FEDERAL ASSISTANCE PROJECT
METROPOLITAN DENVER SEWAGE
DISPOSAL DISTRICT NO. 1
October 1969 • FEBRUARY 1970
^
-> r I
| Westminst
^ r-j— | Thornton I
"LET
^1 J_
Englewoed ~\
ENVIRONMENTAL PROTECTION AGENCY
WATER QUALITY OFFICE - REGION VII
911 WALNUT, KANSAS CITY, MO. 64106
-------
FEDERAL ASSISTANCE PROJECT
METROPOLITAN DENVER SEWAGE
DISPOSAL DISTRICT NO. 1
OCTOBER 1969 - FEBRUARY 1970
By
Bob A. Hegg
And
John R. Burgeson
ENVIRONMENTAL PROTECTION AGENCY
WATER QUALITY OFFICE - REGION VII
911 WALNUT, KANSAS CITY, MO. 64106
-------
The Superintendent of Documents
classification number is:
EP 2.2: D43
-------
TABLE OF CONTENTS
PAGE NO.
I. Introduction 1
II. Purpose and Scope 2
III. Description of Plant and Area 3
IV. Summary of Assistance Project 5
A. Control Testing - Procedures and Results 5
B. Performance Evaluation - Procedures and Results 6
C. Data Analysis - Procedures and Results 9
D. Control of Areas - Procedures and Results 11
E. Control of Sludge Characteristics - Procedures and Results. ... 12
V. Data Analysis 15
A. Analysis of Sludge Production 15
B. Analysis of Secondary Clarifiers 21
VI. Summary and Conclusions 29
VII. Recommendations 32
VIII. Appendices 33
Appendix A - A Resolution: "Concerning the Federal Government's
Responsibilities in Constructing and Operating Sewage
Disposal Facilities" 34
Appendix B - References 36
Appendix C - Determination of Substrate Removal Rate (q) and
Net Growth Rate (l/ec) 38
-------
LIST OF TABLES
TABLE NO. TITLE PAGE NO.
1 A Summary of Various Parameters Associated with the Selected
"Steady State" Periods 17
2 Calculated Values of ec and quODs ~ Selected Periods of
Operation - Areas #2 and #3 • 19
3 Average Settled Sludge Volumes for "Steady State" Periods 22
4 Zone Settling Rates (V$) and Equivalent Surface Overflow
Rates (Or) for "Steady State" Periods 25
5 Waste Sludge Flow Required to Remove an Equivalent Amount of
Solids with Varying Underflow Concentrations 28
ii
-------
LIST OF FIGURES
FIGURE NO. TITLE PAGE NO.
1
2
3
4
5
6
Plant Flow Schematic
Influent BODs Loadings and Seven Day Moving Average, Effluent
B005 and TSS Concentrations vs Time ~
Weekly Average Percentage Reduction of BOOs and TSS
Determination of Zone Settling Rate (Vs) - Height of Sludge
4
8
10
14
20
Interface vs Time - Area #3 Period: 2/9 - 2/16/70 Average
9:00 A.M 24
iii
-------
I. INTRODUCTION
The Metropolitan Denver Sewage Disposal District #1 (Metro Denver) plant was designed mainly as a
secondary treatment facility (activated sludge) to treat wastes from the cities and sanitary districts
in the Metropolitan Denver Area. The plant is administered by a Board of Directors who represent the
various communities and districts that are served by the facility. The largest source of flow to the
plant is the primary effluent from the City and County of Denver's North Side Sewage Treatment Plant.
The Metro Denver plant began operation in 1966 and since that time has continually experienced
difficulties. Odor problems, insufficient sludge handling facilities, air pollution from sludge
incineration; unavailability of land for sludge disposal sites, management, labor, and maintenance
problems are the more significant of the difficulties that the plant has encountered. These problems
have served to further increase the public's awareness of the Metro Denver plant.
In an effort to resolve this situation, the Board of Directors of the Metro Denver plant passed a
resolution (see Appendix A) entitled "Concerning the Federal Government's Responsibilities in Con-
structing and Operating Sewage Disposal Facilities." In the resolution, Metro Denver petitioned the
Congress of the United States and the appropriate Federal agencies to make available to the district
a special team of scientists and engineers to serve as a task force to inspect the district's acti-
vated sludge treatment plant and make appropriate recommendations. As a result of this resolution, a
three-man team from what was then the Federal Water Quality Administration was assigned to the Metro
Denver treatment facility from October 1969 through February 1970. The project officer was
Mr. Alfred West from the National Field Investigation Center (NFIC) in Cincinnati. He was assisted
by Mr. Joseph Jos 1 in and Mr. Bob Hegg of the Kansas City Regional Office.
-------
II. PURPOSE AND SCOPE
The most significant problem areas at the Metro Denver plant, leading to the request for assis-
tance, were the sludge handling and sludge disposal problems. The major sludge handling problem was
processing the volume and type of waste activated sludge generated by the secondary treatment process
employed at the plant. The sludge disposal problem occurred because of the plant's inability to
incinerate all of the sludge that was processed. It was decided at the on-set of the Federal Assis-
tance Project to concentrate efforts on the sludge handling problem by attempting to effect the mass
t
and characteristics of the sludge produced by the secondary treatment process.
Operational changes in the secondary treatment process, training in conducting various control
tests and data evaluation were the major tasks performed during the assistance project. These
functions were coupled with various operational recommendations for both short term and long term
plant operation and control.
This report documents the findings of the Federal team. Also presented are the results of
additional analyses of the data conducted after the conclusion of the project.
-------
III. DESCRIPTION OF PLANT AND AREA
The Metro Denver activated sludge plant Is located north of Denver in Commerce City, Colorado.
The effluent from the plant is discharged to the South Platte River, an interstate stream. The State
Water Quality Standards require a minimum of 8056 removal of five-day 20°C BOD by the Metro Denver
plant before discharge to the South Platte River. Since the plant began operation in 1966, it has
generally achieved this required eighty percent reduction.
The Metro Denver plant is comprised of primary and secondary sewage treatment facilities and
includes sludge processing facilities. A flow schematic is presented in Figure 1.
The primary treatment facilities were designed to treat an average flow of 27 million gallons per
day (MGD) and a maximum flow of 50 MGD. These facilities consist of an inlet structure, bar screens,
grit and grease removal units, sedimentation basins and a grease and scum incinerator.
The secondary treatment facilities were designed to treat an average flow of 117 MGD with a
maximum flow of 234 MGD. The design (8005) load is 166,350 pounds per day or an average influent
concentration of 170 mg/1 8005. The secondary treatment facilities consist of aeration basins, the
blower building, sedimentation basins and chlorine contact chambers.
The sludge processing facilities were designed to treat 37,400 pounds per day of raw primary
sludge and 131,000 pounds per day of secondary sludge from the Metro Denver plant; and 92,700 pounds
per day of digested primary sludge from the Denver North Side plant. These facilities consist of the
waste activated sludge concentrators, sludge holding tanks and the sludge processing building which
housed the vacuum filters and incinerators.
Pertinent design information about types and sizes of equipment is discussed, as necessary, in
the following sections.
-------
NORTH SIDE EFFLUENT GATE
METERING FLUME STRUCTURE
FEDERAL ASSISTANCE PROJECT
METROPOLITAN DENVER SEWAGE TREATMENT PLANT
OCTOBER 1969 - FEBRUARY 1970
PLANT FLOW SCHEMATIC
-------
IV. SUMMARY OF ASSISTANCE PROJECT
The major emphasis during the Federal Assistance Project was aimed at the biological (secondary)
portion of the Metro Denver plant. As shown in Figure 1, the secondary portion is comprised of
twelve aeration basins each of two million gallon capacity and twelve 1.16 million gallon final
clarifiers. These twenty-four structures were equally divided into four separate areas by piping,
pumps and other control devices. Throughout the project these four areas demonstrated characteris-
tics of four different plants possibly due to undetected loading differences, flow characteristics,
etc. For this reason, operational control of each of the areas was different and was based on the
individual characteristics exhibited. Because excessive grease was contained in the influent to the
Metro Denver plant, aeration basins No. 1 and No. 2 (located in Areas #1 and #2) were used as grease
flotation units. This required that Areas #1 and #2 be operated using only two aeration basins in
combination with their respective three clarifiers. Areas #3 and #4 were operated using all three
aeration basins and three clarifiers in each area.
A. Control Testing Procedures and Results
The initial phase of the project involved instigating process control testing, as outlined by
West (1), to monitor process performance. The basic control tests are the centrifuge test, the
settleometer test, blanket depth measurements, turbidity measurements and dissolved oxygen concen-
tration determinations. The main function of each of these procedures is:
1. Centrifuge tests were conducted on the effluent from the aeration basins and on the return
sludge drawn from the final clarifiers. This test indicates the relative concentrations (by
percent volume) of solids needed for determining the solids distribution in the activated
sludge process. The results from the centrifuge test can also be used for other determina-
tions. For example, the secondary clarifiers at the Metro plant are the "vacuum" type with
twelve draw-off tubes in each clarifier. By using the centrifuge to determine the suspended
solids concentrations, the height of each draw-off tube can be adjusted so that a uniform
concentration of sludge can be drawn from the clarifier bottom.
