Upgrading
Existing Wastewater
Treatment Plants
Case Histories
EPATechnology Transfer Seminar Publication
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X£iPA-625/4-77-005a
UPGRADING EXISTING WASTEWATER TREATMENT
PLANTS—CASE HISTORIES
ENVIRONMENTAL PROTECTION AGENCY* Technology Transfer
August 1973
For sale by the Superintendent of Documents, U.S. Government
Printing Office, Washington, D.C. 20402
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ACKNOWLEDGMENTS
This seminar publication contains materials prepared for the
U.S. Environmental Protection Agency Technology Transfer Program
and has been presented at Technology Transfer design seminars
throughout the United States.
The information in this publication was prepared by Warren R.
Uhte and Richard J. Stenquist, representing Brown and Caldwell,
Consulting Engineers, Walnut Creek, Calif. Case histories 1 and 2
appeared in the original version of this publication, prepared by
David Walrath, representing Hazen and Sawyer, Engineers, New
York, N.Y.
NOTICE
The mention of trade names or commercial products in this publication is
for illustration purposes, and does not constitute endorsement or recommenda-
tion for use by the U.S. Environmental Protection Agency.
Revised May 1977
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CONTENTS
Page
Part I. Upgrading Through Biological Process Modification 1
Activated Sludge Plants 1
Operating Parameters 1
Operational Upgrading 3
Conclusions 6
Trickling Filter Plants 7
Modifications to Existing Filters 7
Additional Treatment Processes 8
References . . 9
Part II. Case Histories 11
Case 1. Greensboro, N.C., South Buffalo Creek Wastewater Treatment Plant ... 11
Case 2. Livermore, Calif., Wastewater Treatment Plant 17
Case 3. Stockton, Calif., Regional Waste Water Control Facility 22
Plant Modifications 23
Plant Performance 23
Cost and Schedule 28
Case 4. Sacramento, Calif., Central Wastewater Treatment Plant 28
Description of Plant 29
Operations Before Modifications 30
Major Problems With Existing Operations 30
Activated Sludge Process Problems 30
Process Modifications and Immediate Subsequent Operation 35
RAS Control 37
WAS Control 42
Aeration Air Control . 45
Final Clarifier Operation . . . . 46
Recent Operation Experiences 46
Summary and Conclusions 48
Case 5. Upgrading With Automatic Dissolved-Oxygen Control in the Activated
Sludge Process 49
A. Renton, Wash., Wastewater Treatment Plant 50
B. Palo Alto, Calif., Water Quality Control Plant 53
C. San Jose-Santa Clara, Calif., Water Pollution Control Plant 58
in
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Parti
UPGRADING THROUGH BIOLOGICAL-PROCESS MODIFICATION
Two of the principal biological wastewater treatment processes used in the United States are
activated sludge and trickling filtration. In many cases, upgrading such facilities may involve addi-
tional treatment methods, but in other instances, improvement through modification of the existing
biological process may be possible. For an activated sludge plant, this may involve changing the
influent feed arrangement or modifying the aeration air supply. In trickling filtration technology,
replacement of rock media with deep, plastic media filters is a recent innovation. Part I presents
some of the techniques that can be used to upgrade biological treatment processes.
ACTIVATED SLUDGE PLANTS
Before discussing specific improvements that can be taken to upgrade activated sludge plants,
it is useful to review some of the parameters that relate to the operational efficiency of the treat-
ment plant. Recognition of these parameters and an understanding of their importance will provide
assurance that planned improvements will make available the tools necessary for operational up-
grading.
Operating Parameters
The following five operating parameters are all important in the successful operation of an
activated sludge plant: food/micro-organism ratio (F/M), solids retention time (SRT), sludge volume
index (SVI), solids yield (^BOD)> and dissolved oxygen (DO). While all of these parameters are
directly involved with the biological processes taking place within the aeration tanks, it must be
noted that the interrelationship between the aeration tank and secondary clarifier is such that
operation of one cannot be optimized independently of the other. The physical conditions that will
improve secondary clarifier performance, along with the relevant operating parameters involved, are
discussed in detail in the Technology Transfer Process Design Manual for Upgrading Existing Waste-
water Treatment Plants.1
Food/Micro-organism Ratio. The F/M ratio (pounds 5-day biochemical oxygen demand (BOD)
added per day per pound of volatile suspended solids (VSS) in system) is used to determine the
loading characteristics of the activated-sludge process. The engineer who wishes to assure the main-
tenance of this ratio within a plant's treatment limits will incorporate into the design the following
tools:
• Process uniformity—ability to operate the process as a single treatment system
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• Process flexibility—ability to rearrange aeration tank feed points to operate as conventional
plug flow, step aeration, contact stabilization, or complete-mix systems
• Adequate return activated sludge (RAS) control-variable, independent, and reliable
• Adequate waste activated sludge (WAS) control-variable, independent, and reliable
• Adequate primary sedimentation—removal of debris and leveling of loadings
• Inplant laboratory control—ability to provide fast, accurate determination of system solids
and biological activity
Unit removal, QBOD (Pounds BOD removed per pound VSS in the system), is often used
instead of the F/M ratio to measure system loading. It uses the same determinations, and it requires
the same tools. The volumetric loading (pound BOD added per day per 1,000 ft3 aeration volume)
may be of interest to the engineer but seldom is of much value to the operator, as it does not reflect
the system's condition at the time of loading.
Solids Retention Time. The SRT (pounds VSS in system per total pounds VSS lost from
system) is often referred to as the mean cell residence time, and is used to maintain process sta-
bility. A short SRT indicates the process is being limited to carbonaceous oxidation, while an ex-
tended SRT usually indicates that the process is providing both carbonaceous oxidation and at least
some nitrification. A minimum SRT also means that a major portion of the biological treatment has
been shifted to the solids-handling system. To achieve the optimum control of the system's SRT,
the operator must have full, independent control of both RAS and WAS systems and complete
process flexibility. An adequately sized, independent solids-handling system is also essential.
Sludge Volume Index. The SVI is a measurement of the settling characteristics of the mixed
liquor. It is defined as the volume in milliliters occupied by 1.0 gram of mixed liquor suspended
solids after settling for 30 minutes in a 1,000-ml graduated cylinder. While it is possible for these
settling characteristics to be affected by excessive nitrification or formation of excessively light
pinpoint floe, major changes in SVI are usually caused by the varying quantities of developing fil-
amentous micro-organisms. Provision of the following will help insure maintenance of low SVI's:
• Quality microscopic examination equipment, which will provide the ability to directly
observe micro-organism activity and maintain historic records. Daily examinations can
anticipate trouble.
* Chemical-correction capabilities—provisions for oxidation, flocculation, and weight-
producing chemical treatment.
Solids Yield. The FBOD (pounds waste VSS plus pound effluent VSS per pound BOD re-
moved) value provides a measure of the solids that must be removed from the system and processed
by the waste activated sludge thickeners and solids treatment facilities. To keep track of this param-
eter, the operator must have all the tools required for determination of the F/M ratio. The ^BOD
value also indicates the degree of endogenous respiration taking place within the aeration system.
Endogenous respiration is directly affected by the SRT of the process.
Dissolved Oxygen. DO levels in the aeration and reaeration tanks are directly related to the
biological activity taking place. The activated sludge process is an aerobic process; therefore, suf-
ficient oxygen must be present in all parts of the process to support aerobic biological activity
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necessary to handle the incoming loadings. To assure adequate DO levels throughout the process,
the operator must have an adequate air supply, a positive means of adjusting that supply, and a
rapid means of determining DO levels within the various parts of the system.
Operational Upgrading
In the discussion of operating parameters, some of the operating tools or requirements neces-
sary to achieve maximum efficiency and reliability were mentioned. A detailed description of these
tools can show how they result in upgraded operation.
Process Uniformity. Process uniformity, i.e., the ability for the process to be run as one
system, is needed especially in those facilities that were originally single aeration tanks and clar-
ifiers, and have subsequently grown to multiple units as the flow has increased.
If process uniformity were not a major concern of the designer of these enlarged plants, a
likely result would be two or more systems, each with its own idiosyncrasies. Such a condition usu-
ally results in demands for operating flexibility and control far beyond the facility's capabilities.
In upgrading existing and in designing new activated sludge plants, the designer should care-
fully analyze the process to assure that the system brings all of the activated solids together at least
once. This can be accomplished by the common feed of mixed liquor to the secondary clarifiers or
the common mixing of the RAS from multiple clarifiers. If the plant is very large, it may be neces-
sary to force solids mixing by using an extra pump to circulate the solids within a large common
RAS channel or other type of feeder system.
Process Flexibility. The EPA Technology Transfer Design Manual for Upgrading Existing
Wastewater Treatment Plants1 discusses directly the advantages of process flexibility in upgrading
the activated sludge process. The need for process flexibility is also a must for all new facilities if
they are to achieve reliably the degree of treatment required. It is impossible for the designer to
anticipate every condition that may affect the efficiency of any particular activated sludge process.
The operator of that process must be able to find the operating mode that best fits his conditions.
These conditions may change during the year or from year to year. A truly upgraded activated
sludge plant will provide the means of operating the aeration process in any one of several modes
without sacrificing any of the hydraulic distribution criteria so essential to satisfactory settled
sewage, mixed liquor, and RAS control.
Return Activated Sludge Control. Variable, independent, and reliable RAS control is necessary
for several of the listed operating parameters. Without this tool, it is impossible to exercise adequate
control over the amount of suspended solids in the system and their location within the system.
Ideally, they should be where they will do the most good, in the aeration tank. To upgrade this part
of the activated sludge system, the designer should make sure that each secondary clarifier is pro-
vided with an RAS system, which can be regulated over a reasonable range (usually 20 to 100 per-
cent of clarifier design average dry weather flow capacity) and set so that it will maintain its flow
independently of other process variables. If airlifts are used, their air system must be independent of
all other process air systems.
If the hydraulics of RAS-system suction and discharge are relatively unaffected by varying
process flows, a pair of simple, low head pumps with variable-speed-drive units will suffice. A stable
system can still be produced, however, even when process flows do affect RAS-system hydraulics,
by simply installing a flow-measuring device in the system and setting up a closed-loop control with
the pump's variable drive units.
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There is some indication at the Municipality of Metropolitan Seattle's Renton, Wash., activated
sludge plant that such closed-loop controls can also be used to achieve additional stability by
varying the RAS flow in conjunction with the diurnal variation of wastewater flows. Such a system
has been in operation for several years at this plant, and it is believed partly responsible for the
plant's very efficient (96- to 97-percent BOD removal) and reliable (no system bypasses) operation
during this period. The correlation between these flows maintains constant levels of mixed liquor
volatile suspended solids within the reaeration and contact aeration tanks, thereby assuring the
micro-organisms a highly stable, uniform environment.
Waste Activated Sludge Control. It is very difficult to maintain an activated sludge process
without accurate control of its sludge-wasting rate. The wasting rate must be known in order to
determine the quantity of solids wasted from the system. Further, it is desirable to maintain the
wasting rate at a reasonably constant value to insure proper operation of WAS thickeners. Because
required variations in wasting rates can result from changes in both system hydraulics and process-
solids concentrations, it is recommended that this control be achieved by providing positive, var-
iable removal systems and accurate means of measuring the wasting-flow rate. By tying these two
systems together with a closed-loop control, the operator is provided with complete assurance of
preset dependability and a quick and easy means for making wasting-rate changes as necessary. Of
course, if the wasting is made from one location in the RAS system and solids uniformity is pro-
vided by varying RAS flows diurnally with plant flow, an even more uniform rate of wasting solids
is assured. This is especially true when the process is subject to large fluctuations in flow and solids
loadings as a result of intermittent industrial activity.
Adequate Primary Sedimentation. Numerous activated sludge treatment plants have been in-
stalled without primary sedimentation facilities, often leading to poor performance and many
operational headaches. The difficulties of maintaining the biological activated sludge process are
many, and the absence of primary clarifiers adds unnecessarily to the problems of handling grit,
scum, and debris-ladened raw sewage. Hourly examinations of BOD, suspended solids (SS), and
chemical oxygen demand (COD) at the Sacramento County Central Treatment Plant during the
early summer in 1973 indicated that the highly overloaded primary clarifiers (often less than 30
minutes' detention time) still managed to remove significant quantities of waste solids and signif-
icantly reduced the peak loadings tributary to the plant. Hourly peak COD loadings of over 2,000
mg/1 were always reduced to the 500-mg/l level after passing through the primary clarifiers.
Those activated sludge plants that are now without primary treatment can be upgraded by
providing reliable primary sedimentation facilities. Overall treatment efficiency and reliability could
be increased by 10 to 20 percent using this time-tested tool.
Addition of chemicals to existing primary sedimentation facilities can also result in significant
upgrading of activated sludge systems. An excellent example of the effect on an activated sludge
process of lime and ferric additions to primary sedimentation can be found in the work done on the
full-scale testing of a water reclamation system at the Central Contra Costa Sanitary District in
California. This full-scale test facility confirmed that an activated sludge system immediately fol-
lowing primary sedimentation with chemical addition is extremely stable and can consistently
maintain complete nitrification and a high degree of organic removal.2
Independent Solids-Handling System. Even with the best activated sludge process, occasional
solids upsets or the need for rapid F/M adjustments will occur and cause excessive waste-solids dis-
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charges to the solids-handling system. If the handling, treatment, and disposal facilities do not have
the capacity to accommodate these peak quantities, they will be overloaded and return a high per-
centage of the solids directly to the activated sludge process. Such recycling can only aggravate the
already upset conditions or defeat the corrective adjustments.
