United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
EPA-600/2-79-159
November 1979
Research and Development
&EPA
Automatic Sludge
Blanket Control in an
Operating Gravity
Thickener
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution-sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
US EPA - AWBERC LIBRARY
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EPA-600/2-79-159
November 1979
AUTOMATIC SLUDGE BLANKET CONTROL
IN AN
OPERATING GRAVITY THICKENER
by
R.C. Polta and D.A. Stulc
Metropolitan Waste Control Commission
St. Paul, Minnesota 55101
Grant No. S803602
Project Officers
J.F. Roesler
I.J. Kugelman
Wastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
-------
DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for
publication. Approval does not signify that the contents necessarily
reflect the views and policies of the U.S. Environmental Protection Agency,
nor does mention of trade names or commercial products constitute endorsement
or recommendation for use.
-------
FOREWORD
The Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health
and welfare of the American people. Noxious air, foul water, and spoiled
land are tragic testimony to the deterioration of our natural environment.
The complexity of that environment and the interplay between its components
require a concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem
solution and it involves defining the problem, measuring its impact, and
searching for solutions. The Municipal Environmental Research Laboratory
develops new and inproved technology and systems for the prevention, treat-
ment, and management of wastewater and solid and hazardous waste pollutant
discharges from municipal and community sources, for the preservation and
treatment of public drinking water supplies, and to minimize the adverse
economic, social, health, and aesthetic effects of pollution. This publi-
cation is one of the products of that research; a most vital communications
link between the researcher and the user community.
One of the methods of improving the cost effectivenesss of treatment
processes and systems is to employ automation. This report covers a full-
scale demonstration of automation of a sludge thickener.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
m
-------
ABSTRACT
The purposes of this study were to evaluate some of the hardware
required to monitor and control the operation of a gravity thickener and
to identify any benefits associated with improved sludge blanket level
control.
An automatic sludge blanket level control system was installed in one
of the six gravity thickeners at the Metropolitan WWTP. In addition,
optical type solids analyzers were installed to monitor the inflow, overflow,
and underflow streams of two basins - one with automated blanket level
control and one with manual control. The performance characteristics of the
instruments and automation system were documented during a series of five
tests each lasting approximately two weeks.
The solids monitors used were found to be acceptable for monitoring
thickener inflow and overflow but not for monitoring the solids underflow.
Automation maintained a more stable position for the sludge blanket than
manual control but did not result in an increase the solids level of the
underflow. Solids capture, however, was upgraded in that the solids level of
the automated thickener overflow was much lower than that of the manual
thickener. Based on savings in labor for thickener operation, and lower
costs to treat the thickener overflow, the payback period for thickener
automation was estimated at less than 6 months.
This report was submitted in fulfillment of Grant No. S803602 by the
Metropolitan Waste Control Commission under the partial sponsorship of the
U.S. Environmental Protection Agency. This report covers the period from
November 15, 1976 to November 30, 1977, and the work was completed as of
April 14, 1978.
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CONTENTS
Disclaimer ii
Foreword . . .iii
Abstract iv
Figures viii
Tables . . . ix
Abbreviations x
Conversion Factors xi
1. Introduction 1
2. Conclusions 2
3. Facility Description .... 3
General ........ 3
Thickener basin .............. 3
Sludge blanket control ..... 6
Monitoring 8
Suspended Solids 8
General . 8
Inflow . 8
Overflow 12
Underflow 12
Recording .12
Flow 14
4. Methods ... ............ 15
Sampling 15
Analyses 15
Solids 15
Total organic carbon .......... 16
Temperature and pH .16
Capillary suction time ............... 16
Filter leaf 16
Time Study 17
5. Calibration Procedures ... 18
Solids analyzers . ......... 18
General 18
Inflow 18
Overflow 19
Underflow ..................... 19
Flow meters 19
General ........... 19
Overflow .............. 19
Underflow ............. .20
Sludge blanket detector ......... ... 20
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CONTENTS (Continued)
6. Thickener Operation and Control 21
General 21
Existing 21
Sludge blanket control 22
Sludge blanket detector ... 22
7. Chronology ...... 23
General ...... 23
Test run no. 1 23
Test run no. 2 24
Test run no. 3 24
Test run no. 4 26
Test run no. 5 26
8. Discussion of Results 27
Sludge blanket controller ... 27
Level control 27
General 27
Test run 1 29
Test run 2 29
Test run 3 32
Test run 4 32
Test run 5 32
Maintenance 32
Solids Analyzers 35
Inflow and overflow 35
Calibration 35
Maintenance 41
Underflow 41
Calibration 41
Maintenance 46
Thickener no. 2 analyzer 46
Thickener no. 4 analyzer 46
Flow Meters 46
Overflow 46
Underflow 51
Thickeners 51
Underflow 51
Solids content 51
Dewatering characteristics 51
General 51
Capillary suction time 54
Filter leaf tests 54
Overflow 54
Solids content 57
Organic content 59
Solids capture 59
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CONTENTS (Continued)
9. Cost 63
General 63
Operating 63
Treatment 63
References 65
Appendices
A. Technical provisions of specifications for
hardware items 66
B. Detailed chronology-field notes 69
vn
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FIGURES
Number
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
Plan view of existing treatment facilities at Metropolitan
WWTP.
Typical profile for gravity thickeners.
Sludge blanket detector installed in thickener basin no. 4,
Location of solids analyzers and sampling lines.
Solids analyzer mounted in basin influent pipe.
Instrument panel in thickener gallery.
Solids analyzer mounted in sample sink.
Thickener no
Depth to the
test run
Depth to
test run
Depth to
test run
Depth to
test run
Depth to
test run
Calibration
Calibration
Calibration
Calibration
Calibration
Calibration
Calibration
Calibration
Calibration
no.
the
no.
the
no.
the
no.
the
no.
4 with debris dislodged from influent well
sludge blankets for basins 2 and 4 during
1.
sludge blankets for basins 2 and 4 during
2.
sludge blankets for basins 2 and 4 during
3.
sludge blankets for basins 2 and 4 during
4
sludge blankets for basins 2 and 4 during
. 5.
data
data
data
data
data
data
data
data
data
Observed total solids concentration of underflow samples
during test runs 1-5.
Temperature observations for thickeners no. 2 and 4 during
test runs 1-5.
Overflow solids concentration for basins 2 and 4 based on
multiple linear regression analyses.
Daily average overflow suspended solids concentration
basins 2 and 4.
TOC concentration of overflow for thickeners no. 2 and
Page
4
5
7
9
10
11
13
25
28
30
31
33
for
for
for
for
for
for
for
for
for
thickener
thickener
thickener
thickener
model 52H
thickener
thickener
thickener
thickener
no
no
no
no
si
no
no
no
. 2
. 4
. 2
. 4
udge
. 2
. 4
. 2
overfl
influent
influent
overflow
overflow
density
underflow
underflow
underflow
solids analyzer.
solids analyzer.
solids analyzer.
solids analyzer.
analyzer.
solids analyzer.
solids analyzer.
solids analyzer.
ow measurement systems.
34
36
37
39
40
42
44
45
47
48
for
4.
52
53
57
60
61
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TABLES
Number
1 Sludge Blanket Control Tests 23
2 Photocell Resistance Measurements 38
3 Test Plug Readings for Suspended Solids Analyzers 41
4 Test Plug Readings for Sludge Density Analyzers 43
5 Overflow Indicator - Totalizer Comparison 49
6 Overflow Calculations 50
7 Underflow Totalizer Calibration 51
8 CST of Thickener Underflow - Summary 55
9 Filter Leaf Analyses of Thickener Underflow 56
10 Solids Capture - Summary 62
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LIST OF ABBREVIATIONS
sec - second
CST - capillary suction time
F & I no. 2 - Filtration and Incineration Building number two
ft - feet
gpd/sq ft - gallons per day per square foot
gpm - gallons per minute
hr - hour
in - inch
L - average blanket level for day, feet below surface
mgd - million gallons per day
mg/1 - milligrams per liter
min - minute
ml - milliliters
MWCC - Metropolitan Waste Control Commission
NPT - national pipe thread
PID - proportional, integral, derivative
Q - basin inflow rate, million gallons per day
R - coefficient of correlation
S - average daily inflow suspended solids concentration
milligrams per liter
sq ft - square feet
T - temperature of basin inflow, degrees Centigrade
TOC - total organic carbon
Y - average daily overflow suspended solids concentration
for basin, milligrams per liter
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CONVERSION FACTORS
English Unit
multiply by
to yield Metric Unit
Feet
Gallons per day per square foot
Gallons
Inches
Million gallons
Square feet
0.3048
0.352
3.785
2.54
3785
0.0929
Meters
Liters per day per
square meter
Liters
Centimeters
Cubic meters
Square meters
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SECTION I
INTRODUCTION
A cursory examination of the semitechnical literature of the past 2
years demonstrates that when new facilities are constructed and older facil-
ities are upgraded operating agencies, as well as design engineers, are putting
more emphasis on process control. Reasons for this apparent trend include:
(1) to improve the reliability of the treatment facilities in terms of meet-
ing discharge standards as per NPDES permits, (2) to improve process treat-
ment efficiencies and thereby minimize their size and the corresponding capi-
tal investment and (3) to decrease operating and maintenance costs by making
more efficient use of energy, materials, and personnel.
Process control can usually be accomplished by manual or automated means.
However, because of the increasing complexity of modern wastewater treatment
facilities and the advances in the instrument and computer industries during
recent years, it appears that increased emphasis will be placed on automated
process control using both analog and digital techniques. For example, the
Metropolitan Waste Control Commission, MWCC, is currently constructing a new
6 mgd advanced waste treatment plant (AWTP) which will utilize a central com-
puter to monitor and control all of the processes in both the liquid and sol-
ids treatment areas. In addition, the capacity of the Metropolitan Plant is
currently being expanded from 218 mgd to 290 mgd. The expansion includes the
addition of new secondary treatment facilities and expanded sludge handling
and disposal facilities. A distributed digital control system is being in-
stalled to provide for process control of the new facilities. The older ex-
isting facilities may be retrofitted with the necessary hardware at a future
date.
The USEPA sponsored a workshop dealing with research needs associated
with the automation of wastewater treatment facilities in September of 1974.
The summary of the workshop proceedings (1) listed 6 major research needs;
one being, the development of efficient and dependable sensors. If process
control decisions (made by man, analog or digital computers) are to be based
on the output of sensing devices, the significance of their dependability is
perhaps best summarized by the old equation in computer technology parlance,
garbage in = garbage out.
The purpose of the study described in the following paragraphs was to
evaluate the hardware (sensors) required to monitor and control the operation
of a gravity thickener and to determine the benefits associated with im-
proved sludge blanket level control. The field study was conducted during
the period November 1976 through November 1977 at the Metropolitan Wastewater
Treatment Plant.
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SECTION 2
SUMMARY AND CONCLUSIONS
A system for controlling the sludge blanket level in a gravity thick-
ener was evaluated at the MWCC's Metropolitan Wastewater Treatment Plant.
The performance characteristics of an optical type sludge blanket level de-
tector/controller were documented. The reliability and maintenance require-
ments for one manufacturer's optical type solids analyzer were also identifi-
ed for several applications. The following conclusions are based on a series
of five tests during which two of the plants six gravity thickeners were mon-
itored.
[1] The sludge blanket level controller can provide reliable control pro-
vided the capacity of the sludge withdrawal pump is sufficient.
[2] The probes used to monitor blanket level should be inspected and wiped
clean on a weekly basis to preclude malfunctions related to coatings on
the optical surfaces.
[3] No significant difference was observed in the dewatering characteristics
of the sludges discharged from the manually and automatically controlled
thickeners.
[4] Although the labor savings associated with the automated blanket level
control are estimated at only approximately $500 per basin per year,
the enhanced solids capture provided treatment savings conservatively
estimated to be approximately $4000 per basin per year. The pay back
period for the installed control system is estimated to be less than 6
months.
[5] The suspended solids concentration of the thickener overflow was found
to be significantly affected by both the blanket level and the sus-
pended solids concentration of the influent. This overflow character-
istic, however, was found to be relatively independent of both the hy-
draulic loading rate and temperature in the ranges observed.
[6] The soluble organic content of the thickener overflow was found to be
independent of the basin operating parameters.
[7] The solids analyzers used during the study were found to be acceptable
for monitoring the thickener inflow and overflow streams; however, the
performance characteristics of the analyzer used to monitor the solids
concentration of the underflow were determined to be unacceptable.
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SECTION 3
FACILITY DESCRIPTION
GENERAL
The existing Metropolitan Wastewater Treatment Plant, herein identified
as Metro, was designed to treat a flow of 21.8 mgd having biochemical oxygen
demand and suspended solids concentrations of 250 mg/1 and 315 mg/1 respec-
tively. The plant, as initially placed into service in 1938, consisted of
primary treatment and sludge disposal facilities - vacuum filtration and
incineration. Facilities for secondary treatment by the high rate activated
sludge process were put into service in 1966. The solids processing facili-
ties were also expanded at that time. In 1972 the secondary facilities were
expanded to permit operation by the step aeration process. The plant is
presently operated in excess of its design capacity and is being expanded
to treat a flow of 290 mgd and meet secondary effluent standards. A plan
view of the existing facility is presented in Figure 1.