A relationship between percent solids by centrifuge and by weight (milligrams per liter) of
total and volatile suspended solids (TSS & VSS) was obtained by comparing the results of a
centrifuge test and a suspended solids analysis made on the same grab samples. This compari-
son was made on a daily basis throughout most of the project.
2. Settleometer testing was conducted on the effluent from the aeration basins to determine the
settling rate and characteristics of the sludge. Visual observations of the sludge settling
characteristics indicated the relative removals, flocculation properties, etc. of the sludges
from the four areas. Analysis of the settleometer data coupled with centrifuge data also
-------
allowed a determination of the dewatering or concentrating ability of the various mixed
liquors. Settleometer data were normally collected four times per day at 5:00 A.M., 9:00
A.M., 1:00 P.M. and 9:00 P.M. During the last portion of the project, settleometer tests
were run every two hours. Readings of the settled sludge volume (SSV), as indicated from the
settleometer, were taken every five minutes for the first one-half hour and every ten minutes
for the second one-half hour.
3. Blanket depth determinations (depth of sludge interface from surface) were taken on each of
the final clarifiers to aid in determining the solids distribution and solids mass in the
final clarifiers. During the last portion of the project, blanket readings were taken every
two hours, twenty-four hours per day.
4. Turbidity measurements were taken on samples of settled and skimmed effluent from the final
clarifiers and were used to indicate the relative effectiveness of the activated sludge
process in producing a clarified effluent. The samples were settled and skimmed before
turbidity measurements were made so that clarifier limitations could be eliminated from the
analysis and only the relative effectiveness of the biological system could be judged.
5. Dissolved oxygen measurements were taken to assure that an adequate oxygen supply was avail-
able to support the process.
Plant operators were trained during the project to make the above control tests and to analyze
and interpret the obtained data. Process control adjustments could then be made on a routine basis.
In addition to conducting the control tests, the operators were required to take readings of various
flow meters and to collect grab and composite samples so that the plant performance could be
monitored.
B. Performance Evaluation-procedures and Results.
The Metro plant laboratory conducted various analyses on the collected samples to provide addi-
tional data for the project. Influent and effluent samples for the secondary treatment portion of
the plant were composited and determinations were made for 8005 and TSS. In addition to overall
plant influent and effluent samples, Influent and effluent samples were collected and composited on
each of the individual areas. Figure 2 illustrates the loading in pounds of BOD5 applied to the
secondary treatment (activated sludge) portion of the Metro Denver plant, as well as the seven day
moving average of the overall plant effluent concentrations of 6005 and TSS.
The seven day moving average 8005 and TSS effluent concentrations are depicted on the lower por-
tion of Figure 2. The BOD5 in the effluent is closely.related to the TSS concentration. This
relationship emphasizes the effect of the difficulties encountered with final clarifiers at the Metro
Denver plant. Without exception, each peak on the graph can be correlated with "bulking" problems in
-------
one or more of the areas. A portion of the "bulking" problem was due to a poor-settling sludge
caused by process imbalance. However, many times an apparent good-settling sludge in settleometer
testing was hydraulically "flushed" over the effluent weirs.
It is believed that the peaks or poor effluent quality depicted in ,Figure 2, prior to and during
the initial phases of the Federal project, were caused by the above-average flow and BODg loadings
(See upper portion of Figure 2) that were received at the plant during the month of October 1969.
The large peaks of effluent TSS and BODg experienced in the latter part of November and in December
were caused by a loss of process balance in Areas #1, #2 and #4. The exact reasons for these changes
are not known. However, it may have been the type of loading being used, temperature effects, meter
problems, etc. When Areas #2 and #4 were subsequently converted so that all the sewage was applied
at the head of each aeration basin on December 12, 1969, the trend in the effluent concentrations of
BODjj and TSS decreased. Area #1 was converted to this type of loading on January 5, 1970.
The peaks depicted in the month of January were caused by loss of control of Area #3. Excessive
wasting of sludge and the breakdown of a clarifier were the main causes of this failure.
The peaks in February were caused by "bulking" problems in Areas #1 and #4. Area #1 was bulking
because an attempt was made to rapidly build up solids while Area #4 was bulking because excessive
solids had accumulated due to inaccurate flow meters on the waste sludge stream.
The effluent quality toward the end of the project (excluding the peaks in late February) was
definitely on an improving trend. The only other period of corresponding quality was experienced
during the first part of November 1969.
The effluent quality depicted in Figure 2 represents a composite of all of the areas and, there-
fore, the performance of the individual areas is not reflected directly. Areas #1 and #4 generally
had the poorest quality effluents, while areas §2 and #3 gave the most consistent high quality
effluents. The reasons for this may have been undetected differences in loading, the effects of
different operational modes or undetected difficulties with flow meters.
Also shown in Figure 2 is the loading to the secondary process in pounds of BODs per day. The
dotted line represents the design average day loading (166,350 Ibs. BODg per day) which was exceeded
on various days of all weeks during the project. The average loading for the entire period of study
was 161,560 pounds BODs per day. However, two aeration basins were not in service as activated
sludge basins but rather as grease removal units. Thus, the aeration capacity to handle the design
load was reduced.
The BOD5 load was high during the month of October because of the effects of runoff from early
snows that had occurred in the Denver area. There is no apparent explanation for the higher load-
ings in the middle of January and especially the peak load on January 15, 1970.
Another trend that is not as apparent is the relationship between loading and effluent quality.
-------
i
320
280
240-
£ 120-
UJ
3 80-
90
f 8°
- 20
10
FIGURE 2
FEDERAL ASSISTANCE PROJECT
METROPOLITAN DENVER SEWER TREATMENT PLANT
OCTOBER-1969 to FEBRUARY -1970
INFLUENT BODS LOADINGS AND
7 DAY MOVING AVERAGE, EFFLUENT BOD5
AND TSS CONCENTRATIONS
VS
TIME
A
I *A 7 DAY MOVING. AVG. TSS
./ .A
,^*»'^% 7 DAY MOVING AVG. BOD5
s/ V^\v^/~*/
1 KTOUI 1969
1 HOVEMIEIt 1969
AVERAGE DAY DESIGN
LOADING 166,350 LBS. BOI
TIME IN DAYS
-------
The low loading in December is reflected by a consistent high quality effluent during the first one-
half of the month. The consistent steady loading during the last half of January and the month of
February is reflected by consistently improving effluent BODg and TSS concentrations. The higher
loadings in October and in January demonstrate the adverse effect of decreasing effluent quality.
Figure 3 illustrates the percentage reduction (weekly average) of BOD,- and TSS achieved by the
Metro Denver plant. The percentage removal of BODg is a better indicator than effluent BODg concen-
trations of the benefits of process control. This fact is shown by the gradual increase in percen-
tage removal throughout the project. The percentage removal of TSS declined during the initial phase
of the project and then increased rapidly in December to a somewhat stable percentage reduction
during the final phases of the project.
The increasing trend in percentage BODg reduction in conjunction with the fluctuating effluent
BOD5 concentrations can be explained by the variations in the incoming BODg load. An increasing
BOD5 load was accompanied by increased effluent concentrations and thus a relatively constant rela-
tionship as far as percentage removal.
The average reduction of BOD5 for the entire period was 85% and for TSS it was 60%. These are
reductions by the secondary treatment portion of the plant only and do not include the reductions of
BOD5 and TSS that were achieved by primary treatment. Therefore, the reduction of BODg for the pri-
mary and secondary processes averaged greater than for the secondary treatment process only and
adequately met the 80% minimum reduction of BOD5 required by Colorado's Water Quality Standards.
C. Data Analysis - Procedures and Results
Large volumes of data were obtained from the numerous control tests that were conducted and the
various monitoring or performance determinations that were made. These data were analyzed daily to
determine trends which were indicative of process performance and from these various trends process
control decisions were made. (i.e. increase or decrease return sludge flow, increase or decrease
wasting flow rate, etc.) Metro Denver plant personnel were trained in analyzing the data and
deriving the various trend relationships. Training was also provided in interpreting the various
trend curves so that control adjustments could be made.
A large number of relationships were established to determine the best parameter or combination
of parameters to use for controlling the activated sludge process. At the conclusion of the project
many of these relationships were abandoned and only those that appeared most beneficial were recom-
mended for continual use.
A summary of the more pertinent analyses performed are presented below.
The relationship between the settled sludge volume (settleometer readings) and time was plotted
to indicate the trends in settleability of the sludge. Also established was the trend outlining the
-------
100
80-
TO-
GO-
z
o
/\A
- i so-
li!
.X
40>
30<
20-
FIGURE 3
FEDERAL ASSISTANCE PROJECT
METROPOLITAN DENVER SEWAGE TREATMENT PLANT
OCTOBER - 1969 to FEBRUARY - 1970
WEEKLY AVERAGE PERCENTAGE REDUCTION OF BOD s
AND TSS ( SECONDARY ONLY )
VS
TIME
IO-
26 I Z'l I 1 * • 2/16 I 2/23
WEEK oF^joTToTTTaTjr^zoIiolTl 11 3 111 10 III i7«ll 24l .1 I • 12 8 112 IS! II 21112 291 I S
END MARCH I. 1970
-------
ability of the sludge to concentrate or dewater. Many of the relationships were based on data from
the daily control test values. Sludge blanket depths were determined as many'as twelve times per
twenty-four hour period as well as aeration tank concentrations, return sludge concentrations and
flow measurements. These values were averaged on a daily basis and such parameters as sludge age,
total sludge mass in the system, clarifier overflow rates, sludge detention time in the clarifier,
mass of sludge returned per gallon of sewage, etc. were calculated. Additional trends developed
were effluent quality versus time as described by BODg and TSS concentrations.