Each part of the solids-handling system must have sufficient capacity to absorb such shock
loadings without deterioration of its liquid return. As most activated sludge solids upsets involve
bulking, flotation is usually the most practical thickening process. Flotation thickener design should
be based on low solids-loading rates and should be provided with chemical feed capabilities to assure
maximum solids capture under the worst conditions.
When anaerobic digestion is used for solids treatment, return of digester supernatant directly
to the process flows should be avoided. If some means of lagooning or other liquid-solid storage is
not available, then such liquid should be treated for both settleable and soluble solids removal
before its return to the process flow.
The importance of the interdependence of an efficient, reliable activated sludge process
operation and a successful waste-solids processing and disposal must be fully recognized. In many
cases, an activated sludge process may be upgraded by simply improving the solids-handling system
and isolating its operation from the liquid-process stream so that neither lack of capacity nor im-
proper operation limits the amount of wasting required to maintain the most efficient F/M, SET, or
YBOD values.
Laboratory Control. For reliable, efficient operation of an activated sludge process, it is essen-
tial that provision be made for fast, accurate measurements of system loadings, efficiency, solids
levels, and biological activity. This is especially true of systems that are subjected to industrial
wastes or other varying flow or strength conditions. Even the small package facilities must be mon-
itored and operated according to measured results.
In addition, every activated sludge plant should also be equipped with a microscope of suf-
ficient power and flexibility to allow observation of micro-organism activity within the system.
Routine observations can provide the operator with information that will allow him to anticipate
problems and take steps to prevent them. Recent work has led to the conclusion that such a micro-
organism examination will detect the deterioration of a plant's SVI (caused by bulking filamentous
organisms) as long as a day before it begins to show up in the routine cylinder and solids tests. Such
action will permit corrective measures to be undertaken before upset conditions occur within the
plant.
Proper operating techniques, in combination with fast, accurate laboratory determinations of
process parameters and routine microscopic examinations of process activity, may in themselves
bring about significant upgrading of many existing activated sludge processes.
Chemical Correction Capabilities. Some activated sludge processes require chemical correction
periodically to overcome externally imposed conditions that defy any other solution. Sludge
bulking often falls in this category and can be caused by either hydraulic or biological upsets.
Hydraulic bulking is usually the result of peak wet-weather flows that overload the secondary
clarifiers. This condition causes excessive loss of solids in the effluent and is usually accompanied by
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an increase in filamentous organism activity within the activated sludge biomass. At the Ren ton
36-mgd activated sludge treatment plant, it has been found that by applying intermittent doses of
alum during periods of hydraulic overload, it is possible to reduce this loss of solids greatly and
overcome any resulting development of filamentous activity. SVFs under such conditions have been
kept below 120, and removal efficiency has remained high. The total cost for a wet season's appli-
cation averaged only $1.65 per mgd.3
Biological bulking is more complex. Several facilities in California have recently experienced
great success in controlling bulking caused by filamentous organisms with the continuous appli-
cation of chlorine to the RAS. Hydrogen peroxide has also been used for this purpose. Use of chlo-
rine for such biological filamentous control at the Sacramento County Central Treatment Plant is
documented as a case history in part II (case 4).
Use of chlorine can also control nitrifying bacteria in plants designed for carbonaceous BOD
removal only. Nitrification in the aeration tank can result in subsequent denitrification in the sec-
ondary clarifier sludge blanket. Clumps of sludge may be buoyed to the surface by nitrogen gas and
leave the clarifier by the overflow weir. Because chlorine is toxic to the nitrifying bacteria, its addi-
tion to the RAS will prevent nitrification and consequent denitrification.
The important point is not which chemical is used for such controls, but that the upgraded
activated sludge plant should be provided with the capabilities of applying the corrective procedures
if and when a bulking problem arises. Experience indicates that there are few activated sludge plants
that do not experience some bulking problems during a normal operating year.
Aeration Air Supply and Control. To maintain healthy micro-organisms in the activated sludge
system, aerobic conditions are required under all loadings. Many plants can be upgraded by simply
providing a great enough air supply, supplemental mechanical aeration, or other type of oxygen-
ation capacity. Small plants are especially affected by such limitations because they usually expe-
rience very high oxygen demands during peak flow and loading periods.
Merely providing sufficient oxygen, however, is not enough to maintain the higher levels of
reliability and efficiency demanded by today's treatment requirements. To assure the best results,
the DO level must be maintained at the best level of support for the micro-organism activity. Even
periodic drops to low DO levels can cause problems such as sludge bulking. Control tools are now
available that can provide continuous monitoring for determining and maintaining DO levels. The
Renton plant has used polargraphic DO probes for aeration air control for many years. Information
is available on the value of such controls and on the amount of maintenance required to keep the
system operable (pt. II, case 5A).
Such controls assume that variable-capacity oxygenation systems exist. As with many of the
other operating tools mentioned, such flexibility must be provided to maintain optimum DO levels
in the system. The final upgrading step to provide the DO-measuring equipment and the closed-loop
control system to automatically control the variable-capacity oxygenation system is neither expen-
sive nor complicated.
Conclusions
All too often upgrading of activated sludge plants has consisted either of simply providing
more hydraulic capacity or of changing process modes; the importance of the so-called support
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systems and equipment has been overlooked. The design engineer who truly wants to upgrade an
activated sludge plant to meet today's level of treatment reh'ability and efficiency will make sure
that he has process uniformity and flexibility, positive and independent RAS and WAS control,
adequate primary treatment, independent solids handling, efficient laboratory control, chemical
correction capability, and sufficient aeration air supply and control. If all of these tools are avail-
able, he can fully expect the plant operator to use the relevant operating parameters to produce
an effluent that will always comply with the new EPA secondary treatment effluent requirements.
TRICKLING FILTER PLANTS
Procedures for upgrading trickling filter plants fall into two main categories.
• Modifications to the existing trickling filter or operating procedures
• Provision of additional treatment in conjunction with the existing filters
Upgrading techniques can also be categorized according to whether the principal objective is
handling increased loadings or improving effluent quality. Exceptions can be cited, but internal or
operational modifications are generally used for increased loadings, and additional treatment proc-
esses (other than simple plant expansion) are used to provide improved effluent quality.
Modifications to Existing Filters
Three methods of improving trickling filter performance by modifying existing facilities are
• Use of a different filter media
• Increased recirculation
• Forced-draft ventilation
A fourth, less obvious, procedure involves review of present plant operation with the purpose of
improving performance through better maintenance, proper operating techniques, or equipment
repair. Although this last topic is not covered in this document, it should be one of the first possi-
bilities considered when faced with inadequate performance from a trickling filter plant.4
Alternative Types of Media. A means of greatly increasing the capacity of a trickling filter
plant is to replace rock media with synthetic (plastic) media. Two principal reasons account for the
increased use of plastic media in recent years.
• A large specific surface area, approximately twice that of rock media (27 ft2/ft3 for the
most commonly used material), allows greater slime surface per unit volume, which permits
a higher volumetric BOD loading than can be achieved with rock media.
• The light weight of the media permits construction of filters up to 25 feet deep. The depth
of rock filters is usually limited to approximately 5 to 6 feet.
The high loading capacities and great depth make the use of plastic media advantageous in
plant expansions where space is often limited. In addition, the high allowable organic loading makes
plastic media particularly applicable to treatment of strong industrial wastes. At Stockton, Calif.
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(pt. II, case 3), three of six existing rock filters have been converted to plastic media filters that are
designed to remove carbonaceous BOD from combined domestic and cannery wastes during a
portion of the year and oxidize carbonaceous BOD and ammonia-nitrogen during the remainder of
the year.
Principal U.S. manufacturers of synthetic media consisting of corrugated sheet modules are B.
F. Goodrich, Envirotech, Munters, and the Enviro Development Co.
Redwood slats can also be used to replace rock media. Marketed by Neptune-Microfloc, red-
wood media has many of the same advantages as plastic media. Redwood media is often used in the
Activated Biofilter, a proprietary coupled trickling filter/activated sludge process in which the RAS
is returned to the trickling filter influent line.
Upgrading through media substituion may require other plant modifications. Use of plastic
media requires a greater filter depth, which necessitates raising the height of the filter walls. In-
creased pumping capacity or a higher static head will often result, and modifications to the filter
circulation sump may be required.
Recirculation. Increasing recirculation may be another means of improving filter performance.
For lightly loaded filters, increasing circulation means increasing the contact time of the waste in
the filter, and thereby increasing BOD removal. For heavily loaded filters, particularly where high-
strength wastes occur, increased recirculation can provide dilution to maintain aerobic conditions as
well as produce increased flow through the filter to slough biological growth and prevent media
clogging.
Increased recirculation involves installing additional pumps as a minimum. Modified recircula-
tion piping, a modified circulation sump, and increased distributor capacity possibly will also be
required.
Forced-Draft Ventilation. Forced-draft ventilation can be used to prevent anaerobic conditions
in trickling filters. Usually associated with deep plastic filters and strdng industrial wastes, positive
ventilation can be used where natural draft is not adequate to provide sufficient oxygen transfer or
where dead zones occur.
Additional Treatment Processes
Providing additional biological treatment capacity ahead of or behind the existing trickling
filter can mean a substantial improvement in performance. Other types of treatment processes that
can be used with trickling filters are chemical addition and effluent-polishing filtration.
Biological Treatment. Installing a roughing filter ahead of the existing filter can both improve
performance (particularly if plastic or redwood media are used) and increase capacity. Normally, an
intermediate clarifier would not be used, and all solids would be removed in the final clarifier.
A polishing filter following the existing clarifier can be used to provide separate-stage nitrifica-
tion. Because solids production in such tertiary applications is low, the effluent can often be applied
directly to a multimedia gravity filter without resultant additional sedimentation.
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Installing an activated sludge system downstream from the trickling filter (ahead of the sec-
ondary clarifier) can greatly improve performance. This approach was used at Livermore, Calif, (pt.
n, case 2), to attain nitrification. The roughing filter used ahead of the aeration-nitrification tank
provides a reliability and an operational stability that cannot be achieved with the activated sludge
process alone.
Chemical Addition. Chemicals (aluminum and ferric salts, lime, and poly electrolytes) are being
used increasingly to supplement trickling filtration.5 Chemical addition to the primary clarifiers
will, in addition to removing most of the incoming phosphorus, substantially increase BOD and SS
removal. This technique can relieve overloaded trickling filters, or, where existing filters are lightly
loaded, may allow nitrification to take place.
Alternatively, chemicals may be added before the final clarifiers to reduce secondary effluent
BOD and SS levels. Chemical coagulants can also be used after the secondary clarifiers to improve
final effluent quality. An additional clarification process will be required in this case.
Two factors that should be carefully evaluated when chemical addition is contemplated are
wastewater alkalinity changes and solids-handling capacity.4 Addition of metallic salts may deplete
the wastewater's buffering capacity and reduce the pH to the level where biological activity is
impaired. Caustic soda may need to be added ahead of the trickling filter (for primary chemical
addition) or before effluent disposal to meet discharge requirements. Adding lime to the primary
clarifiers will raise the pH and inhibit biological activity. Recarbonation through addition of CO2
gas can be used to reduce pH.
Adding coagulant chemicals in trickling-filter plants will generate increased quantities of solids.
Further, the sludge will have different characteristics from the sludge produced previously by
primary sedimentation and trickling filtration. Care must be taken that the solids-handling,
-treatment, and -disposal facilities are adequate for the biological-chemical sludge produced.
Tertiary Filtration. Multimedia, rapid sand filtration, originally developed for water treatment,
is being used increasingly as a tertiary wastewater treatment process. Baumann and Cleasby6 have
done extensive work on the design requirements of wastewater filtration as opposed to water filtra-
tion. Deep filters, large grain sizes, and high loading velocities permit storage in the filters of a
sufficient quantity of solids to allow reasonably long filter runs. The higher influent-solids level in
wastewater treatment makes more storage capacity necessary.
Effluent quality requirements are usually not as strict for wastewater treatment as for water
treatment, allowing higher effluent turbidities and SS levels to be tolerated. Effluent BOD and SS
levels of about 5 mg/1 can be expected with effluent filtration. Chemical coagulation is often used in
conjunction with tertiary filtration.
REFERENCES
1U.S. Environmental Protection Agency, Technology Transfer, Process Design Manual for
Upgrading Existing Wastewater Treatment Plants, Cincinnati, Ohio, Oct. 1974.
2G. A. Horstkotte, Niles, D. G., Parker D. S., and Caldwell, D. H., "Full-Scale Testing of a
Water Reclamation System," J. Water Pollut. Cont. Fed., 46, 181, 1974.
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3R. E. Finger, "Solids Control in Activated Sludge Plants With Alum," J. Water Pollut. Cont.
Fed., 45, 1654, 1973.
4R. C. Brenner, Upgrading Trickling Filters, EPA AWTRL/NERC, prepared for EPA Tech-
nology Transfer Design Seminar Program, Cincinnati, Ohio, rev. Mar. 1974.
5 J. E. Laughlin, Upgrading Trickling Filter Units and Upgrading by Chemical Addition,
prepared for EPA Technology Transfer Design Seminar Program, Cincinnati, Ohio, Nov. 19-20,
1974.
