems in meeting the State's
minimum requirement of 80 percent reduction of 8005. A local turkey
processing plant which is in operation for most of the year (May to January
or February) has, in the past, increased the load of BODs to the Longmont
plant considerably. At these higher loadings the plant has been able to
achieve the minimum 80 percent reduction of BODs. However, the effluent
discharged to the river at these higher loading rates is of poor quality.
B. Plant Facilities
Figure 1 shows the basic flow diagram for the Longmont plant as outlined
in the operator's manual provided by the consulting engineer. A brief
discussion of the various units will be outlined below.
Flow entering the plant passes through a mechanical bar screen and is
then split into two separate streams. By design, fifty-seven percent of the
flow is diverted to the "old" rock filter side of the plant and forty-three
percent goes to the new ABF filter side, however, these percentages can be
adjusted by changing gates.
The flow to the rock filter side of the plant passes through a primary
clarifier and into a wet well. The sewage is pumped to the rock filter and
the effluent from the filter is partially returned to the wet well to be re-
circulated while the remainder of the flow is settled in the final clarifier.
Settled sludge from the secondary clarifier is pumped back to the head of the
plant to be settled out in the primary clarifiers.
The sewage to the ABF side of the plant passes through a primary clarifier
and then into a wet well. Sewage is then pumped to the ABF filter (tower). A
portion of the "mixed liquor" from the bottom of the ABF tower is returned to
the wet well to be recirculated with the incoming sewage. The remainder of the
flow is settled in the final clarifier. Settled solids from the final clari-
fier (return sludge) are also returned to the wet well to be recirculated over
the tower with the incoming sewage. A portion of these settled solids are
"wasted" each day back to the head of the plant where they are removed in the
primary clarifier.
Disinfection at Longmont is accomplished by chlorination. A portion of
the final clarifier is used to provide contact time for the chlorine. No
chlorine contact tank is available.
Sludge from the primary clarifiers is pumped at a low solids concentra-
tion through a cyclone type grit separator and then flows to a sludge thickener.
Sludge from the thickener is pumped to the digester or at times it has been
centrifuged and the centrate returned to the sludge thickener. Centrifuged
sludge is hauled to a land fill. The overflow from the sludge thickener is
returned to the head of the plant.
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FIGURE 1
FEDERAL ASSISTANCE PROJECT
LONGMONT WASTEWATER TREATMENT FACILITY
MARCH 1972 TO MAY 1972
PLANT FLOW SCHEMATIC
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Sludge pumped to the digester is treated in the gas r'ecirculation primary
digester and is then drawn to the secondary digester. Under normal operation,
the supernatant from the secondary digester is returned to the head of the
plant and the sludge is either drawn to drying beds or centrifuged. Presently
no flocculating agents are used during the centrifuging of digested sludge.
Unit sizes and some design criteria are shown on Figure 1. It is noted
that very little interconnection exists between the two sides of the plant.
Figure 1 shows the interconnection available at Manhole #7 (Junction Box).
At this point, effluent from the rock filter could be introduced to the ABF
side of the plant. This junction box is also the location where the return
sludge and return mixed liquor from the ABF filter are mixed before they are
returned to the wet well for recirculation.
The return mixed liquor line (labeled on Figure 1) is a 10" unvalued
line through which the mixed liquor flows by gravity to the junction box.
IV. SUMMARY OF PROJECT
A. Summary of Laboratory Assistance
1. Review of Laboratory Procedures
At the time of the laboratory assistance program the plant chemist was
analyzing eight composited samples each day. These samples were composites
of seven grab samples of uniform volume collected every two hours over a
period of fourteen hours (0800 hrs to 2200 hrs). Samples were not composited
in relation to the incoming flow. One of the composite samples was raw
influent waste water collected upstream from the bar screen. The next four
composite samples were collected from each of the two primary clarifier
effluents and each of the two filter effluents (rock filter and ABF tower).
The last three samples were collected from each of the two final clarifier
effluents and the combined final effluent. These effluents had been chlori-
nated. The final effluent sample was collected at a junction box where the
flows from each of the two filters came together. Although every effort was
made to collect a representative sample, it is possible that this final effluent
sample was not a well mixed representative sample.