All of the above-outlined analyses, as well as others, were conducted on each of the areas.
D. Control of Areas - Procedures and Results
Prior to this project, Metro Denver plant personnel were operating the secondary treatment
facilities as one large unit. All four areas were using a two aeration basin, three clarifier com-
bination and were step loading the sewage to the aeration basins. Sewage could be introduced at four
gates along the aeration basin: Gate A at the head end of the tank, Gate B approximately one-fourth
of the length from the head of the basin, Gate C approximately one-half the length from the head of
the basin and Gate D approximately two-thirds of the length of the tank from the head of the basin.
Metro Denver personnel were loading one-half of the sewage at Gate B and one-half at Gate C. Return
sludge was introduced at Gate A.
A short summary of the major operational changes made in each area will be described below. The
majority of the operational changes were made to determine the operational mode which would allow
maximum removal of TSS and BODg and would improve the sludge characteristics to facilitate sludge
handling.
1. Area #1 was operated using two aeration basins and three clarifiers throughout the project,
except for a short time (one week) when one of the final clarifiers was inoperable. Only two
aeration basins were used since the third aeration basin was required to remove the excessive
grease received at the plant. This area was operated using step loading (one-half flow at
Gate B and one-half at Gate C) from the start of the project until January 5, 1970, when
loading was converted to introducing all the flow at the head of each aeration basin (Gate
A). This loading procedure was used until the end of the project. All the return activated
sludge was introduced at Gate A.
2. Area #2 was operated in a manner similar to Area #1. However, Area #2 was converted to
loading all sewage at Gate A on December 12, 1969. Performance in Area #2 was generally
superior to that of Area #1 throughout the project. Although the meters didn't indicate a
difference, it appeared as though Area #2 was receiving less flow than Area #1. It was
attempted to equalize the flow to all of the areas throughout the project. However, this was
11
-------
difficult to achieve because of the plant's hydraulics and, therefore, equal splitting of the
flow to each of the four areas was not successful.
3. Area #3 was converted to a three aeration basin, three clarifier basin operation within a
week after the project started. All sewage was loaded into the aeration basins at Gate A, as
well as return sludge. Area #3 provided the best overall performance during the project, as
measured by effluent BODg and TSS concentrations.
4. Area #4 was converted to a three aeration basin, three clarifier basin operation within a
week after the project started. However, a variety of methods of introducing loads was tried
on Area #4. Initially all return sludge was introduced at Gate A and the loading of one-half
the sewage flow to Gate B and Gate C was maintained. However, this was changed to loading all
the sewage at Gate D on November 12, 1969. (Contact stabilization) This loading was main-
tained until December 12, 1969 when all sewage was introduced at Gate A. Area #4, at times,
showed excellent reductions but the area was generally sporadic in its performance because of
difficulties in retaining the sludge in the final clarifiers.
The major operational changes above were affected by a variety of operational problems. Unreli-
able meter readings on the waste sludge flow, uneven flow distribution to the various areas, mechani-
cal failure of three clarifiers during the project, and a continual problem with solids "flushing" out
of the final clarifiers are but a few of the operational problems that added to the complexity of the
project.
E. Control of Sludge Characteristics - Procedures and Results
The two major problems encountered at Metro Denver were the "flushing" of solids that occurred
out of the final clarifiers and the sludge processing and handling problem. Since the initial
emphasis was to work in the secondary treatment portion of the plant, improving removal efficiencies
and effluent quality became primary considerations in operating the facility. However, a high
quality effluent representing increased removals of BOD5 and TSS also is associated with increased
sludge production, which served to magnify the sludge processing and handling problems. To compensate
for the increased sludge production accompanying the increased treatment efficiencies an attempt was
made to develop a sludge that would concentrate or dewater better than previous sludges. The end
result would be a lesser volume but increased mass of sludge being removed from the system.
At Metro Denver, the waste activated sludge is further concentrated by the use of chemical
coagulants in air flotation units. Therefore, it was also attempted to develop a sludge more amenable
to chemical coagulation.
Figure 4 illustrates the concentration of sludge wasted from the secondary treatment process. No
data on the waste sludge total suspended solids concentration are available for the early phases of
12
-------
the project. Consequently, no comparison Is made of the overall changes In waste sludge concentra-
tions for the entire project period. The trends developed for the period of record are shown in
Figure 4. A decrease in waste sludge concentration was initially noted paralleling the operational
difficulties encountered with Areas #1, #2 and #4 in December (See IV-D above). Later in the project
(January and February) the waste sludge concentration increased steadily to a weekly average of
approximately 7,000 mg/1, representing a substantial increase over the low weekly average of 4,500
mg/1 experienced during the last week of December. Figure 4 indicates that one of the goals in con-
trolling sludge characteristics, that of increased waste sludge concentration, was achieved. However,
the benefits derived from increasing the waste sludge concentration were partially overshadowed by the
increased sludge production resulting from increased removal efficiencies of BODg.
13
-------
Si
g
o
h-
111
U
z
o
3>
2-
• DAILY CONCENTRATION
O WEEKLY AVG. CONCENTRATION
FIGURE 4
FEDERAL ASSISTANCE PROJECT
METROPOLITAN DENVER SEWAGE TREATMENT PLANT
OCTOBER-I%9 to FEBRUARY - 1970
WASTE SLUDGE CONCENTRATION IN MG /L vs TIME
17 NOV., '69-
1DEC.
T!
JAN.
1 FEB.
TT
1 MARCH.
-------
V. DATA ANALYSIS
Tlie objective of the assistance project was to operate the activated sludge process so that the
waste sludge characteristics could be controlled, thereby alleviating at least a portion of the
sludge handling problems. While trying to achieve this goal a large amount of data were collected.
At the conclusion of the project portions of these data were analyzed to further evaluate the major
problems encountered at the Metro Denver plant, namely the sludge handling problem associated with
sludge produced in the activated sludge process and the problem of solids loss from the final
clarifiers.
Certain portions of the data obtained during the project were selected so that smaller and more
workable portions could be investigated. It was decided to evaluate only Areas n and #3, since
these two areas covered most of the operational modes investigated and demonstrated the best
response to operational controls. Area #2 was operated with both step loading and conventional
loading and with two aeration basins in combination with the three clarifiers. Area #3 was operated
with three aeration basins in combination with the three final clarifiers. Both Areas #2 and #3
gave the most consistently good quality effluents and responded favorably to operational controls.
A. Analysis of Sludge Production
The sludge handling problems at the Metro Denver plant were affected by the amount of sludge
produced in the secondary unit. To evaluate the sludge production per pound of 8005 removed, an
application of the kinetic model which has been used and frequently outlined in the literature to
describe biological treatment systems was used. Papers by Lawrence and McCarty (2), Jenkins and
Garrison (3), Pearson (4) and McKinney (5) are a few that have discussed and presented the kinetic
model. The assumptions made in relating the data collected during the project to the analysis made
using the kinetic model are outlined in a sample calculation presented In Appendix C.
Since the kinetic model has been well documented in the literature, the following equations will
be used without a formal presentation of their theoretical basis.
Basic Kinetic Equations
q = F(S0 - ST) = Substrate removal rate Equation 1
X-|V
v -
F*2 * HX|r Equation 2
vxl
v = Yq * Specific Growth Rate Equation 3
IIV.
Mean cell residence time Equation 4
FX2
l/ec = Yq - Kd = 2 * r = Net growth rate Equation 5
v*l
15
-------
WHERE:
q = substrate removal rate, pounds of substrate removed per pounds of cells
in the system per day
S0 = influent substrate concentration
S-j = effluent substrate concentration
F = influent flow rate
X] = MLSS or MLVSS concentration
V = volume of aeration plus secondary sedimentation basins
v = specific growth rate, pounds of cells produced per pounds of cells in
the system per day
Y = yield coefficient, pounds of cells produced per pounds of substrate
removed
Kd = endogenous decay coefficient, pounds of cells lost per pounds of cells
in system per day
X- = effluent TSS or VSS concentration
W = waste sludge flow
Xr = return sludge and waste sludge TSS or VSS concentration
e = mean cell residence time (sludge age), days = pounds of cells in system
per pounds of cells lost from system per day
To derive a kinetic description of a particular waste source requires the development of a
series of steady state conditions. In other words, the rate of change of substrate removal with
time is assumed to be zero. Although steady state is never achieved in a large dynamic activated
sludge plant such as Metro Denver's, certain periods of operation approach this condition. For
Areas #2 and #3 time periods were selected based on uniformity of aeration basin solids concentra-
tion and of sludge settling and concentration characteristics. The uniformity of these characteris-
tics best described a period of "steady state." Table 1 summarizes briefly the periods selected and
the average of selected parameters for each period.
The reciprocal of the mean cell residence time (ec) is the net growth rate. Equation 5, above,
outlines the relationship between the net growth rate (l/ec) and the substrate removal rate q.