6U.S. Environmental Protection Agency, Techndlogy Transfer, Wastewater Filtration: Design
Considerations, Cincinnati, Ohio, July 1974.
10
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Part II
CASE HISTORIES
CASE 1. GREENSBORO, N.C., SOUTH BUFFALO CREEK
WASTEWATER TREATMENT PLANT
The original South Buffalo Creek Wastewater Treatment Plant was built in 1931 for an ultimate
population of 37,000, an average flow of 3.25 mgd, and a wet-weather flow of 6.5 mgd. The plant
was adequate generally for the design load, except that the final settling tank, sludge-digestion tank,
and sludge-drying beds were considerably smaller than would be called for by modern practice. These
deficiencies resulted in somewhat less satisfactory treatment than ordinarily would be expected with
this type of plant.
Facilities in the original plant consisted of screens, detritors, single primary and final settling
tanks, two fixed-nozzle trickling filters, a digester, and sludge-drying beds.
By 1956 the flow to the plant had increased to an average of 4 mgd, of which 1.5 mgd were
industrial waste. The total load to the plant was equivalent to that from 52,000 people, and the
treatment efficiency was inadequate. Among the industries contributing to the plant were textile-
dyeing and -finishing plants, an abattoir, a meatpacking plant, a chemical-manufacturing plant, poultry-
processing plants, and metal-plating plants.
The treatment plant was upgraded in 1957-58 by replacement of screening and grit-removal
facilities; addition of one primary settling tank, two trickling filters, two final tanks, and one digester;
conversion of the existing final tank to chlorine contact; and replacement of the drying beds with a
vacuum-filter installation.
Between 1958 and 1964, the industrial biochemical oxygen demand (BOD) contribution to
the plant increased much more rapidly than expected. Industrial BOD rose from 35 percent to over
65 percent, and the overall plant BOD-removal efficiency fell from 90 percent to 70 percent.
To improve handling of the industrial loads one of the original fixed-nozzle trickling filters was
converted into two aeration basins in 1964-66. Conversion involved removal of media and installation
of platform-mounted mechanical aerators. The trickling filters installed in 1957 were retained as
roughing filters ahead of the activated-sludge facilities.
Since the initial conversion, BOD removals have averaged above 90 percent, and have never
dropped below 80 percent.
In 1970, to meet increased loads, the remaining fixed-nozzle filter was converted to aeration
basins using floating aerators.
The current upgrading project includes preaeration, chemical addition for phosphorus removal,
special odor-control measures, improved sludge handling, and effluent polishing with deep-bed filters.
BOD and suspended-solids (SS) removals in excess of 98 percent are expected.
Design parameters of the various plant additions are detailed in table II-l, typical plant-operating
data are given in table II-2, and capital costs of plant upgrading are outlined in table II-3. Flow dia-
grams for various stages in development of the plant are shown in figures II-l to II-5.
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Included is a possible future upgrading to provide nitrogen removal, should this be required.
This upgrading might involve conversion of the 1957 trickling filters to provide nitrification after the
activated-sludge treatment and addition of biological denitrification.
Table H^-Upgrading of South Buffalo Creek Wastewater Treatment Plant, design parameters
Item
Design parameters:
Design average flow,
mgd
Design peak flow,
mgd
Design BOD,
Ib/day
Design suspended
solids, Ib/day
Phosphorus,
Ib/day
Wastewater treatment:
Screens
Grit chambers
Preaeration tank
Primary settlingtanks
Phosphorus removal
1931 (original
plant design)
3.25
6.50
—
-
-
Mechanical bar
screen.
Two detritors.
—
Square tank.
Dimensions:
70 ft x 70 ft x
10ft SWD.
Surface area:
4,900 ft2
Overflow rate:
670 gpd/ft2
-
1957
8
16
21 ,700
18,800
-
Existing bar screen
replaced by back-raked
mechanical bar screen
with hand-raked bypass
screen.
Detritors replaced by
aerated grit chamber.
Available air supply:
50 ft3/min.
Grit removal: tubular
conveyor discharging
to grit box.
—
Old square tank retained.
New round tank (80 ft
diameter x 9 ft SWD)
added. Flow equally
divided between tanks.
Total surface area:
9,900 ft2 Overflow
rate: 800 gpd/ft2
—
1964-66
8
16
-
-
-
No change.
No change.
—
No change.
—
1970
8
16
-
-
—
No change.
No change.
_
No change.
—
1972 proposals
10.7
21.4
35,500
27,000
900
Replace mechanical
bar screen by new
heavy-duty unit.
Replace grit-removal
mechanism with
new heavy-duty
bucket-type grit-
collecting-and-
elevating mecha-
nism.
Volume: 1 .1 million
gallons. Available
air supply: 4,000
ft3/min.
Enclose primary
tanks with positively
ventilated building
to exhaust air
through odor-
control units.
Addition of facilities
to add lime, alum,
or iron salts
(depending on
results of pilot-
plant studies) ahead
of primary settling
tanks.
12
-------
Table 11-1 -Upgrading of South Buffalo Creek Wastewater Treatment Plant, design parameters-Continued
Item
Secondary
Teatment
Final settling
Chlorination
Effluent filtration .
Residue treatment:
Sludge thickening .
Sludge digestion
1931 (original
plant design)
Two square (200
ft x 192 ft x 7 ft
deep) fixed-
nozzle, stone
trickling filters.
Media volume:
540,000 ft3
One rectangular
tank (50 ft x 50
ft x 10ftSWD).
Total surface
area: 2,500ft2
Overflow rate:
1,300gpd/ft2
None.
—
—
Fixed-cover
digester 60 ft
diameter x 23ft
SWD.
1957
Square filters retained,
accepting flow from
old ordinary tank.
Loading: 15-lb BOD/
1,000 ft3 of media per
day. Two new round
filters (155 ft diameter
x 7 ft deep) added to
accept flow from new
primary tank. Media
volume 260,000 ft3
loading 36 Ib BOD/
1,000 ft3 of media per
day. New filters have
recirculation capacity
of 9 mgd.
Old final settling tank
converted to chlorina-
tion contact tank (see
below). Two new
round (1 05 ft diameter
x 9 ft SWD) tanks
added to handle total
plant flow. Total sur-
face area: 17,300ft2
Overflow rate: 460
gpd/ft2-
Contact tank volume:
25,000ft3
Chlorinator capacity:
4,000 Ib/day.
_
-
New floating-cover
digester (80 ft diameter
x 30 ft SWD) added as
primary digester.
Existing fixed-cover
digester retained as
secondary unit.
1964-66
Use of square fil-
ters discontin-
ued. Round fil-
ters operated as
roughing filters.
One square filter
converted to two
aeration basins
with combined
volume of
326,400 ft3 and
combined aera-
tion capacity of
200 hp of plat-
form-mounted
aerators.
No change.
No change.
—
-
Digestion discon-
tinued. Digesters
retained as
sludge-holding
tanks.
1970
Second square
filter converted
to provide addi-
tional two aera-
tion basins with
360 hp aeration
capacity, giving
total aeration
volume of
655,000 ft3 and
total aeration
capacity of
560 hp.
No change.
No change.
_
-
No change.
1972 proposals
No change.
No change.
No change.
Preaeration followed
by dual media fil-
tration (design
dependent on
results of pilot-
plant studies).
Flotation thickening
of activated sludge
before mixing
with primary-sludge
flotation unit. Sur-
face area: 250ft2
No change.
13
-------
Table 11-1.— Upgrading of South Buffalo Creek Wastewater Treatment Plant, design parameters—Concluded
Item
Sludge dewatering
Ultimate disposal . .
1931 (original
plant design)
Sludge-drying
beds, area:
32,600 ft2.
Trucked to
landfill.
1957
New 11 ft-6-in.x 12ft
large coil spring
vacuum filter (430 ft2
surface area) added.
Drying beds relegated
to standby role.
Trucked to landfill.
1964-66
No change.
Trucked to
landfill.
1970
No change.
Trucked to
incinerator.
1972 proposals
Recondition existing
vacuum filter and
accessories. Replace
filter media. Pro-
vide odor control
for sludge-handling
building
No change.
Table 11 -2.- Typical performance of South Buffalo Creek Wastewater Treatment Plant
Item
Characteristics of raw waste:
Flow:
Average day, mgd
Peak hour, mgd
BOD:
Average day, mg/l
Average day, Ib/day
SS:
Average day, mg/l
Average day, Ib/day
Performance of treatment units:
Average BOD removal, lb/day:b
Primary
Trickling filter
Activated sludge
Effluent filter
Total.
Average SS removal, lb/day:a
Primary
Secondary.
Effluent filter
Total
BOD loading on units (1,000 Ib per ft2):
Trickling filter
Activated sludge
Operating costs, dollars per million
gallons
1957-58
Before
upgrading
3.7
10
380
11,700
350
10,800
2,300(20)
4,700(50)
7,000(60)
4,300(40)
2,200(35)
6,500(60)
17
After
upgrading
4.5
11.5
310
1 1 ,600
240
9,100
3,000(25)
7,400(85)
10,400(90)
4,500(50)
3,200(70)
7,700(85)
b15/36
1964-66
Before
upgrading
4.9
11.5
290
1 1 ,800
250
10,200
3,000(25)
5,200(60)
8,200(70)
5,100(50)
2,500(50)
7,600(75)
b15/36
38.50
After
upgrading
6.6
16
300
16,500
260
14,100
2,500(15)
5,800(40)
6,500(80)
14,800(90)
4,200(30)
7,100(70)
11,300(80)
54
25
52.93
1970
Before
upgrading
6.8
16
390
22,200
290
16,500
3,200(15)
6,600(35)
9,000(75)
18,800(85)
5,000(30)
7,400(65)
12,400(75)
73
38
58.91
After
upgrading
8.9
18
390
29,000
300
22,300
4,400(15)
7,600(30)
14,100(35)
26,100(90)
7,800(35)
8,900(60)
16,700(75)
95
26
78.28
Proposed
After
upgrading
10.7
21
400
35,700
300
26,800
9,000(25)
8,000(30)
16,000(85)
2,000(75)
35,000(98)
13,400(50)
10,500(80)
2,300(80)
26,200(98)
103
29
aFigures in parentheses indicate percent.
bTwo-stage; see table 11-1.
14
-------
Table 11-3.—South Buffalo Creek Wastewater Treatment Plant, capital costs of upgrading
Date of upgrading
1957
1964-66
1970 .
1972 proposal
Item
Screening and degritting . .
Rehabilitation of pump and control building ....
Primary settling ....
Secondary process
Final settling .
Chlorination facilities
Sludge handling
Miscellaneous
Total, 1957 .
Modifications to secondary process.
Modifications to secondary process.
Renovation of screens and degritting equipment. .
Preaeration and odor control
Enclosure of primary tanks and odor control .
Phosphorus-removal facilities ...
Effluent filters and aerators
Sludge handling . . ...
Miscellaneous ...
Total, 1972 proposal
Capital cost
Cost at time
of upgrading
1972 cost
Dollars
45,000
150,000
65,000
330,000
135,000
25,000
300,000
150,000
1 ,200,000
200,000
100,000
110,000
365,000
160,000
800,000
330,000
60,000
730,000
365,000
2,920,000
367,000
133,000
60,000
160,000
200,000
75,000
450,000
250,000
55,000
a 1,250,000
aExclusive of engineering and contingencies.
-C7"
DRYING
BEDS
EFFLUENT
OUTFALL LINE
SOUTl70BUFFALO
CREEK
Figure 11-1. South Buffalo original plant, flow diagram.
15
-------
SLUDGE CAKE
TO
LANDFILL
EFFLUENT
OUTFALL LINE
TD
SOUTH BUFFALO
CREEK
Figure 11-2. South Buffalo, following first upgrade, flow diagram.
NFLUENT SCREENS a , . ^ PRIMARY -J TRICKLING \
—.,-.,— ...fr— DEGRITTING -( r *> SETTLING ^FILTERS 1
SEWAGE FACILITIES ^ ^ TANKS \ y
^
Q
3
/ft.r-eTnA SLUDGE .
DIGESTONU ta DFWATERING A ?
^ TANK y FACILITIES
SLUDGE CAKE
TO LANDFILL
^
>
j. AERATION fc FINAL
9 BASINS "* SETTLING ' '
SLUDGE
1
CHLORINE
CONTACT
TANK
I
EFFLUENT
OUTFALL LIN
TO
SOUTH SUFFA
CREEK
Figure 11-3. South Buffalo, following second upgrade, flow diagram.
PREAERATION
TANK
PRIMARY
SETTLING
, OPTIKAL
EQUALIZATION
AERATION
OASINS
FINAL
SETTLING
TANKS
EFFLUENT
AERATORS
SLUDGE CAKE TO
NORTH BUFFALO
FOR INCINERATION
EFFLUENT
OUTFALL LINE
TO
SOUTH BUFFALO
CREEK
TO
I NFLUENT
BACKWASH WASTE
Figure 11-4. South Buffalo, current upgrade, flow diagram.