Approximately 50% of the primary sludge is thickened, dewatered by
vacuum filtration and incinerated. The six filters and four incinerators
used to treat the primary solids are located in Filtration and Incineration
Building (F & I) No. 1. The remaining primary sludge and all of the waste
activated sludge are mixed, diluted with plant effluent, and discharged to
six gravity thickeners. The thickened sludge is discharged to two sludge
holding tanks, 260,000 gal capacity each, and subsequently dewatered by
vacuum filtration and incinerated. The 12 vacuum filters and four incinera-
tors used to treat the mixed sludge are located in F & I No. 2. The control
room for the gravity thickening, vacuum filtration, and incineration processes
is located in F & I No. 2. Under normal conditions most of the operating
parameters are monitored from this location.
The overall treatment efficiency at Metro, in terms of BOD removal, is
currently limited by the capacity to process solids. Insufficient incinera-
tor capacity is the physical constraint which limits solids throughput for
both the filters and thickeners and eventually dictates sludge wasting rates
and the performance level of the secondary treatment facility.
THICKENER BASIN
The six gravity thickeners are arranged in two banks of three thickeners
each. Basins no. 1,3 and 5 form the west battery and basins no. 2,4 and 6
form the east battery. Each battery is served by a common feed line to dis-
tribute the mixture of primary and secondary sludge and dilution water. Each
basin is 65 ft in diameter with a 10 ft sidewall depth and is equipped with
a rotating sludge collector, skimmer and peripheral launder as illustrated
in Figure 2. The units were designed for loadings of 300 to 1000 gpd/sq ft
0 to 3.3 mgd).
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Pr i m a r y
sludge
storage
tanks
i r
Scr een and gri t
F 8 I no. 2
F S I no. I
Administration
\
Final sedimentation tanks
Control room
Primary sedimentation tanks
Figure 1. Plan view of existing treatment facilities at Metropolitan WWTP.
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-Blanket level controller
Guard rail
Overt I o w weir-
Figure 2. Typical profile for gravity thickeners,
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The thickened sludge is pumped from the center well of each basin and
discharged to one of the holding tanks by way of a common discharge manifold
for each battery. Each basin is served by two sludge pumps, a variable speed
centrifugal pump and a fixed speed positive displacement pump. The centrifu-
gal pump is normally used and operated at speeds to yield discharges in the
range of 50 to 200 gpm. The sludge pumps, influent and thickened sludge flow
meters, local control panels and all piping and other appurtenances are lo-
cated in the thickener gallery between the two batteries below grade.
Each basin has three sidewall sample taps at depths of 4, 6.5, and 9 ft.
One inch lines are used to transport the samples to sinks located in the cen-
ter of the gallery and allow the operator to monitor sludge blanket height
from that location.
SLUDGE BLANKET CONTROL
A Biospherics Model 56 sludge blanket detector was installed in basin
no. 4. The unit, pictured in Figure 3, consists of two submersible probes
suspended from a control box by waterproof cables. Each probe consists of a
light source and photocell separated by a 3/4 in. sensing gap. The light
transmitted to the photocell is a function of the concentration and charac-
teristics of the solids slurry in the sensing gap. In service each sensor
is suspended in the thickener with a cable which also provides power to the
light emitting diodes and photocell and transmits the photocell output to the
control unit.
The control unit is housed in a NEMA 4 enclosure mounted on the thicken-
er handrail and consists of control circuits, power supplies and output re-
lays. A two position response switch is provided for fast or delayed re-
sponse to changes in photocell output. Three indicating lights labeled,
LOW, MEDIUM, and HIGH, are mounted on the enclosure to indicate the sludge
blanket position as below, between or above the two sensors. The unit is
powered from a 120 VAC source and the output relays are rated at 10 amp at
120 VAC.
When probe 'A' is located above probe 'B' in the thickener basin the
control unit functions as follows. The energy source in each probe emits
radiation through the optical sensing gap to the photocell which senses the
amount of energy transmitted through the liquid and produces a signal related
to the suspended solids concentration of the liquid in the gap. This signal
is compared, in the control circuit, to an adjustable density threshold set-
ting. When the signal representing the concentration of solids in the liquid
at probe 'A' exceeds the threshold setting an electronic 15 min timer is
activated. If the sludge blanket level remains at or above probe 'A1 at the
expiration of the 15 min interval,the output relay is energized and the HIGH
lamp on the control box illuminates. The output relay is used to energize
the starter of the thickener underflow pump motor. After a period of sludge
withdrawal the sludge blanket level drops below probe 'A' and the photocell
output drops below the threshold setting. At this time the HIGH lamp is
extinguished and the MEDIUM lamp illuminates, After continued sludge with-
drawal the sludge blanket drops to the level of probe 'B1 and the signal
representing the concentration of solids falls below the threshold setting.
-------
Figure 3. Sludge blanket detector installed in thickener basin no. 4.
-------
At this point in time a second 15 min interval timer is activated. If the
sludge blanket level remains below probe 'B' at the expiration of the 15 min
interval the output relay is deenergized and the MEDIUM lamp is extinguished
and the LOW lamp illuminated. The underflow pump starter is disengaged when
the output relay is deenergized. When the sludge blanket level rises to
probe 'B1 the photocell output increases above the threshold setting and the
LOW lamp is extinguished and the MEDIUM lamp illuminated. The above cycle
is repeated as the sludge blanket level rises to probe 'A1.
A three position switch (HAND, OFF, AUTO) was installed in the control
room to allow the operator to establish the mode of operation. This flexi-
bility was required to minimize problems associated with instrument failures
and high sludge levels in the holding tanks downstream of the thickeners.
MONITORING
Suspended Solids
General--
Optical type solids analyzers were installed in the inflow, overflow ana
underflow streams of thickeners no. 2 and 4 to aid in the collection of data
on thickener performance and to more fully evaluate their own performance
characteristics. During the early months of 1976 MWCC staff evaluated the
short term performance characteristics of analyzers which employed several
operating principles. Based on the results of these tests the decision was
made to utilize the self cleaning optical type analyzers for both the high
and low concentration ranges. Specifications were prepared (Appendix A) and
advertised. Two proposals were received. A contract was subsequently
awarded to supply four Biospherics model 52LE suspended solids analyzers and
two Biospherics model 52H sludge density analyzers.
The analyzers were installed in the influent and underflow pipes through
a pipe insertion adapter mounted in a 2 in. NPT threaded hole. The overflow
analyzers were mounted in the sample sinks in the thickener gallery. The
installation locations are illustrated in Figure 4.
Inflow--
The influent solids analyzers consist of a one foot long sensing head
housing a glass sampling chamber, a motor drive mechanism that moves the
plunger which is positioned in the sampling chamber, and a control unit
connected to the sensing head by a multi-conductor cable. The analyzers were
mounted in the 12 in. influent lines as illustrated in Figure 5. The control
units were mounted in an instrument panel located in the thickener gallery
as illustrated in Figure 6.
The sensing head of the analyzer contains a light source that transmits
a light beam across the sample chamber to a photocell. The plunger is
equipped with a wiping seal that cleans the optical surfaces of the photocell
and light source with each operating stroke. The signal from the photocell
is linearized in the control unit producing a meter reading that is designed
8
-------
-Blanket level controller
e
3
.C
at
!
o |
w *
o .
O
",1
U
Guard roil
/ / //////7/////
Blanket level sensors
Underflow analyzer
^7=W3o>
y—1 i ( / ^~/ v
.I"!1?,". .s£,m£ie. _.j^-Inflow analyzer
Underflow sample
Sample sink
Underf Ipw
/ / / / /~7^7~7^~7~/ / / / / / / / / / / ////// / / /
Figure 4. Location of solids analyzers and sampling lines.
-------
Figure 5. Solids analyzer mounted in basin influent pipe.
-------
Figure 6. Instrument panel in thickener gallery.
11
-------
to be proportional to the suspended solids concentration of the sample.
The unit completes a sample and analysis cycle every 15 sec. The plunger
retracts and draws a sample into the sampling chamber similar to the operation
of a common laboratory syringe. The light transmission measurement is made
and the sample is expelled. The instrument output (meter and/or recorder) is
maintained constant during each cycle. A range switch on the face of the
control unit provides for operation in the range of 0 to 3000, 0 to 10,000,
and 0 to 30,000 rag/1. An ON/DAMP switch allows for recording of the actual
output fluctuations at 15 sec intervals or only a portion of the step change
when the fluctuations are large.
Zero and span controls are located at the rear of the control unit. The
zero control is used to adjust the output with clear water present in the
sampling chamber. The span control is used to adjust the slope of the cali-
bration curve. A test plug which consists of two fixed resistors and a
high/low switch can be attached to the control unit in place of the signal
cable. The resistors simulate photocell output in the 0 to 10,000 and 0 to
30,000 mg/1 ranges. The meter readings with the test plug in place can be
used to identify malfunctions in both the control unit and sensing head.
Overflow--
The overflow analyzers are functionally identical to those used to moni-
tor inflow, however, the sensing heads are four feet long. The units are
mounted in the sample sinks serving thickener basins 2 and 4 as illustrated
in Figure 7. Thickener overflow is piped from the overflow flumes and dis-
charges to one gal containers located in the sample sinks. The analyzers
are supported above the sinks and the sensing heads extend into the sample
containers which overflow continuously.
Underflow--
The underflow monitor is externally identical to the inflow unit; how-
ever, the underflow analyzers contain three photocells in the sensing head,
one each for measuring light transmittance, 90° light scatter and color com-
pensation. The parallel combination of the light transmittance and light
scatter photocells provides an output related to the suspended solids con-
centration in the sludge sample. The color compensation photocell adjusts
the input current to the control unit amplifier to reflect changes in sludge
color. A linerization module in the control unit corrects the nonlinear
response of the transmittance and light scatter photocells. The unit com-
pletes a sample analysis cycle every 40 sec.
The control units are equipped with the ON/DAMP switches and span adjust-
ment controls. The instrument range is fixed at 0 to 10% solids and is
calibrated in terms of total solids. A test plug is used to simulate the
output of the sensing head and check for drift in the control circuitry.
Recording--
The outputs of the six solids analyzers along with the output of the
12
-------
Figure 7. Solids analyzer mounted in sample sink.
13
-------
thickened sludge (underflow) flowmeter serving basin 4 were recorded on a
Leeds and Northrup Speedomax W multipoint recorder - Figure 6.
Flow
The rate of flow to each thickener is monitored by a 12 in. by 8 in.
venturi meter as illustrated in Figure 2. The flow rate is indicated local-
ly and is recorded and totalized in the control room in F & I No. 2. A PID
controller is used to compare the metered inflow with an operator entered
set point and adjust the pneumatically operated control valve accordingly.
The overflow from each basin is metered with a 9 in. Parshall flume. A
standard bubbler arrangement is used to determine the discharge head. The
output of the pressure transmitter and signal characterizer is fed to a local
flow indicator and to a recorder and totalizer in the control room. In addi-
tion, the individual flows for the six basins are summed and displayed on one
totalizer.
Four inch diameter magnetic flow meters are used to monitor the flow of
thickened sludge discharged from each of the basins - Figure 2. The flow
signal is indicated locally and recorded and totalized in the control room.
The flow metering and indicating equipment is calibrated periodically by
the plant instrumentation crew on request of the engineering staff when
problems are encountered or suspected.
14
-------
SECTION 4
METHODS
SAMPLING
The location of the solids analyzers and sampling lines are illustrated
in Figure 4. Samples for analyzer calibration and dewatering tests were
collected at the sample sinks in the thickener gallery. The lengths of the
inflow and underflow sample lines are approximately 25 ft and 35 ft respect-
ively. The overflow sample line extends from the Parshall flume to the
sample sink, approximately 70 ft. All thickeners have the same sampling con-
figuration.
The following procedure was used to collect the inflow and overflow
samples used to calibrate the solids analyzers.
[1] Allow sample line to discharge for several min.
[2] Record meter reading of solids analyzer.
[3] Collect sample (approximately 40 ml) and transfer to 250 ml plastic bot-
tle.
[4] Repeat steps [2] and [3] four times at 15 sec intervals.
The suspended solids content of the composite sample was determined and
compared to the average of the 5 meter readings.
Underflow samples were collected only when the underflow pump was opera-
ting. The procedure outlined above was used. The total solids concentration
of the composited samples was determined along with the pH and capillary
suction time.
Underflow samples for filter leaf tests were collected from the underflow
sample lines in 5 gal plastic carboys. Approximately 6 1 of sample were
collected to provide for duplicate tests. Total solids analyses and filter
leaf tests were conducted on the day of collection.
ANALYSES
Solids
Samples collected for suspended and total solids analyses were thoroughly
mixed prior to removing portions for analysis. Suspended and total solids
analyses were conducted on the day of collection according to procedures
described in Standard Methods for the Examination of Water and Wastewater (2).
Suspended solids concentrations between 0 and 1000 mg/1 were recorded to the
nearest 10 mg/1. Concentrations greater than 1000 mg/1 were recorded to the
nearest 50 mg/1. Total solids analyses were recorded to the nearest 0.1%
solids.
15
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Filter leaf cakes were dried overnight at 103° C and checked for constant
weight. Results were recorded as total dry weight.
Total Organic Carbon
Portions of the samples collected for TOC analysis were filtered through
glass fiber filters to remove suspended solids. The, unfiltered and filtered
fractions were acidified to a pH of 2 with concentrated hydrochloric acid.
The samples were refrigerated at 4° C until the analyses were performed.
After removal from storage, the samples were diluted with deionized water,
acidified as required and blended if suspended solids were present. The
analyses were performed on an Astro Ecology Model 1200 TOC Analyzer after
purging the samples for 5 min with air to remove inorganic carbon in the form
of carbon dioxide.