These values are related by the yield coefficient (Y) and the endogenous respiration coefficient
(Kd). For normal domestic wastes, values for Y and Kd have been determined. Heukelekian, Oxford
and Manganelli (6) have presented values of Y = 0.5 milligrams volatile suspended solids produced
per milligram of waste (BODg) removed and values of Kd = -0.055 as being representative, while
Middlebrooks and Garland (7) have presented values of Y = 0.67 milligrams volatile suspended solids
16
-------
TABLE 1
FEDERAL ASSISTANCE PROJECT
METROPOLITAN DENVER SEWAGE TREATMENT PLANT
OCTOBER 1969 - FEBRUARY 1970-
A Suirmary Of Various Parameters Associated
With The Selected "Steady State" Periods
• Parameter-Average For Period
Influent Flow - MGD
Return Sludge Flow - MGD
Waste Sludge Flow - MGD
Aeration Tank Concentration (ATC)
% Volume Concentration by Centrifuge
Return Sludge Concentration (RSC)
% Volume Concentration by Centrifuge
Ratio TSS/ATC *
Ratio VSS/TSS *
Influent (To Secondary) 8005
Concentration - mg/1
Effluent BODs Concentration - mg/1
Effluent TSS Concentration - mg/1
AREA #2
1/5/70
to
1/11/70
25.62
13.85
.611
2.96
8.27
706
0.804
197
45
68
1/29/70
to
2/12/70
27.78
11.50
.262
4.46
14.89
585
0.811
197
24
36
AREA #3
12/15/69
to
1/5/70
29.64
14.41
.593
3.71
12.00
616
0.840
188
21
40
1/10/70
to
1/13/70
29.85
11.77
.490
1.36
4.61
817
0.846
194
36
44
1/20/70
to
1/25/70
26.30
23.00
.458
5.44
13.19
482
0.793
199
37
79
2/9/70
to
2/16/70
29.47
13.01
.445
3.92
14.05
684
0.782
207
13
26
* The relationship between % volume concentration by centrifuge and TSS and VSS was established
by comparing results conducted on grab samples - normally daily grab samples.
17
-------
produced per milligram of waste (BODg) removed and values of Kd = -0.048.
The value of Y (slope) and Kd (intercept) can be graphically determined by determining the value
of ec (Equation 4) and q (Equation 1) and plotting 1/8C versus q. Values of the removal rate (q)
and the mean cell residence time (ec) were calculated using the Metro Denver data for the selected
"steady state" periods. (See Appendix C for example calculations) These data are presented in
Table 2. Values derived for ec indicate a relatively low cell residence time. Normal residence
times for conventional activated sludge are five to fifteen days, with a mean of ten days [See
Jenkins (3)]. When considering ec and normal values obtained for Y and q during the period, Kd
values were not within the recognized range (i.e. -.05, -.06), which could reflect a lack of aera-
tion capacity, complete mixing, etc.
The values of qBO|v and 1/flc determined from the project data have been plotted in Figure 5.
Also plotted is the line representing the relationship between l/ec and q for a typical domestic
sewage using an average of the values presented in the literature (6) (7). (Y = 0.60 K
-------
TABLE 2
FEDERAL ASSISTANCE PROJECT
METROPOLITAN DENVER SEWAGE TREATMENT PLANT
OCTOBER 1969 - FEBRUARY 1970
Calculated Values of ec and qBOD
Selected Periods of Operation
Areas #2 and #3
AREA #2
Day
Mon
Tues
Wed
Thurs
Fri
Sat
Sun
Thurs
Fri
Sat
Sun
Mon
Tues
Wed
Thurs
Fri
Sat
Sun
Mon
Tues
Wed
Date
1/05/70
1/06/70
1/07/70
1/08/70
1/09/70
1/10/70
1/11/70
1/29/70
1/30/70
1/31/70
2/01/70
2/02/70
2/03/70
2/04/70
2/05/70
2/06/70
2/07/70
2/08/70
2/09/70
2/10/70
2/11/70
1BOD5
Ib/lb
0.412
0.495
0.373
0.344
0.424
0.352
0.275
0.365
0.404
0.341
0.259
0.402
0.457
0.449
0.360
0.343
0.351
0.242
0.454
0.377
0.372
6C
Days
2.533
2.160
2.510
4.263
2.736
3.128
3.947
6.514
6.250
6.714
6.750
6.706
7.586
5.409
5.261
5.311
5.273
4.818
4.952
5.561
6.053
1/8C
Days-1
0.395
0.463
0.400
0.235
0.365
0.320
0.253
0.154
0.160
0.149
0.148
0.149
0.132
0.185
0.190
0.188
0.190
0.208
0.202
0.180
0.165
AREA #3
Day
Mon
Tues
Wed
Thurs
Fri
Sat
Sun
Mon
Tues
Wed
Thurs
Fri
Sat
Sun
Mon
Tues
Wed
Thurs
Fri
Sat
Sun
Mon
Sat
Sun
Mon
Tues
Tues
Wed
Thurs
Fri
Sat
Sun
Mon
Tues
Wed
Thurs
Fri
Sat
Sun
Mon
Date
12/15/69
12/16/69
12/17/69
12/18/69
12/19/69
12/20/69
12/21/69
12/22/69
12/23/69
12/24/69
12/25/69
12/26/69
12/27/69
12/28/69
12/29/69
12/30/69
12/31/69
1/01/70
1/02/70
1/03/70
1/04/70
1/05/70
1/10/70
1/11/70
1/12/70
1/13/70
1/20/70
1/21/70
1/22/70
1/23/70
1/24/70
1/25/70
2/09/70
2/10/70
2/11/70
2/12/70
2/13/70
2/14/70
2/15/70
2/16/70
iBODs
Ib/lb
0.592
0.483
0.606
0.629
0.492
0.299
0.217
0.408
0.407
0.323
0.271
0.351
0.276
0.220
0.304
0.252
0.285
0.187
0.312
0.276
0.305
0.443
0.956
0.658
0.902
0.512
0.366
0.325
0.207
0.290
0.224
0.166
0.348
0.330
0.339
0.354
0.307
0.354
0.302
0.458
ec
Days
1.72
2.03
2.29
2.59
3.38
3.78
3.50
2.94
3.53
3.84
4.75
3.88
3.44
3.51
11.42
3.17
2.59
3.66
3.40
2.31
2.84
2.27
2.02
4.94
5.62
6.22
6.66
5.09
2.98
7.55
2.97
2.36
2.70
3.68
3.75
4.38
4.75
4.29
4.24
4.88
V8C
Days-1
0.581
0.493
0.437
0.386
0.296
0.265
0.286
0.340
0.283
0.260
0.211
0.258
0.290
0.285
0.088
0.315
0.387
0.273
0.294
0.433
0.352
0.441
0.495
0.202
0.178
0.161
0.150
0.196
0.336
0.132
0.338
0.424
0.370
0.272
0.267
0.228
0.211
0.233
0.236
0.205
19
-------
0.9
1
i
s
0.8-
0.7-
0.6-
e 0.5-
= i 0.4-
0.3-
0.2-
0.1-
.0. r'
0 /
• S .v*
FIGURE 5
FEDERAL ASSISTANCE PROJECT
METROPOLITAN DENVER SEWAGE TREATMENT PLANT
OCTOBER 1969 TO FEBRUARY 1970
NET GROWTH RATE (%c)
VS
SUBSTRATE REMOVAL RATE (qBODs)
• AREA-3
O AREA - 2
0.1
0.2 0.3
0.4 0.5 0.6 0.7
SUBSTRATE REMOVAL RATE 4BOD,
•
0.9
0.8 0.9 1.0 1.1
Ib. BOD5 REMOVED PER DAY
Ib. VSS IN SYSTEM
-------
sludge produced In the secondary would have been 82,800 pounds per day. To maintain a specific cell
residence time (sludge age), this amount of sludge should have been wasted. If the estimated yield
coefficient Y » 0.72 (See Figure 5) 1s used. 99.300 pounds per day would have been produced and
would have had to be wasted. These values are dally average values and do not represent the peaks
in loading and sludge production that occur. Both values are less than the 131,000 pounds per day
which was the design basis for the Metro Plant secondary sludge handling facilities. Although this
design loading was not exceeded on an average basis, problems did occur with the sludge handling
facilities (i.e. concentrators and incinerators).
B. Analysis of Secondary Clarlfiers
Eckenf elder and O'Connor (8) have stated that the size of secondary clarlfiers In biological
systems Is.related to three design factors. These factors are: (1) The permissible retention of the
settled sludge 1n the basin as dictated by Its biological properties, (2) The area required fo'r
clarification over the operating mixed liquor suspended solids range, and (3) The area and volume
requirements to produce by thickening an underflow of a desired concentration.
At Metro Denver sludge retention In the final claHfiers should be minimized; possibly to one
hour or less. The value of the sludge detention time, SDT, 1n the final clarifiers was determined
during the project on a daily average basis and normally was easily controlled by adjusting-the
return sludge pumping rates. This fact Implies that the volume of the clarifiers and the .return
sludge pumping capacity was generally satisfactory to allow rapid removal of the' sludge.
The clarification and thickening capacities for a secondary clarlfler can be estimated from
,1 •
batch settling tests. A great number of batch settling tests were conducted during the project, and
these results were used to evaluate the clarification and thickening capacities of the Metro Denver
plant.