The analyses performed on these samples consisted of: 6005, total solids,
total volatile solids, total suspended solids and pH. Also, residual chlorine
was determined on the two final clarifiers and final effluent samples at least
once a day. Dissolved oxygen concentrations were determined in the effluents
from the ABF and rock filters once each day.
Generally, every other day a grab sample was collected from each of the
anaerobic digestors. These samples were analyzed for volatile acids, alkalinity
concentration and pH.
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The technique used to set up the individual dilutions for the five day
Biochemical Oxygen Demand analysis was a simplified cylinder dilution tech-
nique. The initial and five day dissolved oxygen concentration was determined
by the Azide Modification of the Dissolved Oxygen Method listed in Standard
Methods for the Examination of Water and Wastewater, 13th Ed.
The sodium thiosulfate titrating solution used in the determination of
the dissolved oxygen was prepared properly, when needed. However, this
unstable solution was not properly preserved. As a result, the concentration
of the solution gradually decreased. Therefore the accuracy of each day's
titration values were a function of the freshness of the sodium-thiosulfate
solution.
An important change initiated during the laboratory assistance program
was the institution of the policy of preserving the sodium thiosulfate solu-
tion with chloroform and periodically checking the concentration using
standard potassium biniodate solution. In addition, the full bottle technique
of the Azide Modification of the Dissolved Oxygen Method was introduced to the
chemist; i. e. the entire volume of solution in the BOD bottle (305 ml) is
titrated with 0.0375N sodium thiosulfate solution and therefore the relation-
ship of one ml of titrating solution to one mg/1 of dissolved oxygen remains
the same.
A few days after this technique was taught to the chemist, the City of
Longmont decided to purchase a dissolved oxygen meter and probe. The chemist,
as well as other plant personnel, was instructed in the proper calibration
and operation of the dissolved oxygen meter and probe for determining 8005.
Other changes initiated consisted of using a large fish tank aerator
pump and a stone (fine bubble) diffusor to aerate the dilution water at
20°C. Also a siphon was used to transfer the diluted sample from the grad-
uated cylinders to the BOD bottles with a minimum amount of agitation.
Samples were previously poured from the graduated cylinders. Wide tipped
serological pipets were purchased to aid in the accurate measurement of
waste water samples for both BOD and suspended solids analytical procedures.
Dissolved oxygen concentration of waste water samples were determined
by a modification of the Copper Sulfate-Sulfamic Acid Flocculation Modifi-
cation for the determination of dissolved oxygen. This procedure was
modified and the operator was instructed in the proper calibration and
operation of the dissolved oxygen meter and field probe. The measurement of
dissolved oxygen using the meter and probe became part of the control
testing program performed by the plant operators.
Chlorine residual was determined on grab samples collected from both
final clarifiers and the final effluent by the orthotoledine method. No
change was suggested in this method.
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Prior to assistance no attempt was made to remove the residual chlorine
from the composited sample before setting up the BOD sample bottles. This
procedure was modified by instructing the chemist in the proper pretreatment
procedure to remove the effects of residual chlorine on samples.
The plant chemist was performing total suspend solids on the composited
samples by filtering a constant volume (50 ml) of sample through a gooch
crucible containing a Whatman ashless filter paper. This method produced
reasonably good results; however, the use of the paper filter prevented the
determination of volatile suspended solids. To enable the chemist to deter-
mine VSS, Reeves Angel 934AH glass fiber filters were substituted. Also, to
increase precision, the volume of sample filtered was varied inversely with
the suspended solids concentration.
Total solids and total volatile solids were determined by satisfactory
micro-methods, therefore no change in this procedure was suggested.
2. Accomplishments
BOD data was improved from unreliable and extremely variable to precise
and reliable analytical results. This improvement was achieved by changes
in laboratory technique such as the preservation of the sodium thiosulfate
solution and the removal of the residual chlorine from the chlorinated samples.
The change from variable to consistent effluent 6005 values is shown on
Figure 9. Prior to March 22 (the first day of improved BODs results) the
effluent BODs values were extremely variable. On March 16th, the 8065 was
98 mg/1, on the 17th it was 22 mg/1 and back up to 87 mg/1 the 18th. After
March 23rd, most of the effluent BODs results were in the 30 to 70 mg/1 range.