Temperature and pH
Temperature measurements were made on samples from the inflow, overflow
and underflow sample lines using a 0-100° C thermometer.
pH measurements were recorded at the time of sampling using a portable
pH meter previously calibrated with pH 4 and 6.86 buffer solutions.
Capillary Suction Time
Capillary suction time (CST) determinations were made on pumped thickener
underflow samples at the time of collection using a Triton Type 92/1 CST
Timer as described by Baskerville and Gale (3). All CST determinations were
conducted with a 1.8 cm x 2.5 cm (diameter x height) cylinder. Whatman no.
17 chromatography paper was used as the absorbent medium. CST tests were
repeated several times on the same sample to provide a measurement of pre-
cision.
Filter Leaf
Filter leaf tests were conducted on underflow samples on the day of col-
lection. Selected dosages of ferric chloride and lime were used to condition
the sludge samples prior to performing the filtration. Chemical dosages
were based on the total solids content of the sludge samples.
The ferric chloride conditioning reagent was analyzed for FeClo content
using a dichromate-stannous chloride acid titration (4). Lime concentration
(as CaO) in the lime slurry conditioning reagent was determined by titrating
a measured quantity of mixed slurry with standard hydrochloric acid to a
phenolphthalein end point.
A measured quantity of the ferric chloride reagent was blended into a
2000 gram sludge sample for 30 sec at moderate speed using an electric cake
mixer. A measured quantity of lime slurry was added and blended into the
ferric chloride conditioned sludge for an additional 30 sec. The conditioned
sludge was filtered immediately using a 0.1 sq ft filter with polypropylene
16
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filter medium. The filtration was conducted at 15 in. of Hg vacuum for 90
sec. After a drying time of 160 sec, the cake was removed and dried over-
night at 103° C. Filter yields were calculated based on total dry cake weight
and a filter cycle time of 6 min. Duplicate filter leaf tests were performed
on all samples. The filter medium was acid cleaned and a trial filter leaf
test was discarded before proceeding with the actual filter leaf tests.
TIME STUDY
On three occasions the movements of thickener operators were monitored
for an 8 hr shift to determine the actual time required to perform their
duties. Each specific task was identified and the time required recorded in
minutes. Three operators were observed, each during the day shift. The mon-
itoring was accomplished without the knowledge of the operators.
17
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SECTION 5
CALIBRATION PROCEDURES
SOLIDS ANALYZERS
General
The six analyzers used for monitoring thickener solids were installed in
December, 1976. The inflow and overflow suspended solids analyzers were
zeroed with water using the zero adjustment. Zero readings on the underflow
analyzers were checked using water but no adjustments could be made. Initial
calibrations were completed prior to initiating the sludge blanket control
in basin 4. Calibration samples, however, were collected during each of the
5 test runs to establish the reliability of the instruments. Several of the
instruments were recalibrated after component failures. In addition the test
plugs supplied with each instrument were used to identify the drift charac-
teristics of the individual control units.
Inflow
Inflow suspended solids analyzers were calibrated using the zero and span
adjustments on the control unit to match meter readings with inflow suspended
solids concentrations. Because the analyzers were pipe-mounted and could
not be conveniently calibrated directly, a trial and error procedure was
used in which the zero and span adjustments were made according to the pre-
vious day's sample analysis and meter readings. Although the analyzers were
zeroed with water when installed, a linear calibration plot spanning a range
of suspended solids concentrations 0000-5000 mg/1) could not be obtained
after several trials. After the zero adjustment was offset to lower the
readings obtained for samples with low suspended solids concentrations, bet-
ter calibration linearity was obtained. Several samples were required to
determine if the calibration adjustments produced a linear response over a
given suspended solids concentration range.
The samples were collected as previously described with the control unit
in the ON, undamped, position. The average meter reading and the correspond-
ing suspended solids concentration of the composited sample represented one
point on the instrument calibration curve. After several points were identi-
fied, covering a range of suspended solids concentration, new zero and/or
span adjustments were made as required to improve the linearity of the sus-
pended solids - meter reading relationship. Two or three adjustments were
usually required to establish a satisfactory calibration plot.
The meter scales were read to the nearest 25 mg/1 using the 0-3000 mg/1
range and to the nearest 50 mg/1 when the 0-10,000 mg/1 range was used. The
inflow analyzer control units were set on the 0-10,000 mg/1 range and left
18
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in the DAMP position during normal operation to reduce meter fluctuation.
Overflow
The overflow suspended solids analyzers were calibrated using the same
procedure described for the inflow analyzers. Both the 0-3000 and 0-10,000
mg/1 ranges were used during test runs. The control units were left in the
DAMP position during normal operation to reduce readout fluctuation.
Underflow
With the underflow sample flowing continuously, six grab samples were
taken at 40 sec intervals corresponding to six consecutive meter readings.
A composite sample was constructed and analyzed for total solids. The solids
concentration along with the average of the six meter readings represented
one calibration point.
Meter readings were recorded to the nearest 0.1% solids on the 0-10%
solids scale. The ON, undamped, position was used during calibration and the
DAMP position used during normal operation to reduce meter fluctuation.
Because the thickener underflow solids concentration did not change ap-
preciably during the test runs, calibration plots covering a wide range of
solids concentrations were not obtained. A direct calibration was, however,
conducted on the thickener 2 underflow solids analyzer following the conclu-
sion of test run 5. The calibration was performed over a wide range of solids
concentration by diluting an underflow sample with selected volumes of over-
flow. The analyzer was removed from the underflow pipe and placed in the di-
luted underflow. The solids suspension was mixed gently to prevent sedimen-
tation and the meter readings recorded for several consecutive sampling/ana-
lysis cycles.
FLOW METERS
General
Flow totalizers on thickeners 2 and 4 were checked against flow rate
measurements in order to assure reliable flow information for the thickener
control study. Overflow and underflow totalizer values were compared to
calculated flows based on discharge head observations and basin volume mea-
surements respectively. The flow checks were repeated to test the reproduc-
ibility of the comparisons.
Overflow
A two step procedure was used to calibrate the overflow measuring and
recording system. Flow values, displayed on the local indicator, were com-
pared to calculated flows - based on observations of flume discharge head.
Subsequent comparisons were made between the local indicator and the total-
izer over known time increments. The calibration data for the local indicator
were obtained as follows:
[1] Install point gage above Parshall flume and establish elevation.
19
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[2] Record local indicator reading - mgd.
[3] Determine discharge head - average of 3 observations.
[4] Record local indicator reading a second time.
[5] If the readings in steps [2] and [4] differed by more than 0.03 mgd the
observations were discarded. If the readings were within the above
limit the calculated flow and the average observed flow established one
point on the calibration curve.
The local indicators and flow totalizers were compared on three occasions as
follows.
[1]
[2;
[3;
[4]
Record local indicator reading - mgd
Record totalizer reading - gal, and time
Record local indicator reading - mgd
Record totalizer reading approximately 10 to 15 min after step [2],
record time
[5j Record local indicator - mgd
[6] If the three values for the local indicator varied by more than 0.05 mgd
C max-min ) the observations were discarded. If the values were within
this limit the average of the 3 readings of the local indicator and the
flow rate, calculated from the two totalizer readings and known time
interval, established one point on the calibration curve.
Underflow
The calibration status of the underflow discharge totalizers was checked
by measuring the basin drawdown rate with the influent valve closed. Draw-
down measurements were made with a point gage and the underflow pumping rate
was in the range of 100 to 150 gpm.
SLUDGE BLANKET DETECTOR
The sludge blanket control system was checked prior to test run no. 1
to determine if the blanket control hardware started and stopped the under-
flow pump as designed. With the relay switches in the DELAY position and the
pump control switch in the AUTO position, the probes were placed in covered
containers of sludge and water to simulate both a rising and falling sludge
blanket in the thickener. It was observed that the pump started 30 min after
both probes were placed in the sludge sample and that the pump stopped 30 min
after both probes were placed in clear water. The manual supplied with the
instrument indicated the time delay was 15 min in each case. No attempt was
made to modify the timers supplied.
20
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SECTION 6
THICKENER OPERATION AND CONTROL
GENERAL
The thickener influent is a mixture of primary sludge, waste activated
sludge and dilution water with volume ratios of approximately 1:1:2.5. For
the four month period August through November 1977 the influent averaged
1.9 mgd per basin and the solids loading averaged approximately 22 psf/day.
The ratio of primary solids to waste activated solids was approximately 1:1
for that time period.
The underflow solids concentration ranges from 3% in the summer months to
6% in the winter and spring. The observed increased thickening efficiency
in the winter and spring is most likely due to an increase in the primary to
waste activated solids ratio. This may be caused by decreased activated
sludge production at the lower temperature and the increased inorganic solids
loading on the plant associated with spring runoff.
As previously indicated the solids handling capacity of the Metro Waste-
water Treatment Plant is currently limited by the capacity of the incinerators
Because of this constraint it is often not possible to operate the thickeners
so as to obtain maximum efficiency in terms of thickening and solids capture.
Chlorine is added to the dilution water flow to control both slime growth
and floatation caused by gasification. The gaseous chlorine dose falls in
the range of 10 to 20 mg/1 based on total basin inflow.
The operation of the thickeners is controlled by one operator per shift
stationed in the control room in F & I No. 2. The operator's duties include:
recording all flow totalizers on an hourly basis, adjusting inflow set points
as required, monitoring sludge level in holding tanks and controlling under-
flow pumping. In addition several times each shift the operator makes sludge
blanket measurements, collects samples and conducts the normal housekeeping
tasks around the thickeners and in the gallery.
EXISTING
The operators determine the location of the sludge blanket in each
thickener six times each day as follows, 0100 hr, 0500 hr, 0800 hr, 1300 hr,
1700 hr, and 2100 hr. A portable photoelectric device, consisting of a light
and photocell separated by a fixed gap and an audio output, is used to make
the depth measurements. The photocell output is converted to an audio out-
put (intensity of high pitched tone decreases as light transmission decreases
and end point is not defined) rather than a meter. Thus, the measurement
precision is not great because each operator may interpret the sound of the
21
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end point differently- The accuracy of the measurement is also a function
of the interface characteristics (defined or diffuse). Additional sludge
blanket observations are conducted, using the sidewall sample taps, at four
hour intervals.
The operator bases the control of the underflow pumps on the sludge
level in the holding tanks, and on the status of the incinerators and vacuum
filters. No attempt is made to control the sludge blanket level other than
to maintain sufficient underflow pumping to prevent the discharge of high con-
centrations of suspended solids in the overflow and to prevent the develop-
ment of extreme septic conditions in the sludge. The basin with the highest
sludge blanket has the first priority for pumping.
The underflow pumping rates are normally maintained in the range of 50
to 200 gpm. The rate used by the operator is based again on the blanket lev-
els and the available capacity in the holding tanks. Rates above 200 gpm are
not used because of what appears to be 'ratholing' and the associated dis-
charge of more dilute sludge.
SLUDGE BLANKET CONTROL
A total of five tests, each lasting approximately 2 weeks, were con-
ducted to document both the operating characteristics of the blanket control
system and effects of blanket control on process performance. The thickener
operators were not directly involved in the control scheme used for basin no.
4. During each test run they were instructed to maintain the sludge pump
control for basin no. 4 in the AUTO position (controlled by blanket detec-
tor) and maintain a reasonable pumping rate. No. 4 underflow pump was
operated manually periodically during the test runs to collect underflow
samples for analyzer calibration. On several occasions the pump control was
not returned to AUTO immediately and the sludge blanket level was lowered
substantially.
A test run was not initiated if it was anticipated that maintenance re-
quirements would decrease the incinerator or vacuum filter capacity. On
several occasions, however, when problems developed downstream of the
thickeners the operators deactivated the sludge blanket control system.
SLUDGE BLANKET DETECTOR
Because of the nature of the application it was anticipated that the
optical surfaces of the probes would be fouled with coatings of grease and/or
slime. If these coatings were allowed to accumulate they would eventually
become equivalent to the sludge blanket in terms of light transmission from
the source to the photocell. For this reason the photocell output was
determined periodically, with the probes in clean water, to establish the
cumulative effect of the coating and the associated cleaning requirements.
The manufacturer reported that the photocell resistance with the probe
in clean water falls In the range of 5,000 to 10*000 ohms. With the probe
in a solids suspension of 2000 rag/1 the resistance was reported to fall in
the range of 50,000 to 60,000 ohms.
22
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SECTION 7
CHRONOLOGY
GENERAL
The sludge blanket detector and control system was evaluated by a series
of five tests during which inflow, overflow, underflow, sludge dewatering,
blanket level and instrument maintenance data were collected. The tests ran
for periods of 10 to 16 days and were conducted during the period August
through November, 1977 - TABLE 1. The data were collected to determine the
precision and reliability of several types of hardware and to establish the
relationship between blanket level control and basin performance in terms of
thickening, solids capture and sludge dewatering characteristics.
TABLE
SLUDGE BLANKET CONTROL TESTS
Blanket control points
feet from surface
Test run
1
2
3
4
5
Dates - 1977
August 1-16
August 31 - September 16
September 30 - October 14
October 17-27
November 8-18
Upper
5.5
7.5
5
3
5
Lower
6
8
5.5
3.5
5.5
The events, operational problems and observations recorded during the
field inspections are summarized in the following paragraphs. The field
observations are presented in more detail in Appendix B. Inspections were
made as frequently as possible during the work week and both during and
between test runs.