The limitation of this type of analysis 1s in the determination of a representative batch
settling test. The previously selected "steady state" periods for Areas #2 and #3 were selected for
analysis. These periods were Initially selected based on uniformity of sludge settling and sludge
concentration characteristics, as well as uniformity of solids concentration. In addition, these
periods were generally the best periods of control and operation and therefore were representative
of sludge settling characteristics that were experienced during the project.
During most of the project four batch settling tests were conducted on a dally basis at 5:00 A.M.,
9:00 A.M., 1:00 P.M. and 9:00 P.M. Values for settled sludge volume for each hourly test were
averaged for the various "steady state" periods. These values are presented 1n Table 3. Table 1
gives the associated average parameters and average flow values for these same periods. The period
January 10, 1970 to January 13, 1970 for Area #3 was omitted from this analysis because of the low
mixed liquor solids concentration and resulting rapid settling.
21
-------
TABLE 3
FEDERAL ASSISTANCE PROJECT
METROPOLITAN DENVER SEWAGE TREATMENT PLANT
OCTOBER 1969 - FEBRUARY 1970
Average Settled Sludge Volumes For
"Steady State" Periods
Area and "Steady
State" Period
n
January 5, 1970
to
January 11 , 1970
#2
January 29, 1970
to
February 12, 1970
#3
December 15, 1969
to
January 5, 1970
#3
January 20, 1970
to
January 25, 1970
#3
February 9, 1970
to
February 16, 1970
Settling
Time (Min)
5
10
20
30
60
5
10
20
30
60
5
10
20
30
60
5
10
20
30
60
5
10
20
30
60
Average Settled Sludge Volumes for
"Steady State" Period - cc/1
5:00 A.M.
496
346
261
224
171
666
489
403
343
276
805
660
510
431
320
922
870
742
642
480
541
413
334
294
238
9:00 A.M.
523
383
279
239
184
737
545
433
376
301
855
725
585
495
360
953
914
828
717
542
604
476
382
335
264
1:00 P.M.
424
286
211
179
144
453
332
261
228
184 '
650
521
408
347
260
903
752
582
482
367
470
368
294
259
207
9:00 P.M.
517
332
246
213
161
.617
452
362
320
249
677
523
401
. 348
265
936
829
598
489
381
539
421
337
308
237
22
-------
The clarification capacity required In a clarlfler can be estimated from the Initial rate at
/
which the solIds liquid Interface subsides as outlined by Eckenfelder (8), Rich (9) and Smith and
Loveless (10). The zone settling rate (Vs) can be calculated by determining the slope of the Initial
straight line portion of the sludge settling curve. This settling rate-can then be expressed as the
equivalent surface overflow rate since solids will be lost 1n the plant effluent 1f the settling rate
is exceeded by the clarlfler overflow rate.
Or • V x 7.5 gallons per cubic foot x 24 hours per day
r s Equation 6
= Vs x 180
WHERE:
Or = Equivalent Surface Overflow Rate (gal/sq ft/day)
Vs = Zone Settling Rate (ft/hr)
Curves were drawn from each set of average settled sludge volume values for the selected periods.
The slope of the Initial straight line portion of the curve was determined and thus the zone settling
rate (V.) was established. An example determination of V, Is shown In Figure 6. The values of the
d 9
zone settling rates (V$), as well as the associated equivalent overflow rates (Or), are shown 1n
Table 4.
The zone settling rate (V$) varied throughout the "average" day for the selected periods. This
Is to be expected since the zone settling rate Is a function of the Initial MLSS concentration and of
the loading rate, I.e. pounds of BOD per pounds of MLSS. [Smith and Loveless (10)]. Flow variations
throughout the day caused the MLSS and the loading rates to fluctuate, causing the observed varia-
tions In the values of Vs. No attempt was made to distinguish between the portion of the change 1n
Vs due to changing load and that due to change of Initial MLSS concentration or 1n response to any
possible variances 1n growth rates. In addition, as mentioned earlier, cell residence time seldom
exceeded five to six days. Associated effects on settleabillty were also not separable.
It 1s shown 1n Table 4 that the maximum zone settling rate normally occurred at the 1:00 P.M.
test. However, It was observed that this was also the time of the day when most of the solids
flushing occurred. Table 4 shows the calculated overflow rates based on the dally average flow for
each area during the periods. Each area at Metro Denver had three 130 foot diameter secondary
clarlflers which gave a total surface overflow area of 39,900 square feet. Generally the 1:00 P.M.
equivalent surface overflow rates exceeded the average clarlfler overflow rates for the periods
Investigated. However, this 1s based on maximum zone settling rates compared with average clarlfler
overflow rates. If the maximum flow Is assumed to occur at 1:00 P.M. and the design ratio of
l"averagehday1raSte a 2 ^See Hennlngson, Durham and Richardson (11)] Is applied to the clarlfler
overflow rates, then In every case the equivalent surface overflow rate derived from Vs values at
23
-------
FIGURE 6
FEDERAL ASSISTANCE PROJECT
METROPOLITAN DENVER SEWAGE TREATMENT PLANT
OCTOBER. 1969 - FEBRUARY. 1970
DETERMINATION OF ZONE SETTLING RATE ( Vs )
HEIGHT OF SLUDGE INTERFACE vi TIME
AREA #3 PERIOD: 2/9-2/16/70 AVG.9:OOAM
15 20
SETTLING TIME
25 10
( MINUTES )
24
-------
TABLE 4
FEDERAL ASSISTANCE PROJECT
METROPOLITAN DENVER SEWAGE TREATMENT PLANT
OCTOBER 1969 - FEBRUARY 1970
Zone Settling Rates (Vs) And Equivalent Surface
Overflow Rates (Or) For "Steady State" Periods
Area and "Steady
State" Period
n
January 5, 1970
to
January 11, 1970
#2
January 29, 1970
to
February 12, 1970
#3
December 15, 1969
to
January 5, 1970
#3
January 20, 1970
to
January 25, 1970
#3
February 9, 1970
to
February 16, 1970
Zone Settling Rates (Vs)
- ft/hr and Equivalent Surface Overflow Rates (Or) -
gpsfd for "Steady State" Periods *
5:00 A.M.
3.43
620
1.72
310
1.13
204
<1
<180
3.00
544
9:00 A.M.
6.80
595
1.45
262
0.88
159
<1
<180
2.70
488
1:00 P.M.
3. '30
1,225
4.93
890
2.72
490
<1
<180
6.00
1,080
9:00 P.M.
3.83
690
3.03
546
2.53
456
<1
<180
3.24
585
Daily Average
Overflow Rate
For "Period"
gpsfd
644
698
744
660
740
Vs values are given on top and Or values on bottom.
25
-------
1:00 P.M. is exceeded by the clarifier overflow rate and flushing of solids could be expected to
occur. Additionally, a portion of this flushing may be attributable to the normal high return sludge
pumping rates that were utilized in the operational controls.
This problem was further aggravated by locating the effluent weirs for the 130 foot diameter
clarifiers at the outer edge of the clarifiers. This allowed localized high upflow velocities to
occur in the final clarifiers. These localized high velocity currents could have been avoided if
weir placement had been such that more of the surface area in the final clarifiers was developed to
provide a more uniform upflow velocity. However, even if the additional weirs were located to
develop more of the surface area of the final clarifiers, the data shown in Table 4 indicates that
problems with flushing of solids still could occur.
Therefore, either more surface area must be provided or the settling characteristics must be
altered such that the zone settling rate is increased (i.e. a faster settling sludge}. The zone
settling rate is dependent upon the initial MLSS concentration and the loading rate (which directly
affect the sludge flocculation characteristics). [See Eckenfelder (8) and Smith and Loveless (10)]
These factors are dependent upon the influent flow rate, which is highly variable and therefore makes
a positive control of the zone settling rate difficult to achieve. For ease of operation it appears
that more effective surface area, which is better developed by weir placement, is required at Metro
Denver to provide adequate clarification capacity.
The thickening capacity required in a final clarifier can also be estimated from a batch settling
test (8) (9). The average 1:00 P.M. settling test (See Table 3) was selected for analysis since this
time was assumed to coincide with normal daily peak flows which are approximated by twice the average
daily flow (11). The most rapid 1:00 P.M. zone settling rate (See Table 4) was selected to determine
a desired thickening capacity since the value determined would represent a minimum thickening area
required, (i.e. any settling rate with a lesser value would require more thickening area.) The peak
zone settling rate for Area #2 at 1:00 P.M. was 4.93 feet per hour and for Area #3 it was 6.00 feet
per hour. (See Table 4)
Rich (9) outlines an equation for determining the thickening area required:
1TU
A = yi-ii Equation 7
* o
WHERE:
A = cross section required to obtain a layer of a desired concentration
-- ft2
q = flow rate of the mixed liquor entering the final clarifier -- ftVsec.
26
-------
Z'0 = initial height of the interface in the settling column - feet (The
settleometers used at Metro Denver for the batch settling tests had a
0.5 feet depth.)
TU = settling time required to attain a desired underflow concentration -
sec. [This value is obtained from a graphical analysis of a sludge
settling curve as outlined by Eckenfelder (8) and Rich (9).]