Minor changes in equipment and techniques were made in the analysis of
solids. These changes increased laboratory efficiency but didn't significantly
change the results.
From an overall point of view, this laboratory is well equipped and the
plant chemist has been instructed in proper laboratory techniques and procedures.
A problem that remains unsolved is the location of the laboratory next
to the centrifuge equipment. When the centrifuge is in operation, the vibra-
tion transmitted to the laboratory affects the use of the analytical balance.
A large granite stone balance table would partially solve this problem.
B. Summary of Operational Assistance
1. Control Testing
In addition to laboratory assistance, plant personnel were also given
instruction in conducting and interpreting various control tests. Testing
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to aid in the operation of the ABF tower was emphasized since it represented
the unit of the plant that was most amenable to control. Dissolved oxygen
tests, centrifuge tests, turbidity, settleability tests, and sludge blanket
depth in the final clarifiers were the control tests that were initiated.
The tests outlined above were conducted at least twice a day, seven days a
week.
Dissolved oxygen (D.O.) tests were used to monitor the availability of
dissolved oxygen in the plant. D.O. measurements were taken of the mixed
liquor and of the mixture of mixed liquor and return sludge at the junction
box. (See Figure 1) A dissolved oxygen meter was used to measure the
oxygen concentrations.
Centrifuge testing was used to determine variations in solids concen-
trations from day to day. Tests were conducted on the mixed liquor, the
return sludge and the mixture of the return sludge and mixed liquor in the
junction box. The centrifuge tests result in a percent solids determination.
Although it is not necessary for control, a correlation between percent
solids by centrifuge and solids by weight was made. The results of this
correlation indicate that one percent solids from the centrifuge test is
equivalent to 1286 mg/1 solids by weight. This factor may change as the
sludge characteristics change; however, solids concentrations presented by
weight in this report have been converted using the above factor.
Turbidity testing was done on the effluents from both the rock filter
and the ABF filter to monitor improvements in performance prior to obtaining
a BODs test result.
Settleability tests were conducted on the mixed liquor from the ABF
tower to monitor and observe settling characteristics.
Sludge blanket depth determinations were made on the final clarifier on
the ABF side of the plant to monitor changes in the depth of the blanket and
to determine if the sludge was accumulating in the clarifier.
Longmont has recently purchased the equipment to conduct the above out-
lined control tests and is continuing to use these tests to control their
operations.
2. Data Interpretation
The data obtained from the above outlined control tests were used to
perform calculations and to develop graphs that were used and are continuing
to be used to control the ABF filter.
The dissolved oxygen measurement of the mixed liquor and of the mixture
of return sludge and mixed liquor measured at the junction box was plotted
versus time. It was found that if the concentration of dissolved oxygen at
this junction box fell below 1 mg/1 during the day that performance of the
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ABF tower, as measured by turbidity, decreased. A daily plot was also made
of the mixed liquor solids concentration versus time. It was determined that
a solids concentration of 3 percent or greater (approximately 3800 mg/1) in
the mixed liquor caused a decrease in the dissolved oxygen at the junction
box to less than 1 mg/1. Therefore, optimum performance was achieved by
attempting to maintain suspended solids at approximately 3 percent as measured
by the centrifuge test. Solids concentrations were maintained at a desired
level by selecting and wasting to the primary clarifiers a desired amount of
sludge from the ABF system each day. The amount of sludge wasted was plotted
daily to show the correlation between sludge wasted and mixed liquor concentration.
Turbidity of the effluent from the ABF unit was plotted daily to depict
the quality at various solids concentrations.
The above outlined graphs and calculations are continuing to be developed
at Longmont. As plant loadings change and as temperature and other factors
change the most desirable operational characteristics will change. The com-
bination of the control tests plus the data analysis should allow the plant
operators in the future to recognize the operational changes that are needed.