16
-------
I FUTURE BIOLOGICAL ]
OENITRIFICATION
IF REOUIRED I
—-A——J
r--«—i
| FUTURE |
I NITRIFYING i 1 '
I FILTERS f~~*
I IF REQUIRED I
. 1
SCREENS fl j. v ^ PREAERATION ^_fc PRIMARY , . .. ^ AERATION fc. FINAL H-D4 '
A
W DEGRITTING "~\_J TANK SETTLING ^ BASINS SETTLING | ^
y L
/^x^^
T0— ODOR REMOVAL
INFLUENT OIVERS10N ATMOSPHERE UNITS u
SEWAGE CHAMBER ^ §
1
TO TRUN
\ EXHAUST «
\ AIR
^—^. \
/ SLUDGE \ SLUDGE ,
Y TANK J FACILITIES
SLUDGE CAKE TO
NORTH BUFFALO
FOR INCINERATION
r
' TO
^^ ^-^s^ EFFLUENT
T AERATORS
SLUDGE !_ — L
\s-> ^
,, BACKWASH ^ EFFLUENT
THICKFNER I T
\ i CONTACT
, ^-^ TANK
1
EFFLUENT
OUTFALL LINE
TO
SOUTH BUFFALO
CREEK
Hn, n,MO ^ BACKWASH WASTE
K -SEWER INFLUENT"1 TANK
FOR TREATMENTAT
DOWNSTREAM FACILITY
Figure 11-5. South Buffalo, future upgrade, flow diagram.
CASE 2. LIVERMORE, CALIF., WASTEWATER TREATMENT PLANT
The Livermore Wastewater Treatment Plant was built in 1958 to provide secondary treatment
for a domestic wastewater with an average dry weather flow of 2.5 mgd. The plant included prelimi-
nary treatment and roughing filters followed by 30-day oxidation ponds. Sludge was digested, then
dewatered in lagoons. Effluent was discharged to an intermittently flowing drainage ditch. See figure
II-6 for plant flow diagram and table II-4 for plant design data. Design effluent limitations were 40
mg/1 BOD5 and 40 mg/1 SS. The plant achieved effluents with 45-50 mg/1 BOD5 and 45-50 mg/1 SS,
with no significant nitrification.
The plant was enlarged in 1967 to provide an increased degree of treatment for an average dry
weather flow of 5 mgd. All existing structures were incorporated in the upgraded plant. The new
plant includes preliminary treatment, roughing filters, activated-sludge secondary treatment, and pre-
and postchlorination. The former oxidation ponds have been converted to the emergency holding
ponds. The existing sludge-disposal system was expanded. Part of the effluent (about 20 percent)
is used to irrigate a municipal golf course, an airport, and adjacent farmland. See figure II-7 for plant
flow diagram and table II-5 for plant design data. Effluent limitations (Regional Water Quality Con-
trol Standards) were 20 mg/1 BOD5, 20 mg/1 SS, 1 mg/1 grease, 5 MPN per 100 ml conforms (using
5-day median). The plant easily meets these standards (see table II-6 for operating data). Complete
nitrification must be provided to achieve the low effluent baterial count (if nitrification is not
complete, chlorine demand is inordinate because of chloramine formation, and inadequate free
chlorine remains to effect the requisite high levels of disinfection).
The initial plant had a capital cost of $900,000 (1957 dollars), and an annual operating cost of
$102,000 (1968 dollars). Capital cost of the expansion was $1,300,000 (1968 dollars). Annual oper-
ating cost is $270,000 (1972 dollars). Unit capital costs of upgrading are $520,000 per mgd of
increased capacity, or $186 per pound per day of additional BOD5 removal.
17
-------
PLANT BYPASS
__
DIVERSION STRUCTURE FOR
FUTURE CHLORINE CONTACT TANK
METERING
! FLUMES
-*
-*
PREAERATION
SECONDARY
SEDIMENTATION TANK
PRIMARY
A
J
SECONDARY RETURN i
CONTROL VALVE
,/
r
*-H
FILTER CIRCULATION
PUMPS (2)
EFFLUENT T^j
ARROYO
LAS POSITAS
SOUTH TRUNK
GAS CIRCULATION PUMPS (2)
Figure 11-6. Livermore Wastewater Treatment Plant, flow diagram, 1958.
Table 11-4.—Livermore Wastewater Treatment Plant design data, 1958
Design flow, mgd:
Average dry weather
Maximum dry weather
Peak storm rate.
Design loadings:
Population equivalent
SS, Ib/day:
Per capita
Design total.
BOD, Ib/day:
Per capita
Design total.
Preaeration tanks:
Number3
Width, feet
Length, feet
Average water depth, feet
Detention time, hours (one tank).
Air supplied per tank, ft3/minb
Air supplied, ft3/gal
Maximum hydraulic capacity, mgd.
2.5
4.5
10
28,900
0.15
4,500
0.20
6,500
2
19
38
11.7
0.6
200
0.12
10
18
-------
Table \\-4.-Livermore Wastewater Treatment Plant design data, 1958-Concluded
Primary sedimentation tank:
Width, feet 19
Length, feet 124
Average water depth, feet 9
Effluent weir length per tank, feet . . • • • 164
Detention time, hours. ... ... 1-5
Mean forward velocity, ft/min. . ... 1-4
Overflow rate, gal/ft2/day at average dry weather flow 1,050
Maximum hydraulic capacity of tank, mgd 10
Assumed removal, percent:
SS 60
BOD ... 35
o
Secondary sedimentation tankc ( )
Trickling filter:
Inside diameter, feet ... . ... 110
Average depth of filter media, feet. ... . ... 4.25
Size of filter media, inches. ... . 2-4
Net area of filter surface, acres . . 0.218
Volume, acre-ft . .... . 0.92
Circulation ratio to average design dry weather flow 1.5-3
Loading:
Rate per filter, mgd 7.5
Rate per surface acre, mgd. . . 34.5
BOD, Ib/day/acre-ft . . . . 4,600
Assumed removal, percent, filter plus secondary sedimentation:
SS 60
BOD . 75
Oxidation ponds:
Total area, acres. . . . . 37
Average water depth, feet . . 6
Detention time, days. . 30
BOD loading, Ib/acre/day . .... 28
aOne preaeration tank used to aerate trickling-filter effluent prior to secondary sedimentation.
b200 ft3/min supplied to primary aeration tank and 100 ft3/min supplied to secondary aeration tank.
°Details as for primary, except removals.
19
-------
I RR IGAT I ON
PUMP. STATION
I I
GAS CIR
PUMP
>
SLUD3 E
SLUDGE
1
+
Figure 11-7. Livermore Wastewater Treatment Plant, flow diagram, 1968.
Table II-5.—Livermore Wastewater Treatment Plant, design data, 1968
Design flow, mgd:
Average dry weather
Maximum dry weather
Peak storm rate
Design loadings:
Population equivalent
SS, Ib/day:
Per capita
Total
BOD, Ib/day:
Per capita
Total
Preaeration tanks:
Number
Width, feet
Length, feet
Average water depth, feet
Detention time, hours .
Air supplied per tank, ft^/min.
Hydraulic capacity, mgd
Primary sedimentation tanks:
Number
Width, feet
Length, feet
Average water depth, feet
Effluent weir length per tank, feet.
Detention time, hours .
Mean forward velocity, ft/min.
Overflow rate, gal/ft2/day at average dry weather flow
Hydraulic capacity, mgd
5
10
18
62,500
0.20
12,500
0.20
12,500
2
19
38
11.7
0.6
150
10
2
19
124
9
164
1.5
1.4
1,050
10
20
-------
Table \\-5.-Livermore Wsstewater Treatment Plant, design data—Continued
Primary treatment:
Assumed SS reduction, percent
SS reduction, Ib/day
Assumed BOD reduction, percent
BOD reduction, Ib/day
Trickling filters:
Number
Inside diameter, feet
Average depth, filter media, feet
Size of filter media, inches
Net area of filter surface, acres
Volume, acre-ft per filter
Circulation ratio to average dry weather flow.
Loading:
Rate per filter, mgd
Rate per surface acre, mgd
BOD, Ib/day/acre-ft
Assumed BOD removal, percent
BOD reduction, Ib/day , . ...
Activated sludge aeration tanks:
Number . .
Average water depth, feet . .
Width, feet
Length, feet. .
Detention time, hours, based on raw sewage flow
Air supplied, ft3/lb BOD removed, ft2/min
Volume per tank, ft3 . ...
Volumetric loading, Ib BOD per 1,000 ft3 . .
Return sludge, percent
60
7,500
35
4,400
2
110
4.25
2-4
0.218
0.92
1.5-3
7.5
34.5
4,400
50
4,000
2
15
30
160
5.2
1,200
72,000
28
10-100
Secondary sedimentation tanks:
Number
Diameter, feet ...
Side water depth, feet..
Detention time, hours, based on average dry weather flow
Overflow rate, gal per ft2 per day at average dry weather flow
Activated sludge treatment:
Assumed SS reduction, percent
SS reduction, Ib/day . .
Assumed BOD reduction, percent
BOD reduction, ib/day
Overall plant performance:
Assumed SS reduction, percent
Assumed BOD reduction, percent
Effluent SS, mg/l .
Effluent BOD, mg/l
1
90
12
2.75
787
SS
4,400
S6
3,500
96
96
15
15
•21
-------
Table 11-5.—Livermore Wastewater Treatment Plant, design data, 1968—Concluded
Chlorine contact tank:
Volume, ft3 . . .
Detention time at average dry weather flow, hours
28,000
1
Table 11-6.—Livermore Wastewater Treatment Plant, operating data, 1971a
Item
BOD, mg/l:
Mean
High
Low
SS, mg/l:
Mean
High
Low
Ammonia N, mg/l:
Mean
High
Low
Nitrate N, mg/l:
Mean
High
Low
Grease, mg/l:
Mean
High
Low
Coliforrns, MPN per 100 ml:
Mean
High
Low
Raw
influent
213
244
167
229
261
187
Primary
effluent
127
156
106
79
98
60
40.7
51.1
33.8
Trickling-filter
effluent
93
129
46
110
137
74
32.6
44.4
22.5
Secondary
effluent
9.9
16.9
5.2
22
31
15
1.68
6.73
.46
18.5
20.2
16.6
Plant
effluent
7.3
12.4
3.1
13
19
8
.14
1.3
<.1
21.5
30
14
.23
.46
.15
2.5
b7
.4
aMonthly average. If value not given, item not tested for.
bWhen coliform count exceeded 5 MPN per 100 ml, flow diverted to emergency holding pond. Discharge not resumed until
count of less than 5 achieved.
CASE 3. STOCKTON, CALIF., REGIONAL WASTE WATER
CONTROL FACILITY
To receive increased flows and meet new requirements established by the California Central
Valley Regional Water Quality Control Board, the Stockton, Calif., Regional Waste Water Control
22
-------
Facility is currently being upgraded in several ways. Improvements include the construction of three
plastic media trickling filters (fig. II-8), each 166 feet in diameter and 21.5 feet deep. These replace
three of six rock filters, each 166 feet in diameter and 4.2 feet deep. The filters operate in two
modes.
• During the fruit and vegetable canning season (approximately July through September)
when the organic loading is high, the filters oxidize carbonaceous matter only with an
expected 5-day BOD removal of 70 percent.
• During the noncanning season when the organic loading is low, approximately 90 percent of
the carbonaceous BOD is removed, and ammonia-nitrogen is converted to the nitrate form
(nitrified).
The new water quality requirements for the Stockton plant include a provision that restricts
the total nitrogen concentration in the receiving water to less than 3 mg/1. During the summer
canning season, the various forms of nitrogen, primarily ammonia, will be substantially removed by
oxidation ponds through conversion to algal cells with subsequent algae removal in a tertiary facil-
ity. During the noncanning season, when the algal activity level is lower, the nitrate formed in the
trickling filters will be converted to nitrogen gas through microbial denitrification in the ponds.
During the transition periods between the two seasons, breakpoint chlorination will be used for
ammonia removal. A flow diagram for the upgraded Stockton plant is shown in figure II-9 and
design data are shown in table II-7.
Plant Modifications
Conversion of the existing rock media filters to plastic media involved several additional
changes at the plant. To enclose the plastic media, the trickling filter walls were raised from a height
of 5.0 feet to 27.5 feet. To avoid excessive weight on the existing walls and foundation, open-block
construction was used rather than poured concrete. Forced-draft ventilation was used in the up-
graded filters; this approach required enclosing the existing effluent channels and air vents along the
bottom of the filter walls to insure that the air introduced into the filter underdrain area would be
contained and directed up through the filter media. As part of the forced-draft ventilation system,
foul air will be delivered from the headworks (as a part of later modifications) and directed through
the trickling filter for deodorization. A new filter underdrain system was installed, with the bottom
of the plastic media approximately 3 feet above the floor of the filter.
The higher flow capacity of the modified trickling filters required additional modifications in
the form of larger distributors and increased recirculation pumping capacity. Trickling filter influent
supply pumps were installed because, although the previous rock filters had been fed by gravity, the
height of the plastic filters required pumping. The trickling filter distribution structure, which
receives primary effluent, recycles trickling filter effluent, and delivers trickling filter effluent to the
secondary clarifiers, had to be modified substantially to handle the increased flows and the com-
bined operation of rock and plastic media filters. Finally, a second pipeline from the filter distribu-
tion structure to the secondary clarifier distribution structure was constructed.
Plant Performance
The Stockton plant has been operating with three plastic media filters and three rock media
filters since 1973. Table II-8 shows BOD and SS removal for canning and noncanning seasons for
23
-------
Figure 11-8. Plastic-media trickling filters (background), constructed using foundations and structures of existing
rock-media filters (foreground).