TEST RUN NO. 1
The inflow suspended solids concentration varied considerably during
this period. Concentrations greater than 10,000 mg/1 were observed for
periods of 1/2 to 2 hr on August 2, 4 and 6. For the three day period,
August 9-11, the concentration was consistently high, ranging from 4,000 mg/1
23
-------
to 10,000 mg/1. The concentration decreased to the range of 1,000 mg/1 to
4,000 mg/1 for the remainder of the period. Both the sludge blanket level „
and the overflow suspended solids concentration increased during the periods
of high solids loading-.
Relatively large, fibrous clumps of sludge solids accumulated in the over-
flow sample container on several occasions. This material did not have a
significant effect on the overflow analyzer readings; however, on two
occasions the material filled the sample container and could have blocked the
overflow sample line if left unattended.
Problems were encountered with the underflow solids analyzer serving
basin no. 4. The 4 to 20 ma output, used to drive the recorder, did not
correspond to the instrument's meter readings. The control unit was shipped
to the manufacturer for repair.
For the period August 11-16, the overflow totalizer for basin no. 4 in-
dicated flow rates significantly lower than the corresponding inflow total-
izer. Visual observations of the overflow weirs of basins no. 2 and no. 4
indicated that the inflow meter was generating an erroneous signal. On
August 29 debris, which was found to be lodged in the influent well of basin
no. 4, was flushed out by closing the inflow valves to basins no. 2 and no. 6
and thus increasing the flow to basin no. 4. After the influent well was
cleared of the obstruction the inflow and overflow rates corresponded. The
material which had caused the obstruction is pictured in Figure 8.
TEST RUN NO. 2
All of the vacuum filters located in F & I No. 2 were taken out of ser-
vice on September 5 because of maintenance requirements. Because of the
filter shutdown, inflow and underflow pumping, to and from, all thickener
basins was terminated on September 5. All basins were put back into service
late on the following day.
The inflow analyzer serving basin no. 2 failed the first day of run no.
2 and was out of service for the entire period. The motor which drives the
plunger was replaced and the analyzer put back into service on September 27.
Problems were encountered with the underflow analyzer serving basin no.
4. The meter read off scale on the upper end with the motor and plunger
operating normally. The instrument was removed and a grease accumulation
cleaned from the entrance of the sample chamber. The instrument functioned
properly when it was replaced in the underflow pipe.
TEST RUN NO. 3
The underflow analyzer serving basin no. 4 failed off scale several times
during this period. It was finally determined that the light source was
causing the problems and a new lamp assembly was ordered from the manufac-
turer.
The operators were notified to maintain the underflow pumping rate for
24
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ro
en
Figure 8. Thicken No. 4 with debris dislodged from influent well
-------
basin no. 4 at or above 150 gpm. This was required to assure control of the
blanket level during periods of high solids loading. On one occasion the
no. 4 underflow pump control was found in the manual mode. The operator had
switched to MANUAL to adjust the speed and did not return the switch to AUTO.
TEST RUN NO. 4
Basin no. 2 was out of service for two days, October 19 and 20, to con-
duct scheduled maintenance. The blanket level of basin no. 4 was lowered
approximately 4 ft during the evening of October 24 when the operator neg-
lected to return the underflow pump control to the AUTO position. The pump
had been switched to manual control to facilitate sample collection.
Both the upper and lower probe of the blanket detector were cleaned at
the start of test run no. 3. They were cleaned again one month later at the
end of run no. 4. The optical surfaces of the upper probe had become heavi-
ly coated. The photocell resistance of both probes was determined. The
measurements indicated that the coating on the upper probe had caused the
control system to fail and, in turn, caused the underflow pump to run con-
tinuously for 3 days.
TEST RUN NO. 5
The inflow totalizer for basin no. 2 was out of service for the last half
of this period.
On November 17, 20 days after the previous cleaning, the blanket detec-
tor failed due to film accumulation on the probes.
26
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SECTION 8
DISCUSSION OF RESULTS
SLUDGE BLANKET CONTROLLER
Level Control
General--
Two problems were encountered when using the installed hardware to con-
trol sludge blankets during normal operation. The speed of the underflow pump
was not always sufficient to control the sludge blanket elevation during the
periods of high solids loading which were encountered in test runs 1 and 2.
As a result the sludge blanket interface rose above the probes for several
days despite continuous pump operation. Subsequent instructions to maintain
the underflow pumping rate for basin no. 4 at or above 150 gpm, subject to
the level of sludge in the holding tank, helped to control this problem dur-
ing test runs 3 through 5.
Problems with film accumulation on the optical surfaces of the probes
were encountered during test runs 4 and 5, and possibly at the conclusion of
test run 3. The film coating may have caused the underflow pump to continue
running although sludge blanket readings indicated that the blanket was sev-
eral feet below the probe location. A weekly inspection of the probes should
eliminate this problem.
With the exception of the pumping rate and film accumulation problems the
sludge blanket control system operated satisfactorily. On several occasions
the underflow pump on thickener no. 4 was started manually in order to collect
underflow samples. The pump switch was left in the AUTO position during each
test run with the exception of the thickener shutdown on September 6 and on
three occasions when the underflow pump was placed or inadvertently left in
HAND position by the thickener operator (October 7, 12, and 24).
Test Run 1--
The sludge blanket data collected during test run 1 are presented in
Figure 9. For the periods August 2-5 and August 9-12 the solids loading was
significantly higher than normal. During these periods control of the blanket
level was not achieved. The underflow pump for thickener no. 4 operated con-
tinuously during the period August 1-11 except for one hour intervals on
August 55 8, 9, and 11. During this period the pumping rate varied in the
range of 100 gpm to 175 gpm.
The sludge blanket interface was not well defined during the first ten
days of the run, based on the sludge blanket measurements taken during the
27
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IN3
00
High inflow solids cone
High inflow solids cone
O Bosin 4
Bosin 4 by stoff eng
10 II 12 13 14 15 16 17
AUGUST- 1977
Figure 9. Depth to sludge blankets for basins 2 and 4 during test run Ho. 1.
-------
inspections conducted over the same time period. On August 5 the staff
engineer conducting the study found the precision of the measurement to be
particularly poor. A range of 3 ft was observed as illustrated in Figure 9.
The absence of a well defined interface may have contributed to the fluctua-
tions observed in the blanket readings.
The underflow pump maintained an on and off operating pattern beginning
on August 12 and continuing through August 16 with off times ranging from 7
to 13 hr. During this period the sludge blanket was controlled effectively
near the six foot location as illustrated in Figure 9.
Test Run 2 —
Thickener no. 4 underflow pump ran on a continuous basis at 50-125 gpm
for the period September 1-4. Low pumping rates were used because of high
sludge levels in the sludge holding tank. Unfortunately the sludge blanket
readings were not available during the first four days of the run. It does
appear, however, that the sludge blanket elevation was not controlled during
this period based on the continuous pump operation. This lack of control is
attributed to the low pumping rates.
The sludge blanket elevation observations made during test run 2 are pre-
sented in Figure 10. Blanket elevations' well above the control location of
7.5 to 8 ft were recorded for the periods of September 10-12 and 13-16. The
rise of the blanket in thickener no. 4 on September 10 and 11 was caused by
an increase in inflow suspended solids concentration which remained between
4000 and 5000 mg/1 during the period September 7-10, and a decrease in
thickener 4 underflow pumping rate to 75-100 gpm on September 10 and 11.
Thickener 4 pump operation continued uninterrupted from September 6-16 except
for one hr on September 8, during the calibration of the Parshall flume, and
0.5 hr on September 13 when the sludge blanket probes were checked for film
accumulation. As indicated in Figure 10, blanket control was not maintained
using the low underflow pumping rates.
Test Run 3--
The blanket elevation data collected during test run 3 are presented in
Figure 11. The sludge blanket elevation was maintained near the control
range of 5 ft to 5.5 ft during most of the run. The underflow pump serving
thickener no. 4 maintained an on-off pattern throughout the 15 day period
with the off time varying from 1 to 6 hr. The cyclic pattern indicates the
blanket controller was functioning and that the underflow pump had sufficient
capacity to withdraw solids at a rateZthe basin solids loading rate.
The irregular blanket elevations recorded by the operators for basin no.
4 on October 6, 11 and 12 were most likely related to the operation of the
sludge blanket detector as previously described. Instances of inflow sus-
pended solids concentrations greater than 10,000 mg/1 were observed on October
3 and 6. The high loading on October 6 coincided with the high sludge blanket
readings recorded on that date. Both high inflow loading periods produced
overflow suspended solids concentrations greater than 3000 mg/1 in thickener
no. 4.
29
-------
Level detector out of
order
CO
O
Basins out
of service
High inflow solids cone, a underflow pumping rate too low
O Basin 4
• Basin 4 by staff eng.
7 8 9 10
SEPTEMBER - 1977
12
13 14 15 16
Figure 10. Depth to sludge blankets for basins 2 and 4 during test run No. 2.
-------
UJ
CD
O Basin 4
• Basin 4 by staff eng
10 II 12 13
SEPTEMBER - OCTOBER — 1977
15
Figure 11. Depth to sludge blankets for basins 2 and 4 during test run No. 3.
-------
Test Run 4~
The sludge blanket elevation data collected during test run 4 are pre-
sented in Figure 12. Sludge blanket control was achieved in thickener no. 4
near the probe locations of 3 and 3 1/2 ft from October 19-25. The underflow
pump ran continuously from October 17-20 at 100-175 gpm except for a five hr
period on October 18. During this period the inflow suspended solids concen-
trations ranged from 3000 to 5000 mg/1. The solids loading may have been
high enough to maintain the sludge blanket above the probe location and call
for continuous underflow pumping. During the period October 20-24, the
underflow pump operated in an on-off pattern. Inflow suspended solids con-
centrations decreased to approximately 3000 mg/1 over the same time period.
On October 24, the pump control was placed in the manual mode to collect
a sludge sample. The pump was inadvertently left in the manual mode by the
operator and the sludge blanket was lowered to a level well below the desired
location by 1200 hr on October 25. After the pump switch was returned to the
AUTO position on October 25, it pumped continuously through October 27 except
for one 0.5 hr period. The continuous pumping is attributed to the buildup
of film on the optical surfaces of the probes.
Test Run 5--
Sludge blanket elevation data obtained during test run 5 are presented
in Figure 13. Sludge blanket control was maintained in thickener no. 4 near
the probe locations of 5 and 5 1/2 ft except during the period November 12-13
and on November 17. Thickener no. 4 underflow pump operated in an on-off
pattern throughout the test run. Off times ranged from 3 1/2 to 19 hr. On
November 16 and 17 the sludge blanket in thickener no. 4 was pumped down from
6 1/2 to 9 1/2 ft when the pump ran continuously for a period of 29 hr. Film
accumulation on the optical surfaces of the probes appeared to have caused
the problem. The pump remained off for 19 hr after the optical surfaces were
cleaned on November 17.
The variation in sludge blanket readings in thickener no. 4 on November
12 and 13 is not readily explained. A lack of sludge blanket control is
clearly evident with consecutive blanket readings more than one ft above and
below the control location on the same day. The lack of effective blanket
level control may have been due to optical surface coating, inaccurate sludge
blanket readings or a switch to manual pump control by the operator. It was
not evident, however, that any of these problems had occurred. The optical
surfaces of the probes had only a slight visible film accumulation during an
inspection conducted on November 10. Control of the sludge blanket was
achieved on the 13th with blanket readings stabilizing at 5 1/2 to 6 ft.
Maintenance
No mechanical or electrical problems were encountered in the sludge
blanket control system from November, 1976 to the conclusion of test run 5
in November, 1977. Maintenance of the system consisted of inspecting the
optical surfaces of the probes for visible film and checking the light
32
-------
CO
co
Underflow pump left in manual mode
Film accumulation on
probes
A Basin 2
by operators
O Basin 4
• Basin 4 by staff eng.
17 18 19 20 21 22 23 24 25 26 27
OCTOBER — 1977
Figure 12. Depth to sludge blankets for basins 2 and 4 during test run No. 4.
-------
CO
Film accumulation
on probes
8
9
10 II 12 13
NOVEMBER— 1977
15 16 17
A Basin 2
by operators
O Basin 4
• Bas'in 4 by staff eng.
Figure 13. Depth to sludge blankets for basins 2 and 4 during test run No. 5.
-------
sources to verify proper functioning.
The probes were not cleaned during each inspection. This was done to
determine the time period the control system could function without main-
tenance. During test run 4, the probes were left uncleaned for 25 days be-
fore problems (continuous pumping) were encountered due to the buildup of
film on the optical surfaces of the probes. During test run 5 problems were
encountered 20 days after cleaning the probes. No problems were encountered
because of film accumulation for a total of 15 days during test run 1, 17
days during test run 2 and at least 12 days during test run 3. The probes
were inspected but not cleaned during runs 1 through 3.
The photocell resistance of each probe was determined periodically to
monitor the change in output as a function of time and to determine the effect
of film accumulation on the output. A comparison was made between the photo-
cell resistance associated with a film coating and that related to a suspend-
ed solids suspension which closely matched the suspended solids equivalent
of the threshold control setting. The threshold control was maintained at a
setting of 4 which is equivalent to 2000 mg/1 suspended solids according to
the instrument operating manual. Photocell output was determined in clear
water and in a thickener inflow sample with a 1900 mg/1 suspended solids con-
centration. This suspension activated the output relay when the lower probe
was transferred to it from clear water. Each measurement was conducted in a
covered container to eliminate interference from sunlight.