C. Summary of Plant Performance
1. Analysis of Rock Filter
Prior to the initiation of technical assistance at Longmont, the operator
had begun decreasing the flow to the rock filter and increasing flow to the
ABF side of the plant. Normally, 57 percent of the flow goes to the rock
filter side of the plant. Calculations of loading on the rock filter indicated
that prior to flow reduction it was receiving loads in excess of 50 Ibs. of
BODs per 1000 cubic feet. Calculations also showed that the ABF unit was
receiving loads less than design. Therefore, to better distribute the load
in the plant, the flow to the rock filter was further decreased during the
first week of formal assistance (March 20, 1972). After the first week, an
attempt was made to balance the flow equally to each side of the plant. Flow
changes are depicted graphically in Figure 2. Seven day moving average flow
data is plotted to smooth out the daily fluctuations in flow. The changes in
flow indicated after April 9, 1972 were made in an attempt to determine the
optimum distribution of flow to get the best effluent quality from the plant.
This adjusting of the flow between the two sides of the plant should continue
until it is determined that the optimum flow distribution has been achieved.
The effect of the flow reduction on the load to the rock filter is depicted
graphically in Figure 3. Daily average values of load as well as the seven day
moving average is plotted. Data are not available in sufficient quantity to
warrant extension of the curves beyond April 17, 1972. Testing of individual
units was discontinued on a daily basis after that date. The load as depicted
by the seven day moving average in Figure 3 shows a decrease from approximately
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2.6
2.4
2.2
2.0
1.8
1.4
1.2
1.0
FIGURE 2
FEDERAL ASSISTANCE PROJECT
LONGMOKT WASTEWATER TREATMENT FACILITY
MARCH 1972 TO MAY 1972
FLOW TO FILTERS
vs
TIME
MllMMMm..,,.,,,,,!!!!""""11""""
7 DAY AVERAGE FLOW TO A.B.F.
DAY AVERAGE FLOW TO ROCK FILTER
0.8
0.6
0.4
March 10
15
20
30
April 5
10
15
20
25
30
Hay 5
-------
180
160
140
I 120
o 100
CO
O
o
O -o
n
80
60
40
2C
FIGURE 3
FEDERAL ASSISTANCE PROJECT
LONGMONT WASTEWATER TREATMENT FACILITY
MARCH 1972 TO MAY 1972
LOADING - LBS BODr PER 10CD FT-^
vs
TIME
IIIIIIUIII
March 10
15
20
25
30
April 5
10
15
20
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70 Ibs. of BODs per 1000 cubic feet to between 50 to 60 Ibs. of BODs per 1000
cubic feet. It is also noted that during this time period air temperatures
were generally increasing. The combined result of the increasing temperatures
and decreasing load led to an improved effluent quality from the rock filter.
Figure 4 depicts the effluent BODij from the rock filter. The seven day
moving average indicates a reduction from greater than 70 mg/1 BODs to 40 to
45 mg/1 of BODs. Tnis represents a reduction in the effluent strength of
of greater than 35 percent.
Figure 5 shows a plot of the ratio of the effluent BODs to the loading
of BODs per 1000 cubic feet versus time. This graph was plotted to show the
combined effect that the decreased loading had on the effluent BODs and it
shows more dramatically, as indicated by the steeper slope, the improvement
of the rock filter.
2. Analysis of ABF Filter
As shown in Figure 2, the flow was increased to the ABF side of the plant.
This was possible since during the first week or so of March, the pumps for the
ABF wet well were wired so that they could operate simultaneously. This
increased the capacity above that which had been available when the pumps could
only operate alternately. Another modification that had taken place prior to
the formal assistance project was connecting the sludge draw off pump for the
secondary clarifier directly to the hopper in the center of the tank. This
allowed an increased amount of sludge to be removed from the final clarifier.
During the first week of the assistance project (March 20-27, 1972), the
amount of mixed liquor going directly to the wet well was increased to a
maximum. This was done by partially closing the gate that allowed flow from
the bottom of the ABF tower to enter the clarifier and diverting this flow
through the ten inch line available. Closing the gate increased the head in
the splitter box and thus allowed the maximum amount of flow to be diverted
from the bottom of the tower directly to the wet well. Also, all wasting of
sludge (i.e., pumping solids back to the primary clarifiers) was stopped for
the week in an effort to increase the solids concentration in the system.
The mixed liquor solids at the start of the project was 0.5 percent
(approximately 600 mg/1). During the first week the solids concentration
was increased to 2 percent (approximately 2600 mg/1). In the weeks that
followed, solids were varied between 2.5 to 4 percent in an effort to
determine the concentration that produced the best effluent. It was felt
that as the solids increased, one of two things would be the limiting factor.