RECIRCULATION
POND
CIRCULATION
ML
FLUENT
iPOSAL
DECHLORI-
NATION
CHLORINATION
Figure 11-9. Stockton Regional Waste Water Control Facility flow diagram.
2-1
-------
Table \\-7.-Stockton Regional Waste Water Control Facility: Preliminary, primary, and
secondary treatment, design data
Parameter
Value
Flow, mgd:
Noncanning season:
Average dry weather flow (ADWF) 23
Peak storm rate 60
Canning season:
Maximum month 58
Peak rate • • 75
Loadings, 1,000 Ib/day:
BOD:
Noncanning season, maximum month 54
Canning season, maximum month 236
SS:
Noncanning season, maximum month 31
Canning season, maximum month 167
Preliminary treatment:
Bar screens: Number 3
Grift channels:
Number 6
Velocity at 64 mgd, ft/sec 1.4
Metering flumes:
Number 6
Throat width, feet 2.0
Hydraulic capacity, each, mgd 20
Raw sewage pumps:
Number 4
Total capacity, mgd 116
Primary treatment:
Rectangular tanks:
Number 4
Width, feet 37
Length, feet 141
Depth, feet 15
Square tanks:
Number 2
Width/length, feet 70
Depth, feet 14
Detention time, all tanks, hours:
Noncanning season, ADWF 3.4
Canning season, maximum day 1.2
Overflow rate, all tanks, gpd/ft2:
Noncanning season, ADWF 800
Canning season, maximum day 2,200
Removal, percent:
Noncanning season:
BOD 40
SS 65
25
-------
Table \\-l.-Stockton Regional Waste Water Control Facility: Preliminary, primary, and
secondary treatment, design data—Concluded
Parameter
Value
Canning season:
BOD 20
SS 55
Secondary treatment:
Rock-media trickling filters:
Number 3
Diameter, feet 166
Media depth, feet 4.2
Total volume, 1,000 ft3 270
Total hydraulic capacity, mgd 30
Plastic-media trickling filters:
Number 3
Media depth, feet 22
Total volume, 1,000 ft3 1,430
Total hydraulic capacity, mgd 72
Total volume, rock and plastic media, 1,000 ft3 1,700
Recirculation pumping capacity, mgd 76
Unit loading, Ib BOD per 1,000 ft3/day:
Noncanning season, ADWF 20
Canning season, maximum month 110
Secondary sedimentation tanks:
Number 4
Diameter, feet 100
Side water depth, feet 12
Detention time, hours:
Noncanning season, ADWF 2.9
Peak storm rate 1.1
Canning season, maximum day 1.0
Overflow rate, gpd/ft2:
Noncanning season, ADWF 700
Peak storm rate 1,900
Canning season, maximum day 2,100
Secondary treatment performance:
Noncanning season:
BOD removal, percent 90
Effluent BOD, mg/l 17
Effluent SS, mg/l 20
Canning season:
BOD removal, percent 70
Effluent BOD, mg/l 120
Effluent SS, mg/l 35
1974 and 1975. Trickling filter BOD removals averaged 80 percent for the canning season and 89
percent for the noncanning season, based on primary effluent BOD concentrations of 410 and 229
mg/l, respectively.
26
-------
Table 11-8.— Stockton Regional Waste Water Control Facility, performance summary
Loading
condition
and
year
Canning season:13
1974
1975
Average
IMoncanning season:c
1974
1975 . .
Average
Flow,
mgd
38.2
38.3
38.2
17.1
15.3
16.2
BOD, mg/l
Second-
Raw Primary ary
influent effluent effluent
472 413 89
529 407 107
500 410 98
371 257 43
303 200 29
337 229 36
Trickling-
filter
loading
Ib BOD/
1,000ft3/
day3
79
78
78
22
15
18
BOD removal, percent
Primary
Secondary plus
secondary
78 81
74 80
76 80
83 88
85 90
84 89
SS, mg/l
Raw Primary Secondary
influent effluent effluent
488 88 53
510 73 60
499 80 56
220 60 32
243 52 32
232 56 32
Primary
olus
^r i u»
secondary
SS removai,
percent
89
88
88
85
87
86
to
-J
aVolume includes three rock-media plus three plastic-media filters.
bAugust and September.
GJanuary-June, November, and December.
-------
Secondary effluent concentrations in table II-8 represent the combined performance of rock
and plastic media trickling filters. More pertinent data on plastic media trickling filtration were
developed in 1972, when a pilot study was undertaken at Stockton to test the (then) proposed
trickling filters.1 A pilot trickling filter was operated to simulate three full-scale filters receiving
Stockton's waste water, with the purpose of determining BOD removals and nitrification efficiency.
Figure 11-10 shows BOD removal versus loading for the pilot filter under loadings ranging from
about 10 pounds BOD per 1,000 ft3/day to over 200 pounds BOD per 1,000 ft3/day. Observed
removals agreed quite well with design removals; 90 percent at 25 pounds BOD per 1,000 ft3/day
during the noncanning season and 70 percent at 135 pounds BOD per 1,000 ft3/day during the
canning season.
Because the plastic media trickling filters are intended to remove ammonia during the non-
canning season, information was sought during the pilot study on the nitrification efficiency of the
filters. Table II-9 presents pilot study nitrification results for two loading conditions: light organic
loading, 14 pounds BOD per 1,000 ft3/day, and the design organic loading, 22 pounds BOD per
1,000 ft3/day. Effluent ammonia-nitrogen concentrations averaged 1.0 mg/1 for the former loading
and 2.0 mg/1 for the latter. Organic nitrogen removal was low, reflecting the short contact times in
the trickling filter.
Cost and Schedule
The construction cost for modifying the Stockton secondary treatment facilities was about
$3,700,000, and included converting three filters to plastic media, modifying the trickling filter
distribution structure, and adding to piping and pumping capacity. Construction was started in
January 1973, and the first of the three plastic media filters was placed in operation in July 1973
before the start of the canning season. The other two filters were placed in operation near the end
of 1973.
CASE 4. SACRAMENTO, CALIF., CENTRAL WASTEWATER
TREATMENT PLANT
In early June 1973, the County of Sacramento, Calif., requested that the joint venture of the
consulting engineering firms of Dewante and Stowell and Brown and Caldwell determine what could
be done to improve the operation of its Central Wastewater Treatment Plant. This 25-mgd,
complete-mix activated sludge plant, which discharges its effluent into the Sacramento River, had
been unable to meet disposal requirements during the spring of 1973 at flows of less than 15 mgd,
and had indicated a complete inability to handle seasonal wastes from the local canning industry
during the previous years. The major concern in the early summer of 1973 was whether the can-
neries would be forced to curtail their 1973 operations because of the County's inadequate sewage
treatment capabilities.
The description of the problems involved in the activated sludge process, the modifications
undertaken for their correction, the methods employed to complete these modifications before the
canning season, and the operational results of the corrections provide insight into the design and
operation of a successful activated sludge process. The experience related herein provides an oppor-
tunity to observe the reaction of a major activated sludge treatment plant to process changes. It
presents the designer and operator with conclusions that have direct applicability to other full-scale
situations.
28
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100
Q)
a
<
>
O
cc
LO
a
o
ca
80 -
60
40
20
1—I 1—I 1—I"
NONCANNING SEASON
DESIGN VALUE
I—I—r
CANNING SEASON
DESIGN VALUE
Shaded points indicate
nitrification suppressed.
Curve drawn for carbonaceous
BOD removal.
_L
_L
_L
0 20 40 60 80 100 120 140 160 180 200 220
ORGANIC LOADING, Ib BOD5 per 1,000 ft3/day
Figure 11-10. Stockton pilot trickling-filter performance.
Description of Plant
The Central Waste water Treatment Plant began operating in 1960 as a trickling filter plant
with a design capacity of 8 mgd. In 1964, the plant was upgraded when two oxidation ponds for
further treatment of the plant effluent were installed. The plant was expanded in 1969 to its
present capacity and converted to the complete-mix activated sludge process.
On July 1, 1972, one-half of the flow of County Sanitation District No. 1 was diverted from
the city of Sacramento main plant to the central plant. This flow included approximately 4 mgd of
a 5-day-per-week industrial waste, about 23,000 pounds per day (based on dry-solids) of digested
primary sludge and waste activated sludge transferred from other plants within the county's system,
and up to 8 mgd of seasonal cannery wastes. In 1973, these cannery wastes were found to include
over 7 tons per day of mostly inert solids (mud). Much of this mud was infected with filamentous
Thiothrix organisms, which caused bulking in the activated sludge process. The 1972 diversion in-
creased the plant's average annual flow to almost 15 mgd, and marked the beginning of many
operational problems.
Table 11-9.— Steady-state nitrification results, Stockton pilot trickling-filter study
Date
Oct. 23, 1972 to Nov. 21,
1972
Nov. 27, 1972 to Dec. 13,
1972
Loading,
Ib BOD/1,000
ft3/day
14
22
Concentration, mg/l
Influent
NH3-N
16.5
17.5
TKNa
27.8
28.9
Effluent
NH3-N
1.0
2.0
TKNa
9.9
11.0
Removal, percent
NH3-N TKNa
94 64
89 62
aTKN = total Kjeldahl nitrogen = ammonia-nitrogen plus organic nitrogen.
29
-------
Treatment at the central plant in June 1973 consisted of primary sedimentation in two clar-
ifiers, each 100 feet in diameter and 8 feet deep; carbonaceous oxidation in four 120,900-ft3,
complete-mix activated sludge aeration bays; secondary sedimentation in four clarifiers, each 115
feet in diameter and 12.5 feet deep; and effluent polishing (as required) by the two 14-acre tertiary
oxidation ponds. Chlorination was provided for pretreatment odor control and posttreatment disin-
fection. The chlorination system was not enlarged in 1969, but its capacity and flexibility were
upgraded during the summer of 1973 to provide additional treatment capacity and to allow use of
chlorine for bulking control with the return activated sludge system.
Operations Before Modifications
In May 1973, the central plant treated 13.8 mgd with an influent concentration of 297 mg/1
BOD and 321 mg/1 SS. Plant effluent for the month averaged 27 mg/1 BOD and 49 mg/1 SS, for a
90.2- and 84.7-percent removal efficiency, respectively. More important, daily effluent concentra-
tions for BOD and SS exceeded 90 mg/1 three times, and the monthly effluent concentration for SS
exceeded 30 mg/1. SVI's for the month averaged 360 mg/1. Effluent variability made it impossible to
maintain reliable disinfection. On June 4, 1973, the State regulatory agency responsible for mon-
itoring plant effectiveness indicated concern over plant operations and expressed a determination to
set new waste discharge requirements that would necessitate immediate improvements.
Major Problems With Existing Operations
Three major problem areas were apparent. These were
• The unstable activated-sludge process
• The inadequate solids processing and disposal system
• The unknown magnitude of industrial waste loadings
Except to note how these last two items relate directly to the operation of the activated sludge
process, this case history deals only with the successful upgrading of the activated sludge system.
Activated Sludge Process Problems
Six problems affected the stability of the activated sludge system.
• Inability of plant aeration and settling components to operate as a single system.
• Inadequate control of return activated sludge (RAS) rates
• Inability to maintain regulated waste activated sludge (WAS) rates
• Lack of chemical feed facilities to improve mixed liquor settling and control filamentous
growths
• Difficulty in maintaining sufficient dissolved oxygen (DO) in the aeration tanks
• Variability of mixed liquor flows to final clarifiers
The magnitude of each problem was increased by the interrelationship between the process variables
involved.
30
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Single-System Operation. Figure 11-11 presents a schematic layout of the activated sludge
process flow system at the central plant as of June 1973. As the primary effluent flowed into the
aeration bays, distribution was controlled only by the hydraulics of the Y channel and aeration bays
and the position of the inlet slide gates. Once the primary effluent was distributed, there was no
way, as long as all facilities were in service, for this distribution to be changed or for any inter-
mixing to take place. This was true regardless of the position of the H channel isolating slide gates.
In addition to the isolation caused by the hydraulics of the process flows, the RAS flow from
each secondary clarifier was returned only to its related aeration bay (see fig. 11-12). This system
assured that each aeration-bay /secondary-clarifier combination operated as its own separate acti-
vated sludge process. As a result of this loading irregularity and the resulting nonuniformity of the
process, the plant operators were faced with the task of operating not one, but three or four acti-
vated sludge processes depending on the number of bay-clarifier combinations in service. This was a
major obstacle to efficient operation.
RAS Control. In June 1973, RAS flow rates between secondary clarifiers and aeration bays
(fig. 11-12) were controlled by 12 airlift pumps, 3 in each bay. These pumps were designed to
operate directly in the mixed liquor within each bay. The pump discharge weir was located 6 inches
below the nominal working water surface of the bay. A sliding splash cover provided both backflow
protection when the pump was out of service and the diversion necessary to direct the flow out into
the bay when the pump was in operation. No method had been provided to measure the actual
quantity of material being pumped through the system. Thus, there was no way the operator could
really know the RAS flow. Further, the common air supply meant that whenever the aeration air
supply was changed to meet the oxygen requirements within an aeration bay, the airlift pumping
rate was also changed.