To complete the analysis of thickening capacity a desired underflow concentration must be
selected. At Metro Denver the design values for underflow concentration expected ranged between
5,000 to 15,000 mg/1. Therefore, a desired underflow concentration of 10,000 mg/1 was selected.
The settling time (Tu) required to obtain a 10,000 mg/1 underflow concentration for Area #2 for
the selected period January 29 to February 12, 1970 (Vs = 4.93) was determined by a graphical
analysis of the sludge settling curve. This value was used with the average flow for the period to
determine by Equation 7 the area required for thickening. For average flows 42,500 ft2 would be
required for thickening while for peak flows 85,450 ft2 would be required. A similar analysis con-
ducted on Area #3 for the selected period (February 9 to February 16, 1970) showed required areas of
114,000 ft2 and 57,000 ft2 at peak and average flow rates respectively.
The available surface area in Areas #2 and #3 is 39,900 ft2. This is not adequate to provide the
thickening area required to achieve a 10,000 mg/1 underflow concentration with the type of sludge
obtained during the project. The above analysis also indicates the implications of limited thicken-
ing capacity on sludge handling problems. Without sufficient thickening capacity a more dilute waste
sludge flow concentration is realized. The effect of the dilute concentrations is shown by the
relative differences in total sludge volumes to waste 100,000 Ibs. of solids as summarized in Table
5.
The preceding materials were developed to compare actual performance results with the batch
settling data. Most importantly, Rich (9) describes the numerous departures of actual sedimentation
basin performance from that of ideal basins. "The net effect of all the factors that contribute
toward reducing the efficiency of sedimentation in an actual basin is to decrease the clarification
rate and to increase the detention time over that derived from a batch analysis. For the sedimenta-
tion of flocculent particles from dilute suspensions the overflow rate generally will be decreased by
a factor of 1.25 to 1.75 and the detention time will be increased by a factor of 1.50 to 2.00. In
scaling-up thickening operations, a factor of 1.0 to 2.0 is applied to the area required for clarifi-
cation (hindered settling) and a factor of 1.0 to 1.5 to that required for thickening."
Results of the Metro Denver settleability testing should be judged in this light and with the
reported values of loading, residence times, etc. obtained during the period.
27
-------
TABLE 5
FEDERAL ASSISTANCE PROJECT
METROPOLITAN DENVER SEWAGE TREATMENT PLANT
OCTOBER 1969 - FEBRUARY 1970
Waste Sludge Flow Required To Remove
An Equivalent Amount Of Solids With
Varying Underflow Concentrations
Underflow Concentrations ~ mg/1
Waste Volume to Remove
100,000 Lbs. of Solids - Gal.
5,000
2,400,000
10,000
1,200,000
15,000
800,000
28
-------
VI. SUMMARY AND CONCLUSIONS
One of the objectives of the project was to Instigate additional process control testing for the
secondary treatment (activated sludge) portion of the Metro Denver plant. Plant personnel were
trained to conduct process control tests on a routine basis, to evaluate and graph various selected
parameters, and to interpret these data so that adequate daily operational changes could be made.
The full beneficial effect of these process controls was not realized because of various problems
encountered with plant operation, as outlined below:
1. Adjustment of flow to each aeration basin was difficult because each basin was fed by a gate
opening from a common channel. Balancing the hydraulic effects of ten gates to achieve equal
flow to each of the four areas required a great deal of attention. After the gates were
adjusted, determination of actual flow to each aeration basin was questionable because of
occurrences of unreliable instrument readings.
2. Two of the twelve aeration basins provided in the secondary portion of the plant were used as
grease flotation units to remove grease from the influent waste stream and were thus unavail-
able for use as a portion of the activated sludge process. This becomes important since the
average loading to the secondary during this investigation was 161,560 pounds of BOD per day,
which is approaching the design loading of 166,350 pounds of BOD per day.
*)
3. The rate of wasting sludge was difficult to control on a continuous basis because the meters
and control instruments frequently gave erroneous readings. Several times it was discovered
that actual flow and meter readings differed by as much as 100 percent. This definitely
effected the ability to establish a process balance.
4. No reserve capacity was available for final clarification. When a clarifier broke down
(three clarifiers broke down during the project) solids were carried over in the plant
effluent, the effluent quality was degraded, and the process balance in the affected area was
impaired.
Other difficulties encountered were the sludge production in the secondary treatment process and
the flushing of solids from the final clarifiers into the effluent.
The initial emphasis in dealing with the problems at Metro was to control the secondary treatment
portion of the plant. Therefore, removal efficiencies and effluent quality became important consider-
ations in operating the facility. Unfortunately, a high quality effluent representing increased
removals of BOD5 and TSS is associated with increased sludge production, which served to antagonize
•
the sludge processing and handling problem. To compensate for the increased sludge production that
accompanied the slightly increased removals achieved during the project and to relieve the existing
sludge problem, an attempt was made to develop a sludge that would concentrate or dewater better than
29
-------
previously. This would have allowed a lesser volume of a more dense sludge to be wasted. Average
.concentrations of 6,900 to 7,000 mg/1 were obtained in the waste sludge flow toward the end of the
project. However, the benefits derived from increasing the waste sludge concentration were not
realized because of the increased removal efficiencies and the resulting increase in the amount of
sludge produced.
Although slightly greater BOD and suspended solids removal efficiencies were realized through
operational controls, little was accomplished to alleviate the sludge handling problems at the plant.
It is hoped that the increased removal efficiencies will be maintained and the sludge handling proce-
dures modified to alleviate these difficulties. An investigation of the sludge production character-
istics at the Metro plant to compare them with presently available sludge handling facilities was
made.
A kinetic model was applied to the collected data to determine the microbiological character of
the waste stream. At Metro Denver the results of this analysis indicate that the characteristics of
the waste received at the Metro Denver plant do not deviate significantly from those expected from a
typical domestic waste. An attempt was made to determine the amount of sludge production and to com-
pare these results with the sludge handling capacities at the plant. The results indicate that the
design sludge handling capacity (131,000 pounds per day of secondary sludge) could be exceeded during
peak loading periods. It is important when sludge handling procedures or facilities are modified at
Metro Denver that the sludge production during peak loading periods be considered in the design
criteria.
The second major operating difficulty evaluated was the flushing of solids that occurred from the
final clarifiers. Representative zone settling rates were determined for the sludge at Metro Denver
based on the numerous batch settling test data obtained. From this analysis it was determined that
the clarification capacity of the final clarifiers at the Metro Denver plant was not adequate for
the selected periods of investigation. The type of sludge developed proved to have a zone settling
rate (Vs) that was too slow to be held in the final clarifiers. A portion of the flushing problem
was also attributed to the large diameter (130 feet) final clarifiers which had effluent weirs
located at or near the outer periphery. This weir placement allowed excessive velocity currents to
develop further aggravating the solids "flushing" problem. This problem can be alleviated by a
different weir placement arrangement that allows a more uniform use of the surface area on the final
clarifiers. (i.e. another launder of weirs located nearer the center of the tank.)
It was also determined that the thickening area requirements of the final clarifiers were not
adequate to obtain a 10,000 mg/1 underflow concentration with the type of sludge developed during the
project.
Two alternatives can be used to change the effects of the slow zone settling rates of the sludge.
30
-------
The first is to increase the clarifier surface area to reduce overflow rates to less than the settling
velocity established by the zone settling rate. This would provide additional thickening area at the
same time. The second approach would be to increase the zone settling rate of the sludge at the Metro
Denver plant. The zone settling rate is a function of the MLSS concentration and the loading rate.
Because of the constantly changing load (flow) and its effect on the MLSS concentration, it is a con-
tinuous problem to maintain a proper process balance and achieve a desired zone settling rate.
31
-------
VII. RECOMMENDATIONS
The following recommendations are made:
1. It is recommended that control testing established during the Federal Assistance Project be
continued.
2. An effort should be made at the Metro Denver plant to assure the accuracy of all metered
values in order to adequately use control testing procedures.
3. It is recommended that Metro Denver be considered for demonstrating various comparisons.
Because of the unique arrangement of facilities at the Metro plant, four areas with an iden-
tical influent waste are available for evaluation. This arrangement is ideal for conducting
comparisons of various types of equipment (i.e. provide various types of aeration equipment,
evaluate effects of different skimmer arrangements on final clarifiers, evaluate different
weir placement patterns on final clarifiers, etc.).
4. The Metro Denver plant should be operated to achieve the maximum possible reduction of waste
pollutants. To operate and achieve these high removal efficiencies, modifications to the
sludge handling procedures or facilities must be made. Any modification of the Metro Denver
sludge handling facilities should take into account the sludge production characteristics at
the Metro Denver plant which are apparently similar to those of typical domestic sewage and
the clarification-thickening capacity requirements of the secondary clarifiers.
5. Properly located additional weirs are recommended on the secondary clarifiers to develop a
more uniform distribution of flow over the surface area provided in the relatively large
diameter final clarifiers. Surface skimmers are also recommended.
6. Additional clarifier surface area with proper weir placement is recommended or the sludge
settling characteristics must be altered by operational control in order to assure that
solids will not be flushed into the final effluent. Additionally, increased area would
appear to improve sl.udge thickening, thereby reducing waste sludge volumes. More reliable
control would also be obtained by increased clarifier surface area.