The solids might increase to the point that they would accumulate in the
final clarifier and go septic or bulk due to the long detention time, or the
other possibility might be a limited concentration of dissolved oxygen
would be available as provided by the ABF tower. In practice it was found
that the dissolved oxygen became critical long before high solids concentrations
became critical in the final clarifier.
11
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180
160
140
FIGURED
FEDERAL ASSISTANCE PROJECT
LONGMONT WASTEWATER TREATMENT FACILITY
MARCH 1972 TO Nw 1972
ROCK FILTER EVALUATION
EFFLUENT BCD5 - MG/L
vs
TIME
120
CD
o
o
100
80
60
40
20
DAILY AVERAGE
March 10
15
20
25
30
April 5
10
15
20
-------
o
O
O
-tl
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.1
0.2
0.0
DAILY AVERAGE
.,.•.»•*"'',
7 DAY MOVING AVERAGE
FIGURE 5
FEDERAL ASSISTANCE
LONGMONT WASTEWATER TREATMENT FACILITY
MARCH 1972 TO MAY 1972
ROCK FILTER EVALUATION
RATIO OF EFF BQDs TO LOADING PER 1000 FT
Tvs
TIME
\x\ V
March 10
15
20
25
30
April 5
10
15
20
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Figure 6 shows the loading applied to the redwood (ABF) filter in Ibs.
of BOD5 per 1000 cubic feet. The graph shows that the seven day average load
was well below the design value of 175 Ibs. of BODs per 1000 cubic feet. The
seven day average load leveled out at around 110 Ibs. of BODs per 1000 cubic
feet. The daily average loading exceeded the design load only once - on March
20, 1972. Despite the loadings lower than design and the increase in mixed
liquor solids, the performance of the ABF unit did not improve dramatically.
Figure 7 shows the effluent BODs for the ABF side of the plant. The reduction
of effluent BODs is indicated by the seven day average from greater than 80
mg/1 to a low of 60 mg/1 and then an increase to 70 mg/1 of 6005.
The effect of improved operational controls is more dramatically shown
in Figure 8. Figure 8 is a plot of the ratio of effluent BOD^ to loading per
1000 cubic feet versus time. A plot of this type helps to eliminate some of
the effect of a fluctuating load on effluent BODs and shows that improvement
occurred despite the increasing load. However, the improved controls and
control testing seemed to have an effect only to a certain level and then no
further improvement is indicated.
In interpreting Figure 8, it appears that a ratio of 0.6 is the best that
can be achieved at the Longmont plant. This would mean (assuming a straight
line relationship) that in order to achieve an effluent BODs concentration of
25 mg/1, the ABF tower would have to be loaded at no more than 40-45 Ibs. of
BODs per 1000 cubic feet. This is far below the design loading of 175 Ibs.
per 1000 cubic feet.
Based on the above analysis, it appears that the ABF tower is not capable
of producing a satisfactory effluent even with good, or at least improved,
operational controls. Three things concerned with the design of the unit
appear to be the limiting factors in improving the effluent from the ABF unit.
The final clarifier appears to be too large. Detention time in the
final clarifier at a flow of 1.5 MGD (See Figure 2) is 6.5 hours with an
overflow rate of 210 gallons per day per square foot. Although peak flows
exceed these values, they do not approach commonly used design values (i. e.,
2 hours detention or 800 gallons per day per square foot overflow rates).
It is noted that design flow is 2.24 MGD, however 1.5 MGD was actual flow.
The effect of the large clarifier is that it serves, simply by its sheer
size in relation to the rest of the system, as a reservoir of the solids.
The environment of the clarifier which is generally anaerobic, has an ad-
verse effect on the aerobic bioligical organisms. Another feature of the
final clarifier that may have affected performance is the scraper type
sludge collector mechanism. This type is generally considered inferior to
the suction type collector for removing aerobic organisms from a clarifier.