The measured photocell resistance values for the probes are presented in
TABLE 2. On one occasion [October 28) the coating on the optical surfaces
was sufficient to produce a resistance reading exceeding the resistance re-
corded for the 1900 mg/1 suspension. The 800 kiloohm resistance was evidence
that the cause of the continuous underflow pumping observed from October 25
to 27 was due to the coating of the optical surfaces. When inspecting and
cleaning the probes it was observed that the upper probe usually had a great-
er film accumulation than the lower probe. Both photocells exhibited a slow
increase in resistance in clear water as a function of time.
Monitoring the photocell resistance was not necessary for operation of
the blanket control system and was not considered to be required maintenance.
It appears that a weekly inspection and cleaning of the probes would be suf-
ficient to maintain trouble free operation.
SOLIDS ANALYZERS
Inflow and Overflow
Calibration--
The calibration data collected for the inflow suspended solids analyzers
are presented in Figures 14 and 15. Two calibration curves are presented
for each instrument; one for each setting of the calibration control. The
values of the correlation coefficients, r, determined by linear regression
analyses, indicate that the solids analyzers did provide reasonably good
estimates of the actual suspended solids concentrations. Analyses of the
35
-------
8000
GO
6000
en
i
CD
4000
OL
GO
2000
Calibration 1
Y - 1.23 X - 128
r = 0.998
1
2
3,4
5
Calibration 2
Y = 0.88 X + 416
r = 0.967
Test Calibration Symbol
1 •
out of service
2 A
2 a
2000 4000 6000 8000
X = INFLUENT SUSPENDED SOLIDS - MG/L
10000
Figure 14. Calibration data for thickener no. 2 influent solids analyzer.
-------
co
6000
-
4000
oo
2 2000
Calibration 1 —
Y = 0.96 X + 771
r = 0.973
Calibration 2
Y = 0.91 X + 373
r - 0.952
Calibration
2000
4000
6000
Symbol
•
O
A
D
8000
X = INFLUENT SUSPENDED SOLIDS - MG/L
Figure 15. Calibration data for thickener no. 4 influent solids analyzer.
-------
data obtained during each test run did, however, indicate that the values of
the correlation coefficient decreased regularly during the 4 month period.
This observation may not be significant, however, because of the limited num-
ber of calibration points generated during the individual test runs and the
limited range of suspended solids encountered.
TABLE 2, PHOTOCELL RESISTANCE MEASUREMENTS
Photocell resistance
Probe optical in kiloohms
Date Sample surface condition Upper probe Lower probe
5-26-77 water clean 16.0 4.95
1900 mg/1
SS suspension clean 220 65
6- 3-77 water clean 16.5 5.2
water 7 day film accumulation
under operating conditions 21 8.4
8-16-77 water 15 days under
operating conditions 27 8.5
10-28-77 water clean 21.5 11.5
water 28 days under
operating conditions 800 23.5
Cupper probe heavily coated)
The calibration curves for the overflow solids analyzers are presented
in Figures 16 and 17. The values for the correlation coefficients again
indicate that the instruments provided good estimates of the actual sus-
pended solids concentrations.
The instrument readings obtained with test plug are summarized in TABLE
3. According to the manufacturer's literature supplied with the instruments,
the test plug readings should be within the 2000 to 6000 mg/1 range when the
LOW position is used and in the 10,000 to 20,000 mg/1 range for the HIGH po-
sition. Although the readings remained within the specified ranges some
variations were observed. The actual significance of these variations and
the effects of control unit drift were not established.
Based on the calibration data collected over the 4 month period the
solids analyzers were judged to provide acceptable estimates of suspended
solids in the 0 to 10,000 mg/1 range. (The thickener feed was a mixture of
primary sludge, waste activated sludge and dilution water in the ratio
38
-------
4000
Calibration 1 -
Y - 1.18 X + 27
r = 0.997
3000
UJ
D;
LU
Calibration 3 —
Y = 0.86 X + 179
r - 0.991
2000
1000
1000
Calibration 2
Y = 0.90 X - 46
r = 0.999
Calibration
Symbol
•
O
A
A
D
5000
Figure 16.
2000 3000 4000
X = OVERFLOW SUSPENDED SOLIDS - MG/L
Calibration data for thickener no. 2 overflow solids analyzer.
-------
4000
Calibration 1 —
Y = 1.16 X + 208
r = 0.998
3000
Q
-
Calibration 2
Y = 0.90 X - 69
r = 0.999
1000
Calibration 3
Y = 0.83 X + 59
r = 0.967
Calibration
Symbol
•
O
A
A
D
2000 3000 4000
X = OVERFLOW SUSPENDED SOLIDS - MG/L
5000
6000
Figure 17. Calibration data for thickener no. 4 overflow solids analyzer.
-------
1:1:2.5 respectively.) In order to insure adequate performance, however,
routine calibrations should be performed on a semi-monthly basis and the
electronic drift documented on a weekly basis with the test plug.
TABLE 3. TEST PLUG READINGS FOR SUSPENDED SOLIDS ANALYZERS
Date Thickener
Inflow Overflow
ppm calibration range ppm calibration range
Low Hi gh Low High
12- 3-76
8-31-77
11-18-77
12- 3-76
8-31-77
11-18-77
2
2
2
4
4
4
3600a
3850b
3950b
3900a
3750b
3600b
15,000a
16,900b
17,000b
16,000a
16,500b
16,100b
3000a
3200b
3400c
3100a
3150b
3250c
13,500a
14,000b
14,500c
13,200a
13,500b
14,000c
a-Prior to calibration, b-Calibration 2, c-Calibration 3
Maintenance--
A motor failure occurred on thickener no. 2 inflow analyzer on August
31, 1977, approximately nine months after installation. A new motor was
received on September 26 and installed on September 27. No maintenance was
required on thickener 4 inflow analyzer. Both overflow analyzers were
routinely checked for overflow debris and slime accumulation during each
visit. No other maintenance was conducted on the overflow analyzers.
Underflow
Calibration—
The MWCC conducted evaluations of several types of solids analyzers in
1975 and early 1976. Several instruments on loan from the manufacturers
were mounted in the influent and underflow line of thickener no. 2 and cali-
bration data were collected as in this study. The specifications for the
monitoring instruments used in the subsequent evaluation of the sludge blan-
ket control scheme were prepared, based on the results of these evaluations-
see Figure 18. The technical specifications for all of the instruments used
to monitor solids concentrations and control the sludge blanket level are
presented in Appendix A.
After the purchased instruments were installed and initially calibrated
it became apparent that the span adjustment was not sufficient to bring the
meter reading into agreement with the measured total solids concentrations.
41
-------
ro
a
i—i
_i
o
Q_
II
Data Obtained 12/22/75 - 1/19/76
Y = 0.813 X + 0.51
r = 0.90
234
X = INSTRUMENT READING - PERCENT SOLIDS
Figure 18. Calibration data for model 52H sludge density analyzer.
-------
The two instruments were returned to the manufacturer late in January 1977.
The electronic components were modified and the instruments were returned to
MWCC early in March 1977.
The calibration data are summarized in Figures 19 and 20. The origin
was included as a calibration point in the linear regression analyses. Be-
cause of this, the correlation coefficients are misleading. Although the
calibration data are inadequate in terms of the range of solids concentrations
it appears that reliable estimates of the sludge solids concentration cannot
be obtained with the instruments under the test conditions.
The data presented in TABLE 4 indicate that the scatter"observed'in
Figures 19 and 20 should not be attributed to drift in the electronics mo-
dules. The scatter was more likely due to the changing physical properties
of the sludge solids and the color of the suspension. Regardless of the
cause it is obvious that significant errors can be introduced if the calibra-
tion status is not identified at frequent intervals.
TABLE 4. TEST PLUG READINGS FOR SLUDGE DENSITY ANALYZERS
% Solids
Date
December 3, 1976
August 22, 1977
August 31, 1977
September 22, 1977
October 6, 1977
November 8, 1977
November 18, 1977
Thickener 2
3.0
2.0a
2.2c
2.3d
2.2d
2.3d
2.2d
Thickener 4
3.0
1.4a
2.6b,c
2.6d
2.6d
e
e
a - Range adjusted by the manufacturer during February, 1977.
b - Thickener no. 4 underflow analyzer control unit checked by
the manufacturer between August 23 and August 29, 1977.
c - Calibration 1
d - Calibration 2
e - Out of Service
43
-------
-pi
-pi
GO
Q
O
CO
O
C£
LU
Q_
!
CJ3
Q
-------
oo
o
00
o
o:
UJ
o_
CJ3
oo
Calibration 1
Y = 1 .30 X + 0.159
r = 0.905
Calibration 2
Y = 1.08 X + 0.145
r = 0.924
Calibration
1
2
2
2
Symbol
O
A
4 5
X - PERCENT SOLIDS
Figure 20. Calibration data for thickener no. 4 underflow solids analyzer.
-------
The underflow solids analyzer serving thickener no. 2 was withdrawn from
the sludge line and calibrated using solids suspensions covering a wide range
of concentrations. The data obtained are presented in Figure 21 along with
the four regression lines for the data collected in test runs 2, 3, 4 and 5 -
all at the second calibration setting. These data support the conclusion
that optical instruments of this type are not appropriate for all applications.
Maintenance—
Thickener no. 2 analyzer—
The analyzer was removed from its pipe mounting on November 30, 1977 for
direct calibration and inspection approximately one year after installation.
A thin coating of grease was removed from the analyzer extension tube but the
sample port was not plugged. After calibration the analyzer was re-installed
in the underflow pipe.
Thickener no. 4 analyzer--
In August 1977 it was observed that the instrument outputs, meter and
recorder, were not identical. The control unit was shipped to the manufac-
turer for service. The unit was returned and installed within a weeks time.
On September 21, 1977 the meter was found pegged offscale. The analyzer was
removed from the pipe and inspected. The light source, motor and plunger
were observed to operate normally; however, a thick coating of grease was
removed from the end of the analyzer extension tube. The grease coating was
thought to. be the cause of the malfunction and the analyzer was installed on
the following day. The instrument failed again on October 5, 1977. The
control unit was tested and found to be functioning. The analyzer was re-
moved from the underflow pipe on October 5, 1977 and was found to operate
normally with both clear water and sludge and subsequently reinstalled.
After another failure on the following day it was determined that the light
source was failing intermittently - most likely caused by a poor electrical
contact and vibration. A replacement light source was ordered but was not
delivered until well,after test run no. 5 was completed.
FLOW METERS
Overflow
The res.ults of the overflow indicator-totalizer comparisons are presented
in TABLE 5. The average difference between the indicator and totalizer values
was used to adjust the observed flow indicator readings and, in turn, correct
the observed totalized flows as illustrated in TABLE 6. The head on the
Parshall flume and the corresponding calculated flow rates are presented in
columns 1 and 2 respectively. The flow rates observed at the local indicator
are listed in column 3. The values presented in column 4 were obtained by
adding the appropriate correction factor (TABLE 5) to the values of column
3. The corrections presented in column 5 were obtained by subtracting the
values of column 4 from those of column 2. The calibration data are sum-
marized graphically in Figure 22. In subsequent paragraphs the observed
daily overflow volumes for thickners no. 2 and no. 4 were adjusted by +0.48
46
-------
Calibration Data Obtained 11/22/77
O
OO
LlJ
C_>
CfL
LLJ
Q_
LU
Regression lines for
test runs 2-5
3 4
PERCENT SOLIDS
Figure 21. Calibration data for thickener no. 2 underflow solids analyzer.
-------
2.5
co
Q
UJ
2.0
1.5
1.0
0.5
- Thickener No. 4
- Thickener No. 2
0.5 1.0 1.5 2.0
CALCULATED OVERFLOW - MGD
"2.5
TO
Figure 22. Calibration data for thickener overflow measurement systems.
-------
mil gal and +0.18 mil gal respectively as per TABLE 6 and Figure 22.
TABLE 5. OVERFLOW INDICATOR-TOTALIZER COMPARISON
Date
9-13-77
9-13
9-14
9-14
10-12
Correction
9-13-77
9-13
9-14
9-14
10-12
10-13
Correction
Time
increment
(minutes)
11.0
11.0
9.75
14.75
6.75
factor =
11.33
10.67
10.0
14.75
13.0
12.0
factor =
Thickener no. 2
Difference
totalizer
readings
1342
1346
1436
2180
545
averaged difference =
Thickener 4 overflow
1325
1258
1547
2282
1870
1519
average difference =
overflow
in
totalizer
1.76
1.76
2.12
2.13
1.16
0.03
1.68
1.70
2.23
2.23
2.07
1.82
0.09
Flow
mgd
indicator
1.72
1.72
2.10
2.10
1.15
1.60
1.60
2.15
2.15
1.98
1.73
49
-------
TABLE 6. OVERFLOW CALCULATIONS
Thickener
Date
9- 7-77
9- 7
9- 7
9- 8
9- 8
9- 8
9-26
9-28
10-12
10-14
Thickener
9- 7-77
9- 7
9- 7
9- 8
9- 8
9- 8
9-26
9-28
10-12
10-14
no. 2
T
Fl ume
head
(feet )
0.938
1.068
0.729
1.050
0.921
0.828
0.978
0.969
0.901
0.819
no. 4
0.935
0.708
0.528
0.969
0.776
0.529
0.998
0.945
1.001
0.875
2
Cal culated
flow(a)
(mgd)
1.80
2.19
1.22
2.14
1.75
1.49
1.92
1.89
1.69
1.46
1.79
1.17(b)
0.75(b)
1.89
1.35
0.75(b)
1.98
1.82
1.99
1.62
3
Indicated
flow
(mgd)
1.29
1.69
0.75
1.61
1.25
0.99
1.38
1.37
1.20
0.97
1.51
0.96
0.55
1.62
1.10
0.54
1.70
1.55
1.70
1.37
4
Cal culated
total izer
(.mgd)
1.32
1.72
0.78
1.64
1.28
1.02
1.41
1.40
1.23
1.00
Average
1.60
1.05
0.64
1.71
1.19
0.63
1.79
1.64
1.79
1.46
Average
5
Correction
appl ied
to totalizer
(mgd)
+0.48
+0.47
+0.44
+0.50
+0.47
+0.47
+0.51
+0.49
+0.46
+0.46
+0.48
+0.19
+0.12(b)
+0.11(b)
+0.18
+0.16
+0.12(b)
+0.19
+0.18
+0.20
+0.16
+0.18
a Q = 3.07 H1'53 with Q in cfs and H in feet (5)
b Low flows were not used in calculating the average correction because they
were not encountered during normal thickener operation.