WAS Control. June 1973 WAS flow rates were designed to be controlled by adjustable,
downward-opening slide gates in a common sludge control box (figs. 11-12 and 11-13). Side streams
from each sedimentation tank RAS system were piped to individual compartments within the
control box. Each compartment was provided with an isolating inlet mud valve and an adjustable
BOTTOM OPENING
-INLET SLIDE GATE
(TYPICAL OF 12)
-Y CHANNEL
-PRIMARY EFFLUENT
T T 7
i
r-OVERFLOW WEIRS
T T T
AERATION
2
(TYP EACH BAY)~7
g>«3 r>-j c><]
BAYS
3
C*4 t-a fc»d
4
MIXED
ISOLATING
MUD VALVE
(TYP OF 4 )
— H CHANNEL
-MIXED LIQUOR
^ISOLATING
SLIDE GATE
( TYP. OF 3 )
-MIXED LIOUOR
SECONDARY EFFLUENT
Figure 11-11. Schematic layout, Sacramento process flow system.
31
-------
-RAS AIR LIFT PUMP (TYP. OF 12)
6" (15.24 cm) WAS METER
MAX CAP. 2500 GPM( 157.8 I/sec)
WAS TO GRAVITY THICKENERS
Figure 11-12. Schematic layout, solids flow system.
AERATION
AIR -3
PRIMARY
EFF LUENT
Y
CHANNEL
U
r— I9.O ft ( 5.79 m)
r_ _ WATER SURFACE
•RAS LIFT PUMP
(TYP OF 12)
AERATION
BAY
RAS
MIXED
L IOUOR
H
CHANNEL
MUD
VALVE
(TYP. OF 4
-SLUDGE CONTROL BOX
^EFFLUENT LAUNDER
I 4.33
(4.3
^ADJUSTJABLE
SLIDE
GATE
(TYP OF 4)
ft
m)
J
f
— 16,0 ft (4.88m
—=^. WATER
~~ "" SURFACE
SECONDARY
CLARIFIER
^-HYDRAULIC
/ COLLECT ION
ARM ( 2 / TANK)
RAS
MIXED .
L IOUOR
Figure 11-13. Hydraulic profile, waste activated sludge removal system.
32
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slide gate. Slide gate discharges were directed to a common control box sump from which two
variable speed WAS pumps took suction. WAS pump speed control was regulated by the WAS level
in the control box sump. A 6-inch, 2,500-gal/min capacity, magnetic flowmeter was provided to
record the WAS pump discharge flow.
WAS side streams from the RAS systems could not be stabilized, and consequently waste rates
were impossible to maintain. The hydraulic section shown in figure 11-13 indicates how this problem
occurred. The hydraulic head in the sludge control box varied in relationship to the quantity of
flow being pumped through the RAS system. The friction head loss through the secondary clarifier
hydraulic collection arms was said to approach 1.5 feet at a 100-percent RAS flow rate. With the
additional pipeline friction involved, it is probable that at the 100-percent rate the head losses to
the control box exceeded the 1.67-foot hydraulic limitation necessary to assure WAS flow over the
fixed bottom lip to the adjustable slide gate opening.
These conditions, when coupled with the difficulty in determining the rate of RAS flow and in
maintaining its stability, made it impossible to keep a uniform WAS flow rate to the gravity thick-
eners. This, in turn, made it impossible to determine the most efficient way to operate these
thickeners. Consequently, the solids treatment and disposal systems were frequently overloaded
with dilute waste activated sludge, and the solids retention time within the activated sludge process
was uncontrollable.
Chemical Feed Facilities. Except for a minor iron supplement system, no activated sludge
chemical feeding facilities were provided at the central plant. By early June, however, a contract
had been awarded for the installation of a sludge-bulking-control chlorination system. This work,
however, was part of a major modification to the plant's chlorination system and was not scheduled
to be in operation until the end of August 1973.
Aeration Air Control. Figure 11-14 shows the schematic layout of the air distribution system to
one of the four aeration bays. To adjust the air feed to the aeration bays, it was necessary to
operate 32 air distribution pipeline valves (8 per bay) and from 4 to 12 RAS airlift pump supply
control valves (up to 3 per bay). Unless all valves were adjusted at the same instant, readjustment
for a new air feed rate required several readjustments for each valve. Plant operators reported that it
was not unusual for them to spend up to 4 hours readjusting the entire system when such a feed-
RAS AIR LIFT
(TYP 3/BAY)
Y CHANNEL AGITATION AIR
7
AERATION AIR PIPELINE
SPARGER
DIFFUSION
HEADER
(TYP. 24
PER BAY)
AIR DISTRIBUTION
PIPELINE (TYP 8 PER BAY)
Figure i 1-14. Schematic layout, aeration air distribution.
-------
rate change was required. As the plant loads often changed significantly over a period of several
hours, such feed-rate changes were required often to attempt to operate all systems at optimum
efficiency. That the plant was operating as separate, multiple activated sludge plants compounded
this problem.
Agitation air for the Y channel was also supplied by the aeration air system and, consequently,
was also subject to the variations described above. The Y channel agitation air diffuser was installed
about 2 feet off the bottom of the channel and the area beneath the diffuser seemed to be always
filled with septic debris. The H channel required no agitation air because it usually operated at a
minimum water level.
Final Clarifier Operation. The major immediate problem affecting the plant's effluent quality
was the inability of the circular secondary clarifiers to retain the suspended solids (fig. 11-15).
Several times a week significant quantities of activated sludge were lost with the effluent. This loss,
when coupled with the lack of RAS and WAS control, made accurate control of system solids im-
possible. The clarifiers were sized for a 600-gal/day/ft2 surface loading at average design loadings of
25 mgd. This is a conservative loading and indicates that the clarifier upsets were either the result of
solids overloading within the activated sludge process or that individual clarifiers were receiving
much higher surface loadings as a result of hydraulic imbalance caused by variability of mixed-
liquor flows. These flows, as indicated earlier, were controlled by the aeration tank influent distri-
bution system and could be radically altered by the unequal operation of the RAS system.
Figure 11-15. Secondary Clarifier, June 7, 1973. (Note solids bulking to the surface and overflowing with the
effluent on the outer weir surface. Inner weir surface handling only clear effluent. Effluent suspended solids for
that day averaged 79 mg/l.)
-------
Process Modifications and Immediate and Subsequent Operation
The need to solve the plant's problems before the canning season started dictated a very tight
time schedule and mandated success on the first attempt. Fortunately, most of the modifications
were interrelated and, therefore, one solution often helped to solve more than one problem.
Because the central plant is scheduled to be abandoned when the new regional system is completed
in 1980, any modifications have to last slightly more than 7 years.
Schedule and Cost. Actual construction of the modifications to the activated sludge system
commenced with the installation of the H-channel equalizing weirs on July 2, 1973, and ended
when the entire plant was placed on a new mode of operation on July 28, 1973. Estimates indicate
that the work cost less than $175,000.
Single-System Operation. To assure that the entire plant operated as a single activated sludge
system, it was necessary to have the primary effluent and RAS mixed before distribution to the
aeration bays and to have all secondary clarifier RAS flows thoroughly intermixed before their rein-
troduction into the flow stream. How these criteria were met is shown in figures 11-16 and 11-17.
Aeration bay 1 was transformed into an RAS reaeration bay with new RAS pumps arranged to
discharge from each secondary clarifier to the reaeration bay through a common trough. Mixed
liquor overflow weirs for aeration bay 1 were blocked and the RAS level in the bay forced to rise
until sufficient head was developed to allow the intermixed reaerated RAS to flow out into the
primary effluent in the Y channel via the bay's three bottom-opening inlet gates. These gates thus
became outlet gates. Normally only the gate nearest to the Y-channel intake operates, thereby
assuring maximum mixing with the primary effluent. However, high plant flows or excessive RAS
flow rates have occasionally required all three gates to be opened. Additional gates are opened
whenever the RAS reaeration bay level becomes excessively high. To assure minimum Y-channel
water levels at higher flows, it was necessary to remove the overflow slot weirs from the remaining
complete-mix aeration bays, thereby lowering their normal operating level about 6 inches.
BOTTOM OPENING
OUTLET SLIDE GATE;
(TYPICAL OF 3 )
MIXED
LIOUOR
1
BOTTOM OPENING
INLET SL IDE GATE
(TYPICAL OF 9 )
Y CHANNEL
-f~-T~-
RAS
REAERATION BAY
1
OVERFLOW SLOTS BLOCKED
T T
CONTACT
2
f T T
AERATION
3
OVERFLOW SLOTS OPEN (TYP EACH CONTACT
T T
BAYS
4
BAY)
VFLOW EQUAL 12 ING
OVERFLOW WEIR
(TYP OF 4 )
H CHANNEL
z ISOLATING
SLIDE GATE
REMOVED
(TYP OF 3
SECONDARY EFFLUENT
Figure 11-16. Schematic layout, revised process flow system.
35
-------
RAS COLLECTION
TROUGH
,-10" (25.4 cm) PVC RAS PUMP DISCHARGE
j( PIPING
VARIABLE
SPEED
RAS PUMPS
(TYP OF 8)
SLEEVE PROVIDED
TO ISOLATE SUCTION
FROM TANK CONTENTS
(TYP OF 8)
RAS AIR
LIFT
PUMP
(TYP OF 4)
SLUDGE BULK ING
CONTROL CHLORINE
SOLUTION FEED
POINTS
3" (7.62 cm) WAS METER
MA* CAP 750 GPM(47.3 I/sec)
WAS TO GRAVITY THICKENERS
Figure 11-17. Schematic layout, revised solids flow system.
With aeration bay 1 being used as an RAS reaeration bay, it was necessary to assure that the
remaining three complete-mix aeration bays would distribute their mixed liquor equally to the four
secondary clarifiers. This was accomplished by installing special flow-equalizing overflow weirs at
each clarifier H-channel inlet (figs. 11-18 and 11-19). These weirs were fabricated of steel and shaped
with one flat side to provide the maximum channel width for the mixed liquor that had to be trans-
ferred along the channel to the fourth inlet weir. The mixed-liquor depth within the H channel was
raised to about 3 feet. An air agitation system was installed in the H channel to insure against solids
deposition. The new distribution system works well, taking full advantage of the H channel depth
without causing any backup into the aeration bays.
To insure that the higher water level in the RAS reaeration bay would not cause distribution
problems with the aeration air system, the air diffusers within the reaeration bay were raised
approximately 12 inches. This modification works extremely well, allowing the bay to operate at
peak RAS flows of up to 60 percent of the average daily flow (about 15 mgd) with no distribution
upset within the air supply system.
This new mode of operation has created an activated sludge process that performs as a single
system capable of operating at higher solid levels, thereby maintaining lower food/micro-organism
ratios during peak cannery load periods. The combination of high solids loadings in the RAS reaer-
ation bay and lower normal loadings in the other bays also results in lower solids loadings on the
secondary clarifiers, thereby improving their capability to treat bulking sludge and high hydraulic
loadings.
36
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FLEXIBLE AIR HOSE
X
OPEN OVERFLOW
SLOTS, WEIRS
REMOVED
19.0 ft ^^^^
(5.79 m)\
X ir
CONTACT
AERATION
BAYS 2, 3, 4
k.
C
\
TYPICAL SECONDARY
H
HANNE
TYPIC
WATE
^ SURF/
|
=
L
AL
3
\CE
I ^
I
1
36 in (91.44 cm)
MIXED LIQUOR
TO SECONDARY
CLARIFIER
L^T~^
/
-
FLOW
EQUALIZING
OVERFLOW
WEIR (TYPICAL
OF 4)
SEDIMENTATION
TANK INLET
6 in (15.24 cm) PVC
AIR HEADER
TYPICAL WATER
SURFACE
6 to 8 in
(15.24 to 20.32cm)
— - — ^T F P I fc
ABOVE PLATE
CONTACT BLOCKED
BAYS OVERFLOW
SLOTS
RAS
REAERATION
BAY 1
H
CHANNEL
TYPICAL
WATER
SURFACE
*
&
4
A
f
H-CHANNEL
^- AGITATION
AIR DIFFUSERS
TYPICAL H CHANNEL
Figure 11-18. H-channel cross sections.
RAS Control
Although every attempt was made to determine how the existing airlift pumps could be used
to provide the required RAS control arid flexibility, no solution could be found. Finally it was
decided that, to meet fully the requirements for successful RAS control, at least two independent
RAS pumps would have to be installed for each clarifier. Each pump would have to be variable-
speed driven and capable of pumping up to 1,400-1,500 gal/min against a total head of about 15
feet. This was a difficult decision to make, because it meant that pumps and variable drives would
have to be purchased and installed within a very short time.
Once the decision was made, however, an immediate search tried to locate equipment that
could be delivered on or before the end of July Self-priming, nonclogging contractor pumps were
37
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Figure 11-19. H-channel flow-equalizing weir in place before plant restart, July 1, 1973. (Note anchor clips used to
hold weir in place.)
found in stock, and the V-belt variable-speed drives were acquired from the municipality of Metro-
politan Seattle, which was in the process of removing them from the Renton treatment plant. This
arrangement made it possible to have six of the eight pumps on line by July 28, with all eight on
line by the actual start of the canning season during the second week in August.
The pumps were installed on the far side of the aeration bays. Pump suctions were fabricated
of lightweight steel pipe and were located so as to pull RAS from the airlift pump draft tube (figs.
11-20 to 11-23). The siphon on each suction proved to be no problem to the operation of the RAS
pumps. The diameter of both the suction and discharge piping is 10 inches, purposely large to
minimize friction head loss and thereby achieve maximum capacity from the pumps. A special steel
extension sleeve was set over each modified airlift to prevent the return of the aeration bay contents
to the airlift draft tube.