A method of eliminating some of the adverse effects on the aerobic solids
from the ABF tower would be to decrease the quantity of sludge (organisms)
going to the clarifier by increasing the amount of mixed liquor returned
directly to the wet well. This points out the second item inhibiting improvement
14
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g
200
180
160
140
120
100
80
60
•DESIGN LOADING 175 IBS PER 1000 FT3
FIGURES
FEDERAL ASSISTANCE PROJECT
LONGMONT WASTEWTER TREATMENT FACILITY
MARCH W2 TO MAY 1972
A.B.F. EVALUATION
LOADING - LBS BQD^ PER IflOO FT5
vs
TIME
40
20
March 10
15
20
25
30
April b
10
15
20
-------
180
160
140
120
^ 100
ID
80
60
40
DAILY AVERAGE
mini""1
FIGURE/
FEDERAL ASSISTANCE PROJECT
LONGMONT WASTEWATER TREATMENT FACILITY
MARCH 1972 TO MAY 1972
A.B.F. EVALUATION
EFFLUENT BO - MG/L
vs
TIME
20
1
March 10
15
20
25
30
April 5
10
15
20
-------
o
o
1.6
1.4
o
5
2: 1.0
S
0.8
0.6
0.4
0.2
FIGURES
FEDERAL ASSISTANCE PROJECT
LONGMONT WASTEWATER TREATMENT FACILITY
1972 TO MAY 1972
A.B.F. EVALUATION
RATIO OF EFF BOD5 TO LOADING PER 1000
vs
TIME
March 10
15
25
30
April 5
10
15
20
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of the ABF performance. The amount of flow or mixed liquor that can be re-
turned to the wet well directly is limited by the 10 inch line presently in
use at the plant. A variable flow control for returning a desired amount of
mixed liquor appears to be necessary.
The third and most critical limiting factor at the Longmont Plant appears
to be the depth and/or volume of the redwood media. The relatively shallow
depth of the Longmont filter (i.e.,5 feet) does not allow for sufficient
contact time to effectively remove the soluable BODs from the sewage as it
flows vertically through the tower. This problem is somewhat compounded by
the turbulence caused by the rotary distributor, which literally knocks any
growth off the top 6 inches of filter media and by the inactivity of the
organisms that are returned from the final clarifier.
All of the above items are interrelated. However, it is doubtful if
any significant improvement can be achieved without first approaching the
problem of the contact time in the redwood filter.
The above analysis also leads to another conclusion. The redwood media
that is available at Longmont to replace the rock in the old filter will not
provide a satisfactory effluent since two of the same shortcomings outlined
above would also affect the "old" filter. The depth of the redwood and the
lack of flexible control on the volume of mixed liquor return would be inade-
quate to provide the treatment necessary to handle the waste.
Several other items that would aid in operation of the ABF unit are
outlined below. Flow measuring devices on the return sludge line, the
mixed liquor line and the waste sludge line would be very beneficial in
determining and maintaining a desired solids distribution in the system.
Besides the flow measuring devices, the ability to fluctuate the flow of
each of the three streams outlined above would be desirable. For example, a
variable speed pump for the return sludge would greatly extend operational
flexibility. Another desirable feature would be to install a smaller and
independent waste sludge pump in the ABF system. This would allow more
frequent wasting of lesser amounts which would be more desirable than the
current "slug" wasting methods that are required by the present equipment.
3. Analysis of Other Plant Units
In addition to the two filters, several other portions of the plant
were investigated to evaluate operational practices.
The digesters had not been in operation on a daily basis for several
months prior to the assistance project. The plant superintendent had been
"feeding" the digestors moderately in an attempt to develop active gas-producing
digesters. This goal had been accomplished. Both the primary and secondary
digestors were very actively producing gas and all parameters (pH, alkalinity
and volatile acids) appeared to be satisfactory. However, major difficulties
and numerous plant upsets occurred during the assistance project when the
operators attempted to draw supernatant from the secondary digestor back to
the head of the plant. The reason for the upset was the fact that digestion
was not being completed in the primary gas mixed digestor and active decompo-
sition and gas production was taking place in the secondary digestor. Although
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the secondary digester is unmixed, the mixing caused by digestion was adequate
to prevent any development of a supernatant layer. Testing of the various draw-
off ports on the secondary digestor showed an almost equal solids distribution
throughout the tank. Therefore, the so-called supernatant that was being returned
to the head of the plant was actually partially digested sludge with a solids
concentration of 20,000 to 30,000 mg/1. It is recommended that, until the secon-
dary digestor is capable of developing a supernatant layer, the digested sludge
from the secondary be drawn to the sludge drying beds and/or centrifuged and
hauled to a landfill. It would also be beneficial to use the secondary digestor
as a primary digestor to increase the plant's digestion capacity. Plant per-
sonnel reported they have not had good results in the past trying to centrifuge
digested sludge. It may be necessary to add polymers to obtain satisfactory
results with the centrifuge. In any event, "supernatant" as it presently exists
at Longmont must not be returned to the head of the plant. Instead, sludge
should be centrifuged and the centrate be returned to the thickener or the
sludge should be drawn to the drying beds. In the event that a "good" super-
natant layer can be developed in the secondary digestor, it should be possible
to begin returning supernatant to the head of the plant.