50
-------
Underflow
The results of the underflow totalizer calibration are presented in
TABLE 7. The correction factors in the last column were used to adjust the
observed daily thickened sludge flows.
TABLE 7. UNDERFLOW TOTALIZER CALIBRATION
Time
Date Thickener Drawdown increment
No. (feet) (minutes)
Totalizer Measured -Volume Ratio
Volume Volume Measured
(gal) (gal) Totalizer
9-15-77
9-21
9-15
2
2
4
0.248
0.445
0.411
40
106
73
6390
11590
9070
6155
11044
10200
0.96
1.12
(Avg.)
THICKENERS
Underflow
Solids Content--
The solids content of the underflow samples collected during test runs
1 through 5 are summarized in Figure 23. The concentration increased slight-
ly during the 4 month period for both basins; however, the concentrations
for the two basins did not appear to be significantly different. The stu-
dent's t test (6) was used to compare the means of the two sample popula-
tions. The hypothesis U2 = u^ was tested against the alternative U2 X u4.
The hypothesis was accepted when the type one error (the probability that
U2 = U4 but rejected) was fixed at 5%.
The temperatures observed during the 5 test runs are presented in Figure
24. The temperature drop was significant as expected for this time of the
year. It is possible that the observed increase in underflow solids concen-
tration was related to the decrease in temperature. At lower temperatures
the biological activity in the thickened sludge is decreased and the likeli-
hood of gas formation and subsequent expansion of the sludge blanket is also
decreased.
Dewatering Characteristics--
General--
The dewatering properties of thickened sludge determine the ease in which
51
-------
en
ro
oo
Q
O
GO
0
»2
o
°o
A A
o
A
AO
O
GD
o
o
Thickener No. 2 O
Thickener No. 4 A
j_
I
AUGUST " SEPTEMBER OCTOBER NOVEMBER
MONTH - 1977
Figure 23. Observed total solids concentration of underflow samples during test runs 1-5,
-------
en
CO
26.
25
24
23
22
21
20
19
18
17
16
15
O O
AUGUST
Inflow - O
Underflow - • - Averages for Basins
00 O •
•§• o
O
SEPTEMBER OCTOBER NOVEMBER DECEMBER
MONTH - 1977
Figure 24. Temperature observations for thickeners no. 2 and 4 during test runs 1-5.
-------
water can be withdrawn by subsequent processes such as vacuum filtration or
centrifugation. The dewatering properties have a significant effect on both
the size of the dewatering equipment and the quantities of conditioning chem-
icals required. The dewatering properties also affect the economics of sub-
sequent disposal methods such as incineration or landfilling. CST and filter
leaf analyses were employed to characterize the dewaterability of the thick-
ened sludge from basins no. 2 and 4.
Capillary suction time--
Baskerville and Gale (3) have shown that CST can be correlated to the
specific resistance for filtration of raw sewage sludges. The CST method
is easy to perform and can be conducted in the field at the time of sample
collection. CST determinations, however, have been shown to be a function
of the absorbent paper used, temperature, the surface tension of the liquid
being absorbed, and the suspended solids content of the sample being tested.
Differences in surface tension were not considered because of the similarity
of the inflow characteristics between thickeners no. 2 and 4. The differ-
ence in underflow temperature between thickeners did generally not exceed
0.5 degrees Centigrade and any effects due to differences in temperature were
not considered significant in the CST testing procedure. The same batch of
absorbent paper was used throughout the testing period, which minimized any
effects produced by irregularities in the paper.
The results of the CST determinations conducted during test runs 1
through 5 are summarized in TABLE 8. The CST exhibited a variable correla-
tion with solids concentration; however, in each run the highest CST value
was associated with the highest solids concentration. Because of the rela-
tively small differences in solids concentration between thickeners no. 2
and 4, and the lack of a well defined CST-solids relationship, the CST de-
terminations were treated as distinct analytical measurements regardless of
the solids content of the underflow samples used for CST determinations.
- •)(
The student's t test (6) was again used to test the hypothesis, u2 = u^,
where u2 and U4 are the average CST values for basins no. 2 and 4 respective-
ly for a single test run. The hypothesis was accepted for test runs 1, 2,
4, and 5 when the type one error was fixed at 5%. The hypothesis was re-
jected for test run no. 3.
Filter leaf tests--
Filter leaf tests were conducted on underflow samples to obtain a more
direct indication of the underflow dewatering characteristics. Filter yields
were determined using underflow samples from both thickeners during each
test run to demonstrate any significant difference in dewaterability. The
results of the filter leaf tests are summarized in TABLE 9.
Differences in yield between thickeners were observed during test runs
1, 4 and 5. In each case individual yields were a function of sludge solids
content with the larger filter yield obtained from the underflow sample with
the higher solids content. The average solids content for all of the under-
flow samples taken for filter leaf analysis during the runs was 3.5% for
54
-------
TABLE 8. CST OF THICKENER UNDERFLOW-SUMMARY
Test
run
1
2
3
4
5
Average
percent
solids
2.8
3.2
3.5
3.2
4.1
Thickener
CST
runs
25
15
22
17
18
2 underflow
CST
Average standard
CST deviation
320.0
343.5
175.1
381.9
666.4
68.4
61.6
37.9
141.5
296.7
for all test runs
Test
run
1
2
3
4
5
Average
percent
solids
2.8
3.3
3.6
3.4
3.9
Thickener
CST
runs
28
17
24
22
16
4 underflow
Average
CST
340.5
347.7
210.2
396.0
614.0
for all test
CST
standard
deviation
84.9
35.1
44.3
140.6
178.2
runs
CST/solids
Correlation coef.
0.40
-0.50
-0.72
0.27
0.58
0.41
CST/solids
Correlation coef.
0.70
0.03
-0.15
0.34
-0.63
0.25
both thickeners. The corresponding average filter yield for all the test
runs was 3.2 psf/hr for both thickeners.
Based on the results of both the CST and filter leaf tests it is con-
cluded that there was no significant difference in the dewatering character-
istics of the underflow samples collected from thickeners no. 2 and 4.
Overflow
55
-------
TABLE 9. FILTER LEAF ANALYSES OF THICKENER UNDERFLOW
en
Thickener
underflow
.Date
8- 1-77
8-16
9- 9
9-16
9-30
10-14
10-18
10-21
10-27
11- 8
11-18
Test
run
1
1
2
2
3
3
4
4
4
5
5
Sol°
ids
Thickener
2 4.
1.8
3.3
3.8
3.1
4.0
4.2
3.5
3.6
2.7
3.3
4.9
1.8
3.2
3.8
3.3
3.8
4.7
3.3
4.5
2.8
3.6
4.2
PH
Thickener
2 4.
6.4
6.2
5.9
5.7
6.0
5.9
6.0
6.1
6.2
6.1
6.1
6.6
6.0
5.9
5.8
5.8
5.8
5.7
5.9
6.2
6.1
5.8
Chemical
dosage
Filter yield
psf/hr
Thickener 2
% FeClij
10
9
10
12
8
7.5
8
8
10
8
6
% CaO
30
27
27
40
20
23
24
24
30
24
18
Trial 1
2.0
4.3
3.6
3.2
2.7
4.2
4.3
2.7
2.8
2.8
4.1
Trial 2
1.7
4.1
4.4
2.4
2.0
3.9
3.2
3.0
2.3
2.7
4.4
Thickener 4
Trial 1
1.6
3.7
4.3
2.8
2.6
4.8
3.2
3.8
2.5
2.8
3.0
Trial 2
1.5
3.5
4.1
3.0
2.5
3.7
3.2
4.6
2.5
2.8
3.4
Average
3.5 3.5
3.3
3.1
3.2
3.2
-------
Solids Content—
The overflow suspended solids concentration was highly variable for both
thickeners. The values ranged from 100 mg/1 to over 3000 mg/1 and, during
the tests, appeared to be related to the solids loading rates for the basins.
Multiple linear regression analyses were conducted to define the overflow
suspended solids concentration as a function of: blanket level, hydraulic
loading rate, inflow suspended solids concentration and temperature. The
least squares procedure yielded the following.
[1] Y2 - -1709 - 154 L + 422 Q + 0.784 S + 8.96 T
and
[2] Y4 = -930 - 186 L +479 Q + 0.432 S + 14.0 T
where: Yn = average daily overflow suspended solids concentration for basin
no. n, mg/1
L = average blanket level for day, ft below surface
Q = basin inflow rate, mgd
S = average daily inflow suspended solids concentration, mg/1
T = temperature of basin inflow, °C
The standard error of the estimate for equation [1] is 540 mg/1 and the mul-
tiple correlation coefficient, R, is 0.865. The values of the standard error
and R for equation [2] are 466 and 0.758 respectively.
The value R^, sometime termed the coefficient of multiple determination,
is in fact the ratio of the variation in Y explained by the combined influ-
ence of the independent variables to the total variation in Y. (7) Thus for
equation [Ij, basin no. 2, 75% of the variability of Y is explained and for
equation [2], basin no. 4, 57% is explained.
The significance of each of the four variables, L, Q, S and T,
amined by conducting the regression analysis with one, two, three
four variables. Based on the results obtained the blanket level,
inflow solids, S, are by far the more significant variables. The
equations using only L and S are as follows.
was ex-
and all
L, and the
regression
[3] Y2 = -508 -159 L + 0.761
[4] Yy, = 302 - 200 L + 0.453
S , ff = 0.72
standard error = 535
S , R2 = 0.50
standard error = 479
The variability not explained by the regression analysis is most likely
related to the demonstrated lack of precision in the blanket level determin-
ation. In addition, the settling characteristics of the solids, which were
not documented, no doubt also affected the quality of the overflow stream.
The regression equations, [3] and [4]s are represented by two families
of curves in Figure 25. For identical blanket levels the curves indicate
that basin no. 4 performed better than basin no. 2 when the inflow solids
concentration exceeded approximately 2000 mg/1.
The average hourly inflow and overflow suspended solids concentrations
were estimated using the recorded output of the solids analyzers along with
57
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4000
en
CO
co
Q
o
CO
Q
UJ
QL.
CO
CO
UJ
>
o
2000
1000
Basin No. 2
Basin No. 4
Blanket depth, ft
1000
2000
3000
4000
5000
6000
INFLOW SUSPENDED SOLIDS - MG/L
Figure 25. Overflow solids concentration for basins 2 and 4 based on multiple linear regression analyses.
-------
the calibration data (Figures 14, 15, 16, 17). The recorded values for inflow
suspended solids were off scale,>10,000 mg/1, on five occasions totaling
13 hrs. A value of 10,000 mg/1 was used in subsequent calculations. Like-
wise, the recorded overflow values were off scale,>3,000 mg/1, on five oc-
casions for basin no. 2, totaling 12 hrs, and on six occasions for basin no.
4, totaling 28 hrs. In these instances a value of 3,000 mg/1 was used. The
hourly averages were used to construct average daily concentrations.
The daily average suspended solids concentrations for basins no. 2 and
4 are presented in Figure 26 for four ranges of inflow solids concentration.
In addition, the average concentrations are plotted for each range of inflow
solids concentration. Although the variation of blanket level is not ad-
dressed in this plot, the data indicate that basin no. 4 performed signifi-
cantly better than basin no. 2 when the average daily inflow suspended solids
concentration remained over 4000 mg/1.
Organic Content--
The total organic carbon (TOC) concentrations observed in the filtered
and unfiltered overflow samples are presented in Figure 27- These data
demonstrate that the total TOC of the overflow is directly related to the
suspended solids concentration; however, the soluble TOC concentration ap-
pears to be independent of the solids concentration. It thus appears that
although the quality of the overflow may affect the organic loading of the
primary sedimentation basins, the organic loading at the secondary facilities
should not vary substantially.
Solids Capture
The solids capture efficiency was determined for each thickener during
each of the test runs to establish the significance of blanket level control
on overall basin performance. The calculations are described in the follow-
ing paragraphs.
For the period of time during which the inflow solids analyzer for
thickener no. 2 was out of service the average inflow solids concentrations
were based on the data collected for thickener no. 4. This substitution was
justified because both basins are fed from the same inflow header. The cal-
culated suspended solids content of the underflow was based on the analysis
of the underflow samples used to establish the instrument calibration char-
acteristics.
The observed values for basin discharge, overflow and underflow, were
adjusted as described in a previous section. The thickener influent flow
was assumed to be equal to the sum of the overflow and underflow.
The solids capture calculations are summarized in TABLE 10. During four
of the five tests the average solids capture was greater for thickener no. 4
which utilized the sludge blanket control system. Although the differences
for the individual tests may not be significant the conclusion that the
overall performance of thickener no. 4 was better is substantiated by the
data presented in Figure 25.