32
-------
VIII. APPENDICES
Appendix A - A Resolution: "Concerning the Federal Government's Responsibilities in
Constructing and Operating Sewage Disposal Facilities."
Appendix B - References
Appendix C - Determination of Substrate Removal Rate (q) and Net Growth Rate (l/ec)
-------
APPENDIX A
A RESOLUTION ADOPTED BY METROPOLITAN
DENVER SEWAGE DISPOSAL DISTRICT NO. 1'S
BOARD OF DIRECTORS
ENTITLED
"Concerning the Federal Government's
Responsibilities in Constructing and
Operating Sewage Disposal Facilities"
July 11, 1969
-------
A RESOLUTION
(CONCERNING THE FEDERAL GOVERNMENT'S RESPONSIBILITIES IN
CONSTRUCTING AND OPERATING SEWAGE DISPOSAL FACILITIES)
WHEREAS, the federal government has enacted water pollution control legislation which makes it
incumbent upon states to establish stream quality limits, or to be subjected to stream quality stan-
dards as dictated by the federal government itself, and
WHEREAS, the water pollution legislation adopted by the State of Colorado is not consistent but
rather relates to stream classification, based upon an evaluation of each stream's individual
characteristics, and
WHEREAS, the evaluation process for stream classification relates to a multitude of factors
other than the consideration of protection to health and the abatement of nuisance, and
WHEREAS, sewage treatment to the extent of providing for the development of streams and adjacent
properties into recreational areas does require an additional capital investment for treatment
facilities, as well as substantially increasing operating and maintenance expenses thereof, and
WHEREAS, the arid and semi-arid regions of the western United States have additional burdens for
capital investments and operational and maintenance expenses, due to the lack of dilution water to
the same degree as do the other regions of the United States, and
WHEREAS, the high degree of sewage treatment required to effect water pollution control does
generate additions to solid wastes to be disposed of in the form of sludge, and
WHEREAS, cities, counties and independent samitation districts in the Metropolitan Denver area
recognized in the early 1960's their financial inability as separate political subdivisions to meet
the strict standards being forced upon them by the national Congress and the State Legislature, and
WHEREAS, these independent political subdivisions banded together and created the Metropolitan
Denver Sanitation District No. 1, prevailing upon the Colorado General Assembly to adopt Colorado
Revised Statute 89-15-5 giving them authority so to do, and
WHEREAS, property owning electorate, demonstrating their concern over the pollution threat to
the health and welfare of the total community, by a vote of 25,099 to 2,756, agreed to mortgage
their property so that bonds in the amount of $32.5 million could be issued for the construction of
a modern primary and secondary sewage treatment plant at the confluence of Clear and Sand Creeks
with the Platte River, and
WHEREAS, this plant has been constructed following review and approval of engineering and con-
struction plans by all required federal, state and regional agencies with these bond moneys, supple-
mented by some federal but no state funds, to take care of residential, commercial and industrial
34
-------
wastes with each participating political subdivision, by means of billings to users within their
subdivisions, paying their proportionate shares of all operating costs, and
WHEREAS, this multi-million-dollar plant does bring effluent dumped into the Platte River up to
water pollution control standards it does not dispose of the solid wastes resulting from such treat-
ment for a variety of reasons not the least of which is the fact that our technology has developed a
multitude of consumer goods, paper products, garbage disposal systems and detergents, handle human
waste, and
WHEREAS, resident property owners of Metropolitan Denver recognized their responsibilities to
take the initiative and act to abate practices which contributed to the pollution of Clear Creek,
Bear Creek, Sand Creek and other watercourses that flowed into the Platte River as well as the
Platte River itself, and
WHEREAS, residents and taxpayers of the various political subdivisions that are now participating
in this metropolitan effort to eliminate a pollution problem are being taxed the maximum they can
afford to pay for sewage disposal and do not have the financial capability to pay imminent additional
operating and maintenance costs or to effect the engineering, design and capital construction
necessary to increase the efficiency of this plant so as to halt continuing pollution of our
environment;
NOW, THEREFORE, be it resolved, that the Board of Directors of the Metropolitan Denver Sanitation
District No. 1 hereby does petition the Congress of the United States and the appropriate federal
agencies to:
1. Conduct and finance extensive research to discover new techniques of handling the variety of
waste products now being dumped into the sanitary sewers of America and being carried to
traditional plants that do not have the capabilities of handling them.
2. Make available to this district a special team of scientists and engineers assembled from
appropriate federal departments to serve as a task force to inspect the District's sewage
disposal plant and make appropriate recommendations.
3. Appropriate sufficient funds so that these recommendations can be implemented, since the
Federal government has set up the standards the District is required to meet.
4. Recognize that antipollution standards adopted by the Congress and enforced by federal and
state as well as local government agencies are placing unprecedented and unbearable financial
responsibilities on local governments and their constituents, thus making it mandatory that
the Federal government assist local communities in meeting costs involved not only in con-
structing adequate sewage facilities but of operating them as well.
35
-------
APPENDIX B
REFERENCES
-------
1. WEST, A. W.
Case Histories: Improved Activated Sludge Plant Performance by Operations Control -
Proceedings 8th Annual Environmental and Hater Resources Engineering Conference,
Vanderbilt University. 1969.
2. LAWRENCE, A. W. and McCARTY, P. L.
"Unified Basis for Biological Treatment Design and Operation." Journal of the Sanitary
Engineering Division. ASCE, Volume 96, No. SA 3, Proc. Paper 7365, 1970, pp. 757-778.
3. JENKINS, D. and GARRISON, W. E.
"Control of Activated Sludge by Mean Cell Residence Time," Journal Mater Pollution Control
Federation, Volume 40, No. 11, Part 1, 1968, pp. 1905-1919.
4. PEARSON, E. A.
Kinetics of Biological Treatment. Paper presented at: Special Lecture Series -
Advances in Water Quality Improvement, University of Texas, Austin. 1966.
5. McKINNEY, R. E.
"Mathematics of Complete-Mixing Activated Sludge." Journal of the Sanitary Engineering
Division, ASCE, Volume 88, No. SA 3, Proc. Paper 4362, 1965, pp. 45-61.
6. HEUKELEKIAN, H., OXFORD, H. E. and MANGENELLI, R.
"Factors Affecting the Quantity of Sludge Production in the Activated Sludge Process."
Sewage and Industrial Wastes. Volume 23, No. 8, 1951, pp. 945-958.
7. MIDDLEBROOKS, E. J. and GARLAND, C. F.
"Kinetics of Model and Field Extended-Aeration Wastewater Treatment Units," Journal Water
Pollution Control Federation, Volume 40, No. 4, 1968, pp. 586-612.
8. ECKENFELDER, W. W. and O'CONNOR, D. J.
Biological Waste Treatment. Pergamon Press, New York. 1961.
9. RICH, L. G.
Unit Operations of Sanitary Engineering. John Wiley and Sons, Incorporated, Publishers,
New York, London. 1961.
10. SMITH and LOVELESS
Notes on Activated Sludge, Lenexa. 1969.
11. HENNINGSON, DURHAM and RICHARDSON
Consulting Engineers Report. Metropolitan Denver Sewage Disposal District No. 1 - Metro
Plant Expansion Study, Part 1 - Immediate Requirements. 1969.
-------
12. WEST, A. W.
Listing of Abbreviations Used to Describe Activated Sludge Systems. Lecture Presentation
to Consulting Engineers and Plant Operators Concerning Control Testing for Activated
Sludge Plants Sponsored by Water Pollution Control Division, Colorado Department of
Public Health. 1970.
37
-------
APPENDIX C
DETERMINATION OF SUBSTRATE REMOVAL
RATE (q) AND NET GROWTH RATE (1/eJ
C
-------
It is the purpose of this appendix to present a sample calculation of the determinations made of
the substrate removal rate (q) and the net growth rate (l/ec). Throughout the sample calculation the
assumptions made in relating the data collected and analyzed during the assistance project to the
analysis made using the kinetic model will be stated. Data obtained for Area #3 on December 15, 1969,
will be used for the presentation of the sample calculation.
A. Determination of the Substrate Removal Rate (q)
q = F(S° " S1J [See Jenkins (3)]
V)tj
1. Determination of F(SQ - 5j)
WHERE:
SQ = influent substrate concentration - For Metro Denver a BOD5 value based on a
composite sample was used to represent SQ (12/15/69 for Area #3, S = 198 mg/1).
S. = effluent substrate concentration - For Metro Denver a BOD,, value based on a
composite sample was used to represent S (12/15/69 for Area #3, S. = 16 mg/1).
F = influent flow rate (12/15/69 for Area #3, F = 34.8 MGD) - This value was obtained
from flow meters at the Metro Denver plant.