At Longmont, a thin sludge is drawn from the primary clarifiers, then is
passed through a cyclone grit separator and then to a sludge thickener. Raw
sludge from the thickener normally would go to the digestor. However, a portion
of the time, raw sludge has been centrifuged. It appears that the centrifuge
has the effect of making soluable a portion of the BODs attached to the sludge
solids. This increases the load to the secondary portion of the plant and
therefore, centrifuging of raw sludge should be avoided if at all possible.
Grit removal prior to the primary clarifier and pumping of primary sludge
directly to the digesters might also effectively remove BOD5 that is presently
"washed off" solids, using the present sludge handling facilities. No studies
were conducted to support this aspect of operation and, therefore, the operators
should develop a testing program to analyze the sludge handling facilities of
the plant in the near future.
4. Analysis of Total Plant Performance
Effluent quality at the Longmont waste water treatment facility is depic-
ted in Figure 9. The seven day moving average indicates a reduction of BODs
from 70 mg/1 to values between 50 to 60 mg/1. The majority of this decrease
can be attributed to the decreased loading to the rock filter and to the improved
operational control of the ABF tower. The fluctuations that are indicated in
the effluent BOD-5 at Longmont after assistance began can be attributed to numerous
factors, including: a change in plant personnel, clogging of the sludge thickener
draw-off line, drawing a very poor quality "supernatant" back to the head of the
plant, the effects of centrifuging raw sludge, etc. Many of these difficulties
can be expected in the routine operation at most plants and some variation in
the effluent quality is expected. However, the significant point is that, des-
pite the improvement after assistance, the effluent, even at best, was not of
a satisfactory quality (i.e.,seven day average was never less than 45 mg/1).
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130
r- 80
FIGURE 9
FEDERAL ASSISTANCE PROJECT
LONGMONT WASTEWATER TREATMENT FACILITY
MARCH 1972 TO MAY 1972
TOTAL PLANT EFFLUENT BOD5 - MG/L
vs
TIME
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Figure 10 graphically depicts the total plant performance at Longmont as
measured by the percentage reduction of BOD5. A gradual increase in removal
is indicated to a seven day average high of 77 percent, which was achieved on
April 2, 1972. However, after that peak and a rapid decline in percent reduc-
tion, which was due to some of the problems outlined above, the plant seems
to have leveled off at about 70 percent reduction of BODs. This is below the
required state water quality standard of 80 percent reduction of BODs. It is
noted that some values on a daily basis show an 80 percent reduction. However,
this is not a consistent occurrence and it appears that 80 percent reduction
is a borderline value to be obtained rather than a minimum value that the plant
should achieve. It definitely appears that despite the improvements shown by
the technical assistance and despite the feasibility of improved performance
with the improvement of existing operational practices that the Longmont plant
will not be able to consistently achieve a minimum of 80 percent removal of
without major plant modifications.
V. SUMMARY AND CONCLUSIONS
Since the treatment plant at Longmont, Colorado, was not providing a
satisfactory effluent, an operational assistance project was initiated in an
effort to improve the effluent quality. The concentration of BODs in the
effluent was measurably reduced due to operational changes and control. The
effluent was reduced from a concentration of 70 mg/1 to 50 to 55 mg/1 BODs.