59
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4000
3000
oo
Q
O
00
LiJ
Q
00
ZD
00
o
CM
00
<
QQ
2000
1000
Influent
Suspended
Solids Range
S 3000
3000 - 4000
4000 - 5000
>5000
Average for
above ranges
1000 2000 3500
BASIN NO. 4 OVERFLOW SUSPENDED SOLIDS - MG/L
Figure 26. Daily average overflow suspended solids
concentration for basins 2 and 4.
60
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CJ3
O
03
O
O
CD
a:
o
1000
800
600
400
Unfiltered Samples
Y = 0.248 X + 192
r = 0.85
200
Filtered Samples
Y = - 0.0009 X + 119
r - - 0.026
1000 2000 3000
X = OVERFLOW SUSPENDED SOLIDS - MG/L
Figure 27. TOC concentration of overflow for thickeners no. 2 and 4.
-------
TABLE 10. SOLIDS CAPTURE-SUMMARY
CTi
ro
Test run Basin no.
1 2
4
2 2
4
3 2
4
4 2
4
5 2
4
Total
Inflow
915,000
827,000
978,000
998,000
852,000
1 ,000,000
552,000
706,000
570,000
503,000
Ibs suspended
Overf 1 ow
311,000
229,000
422,000
192,000
133,000
122,000
101,000
163,000
106,000
58,000
solids
Underflow
560,000
578,000
532,000
753,000
711,000
746,000
395,000
519,000
480,000
517,000
% solids capture
(inflow-overflow) (100)
(inflow)
66
72
57
81
84
88
82
77
81
88
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SECTION 9
COST
GENERAL
The decision to utilize a sludge blanket control system on one or more
thickeners should be based on estimates of the costs and benefits associated
with the control system. The data and discussion presented in the following
sections indicate that it would be cost effective to use such a system with
one or more thickeners. For any such system, however, it would be advisable
to include a high level alarm on the thickened sludge holding tanks as
minimum protection against overflows. In the case of the Metro WWTP, however,
the use of such a system is presently precluded by the physical constraint of
inadequate solids processing capacity downstream of the thickeners.
OPERATING
Based on three separate observations, the thickener operators spent ap-
proximately 30 minutes during each eight hour shift monitoring the location
of the sludge blanket. The annual labor requirement for this task per basin
is approximately 90 hours. The maintenance requirement for the operator's
blanket detector averages approximately 10 hours per year according to plant
maintenance personnel. Based on these two labor requirements the annual mon-
itoring cost per basin averages approximately $900 per year using current
salary and benefit rates.
The data obtained during this study indicate that the two probes of the
sludge blanket control system should be inspected and wiped clean once per
week. It is estimated that this task would require approximately 20 minutes
for the six basins or 3 hours per basin per year. An additional 10 hours per
basin per year would be required to determine the location of the blanket
level on a daily basis to verify that the control system is functioning. In
addition, it is assumed that probe repair and replacement and maintenance of
the control modules would require no more than 10 hours per year and no more
than $100 for parts and supplies. The total annual cost for operating and
maintaining each blanket control system is estimated to be approximately
$360 based on current salary scales.
The total installed cost for the sludge blanket control system used on
thickener no. 4 was approximately $2000. Based on the above actual and es-
timated costs approximately four years would be required to recover the cost
of the control system. Based on the assumption that the useful life of the
entire control system is approximately 10 years its installation is economi-
cally justified even though the annual operating cost savings would not be
substantial.
63
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TREATMENT
For the entire study period the solids capture efficiencies averaged_74%
and 81% for thickeners no. 2 and no. 4 respectively - TABLE 10. The solids
loading averaged approximately 61,000 Ibs per basin per day. Thus thickener
no. 2 returned approximately 4,000 Ibs/day more suspended solids to the pri-
mary treatment facilities than thickener no. 4. The annual cost of treating
these additional solids is substantial.
In June of 1976, the MWCC officially adopted a strength charge system as
per a consultant's recommendation. (8) This system is used to calculate
strength charges for industrial dischargers containing above average
concentrations of COD and suspended solids. Although the system was based on
data from all MWCC treatment facilities (22 at the time of analysis) it is
believed to accurately describe costs at the Metro plant because approximately
84% of the system flow was treated and 66% of the system 0 & M budget was
expended there.
If the thickener overflows are treated as industrial discharges and cur-
rent unit costs are employed, the annual strength charges which would be
assessed against basins no. 2 and 4 are approximately $96,000 and $58,000
respectively. These values were based on suspended solids concentrations
onlyi because, the soluble organic content of the overflows was essentially
constant (Figure 27). It is conservatively estimated that 10% of these
strength charges can be directly allocated to the retreatment of these return
solids during their passage through the treatment plant.
An annual saving of $3,800 per basin can thus be attributed to improved
blanket level control. This value is indeed conservative because the average
solids capture efficiencies obtained during this study were used. A review of
Figures 9-13 illustrates that the performance of the automated blanket level
control system was better than the average obtained here once equipment
maintenance was standardized.
Based on the above conservative assumption, the payback period for the
blanket level control system as described is approximately six months.
Additional costs, however, would be incurred to construct an integrated control
system for six basins and two holding tanks.
Based on experience at Metro, the performance of the entire treatment
plant can be affected by the thickener overflow characteristics. The over-
flow is discharged to one of the six primary settling basins. The overflow
suspended solids are not effectively removed by sedimentation and discharge to
the aeration tanks thus increasing the quantity of waste activated sludge.
When the overflow suspended solids concentration remains over 1000 mg/1 for
several days solids cycling within the plant increases and eventually these
solids appear in the final effluent. It has also been observed that the
dewatering characteristics of the primary sludge processes in F & I No. 1
are adversely affected by the overflow suspended solids. Although no attempt
was made to identify the direct costs associated with those operating problems,
they are assumed to be significant because the ability of the facility to meet
its' discharge permit is affected.
64
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REFERENCES
1. Rosenkranz, W.A. Workshop Summary. In: Research Needs for Automation
of Wastewater Treatment Systems. Clemson University, Clemson, South
Carolina, 1975. pp 9-10.
2. Standard Methods for the Examination of Water and Wastewater. 14th Edi-
tion, APWA, AWWA, WPCF, Washington, D.C. 1976.
3. Baskerville, R.C. and Gale, R.S. A Simple Automatic Instrument for De-
termining the Filtrability of Sewage Sludges. Journal of Water Pollu-
tion Control, Vol. 67, No. 2: 233-241, 1968.
4. Fritz, J.S. and Schenk, G.H. Quantitative Analytical Chemistry. Allyn
and Bacon, Inc., Boston, Massachusetts, 1969. 660 pp.
5. Spink, L.K.. Principles and Practice of Flow Meter Engineering. The
Foxboro Company, Foxboro, Massachusetts, 1958. 549 pp.
6. Dixon, W.J. and Massey, F.J. Introduction to Statistical Analysis.
McGraw-Hill Book Co., New York, N.Y., 1957. 488 pp.
7. Nie, N.H., Hull, H.C., Jenkins, J.G., Steinbrenner, K. and Bent, D.H.
Statistical Package for the Social Sciences, 2nd ed. McGraw-Hill Book
Co., New York, N.Y., 1975. 675 pp.
8. Robins, M.C. MWCC Strength Charge System. M.L. Robins Associates, St.
Paul, Minnesota, 1976. 20 pp.
65
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APPENDIX A
TECHNICAL PROVISIONS OF SPECIFICATIONS FOR HARDWARE ITEMS
BID ITEM A SLUDGE DENSITY ANALYZER
The sludge density analyzer shall consist of a photoelectric sensor and a
remotely located control unit.
The sensor shall be of the self cleaning type. The sensing head shall
contain a pyrex glass sampling chamber with a piston, a light source and
photocells. The sensing head shall be mounted at the wet end of a one foot
long tube which connects it with the drive mechanism located at the dry end
of the tube. The drive mechanism operating the piston shall periodically
draw the sample slurry into the sampling chamber and wipe its optical surfaces
at the same time. The sensor shall be supplied with a pipe insertion adapter
for attachment through a 2 inch NPT threaded hole to the process pipe. The
pipe insertion adapter shall be constructed so as to allow removal of the
sensor from the process pipe without dewatering the process pipe. A 100 foot
long multiconductor cable shall be attached to the sensor for connection to
the control unit.
The control unit shall have a meter graduated from 0 to 10 percent sus-
pended solids, a power switch, and a calibration control. The photo cell
output shall be conditioned and/or converted through internal circuitry to
yield a 4-20 ma DC output. The output shall be directly proportional to the
suspended solids content of the sample over the specified concentration
range. The control unit shall be designed for both bench and panel mounting
and all necessary hardware shall be provided. A terminal block shall be
provided for connection to 120V AC power and for connection of an output
recorder. The sludge density analyzer shall be designed to operate in an
ambient temperature range of 5°C to 50°C.
BID ITEM B SUSPENDED SOLIDS ANALYZER - PIPE MOUNT
The suspended solids analyzer shall consist of a photoelectric sensor and
a remotely located control unit.
The photoelectric sensor shall be designed such that the optical surfaces
are continually cleaned by the process flow stream or by a mechanical wiper.
The sensor shall be supplied with a pipe insertion adapter for attachment
through a 2 inch NPT threaded hole to the process pipe. The pipe insertion
adapter shall be constructed so as to allow removal of the sensor from the
process pipe without dewatering the process pipe. A 100 foot long multi-
conductor cable shall be attached to the sensor for connection to the control
66
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unit.
The control unit shall have a meter scale graduated in ppm suspended
solids, a power switch, and a calibration control. The instrument shall have
a_full scale range of 0 to 20,000 ppm. The photocell output shall be condi-
tioned and/or converted through internal circuitry to yield a 4-20 ma DC out-
put. The output shall be directly proportional to the suspended solids con-
tent of the sample over the specified concentration range. A terminal block
shall be provided for connection to 120V AC power and for connection of an
output recorder. The control unit shall be designed for both bench and panel
mounting and all necessary hardware shall be supplied. The suspended solids
analyzer shall be designed to operate in an ambient temperature range of 5°C
to 50°C.
BID ITEM C SUSPENDED SOLIDS ANALYZER - CLAMP MOUNT
The suspended solids analyzer shall consist of a photoelectric sensor and
a remotely located control unit.
The photoelectric sensor shall be designed such that the optical surfaces
are continually cleaned by the sample flow stream or by a mechanical wiper.
The sensor shall be designed to operate in the vertical position. The sensor
shall be supplied with a clamp for attachment to a one inch pipe. The clamp
shall be located approximately 24 inches above the sensor face to allow for
positioning in a sample sink. A 100 foot long multiconductor cable shall be
attached to the sensor for connection to the control unit.
The control unit shall have a meter scale graduated in ppm suspended sol-
ids, a power switch, and a calibration control. The instrument shall have a
full scale range of 0 to 2500 ppm. The photocell output shall be conditioned
and/or converted through internal circuitry to yield a 4-20 ma DC output.
The output shall be directly proportional to the suspended solids content of
the sample over the specified concentration ranges. A terminal block shall
be provided for connection to 120V AC power and for connection of an output
recorder. The control unit shall be designed for both bench and panel
mounting and all necessary hardware shall be supplied. The suspended solids
analyzer shall be designed to operate in an ambient temperature range of
5°C to 50°C.
BID ITEM D SLUDGE BLANKET CONTROLLER
The sludge blanket detector shall consist of two submersible probes sus-
pended, by individual waterproof cables, from a control box.
The probes shall be constructed of non-corrosive materials. Each probe shall
consist of a light source and photocell separated by an optical gap. The
radiant energy emitted from each light source shall be limited to those wave
lengths that do not promote algae growth. Each probe shall be attached to
a 25 foot waterproof cable by means of a waterproof connection made by the
contractor.
The control box shall be a weatherproof NEMA 4 enclosure equipped with
67
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mounting lugs and clamps for attachment to a wall, handrail or vertical post.
Space shall be provided inside the control box for storage of the unused
lengths of the two 25 foot long probe cables. The control box shall contain
the required solid state electronic circuits, power supplies and output
relays. The output relays shall be rated for 10 amps at 120 VAC. A two
position response switch shall be incorporated in the electronic circuitry
to provide fast or delay response to sludge level changes. Three indicating
lights labeled, LOW, MEDIUM, HIGH, shall be mounted on the enclosure to
indicate the sludge blanket position as below, between or above the two
probes. A terminal block shall be provided for connection to 120 VAC,
60 Hz power and for connection of relay contacts to external pump controls.
When probe 'A1 is located above probe 'B1 in a sedimentation basin the
controller shall function as follows. The energy source in the probe shall
emit radiation through an optical sensing gap to the photocell which senses
the amount of energy transmitted through the liquid and produces an electronic
signal related to the suspended solids concentration of the liquid in the gap.
This signal is compared in the control circuit to an adjustable density
threshold setting. When the signal representing the concentration of solids
in the liquid at probe 'A' exceeds the threshold setting an electronic
integration is initiated. If the sludge level remains at or above probe 'A'
at the expiration of the integration interval the output relay is energized
and the HIGH lamp on the control box illuminates. When the sludge level
drops below probe 'A' the photocell signal drops below the threshold setting
and in turn the HIGH lamp is extinguished and the MEDIUM lamp illuminates.
When the sludge level drops to probe 'B1 and the signal representing the
concentration of solids in the liquid is less than the threshold setting an
electronic integration is initiated. If the sludge level remains below
probe 'B1 at the expiration of the integration interval the output relay is
deenergized and the MEDIUM lamp is extinguished and the LOW lamp illuminated.