As shown in figure 11-17, some pump discharge pipe runs were many feet longer than others;
the longest was over 450 feet. All pipes except the discharge header near the pump were plastic. The
steel discharge header near the pump was used to provide the connection for a direct return bypass
to each tank and a thrust anchor for the entire discharge pipe run. Plastic pipe expands at a very
great rate per unit temperature change, and so it was necessary to provide room for that expansion
in each pipe run. Flexible joints for 10-inch pipe presented major cost and delivery problems. There-
fore, it was decided to anchor the end of the pipe near the pump and allow the other end to move
as required while directing its discharge into a common, open-top collection trough.
The RAS collection trough (figs. 11-24 and 11-25) is fabricated of plywood completely encased
in fiberglass. The trough is 3 feet wide and 3 feet deep and is designed to discharge to the center of
38
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10" (25.4 cm)
RAS DISCHARGE
PIPEL INE
TYPICAL
WOOD
SUPPORT
BENT
TYP. EVERY
10 FEET
(3.05 m)
10" ( 25,4 cm)
FABRICATED
STEEL SUCTION-
AGITATION
AIR HEADER
-STEEL
EXTENSION
SLEEVE
(TYP. OF 8 )
6 " (15.24 cm)
RAS PUMP (Typ. of 5)
8" (20.32 cm)
RAS PUMP (Typ. of 3)
'Y CHANNEL
AGITATION
AIR DIFFUSERS
(TYP. OF 45)
Figure 11-20. Y-channel cross section.
AIR LIFT
DRAFT TUBE
Figure 11-21. Complete support system for RAS discharge piping,
October 11, 1973. (Note Y-wall agitation air header and hose
connections along railing at left, and guides to keep PVC
piping in alignment at right.)
39
-------
Figure 11-22. RAS pump 8 at far end of aeration basin 4, August 3, 1973. (Note steel suction and discharge piping
with flexible hose connections to pump. All timber supports were set in concrete.)
Figure 11-23. Aeration basins 2, 3, and 4 during foaming period after cornstarch dump, August 1973. (RAS pump 3
suction and bypass discharge piping can be seen crossing over Y channel in the foreground.)
40
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Figure 11-24. Closeup of 10-inch (25.4 cm) PVC elbows discharging RAS at head end of RAS collection trough,
October 11, 1973. (Small pipe at left is FeCI3 feed.)
Figure II-25. Discharge end of RAS collection trough, October 11, 1973. (Curved section can be reversed if it is
necessary to operate aeration basin 2 as a reaeration basin.)
41
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the RAS reaeration bay. A removable curved section at the discharge point allows for the future use
of aeration bay 2 as a second reaeration bay, should this be required. The trough, in addition to
providing the space for pipeline expansion, has also been used as a point for ferric chloride addition
and is equipped with a removable Cipoletti weir, which can be used to calibrate individual RAS
pumps. Each RAS pump is completely independent of the other pumps and of all other process
variables, and can be calibrated to provide a consistent flow-versus-speed relation.
The new RAS system provides the operator with positive control of the solids circulating
within the system, regardless of other process variables. Although the system is limited in its
maximum capability to about 60 percent of the plant's average dry weather design capacity, expe-
rience so far indicates that this will not be a handicap. Normal operation so far has been in the 25-
35-percent range. One airlift pump has been left in the RAS system from each secondary clarifier.
Although their use for clarifiers 2, 3, and 4 short circuits the reaeration bay, they can be pressed
into service in emergencies.
WAS Control
The modification required to achieve the WAS control necessary for successful operation
proved to be an excellent example of how many of these changes were interrelated. Once aeration
bay 1 was transformed into a reaeration bay, it was possible to use some of the existing WAS dis-
charge piping to provide a WAS system completely independent of all other process variables.
Figure 11-12 indicates how the original WAS system had been provided with the option of returning
WAS discharge flows directly to the aeration bays instead of to the gravity thickeners. Each such
aeration bay WAS discharge was protected against backflow by a tank-mounted flap valve. Once
aeration bay 1 became a reaeration bay, it was a simple matter to remove its flap valve and recon-
nect the former discharge piping to the WAS pump suctions as shown in figure 11-17.
The existing WAS pump discharge header was raised 3 feet, provided with a new 3-inch WAS
magnetic flowmeter (peak capacity 750 gal/min) and a meter bypass piping connection (figs. 11-26
and 11-27). The existing oversized meter was removed and replaced with a valved tee designed to
facilitate the reconnection of the new pump discharge to the gravity thickener pipeline.
The new WAS system allows the operator to pump from either the reaeration bay or the sludge
control box sump. To assure that the new system functions independently of other process var-
iables, it has been provided with a feedback control system that uses the signal from the flowmeter
to control the speed of the pump driver. If the pump should become partially gas bound, or if the
solids-concentration or pipe-carrying capacity changes sufficiently to affect the system's head loss,
the pump speed is automatically adjusted to maintain the preset flow rate. This system allows the
operator to preset his wasting rate with the assurance that the rate will be uniformly maintained
until he wishes to make a change.
The WAS control system has worked extremely well and has made it possible to optimize
gravity thickener operation. The new system uses polymers to thicken WAS from relatively good
settling RAS (SVI 70-120) of less than 1 percent concentration to a concentration of 2-2.5 percent.
This significantly decreases the quantity of liquid being pumped to the digesters.
In addition to providing for increased thickening efficiency, the reliability and adjustability of
the WAS system has also made it possible for the plant operators to control accurately the mean cell
residence time (MCRT) of the activated sludge solids. By keeping this time to between 3 and 5 days,
it has been possible to minimize the formation of unstable denitrifying sludge. Secondary clarifier
nitrate levels, which previously averaged 19-20 mg/1, have been reduced to 3-4 mg/1.
42
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Figure 11-26. WAS suction and discharge piping modifications within the WAS pump building, August 1973. (New
discharge line set 3 feet (0.91 meter) above original discharge line converted to suction. Note suction valves
located on new vertical connections to pump suction system.)
Figure II-27. WAS flowmeter installation with bypass connection, August 1973. (Meter and valves reduced to 3-inch
(7.62 cm) size to assure adequate velocities through system.)
43
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Chemical Feed Facilities. Ferric chloride and chlorine chemical feed facilities designed to
provide flexibility during critical operational periods were constructed as quickly as possible. Ferric
chloride chemical feed facilities were installed as indicated in figure 11-28 and provided for the in-
termittent application of ferric chloride to the mixed liquor to improve its settling characteristics.
Different feed points provide the ability to add heavy slugs of chemicals over short periods of time
or smaller quantities uniformly over long periods of time.
The Y-channel secondary clarifier inlet feed points require equalizing the chemical feed
between the tanks in service but provide the additional benefit of using ferric chloride's increased
weight and coagulant and flocculant capabilities for improved settlement. Normally H-channel feed
points are used whenever the operator anticipates clarity problems within the secondary clarifiers.
Part of this extensive ferric chloride distribution system included a continuous stream of ferric
chloride added to the Y channel at the rate of approximately 700 Ib/day to assure that the activated
sludge process has sufficient iron to support the growth of nonfilamentous micro-organisms. Use of
this nutrient supplement was one of the recommendations of a 1972-73, industry-sponsored study
of plant problems; it was implemmented before June 1973 and was retained during the 1973
canning season. Once the plant's activated sludge system stabilized during the late fall of 1973, this
continuous flow was stopped. In 1975, the periodic ferric chloride additions for bulking and clarity
control were apparently sufficient to satisfy whatever nutrient support was required.
Before the chlorination system improvements to provide permanent sludge-bulking control
were completed, efforts were made to inject chlorine into the RAS system by feeding approx-
imately 100 Ib/day to the mixed liquor inlets to the secondary clarifiers. These efforts met with
some success in late July and early August, but could not cope with the increased filamentous
growths that infected the system as soon as the plant began to receive the heavy cannery wastes.
Late in August 1973, the new chlorine hulking-control system became operable. Chlorine
solution carrying up to 2,000 Ib/day of chlorine could be injected at any of the RAS system
locations shown in figure 11-17. As the bulking problems with the cannery waste sludge continued
and the SVI exceeded 300, the chlorine feed rate to each of the four RAS pipeline injectors was
increased until the system was injecting 400 To/day per injector (about 10 mg/1). Little effect on the
filamentous growths was observed, however, until the heavy solids of the cannery wastes were di-
2500 gal
(9,460 I)
FIBERGLASS
FeC
1 in (2.54 cm)
FeCI3 FEED
TO RAS TROUGH
n PVC FeCI3 FEED TO Y
CHANNEL
RAS |
COLLECTION
TROUGH
|_
Y
CHANNEL
BOTTOM OPENING OUTLET
SLIDE GATE 1
TO H CHANNEL
SECONDARY CLARIFIER INLET (TYPICAL OF 4)
n (1.27 cm) PVC FeCI3 FEED
in (3.81 cm) PVC FeCI3 PIPELINE
Figure II-28. Schematic layout, ferric chloride distribution system.
44
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verted to the city main plant on September 11, 1973. Continued use of the RAS chlorination
system has improved the plant's SVI level until it is now averaging below 80. Bulking control was
successful during the 1974 canning season. A cannery effort to eliminate major quantities of the
7Vi('o£/i roc-infested mud also helped assure this success.
Chlorination of the RAS has proved to be another useful and effective means of controlling
nitrification within the activated sludge process. Chlorine applied at the proper dose is toxic to ni-
trifiers, and its addition has controlled effluent nitrite and nitrate levels and allowed the plant to
stabilize the operation of its disinfection system.
Aeration Air Control
With the increased stability of single-system operation and stable RAS flow rates, the need for
frequent individual aeration air adjustments within the aeration system has practically disappeared.
Once the air system is adjusted to maintain the 0.5-mg/l DO level in the RAS aeration bay and the
4.0-mg/l DO level in the complete-mix aeration bays, it is possible for the plant operators to com-
pensate for increased plant loadings by simply adjusting the single output valve on the aeration
blowers.
To eliminate the possible development of septic conditions that might promote the growth of
filamentous organisms in the Y channel, additional agitation air was added along the bottom of the
channel. Aeration air for the new H-channel (fig. 11-29) and Y-channel agitation air systems was
taken from the main air header pipeline before any other system feeds.
Figure 11-29. H-channel agitation air system in service, August 1973. (Six-
and 4-inch (15.25- and 10.16-cm) header mounted on top of wall with
hose connection to diffuser units.)
45
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Final Clarifier Operation
Minimizing hydraulic losses and operating equalizing overflow weirs within the aeration tank H
channel assures equal distribution of mixed liquor to each of the four clarifiers. The remaining
emergency RAS airlifts can be used for controlled relief of individual tank blowouts. The operators
have discovered these airlifts to be extremely useful tools to retain solids within the system. Short
periods of airlift operation distribute excessive solids from the offending sedimentation tank
throughout the entire final clarifier distribution system and quickly relieve the upset condition.
When the design of the clarifier sludge-collection mechanism was reviewed with the manufac-
turer, it seemed that rearranging certain hydraulic orifice locations would improve operations. The
manufacturer provided sketches eliminating the center orifices and adding new orifices at the outer
end of each collection arm. Two of the four clarifiers were so modified. Tests with density probes
indicate a significant solids-distribution difference between the modified and original units. The
modified clarifiers have a uniform sludge blanket, while the original clarifiers showed a decidedly
higher level of sludge at the tank periphery. The other two clarifiers are being modified as part of
additional interim improvements currently being made to the central plant.
Recent Operation Experiences
For the first 26 months since the central plant (fig. 11-30) began operating with these activated
sludge process modifications, BOD removal efficiencies have averaged better than 96 percent, and
SS removal efficiencies 94.8 percent. BOD daily and monthly average concentrations have never
Figure 11-30. Aerial view of plant looking west across aeration tanks, clarifiers, and digesters at oxidation ponds
and solids storage basins, October 1973. (Note location of basins in relation to rest of plant.)
46
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exceeded 90 mg/1 and 30 mg/1, respectively. Except for 4 days in September 1974 during the can-
ning season, no daily SS effluent concentration has ever exceeded 90 mg/1. The September 1974 and
September 1975 monthly average effluent SS concentrations are the only monthly effluent averages
that have exceeded 30 mg/1. The tertiary oxidation ponds were taken out of service in the spring of
1974.
Figure 11-31 provides monthly average BOD and SS loading comparisons for the typical non-
canning month of June before and after the activated sludge modifications had been completed.
The June loadings also provide an excellent picture of the loading increases resulting from the di-
version of industrial waste and other treatment plant solids to the central plant in July 1972. Both
BOD and SS influent loadings in 1975 are over twice the 1972 loadings, while the monthly average
flow has increased less than 20 percent. The huge 1975 SS jump reflects extra solids discharged to
the plant during the cleaning of digesters in upstream facilities. Note that 1975 removal efficiencies
for both parameters still exceeded 96 percent.
September BOD and SS loadings in figure 11-32 show the plant's ability to treat flows and
loadings. The September 1974 final clarifier blowouts were the result of the inability of the WAS
thickeners to handle the necessary solids loading. The units have a hydraulic limitation of 400
gal/min, which is equivalent to approximately 40,000 Ib/day of WAS. During September 1974, the
monthly average WAS loading was 34,800 Ib/day, with a peak day of 53,600 pounds. Under these
conditions the final clarifiers were forced to waste excess solids.