However, this effluent concentration is not a satisfactory quality. Various
plant facilities appeared to be the factors limiting the production of a sat-
isfactory plant effluent. It was concluded that the main limiting factors on
the ABF unit were the lack of sufficient contact time (i.e., depth) of the
redwood media, the lack of variable return mixed liquor flow and the large
final clarifier. Other factors limiting operational capability were the lack
of flow meters, the lack of variable flow control and the lack of a separate
sludge wasting system. Since similar design features that exist in the ABF
filter and apparently inhibit performance would be incorporated in replacing
the existing rock filter with redwood media, it is concluded that replacement
of the existing rock would not give the desired results.
Despite the improvements in effluent quality shown by the technical
assistance project, and despite the feasibility of improved performance with
continued improvement in operational controls and adjustments, the Longmont
plant will not be able to provide a high quality effluent and probably will
not be able to consistently provide a BODs reduction of 80 percent. Major
plant modifications appear to be necessary to provide an effluent of satisfactory
quality.
Control testing that was initiated during the assistance project proved
to be satisfactory to control the plant's performance. The daily testing and
data interpretation should be continued.
"Supernatant" should not be returned to the head of the plant until such
time that a true supernatant layer can be developed in the secondary digestor.
In the meantime, both digestors could be used as primary digesters with the
sludge being drawn off to the drying beds or the centrifuge and return the
centrate to the sludge thickener. Flocculating agents may be needed to aid
in the centrifuging of digested sludge.
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100
FEDERAL ASSISTANCE PROJECT
LONGMDNT WASTEWATER TREATMENT FACILITY
1972 TO MAY 1972
TOTAL PLANT PERFORMANCE
PERCENT REDUCTION OF BODc
vs
TIME
7 DAY MOVING AVERAGE
40
20
March 10
15
20
25
30
April 5
10
15
20
30
May 5
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Raw sludge should not be centrifuged since it appears to cause an increased
organic load on the secondary portion of the Longmont plant.
The sludge thickener operation and function should be evaluated and an
attempt should be made to compare the present sludge handling system with one
where grit removal is accomplished prior to primary settling and the sludge is
drawn directly from the primary clarifiers to the digesters. It is felt that a
system of this type may provide better BOD5 reductions than the present system.
However, data is needed to evaluate this conclusion.
The laboratory assistance program was initiated to evaluate the methods
of analysis used in the Longmont Laboratory. Precise and reliable analytical
results from this laboratory were achieved by instituting appropriate changes
in laboratory techniques and by the purchasing of a few items of laboratory
equipment. These changes should remain a part of the routine laboratory oper-
ation and every effort should be given to monitoring precision by performing
duplicate analysis on 10-20% of the samples.
VI. RECOMMENDATIONS
The following recommendations are made:
A. Testing and data analysis initiated to control the operation of
the ABF filter should be continued.
B. Laboratory techniques that were instituted should remain a part
of the routine laboratory operation and effort should be given
to monitoring precision by performing duplicate analysis on 10-
20% of the samples.
C. The operational guidelines listed below should be followed:
1. Flow adjustments should be made between the two filters
to determine the optimum flow distribution as measured
by a minimum effluent 6005.
2. Raw sludge should not be centrifuged if at all possible.
3. "Supernatant" should not be returned to the head of the
plant until such time that a supernatant layer can be
developed in the secondary digester.
4. Digested sludge should be centrifuged or drawn to the
sludge drying beds. It may be necessary to use a floc-
culating agent or agents to aid in centrifuging digested
sludge.
5. Analysis of all plant functions should continue in an
effort to "fine tune" the performance of the facility. .
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D. The plant must be modified to provide an effluent of high quality.
Although specific recommendations are not given as to the type of
modifications, the following suggestions are made:
1. The ABF side of the plant will require a redwood tower with
greater contact time (i.e. depth).
2. Flow capacity to the ABF side should be increased to overcome
some of the difficulties presented by the large final clarifier
and its associated detention time.
3. Increase flexibility of control, such as variable mixed liquor
return, variable return sludge pumping, separate sludge wasting
and flow measuring devices should be considered and preferably
incorporated in modifications to the ABF tower.
4. An evaluation of grit removal prior to primary settling and
direct pumping of sludge from the primary clarifiers should be
made in comparison with the existing system.
5. The rock in the old filter should not be replaced with the
available redwood media. If the redwood is used, a new tower
or filter should be constructed more in line with the design
requirements of the ABF process.
6. Since major modifications appear to be required, other available
processes should be considered in the expansion.
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