When the sludge level increases to probe 'B' the photocell signal increases
above the threshold setting and in turn the LOW lamp is extinguished and
the MEDIUM lamp illuminated. The above cycle is repeated.
The sludge blanket controller shall be designed to operate at ambient air
temperatures in the range of -30°F to llOop. The wetted portions of the
sludge blanket controller shall be designed to operate at ambient water
temperatures in the range of 32°F to 85°F.
-------
APPENDIX B
DETAILED CHRONOLOGY - FIELD NOTES
TEST RUN NO. 1
August 1
The sludge blanket probes were adjusted to levels of 5 1/2 and 6 ft
below the surface of thickener 4. The optical surfaces of both probes
were cleaned prior to measuring their photocell resistance in water.
Both initial sludge blanket levels were measured at 5 feet using the
operator's portable sludge blanket detector.
The CST values for the underflow sludges were low, approximately 200
sec because of their low solids content.
August 2
Debris was removed from no. 4 overflow sample container. The debris con-
sisted of fibrous clumps of sludge solids which were observed floating on
the thickener surface. The material somehow was transported under the
scum baffle and discharged to the launder and eventually to the overflow
sample line. The material appeared to be associated with the primary
sludge because of its size and appearance.
Both optical surfaces of both probes were visually checked and found to
be free of film accumulation. The sludge blanket levels of both thickeners
were determined with the operators blanket detector. Both basins exhibited
diffuse interfaces ranging from 2 to 5 ft from the surface.
The recorded value for underflow analyzer no. 4 was approximately 0.5%
solids lower than the corresponding meter reading. Both underflow analyzers
yielded high readings based on sample analyses.
August 8
Clumps of solids were again removed from both overflow sample containers.
The overflow analyzer readout increased by approximately 50 to 100 mg/1
after removing the material.
Because both underflow solids analyzers gave very high readings twice in
succession, their calibration settings were adjusted downward.
69
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Both sludge blanket interfaces were found to be diffuse; thickener no. 2
interface was located 1 to 4 ft from the surface; thickener no. 4 inter-
face was 3 1/2 to 5 ft from the surface.
The optical surfaces of both probes were found free of film accumulation.
August 9
Both sludge blankets were located at 1 to 2 ft below the surface.
Clumps of solids were again removed from both overflow sample containers.
Analyzer output increased by 25 to 50 mg/1 when solids removed.
The recorded value for underflow analyzer 4 was approximately 0.8% solids
lower than the corresponding meter reading.
August 10
Solids were again removed from both overflow sample containers however no
changes were observed in the overflow analyzer readings.
Both sludge blanket interfaces were measured at 1 to 2 ft from the surface.
August 12
Both sludge blankets were located at approximately 6 ft below the surface.
The recorded value for underflow analyzer no. 4 was approximately 1.5%
solids lower than the corresponding meter reading.
August 15
Both blankets had well defined interfaces - no. 2 at 8.5 ft and no. 4 at
5.5 ft.
The recorded output of no. 4 underflow analyzer continued to read 1.5%
solids lower than instrument meter. CST values for underflow increased
substantially compared to previous values.
No. 4 underflow was black and no. 2 was brown. The pH values were 5.8
and 6.3 respectively.
August 16
Test run 1 was terminated. Both probes were checked for film accumula-
tion and photocell resistance. After 15 days of operation, the upper
probe had a noticeable accumulation on the optical surfaces however the
light source and detector were visible.
The head on the overflow weirs of thickener 4 was found to be much lower
than the other thickeners. Although no. 4 overflow totalizer had been
indicating low flow rates since August 11, no. 4 inflow totalizer had not
70
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shown a corresponding decrease and continued to indicate flow rates in
excess of 2 mgd. Based on the observations made on the overflow head on
thickener 4, it appeared that the inflow meter was generating an erroneous
flow signal for several days.
INTERIM
August 22
No. 4 underflow analyzer was examined and it was found that the 4 to 20
ma output did not correspond to meter readings. The control unit was packaged
and shipped to the manufacturer for repair.
August 29
Debris which was found to be lodged in thickener 4 influent well was
flushed out by closing the inflow valves to thickeners 2 and 6 and
increasing the flow to thickener 4. After the influent well had been
cleared of the obstructions the head on the overflow weirs of thickener 4
increased to levels similar to those observed on thickeners 2 and 6.
August 30
The optical surfaces of the probes were wiped clean and placed 7 1/2
and 8 ft below the surface of thickener no. 4.
No. 4 underflow analyzer control unit was returned from the manufacturer
and installed in the control panel. After installation of the unit, the
meter reading matched the recorder reading and the analyzer appeared to
be working properly. The calibration adjustment was not changed
from the previous setting.
The sludge blanket detector used by the operators was out of service and
thus sludge blanket levels were not determined.
TEST RUN NO. 2
August 31
No. 2 inflow analyzer failed during a test plug check. A work order was
issued for inspection and repair.
September 5
Inflow to all thickeners was terminated because the vacuum filters were
shutdown.
September 7
Thickeners in service for full day. Calibration of overflow flumes
started.
71
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September 8
Solids removed from overflow sample containers; however, the analyzer
output was not affected.
September 9
Both probes were visually inspected and the optical surfaces were found
to be free of film accumulation. Both underflow streams sampled for filter
leaf tests. No. 2 inflow local indicator and totalizer out of service.
September 12
The overflow sample containers were cleaned. No effect observed on
instrument outputs.
September 13
The optical surfaces of both probes were found to be free of film
accumulation based on visual inspection.
No. 2 influent metering system was back in service. Both blankets
located at 5 ft.
September 15
No. 2 underflow totalizer calibrated.
September 16
Both no. 2 and 4 underflow totalizers calibrated. Test run 2 terminated,
INTERIM
September 19
Failure of no. 2 inflow analyzer identified as motor failure - motor
drives plunger. New motor ordered from manufacturer.
September 21
No. 4 underflow analyzer output off scale (upper end) with motor and
plunger operating normally.
September 22
No. 4 underflow analyzer output remained off scale.
September 26
Replacement motor for no. 2 inflow analyzer received. Instrument
maintenance crew removed no. 4 underflow analyzer from underflow pipe
72
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and a grease accumulation at the sample entrance was removed. The
analyzer was tested in the laboratory with sludge samples and found to
operate properly.
September 27
Both no. 2 inflow and no. 4 underflow analyzers installed in respective
process lines and placed into service.
TEST RUN NO. 3
September 30
The optical surfaces of both probes were wiped clean. The probes were
positioned at 5 and 5.5 ft below the surface. The blanket levels were
located at 4 ft and 2.5 ft for basins 2 and 4 respectively.
The CST values were lower than normal and the sludge appeared to have
a high fiber content.
No. 2 underflow analyzer output was low based on sample analysis.
October 3
No. 4 underflow analyzer output off scale as previously observed on
September 21.
No. 2 underflow analyzer output again low based on sample analysis.
CST values lower than normal, <200 sec.
October 4
Diffuse blanket interfaces observed in both basins (2 to 5 ft) at 1200 hr.
The operator reported values of 7 ft for both basins at 1300 hr.
The operators were notified to maintain underflow pumping rate for basin
no. 4 -150 gpm to assure control of blanket level during periods of high
influent solids loading.
October 5
Both probes were found to be free of film accumulation based on visual
inspection.
No. 4 underflow analyzer was removed from the underflow pipe after it was
determined the control unit was functioning. The analyzer was operated
in clean water and sludge samples and operated normally. The analyzer
was installed in the underflow pipe of basin 4.
October 6
73
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Diffuse interfaces were observed in both thickeners - 3 to 6.5 ft in
basin 2 and 1 to 5 ft in basin 4. No. 4 underflow failed off scale again.
October 7
No. 4 underflow analyzer was removed from the underflow pipe and the light
source did not appear to be functioning. When the unit was later installed
it functioned properly. All underflow pumps turned off at 1030 hr because
of flooded gallery. No. 4 pump turned on AUTO at 1245 hr.
No. 4 underflow analyzer failed late in day based on recorder chart.
October 11
No. 4 underflow analyzer removed from underflow pipe. It was determined
that the light source had failed and had caused problems in previous days.
Both blanket interfaces were well defined - no. 2 at 9 ft and no 4 at
5.5 ft.
October 12
No. 4 underflow pump was found in MANUAL at 1250 hr. At 1200 hr operator
switched to MANUAL while adjusting speed and did not return to AUTO. Pump
returned to AUTO at 1300 hr.
The optical surfaces of the upper probe had a visible film accumulation
however the lower probe was clean. The blanket levels were observed at
7.5 to 8.5 ft for no. 2 and 5 to 6 ft for no. 4 at 1245 hr. The operator
reported a level of 7 ft for no. 4 at 1300 hr.
October 14
No. 2 underflow analyzer yielded low reading based on sample analysis.
The film accumulation was readily visible on the optical surfaces of
the upper probe but the light source and detector were visible.
The blanket level detector used by the operators was taken out of service
at 0800 hr for repair.
TEST RUN NO. 4
October 17
The probes were placed at the 3 and 3 1/2 ft levels, with the upper
probe retaining the film coating formed during test run 3.
No. 2 underflow analyzer yielded low reading based on sample analysis.
October 18
74
-------
No. 2 underflow analyzer continued to yield low values. The pH of basin
2 sludge was 6.0 and lighter in color than basin 4 sludge which had a pH
of 5.7.
October 19
Thickener 2 was pumped down several feet by the operator to facilitate
an oil change in the sludge collector mechanism. All of the thickeners
were scheduled to be lubricated during the following week. The test
run was terminated until October 21 when thickener 2 would be returned to
operational status. Thickener 4 would be oiled after the next test was
terminated.
The operator's sludge blanket detector was returned from the instrument
maintenance department.
October 21
The blanket levels were observed at 8.5 ft and 4.5 ft for basins 2 and
4 respectively.
October 24
A moderate film was visible on the optical surfaces of the upper probe.
The film on the lower probe was somewhat lighter. The control system
functioned properly despite the coatings. The optical surfaces were
not cleaned since September 30.
Both blanket interfaces were well defined - no. 2 at 7.5 ft and no. 4
at 4 ft.
No. 2 underflow analyzer continued to yield low values based on sample
analysis.
No. 4 underflow pump was switched to MANUAL to facilitate sample collec-
tion.
October 25
The no. 4 underflow pump was not returned to AUTO on the previous day and
operated through the night. The blanket level of basin no. 4 was dropped
to 8 ft before the pump was returned to AUTO.
October 26
Solids were removed from the overflow sample containers. The analyzer
outputs were not affected.
No. 2 underflow analyzer continued to yield low values.
October 27
75
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Test run 4 terminated.
October 28
The optical surfaces of the upper probe were heavily coated. The
photocell resistance of both probes was determined before and after
the optical surfaces were wiped clean.
TEST RUN NO. 5
November 8
The probes were located at 5 and 5.5 ft depths. Both probes had visible
film accumulation on the optical surfaces.
No. 2 underflow analyzer yielded values higher than in previous days
for same solids concentrations.
Blanket levels observed at 5 ft and 3 ft for basins 2 and 4 respectively.
CST values were extremely high.
November 9
No. 2 underflow analyzer again yielded higher values.
November 10
Both probes had slight film accumulation on optical surfaces.
No. 2 inflow totalizer out of service.
November 14
Basin temperatures dropped 2°C since November 8.
November 16
No. 2 underflow analyzer yielded low values based on sample analysis.
November 17
The blanket level detector failed due to film accumulation. The pump
was operating when the blanket level was observed 3.5 ft below the lower
probe. Both probes were wiped clean.
November 18
Test run 5 terminated.
CST value for basin 2 underflow extremely high, >1200 sec.
76
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA-600/2-79-159
3. RECIPIENT'S ACCESSI ON" NO.
TITLE AND SUBTITLE
5. REPORT DATE
AUTOMATIC SLUDGE BLANKET CONTROL IN AN
OPERATING GRAVITY THICKENER
November 1979 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
AUTHOR(S)
R.C. Polta and D.A. Stulc
8. PERFORMING ORGANIZATION REPORT NO.
PERFORMING ORGANIZATION NAME AND ADDRESS
Metropolitan Waste Control Commission
St. Paul, Minnesota 55101
10. PROGRAM ELEMENT NO.
1BC611 SOS#2 Task E.I
11. CONTRACT/GRANT NO.
S-803602
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory-Cin,
Office of Research and Development
U.S Environmental Protection Agency
Cincinnati,
, OH
13. TYPE OF REPORT AND PERIOD COVERED
Final Report- 9/7E-4/7R
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: Irwin J. Kugelman (513) 684-7633
16. ABSTRACT
The purposes of this study were to evaluate some of the hardware required to
monitor and control the operation of a gravity thickener and to identify any
benefits associated with improved sludge blanket level control.
An automatic sludge blanket level control system was installed in one of the
six gravity thickeners at the Metropolitan WWTP. In addition, optical type solids
analyzers were installed to monitor the inflow, overflow, and underflow streams
of two basins - one with automated blanket level control and one with manual control
The performance characteristics of the instruments were documented during a series
of five tests each lasting approximately two weeks.
The performance of the two basins is characterized in terms of underflow solids
concentration and dewatering characteristics along with solids capture. The cost
savings associated with automated blanket level control are presented.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
COSATl Field/Group
Automation
Automatic Control
Instruments
Waste Treatment
Process Control
Sludge Thickener
13B
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
89
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
77
U S GOVERNMENT PRINTING OFFICE 1930-657-146/5504
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