THEREFORE:
F = 34.8 MGD SQ = 198 mg/1 S] = 16 mg/1
34.8 (198-16) (8.33 Ibs/gal) = 52.760 Ibs. BOD5 removed/day
2. Determination of VX,
WHERE:
V = volume of aeration plus secondary sedimentation basins
X = MLSS or MLVSS concentration
NOTE:
VX-| is a number representing the total pounds of cells in the system. Normally in
determining this value mixed liquor suspended solids concentrations by weight are used
(Xj). Instead of MLSS concentrations, sludge concentrations were obtained on a percent
volume basis by using the centrifuge. During most of the project, however, daily rela-
tionships between percent concentration of sludge by volume and concentration by weight
were determined on the basis of a grab sample. These comparisons varied from 1% =
500 mg/1 TSS to 1% = 1,000 mg/1 TSS. However, during "steady state" conditions, the
relationship between spin concentrations and mg/1 remained fairly constant. Therefore,
the average of the relationship between spin concentrations and mg/1 for each "steady
state" period selected was determined and used to convert the spin concentration to
38
-------
mg/1 of total suspended solids. The relationship between total suspended solids (TSS)
and volatile suspended solids (VSS) was also obtained from the analysis of daily grab
samples. The average ratio of VSS/TSS for each "steady state" period was determined.
For December 15, 1969, and the associated "steady state" period the average relationship
between volume or spin concentrations and mg/1 was 1% = 616 mg/1 TSS for Area #3 and the
average VSS/TSS ratio was 0.840. Another refinement was also used in obtaining VX ,
which will be outlined below. • ..'..'
APPROACH:
A value comparable to VX, called total sludge units (TSU) was determined using the Metro
Denver data. Total sludge units are equivalent to the summation of the aerator sludge
units (ASU) and the clarifier sludge units (CSU). A sludge unit is defined as one
gallon of sludge at 100% concentration, based on sludge concentrations obtained by cen-
trifuge testing. One of the differences between TSU and VX-| lies in the fact that a
modification is made in determining the clarifier sludge units.
a. Determination of Clarifier Sludge Units (CSU)
Final Clarifier
CONC = RSC
WHERE: [West's Symbols (12)]
CWD = clarifier water depth (mean depth if bottom is sloped) - At Metro Denver the
mean depth was 11.7 feet.
DOB = depth of sludge blanket - At Metro Denver blanket depth determinations were made
every two hours on each of the three clarifiers in the respective areas. These
values were averaged on a daily basis to obtain DOB (12/15/69 for Area #3, DOB =
9.7 feet).
39
-------
BLT = sludge blanket thickness - This value is equivalent to CWD - DOB (11.7 - 9.7 =
2.0 feet (BLT) for Area #3 on 12/15/69).
ATC = aeration tank concentration - This is the concentration of sludge by percent
volume in the aeration basin. This value was obtained by centrifuging samples of
the effluent from the aeration basins. A daily average of ATC values was obtained
for use in calculations. (12/15/69 for Area #3, ATC = 2.75X)
RSC = return sludge concentration - This is the concentration of sludge by percent
volume drawn off the bottom of the secondary clarifiers. This value was obtained
by centrifuging samples taken from the return sludge wet well. A daily average of
RSC values was obtained for use in calculations. (12/15/69 for Area #3, RSC =
11.25%)
CMC = clarifier mean sludge concentration - This value is obtained by the equation
ATC + RSC ^is equation assumes a sludge concentration at the top of the blanket
equal to ATC and that at the bottom equal to RSC and a uniform distribution of
concentration. (2-75 + 11.25 = 7.0% (CMc) for Area #3 on 12/15/69)
OTHER FACTORS:
CVG = clarifier volume in gallons per clarifier multiplied by the number of clarifiers in
operation. At Metro Denver the volume of each clarifier was 1.165 million gallons
and three clarifiers were in operation. (1.165 x 3 = 3.495 MG (CVG) for Area #3
on 12/15/69)
CSP = clarifier sludge percentage or the portion of the clarifier occupied by sludge
which is determined by the ratio of *£. (2.0 = n.171 (CSP) for Area #3 on
CWD 11.7
12/15/69).
From the above the clarifier sludge units can be determined by the equation: CSU =
CMC X CSP X CVG.
A modification was made in the equation for this analysis in that the CMC was multiplied
by the factor representing the conversion between percent concentration by volume and mg/1
(616 mg/1 TSS = 1% for Area #3 for 12/15/69 and the related "steady state" period).
Therefore the modified clarifier sludge mass can be determined by CSU (modified) =
CMC x 616 x CSP x CVG x 8.33 Ibs/gal.
C = 7.0 x 616 x 0.171 x 3.495 x 8.33
= 21.550 IDS, of total suspended solids or sludge in clarifier
b. Determination of Aerator Sludge Units (ASU)
ASU = AVG x ATC
40
-------
WHERE:
AVG = aeration basin volume in gallons per aeration basin times the number of basins
in service. At Metro Denver the volume of each aeration basin was 2.0 MG and
three basins were in operation in Area #3 (2.0 x 3.0 = 6 MG (AVG) for Area #3
on 12/15/69).
ATC = 2.75% for 'Area #3 on 12/15/69 (See a. above).
From the above the aeration basin sludge units can be determined. However, the per-
centage sludge concentration by volume must again be converted to mg/1 (616 mg/1 TSS =
ITfor Area #3 for 12/15/69 and the related "steady state" period).
Therefore the modified aeration basin sludge mass can be determined by:
ASU (modified) = AVG x ATC x 616 x 8.33 Ibs/gal.
= 6 x 2.75 x 616 x 8.33
= 84.670 Ibs. of total suspended solids or sludge in aeration basin
c. Determination of TSU
Using the modifications outlined above the value of TSU is assumed to be equivalent
to the value VX-j.
THEREFORE:
TSU (modified) = VX-, = ASU (modified) + CSU (modified)
= 21,550 (From a. above) + 84,670 (From b. above)
= 106,220 Ibs. of total suspended solids or sludge in system
NOTE:
The value of TSU, as determined above, was obtained on a TSS basis. Normally in
determining a substrate removal 'rate (q) a VSS basis is used. (VSS/TSS = 0.840 for
Area #3 for 12/15/69 and the related "steady state" period)
THEREFORE:
TSU (modified) x VSS/TSS = VX1 in Ibs. of VSS
= 106,220 x 0.840
= 89.220 Ibs. of volatile suspended solids in system
3. Example Determination of q
VX-|
For Area #3 on 12/15/69:
F(S0 - S-j) = 52,760 Ibs. BOD5 removed/day (1. above)
VX1 = 89,220 Ibs. of volatile suspended solids in system (2. above)
41
-------
q = 52.760
H 89,220
= 0.592 Ib. of BODs removed per day
Ib. of VSS in system
[q for conventional activated sludge normally has a value of 0.2 to 0.5, see Jenkins (3)]
B. Determination of the Net Growth Rate l/ec
1. _ FX2 + WXr [Sefi jenk^s (3)]
c VX]
1. Determination of VX-)
VX, or its assumed equivalent was determined in Part A-2 above. This was determined for
Area #3 for the date of 12/15/69.
VX. = 106.220 Ibs. of total suspended solids in system (A-2 above)
NOTE:
In the determination of l/ec it is not necessary to convert from a TSS basis to a VSS
basis since both the numerator (FX2 + WXr) and denominator (VX]) in the calculation can be
determined on a total suspended solids basis. Therefore, VX] on a total suspended solids
basis is given above and WXr and FX2 will be calculated on a total suspended solids basis
below.
2. Determination of UXr
WXr represents the mass of sludge wasted from the system per day.
WHERE:
W = waste sludge flow rate (12/15/69 for Area #3, U = 0.89 MGD) - This value was
obtained from flow meters at the Metro Denver plant.
Xr = return sludge TSS or VSS concentration - This value was not determined at Metro
Denver on mg/1 basis but rather the return sludge concentration (RSC) was deter-
mined as a percent volume using the centrifuge. This value (RSC) can be related
to Xr using the relationship established between mg/1 and percent concentration by
volume based on daily grab samples. (616 mg/1 TSS = IX for Area #3 and the
related "steady state" period) For Area #3 the daily average RSC concentration on
12/15/69 was 11.25%.
Xr = 11.25 x 616 = 6,930 mg/1
THEREFORE:
WXr = 0.89 x 6,930 x 8.33 Ibs/gal. = 51.310 Ibs. wasted per dav
42
-------
3. Determination of FX2
FX2 represents the cells lost from the system per day in the plant effluent.
WHERE:
X2 = effluent TSS or VSS concentration - At Metro Denver the effluent TSS concentra-
tion was determined for each area based on the analysis of a composite sample
(12/15/69 for Area #3 effluent TSS = 36 mg/1).
F = influent flow rate (F = 33.9 MGD for Area #3 on 12/15/69).
THEREFORE:
FX2 = 33.9 x 36 x 8.33 Ibs/gal.
= 10.180 Ibs. of total suspended solids lost in the effluent per dav
4. Example Determination of Net Growth Rate (l/ec)
FX2 + WX»
•I / gt _ fc. I
For AreaJ3 on 12/15/69:
FX2.= 10,180 Ibs/day (3. above)
WXr = 51,310 Ibs/day (2. above)
VX1 = 106,220 Ibs. (1. above)
THEREFORE:
1/0 „ 10.180 + 51.310
= 61.490
106,220
= 0.581 Ibs. TSS wasted or lost per day
Ibs. TSS in system
The reciprocal 'of l/ec is equal to ec or the mean cell residence time (sludge age). For
Area #3 on December 15, 1969, ec = 1.72 days.
Similar calculations we're made for the other days included in the selected "steady state"
periods for Areas #2 and #3. The results of these analyses are presented in Table 2 in text.
43
------- |