New flotation thickeners were put in operation after the 1976 canning season. To alleviate the
solids-wasting limitation before their completion, some of the WAS was wasted through the reaer-
ation tank drain back into the plant influent. This was done during September 1974 with excellent
results. The excess WAS was removed with the heavier canning solids in the primary clarifiers and
did not recycle through the activated sludge process. When tried again in September 1975, solids
recycling caused a process failure. However, it was discovered that excess WAS could be safely dis-
charged directly to the solids-storage basins (sludge lagoons).
BODR
1972
1973
1974
1975
23,000 (10,440) |jj:j
31,600(14,350)
41,900(19,020)
50,300 (22,840)
2,600(1,180)
|j 1,900 (860)
jjj 1,800 (820)
|| 1,600 (730)
mgd
13.5
14.5
16.2
15.9
FLOW
CO
cc
01
SUSPENDED SOLIDS
1972
1973
1974
1975
29,900 (13,570)
35,400 (16,070)
39,700 (18,020)
11,700(770)
W& 5,200 (2,360)
|[jj 1,400 (640)
67,100(30,460) ||:$
m3/day
(51,100)
(53,700)
(61,300)
(60,200)
2,700 (1,230)
LEGEND
| 1 Influent,
Ib/day
Effluent,
Ib/day
(1,500) kg/day
Figure 11-31. Plant performance, June (noncanning season) monthly averages.
47
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1972
1973
1974
1975
BOD5
36,400(16,530)
51,200 (23,240)
67,500 (30,650)
79,400 (36,000)
Sf>i3 4.000 (1,820)
jjvj 2,100 (950)
2,200 (1,000)113
/| 2-600 (1,200) H|
CO
tr
1972
1973
1974
1975
29,400 (13,350)
SUSPENDED SOLIDS
1133,000(1,360)
mgd
16.4
19.2
22.1
21.4
FLOW
72,900 (33,100)
m3/day
(62,100)
(72,700)
(83,600)
(81,000)
74,100(33,640)
110,300 (50,100)
1,600 (730)
8,100 (3,680)
6,600 (3,000)
LEGEND
Influent,
Ib/day
Effluent.
Ib/day
(33,100) kg/day
Figure II-32. Plant performance, September (canning season) monthly averages.
Summary and Conclusions
While operation of the Central Waste water Treatment Plant has been extremely effective, it has
not been routine. The continuing heavy loading of industrial wastes and treatment plant solids
makes the establishment of steady-state operation very difficult. As soon as stable operation is
achieved, an industrial accident or a change in upstream solids discharge creates a whole new con-
dition. The results shown in figures 11-31 and 11-32 attest to the ability of the operating staff to use
the system modifications to adjust and compensate for these changes. As a result of the success of
these full-scale, online modifications, it is believed the following conclusions hold for activated
sludge systems:
• System design must provide positive hydraulic controls to assure that the entire activated
sludge process acts as one system, regardless of the number of aeration-clarifier units
employed. System design must include intermixing of RAS flows and intermixing of either
all mixed liquor flows or RAS flows with all aeration bay influent flows.
• RAS system design must be independent of other process variables and capable of being
adjusted to meet changing requirements. Closed-loop control with variable capacity pumps
and individual flowmeters provides excellent positively controlled systems.
• WAS system design must also be independent of other process variables, and must be
capable of being adjusted to meet changing requirements. Closed-loop control with variable
capacity pumps and individual flowmeters provides excellent positively controlled systems.
• Chemical feed systems for settling, bulking, and nitrification control are necessary tools to
assure process reliability. Every activated sludge system design should be capable of using
chemicals (chlorine, ferric chloride, or alum) to overcome upsets.
© Design of air control to each independent aeration bay must be positive, simple to effect,
and relatively unaffected by other bay adjustments. Closed-loop control and DO probes,
48
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automatically controlled valves, and simple flowmeters provide excellent and efficient
systems.
• Design of distribution systems for multiple aeration-clarifier systems should be positive and
simple. This is best accomplished by providing flow-equalizing layouts and inlets with self-
equalizing, high head losses. Closed-loop, automatically controlled inlet valves with integral
flowmeters can be used for systems that cannot accommodate the preferred layout and inlet
design.
• WAS thickening facilities must be capable of treating the maximum WAS flow imaginable
under the worst possible operating conditions. WAS thickening facilities must be reliable
and conservatively designed to assure the removal of bulking sludges with high SVI's. Flota-
tion thickeners designed for loadings of approximately 10 lb/day/ft2 and provided with
chemical aids are the most reliable means of meeting these criteria.
• Primary sludge and WAS processing and disposal must not be allowed to recycle significant
quantities of solids through the activated sludge system. Conservatively designed primary
digesters (25-30 days detention) with properly designed facultative type, deep (15 feet)
secondary lagoons (loaded at approximately 20 pounds VSS per 1,000 ft2/day) provide the
most positive means of controlling such recycle.
CASE 5. UPGRADING WITH AUTOMATIC DISSOLVED-OXYGEN CONTROL
IN THE ACTIVATED SLUDGE PROCESS
Control of oxygen dissolution in the mixed liquor is an important parameter in the activated
sludge process. The desired strategy is to add sufficient air or high-purity oxygen to meet the time-
varying oxygen demand of the wastewater. Because electrical energy is one of the major operating
costs of the activated sludge process, there is an economic incentive to minimize unnecessary
aeration blower operation.
If the DO level in the mixed liquor drops below approximately 0.5 mg/1, oxygen becomes rate
limiting and the aerobic bacteria become inactive. Too low a DO level can also lead to the growth of
filamentous organisms and consequent sludge bulking. Too high a DO level, however, indicates
wasted power, which can cause floe breakup if energy levels are too intense. This can result in high
effluent SS levels because of poor settling characteristics. Nitrifying organism activity is reduced at
DO levels below 2.0 mg/1; thus, the DO concentration should be kept above this level to insure nitri-
fication where it is required.
Manually controlled air addition is now used at most activated sludge plants. The operator may
attempt to pace oxygen transfer in proportion to the oxygen demand by turning blowers on and off
or by adjusting variable-speed blowers; to insure adequate oxygenation, however, he usually pro-
vides more aeration than required. Power costs can be minimized if aeration capacity is automat-
ically paced in proportion to the time-varying oxygen demand.
Because automatic DO control involves additional capital and operating expenses, it is gen-
erally only applicable to plants with a flow capacity in excess of 2.0 mgd.2 Automatic DO control
may be appropriate for smaller plants, however, where waste strength varies greatly and rapidly,
because of industrial discharges, for example.
Presented herein are case histories of three wastewater treatment plants that use automatic DO
control: the Renton, Wash., Wastewater Treatment Plant, the Palo Alto, Calif., Water Quality Con-
trol Plant, and the San Jose-Santa Clara, Calif., Water Pollution Control Plant. Data are presented
49
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that compare performance and power consumption for operation under manual and automatic
control modes. These case histories are taken from Design Procedures for Optional Dissolved
Oxygen Control of Activated Sludge Processes by Brown and Caldwell,2 which provides a more
complete discussion of automatic DO control methods and procedures.
A. Renton, Wash., Wastewater Treatment Plant
The Renton plant,2'3 located near Seattle, Wash., commenced operation in June 1965 and was
enlarged in 1973 to an average dry weather flow capacity of 36 mgd. Two oxidation tanks are
provided, each with four passes. Air is supplied by six single-stage centrifugal blowers and in-
troduced through two headers in each tank. Each header serves two passes.
Currently, the Renton plant is equipped with three 12,000-scfm, 500-hp blowers, and three
14,000-scfm, 600-hp blowers; all are supplied with 4,160-volt power. The 500-hp blowers were in-
stalled in 1963, and are driven by synchronous motors. The 600-hp blowers were installed in 1973,
and are driven by squirrel cage induction motors. All blowers are located in a temperature- and
humidity-controlled gallery and receive finely filtered air.
An automatic DO control system is provided and incorporates a pressure control loop in the
blower feed manifold and a DO-regulated flow control loop for each of the four tank headers. Three
probes are installed in each of four passes in each oxidation tank for a total of 12 probes per tank.
An instrumentation and control diagram of the DO control system is shown in figure 11-33 (table
11-10 presents definitions for symbols used in figs. 11-33,11-34, and 11-37). Components include the
following:
• Six single-stage, centrifugal blowers with individual suction throttle valves, and flow-
regulated surge control systems
• One blower discharge manifold pressure control loop with pressure transmitter and pressure-
indicating controller
• 24 DO analyzers and probes
• Two 12-point DO strip chart recorders (one recorder for each tank)
• Four DO probe selector switches (one per air header)
• Four DO controllers
• Four flow control loops for each header including orifice plate, square root extractor, re-
corder, totalizer, indicating controller, and piston-operated butterfly valve
Operation. The Renton plant currently receives insufficient loading to warrant use of both
oxidation tanks, although both tanks are fully instrumented for automatic DO control. Using the
appropriate probe selector switch (HS), the operator selects one DO probe in each two-pass tank
section and uses that probe to control the airflow rate in the corresponding supply header. All DO
probes are continuously monitored on a strip chart recorder (AR), and the selected control probes
may be changed at any time. The output of the selected probe is transmitted to the oxygen con-
troller (AIC), which provides an output to vary the set point of the flow controller (FIC) as re-
quired. The flow controller modulates a butterfly valve in the airflow header in accordance with the
computed set flow rate.
50
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TO OTHER
5 BLOWERS
NOTE: INSTRUMENTA TION OF
SECOND TArlK SAME
AS FIRST TANX
Figure 11-33. Renton Automatic DO control system.
DO in the first two passes is typically maintained at 1.5 mg/1, using one of the last two probes
in the second pass for control. DO in the last two passes is usually maintained at 2-2.5 mg/1, using
one of the last probes in the fourth pass for control.
A constant pressure of 8 psig is maintained in the blower discharge manifold by simultaneous
throttling of all blower suction valves through a pressure-indicating controller (PIC). A discharge-
flow-regulated surge control system with a load control override is provided for each blower. In the
event the PIC calls for the blower-throttling valve to be open for a period of time so that the blower
power draw becomes excessive, the power-indicating controller (JTC) will override the PIC and
throttle the blower.
Performance. The Renton plant has continuously operated under automatic DO control since
1967, except for a 13-month period beginning March 1970, when the initial oxidation tank was
modified from a two-pass to a four-pass structure. During this period, all influent was diverted to
the new oxidation tank. DO probes had not yet been installed in the new tank; therefore, DO data
collection was performed by the plant laboratory staff. Set points on the header FIC's were deter-
mined by the laboratory using the copper-sulfate/sulfamic-acid flocculation modification to the
Winkler test for DO. The FIC set points were varied once or twice a day according to directions
from the laboratory. Other analyses performed in the laboratory included influent and effluent
BOD and COD and sludge SVI and 30-minute settleability. These analyses were completed once a
day from composite wastewater samples. Detailed data have been reported on 3 months of manual
DO control in 1970 and compared with data for the same 3 months in 1971 using automatic DO
control.3 Frequency distribution plots of BOD-removal efficiency indicate that efficiency was con-
51
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Table 11-10.—DO control symbols and definitions
Symbol
AE
AIT
AIC
AR
FE
FIT
Fl .
Fic
FQI
FY
FR
FT
FSL ....
HS
I/P
JIC
JIT
M
PIC
PIT
S
SP ....
ST ... ....
SI ...
ZY
Definition
DO probe
DO level transmitter
Oxygen controller
Strip chart recorder
Flowmeter
Flow signal transmitter
Flow indicator
Flow controller
Flow integrator
Square root extractor or current/power conversion
relay
Flow recorder
Flow transmitter
Low flow switch
Probe selector switch
Current/pressure conversion
Power indicating controller
Power indicating transmitter
Motor
Pressure indicating controller
Pressure indicating transmitter
Switch
Pressure switch
Speed transmitter
Speed indicator
Current/pressure conversion relay
Table 11-11.—Performance comparison of manual and automatic DO control,
Renton Wastewater Treatment Plant, October-December, 1970 and 1971
Parameter
BOD removal efficiency,
percentc
Sludge volume index01 . . . .
Air supplied:
ft3 /gal influent
ft3/lb BOD removedd
BOD removed, d Ib/kWh .
aAverage daily flow— 24.5 mgd; average BOD loading— 21.3 pounds per 1,000 ft3/d
hAverage daily flow— 27.1 mgd; average BOD loading-31 .6 pounds per 1 ,000 ft3/d
Manual3
85
e332
1 25
2 190
0 882
ay.
ay.
Automatic13
96
86
1 10
1 380
1 39
Percentage
improvement
13 0
286
12 0
37 0
57 6
Arithmetic mean.
eBu!king problems occurred.
52
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sistently high under automatic control and varied considerably under manual control. Data com-
piled on BOD-removal efficiency and other performance parameters are shown in table 11-11.
It has been reported that automatic DO control significantly reduces the air required for sec-
ondary treatment. Table 11-11 shows that for the tests at the Renton plant not only was the air
required under automatic DO control significantly less than under manual DO control, but BOD-
removal efficiency improved as well.
Maintenance. Maintenance of the DO control system with associated blowers has been judged
by the plant maintenance superintendent to require minimal labor and material costs. However,
some problems have been experienced with DO probe drift and moisture accumulation in the probe
plugs. At least 2 of 12 probes in the operating oxidation tank have displayed excessive drift, un-
correctable by recalibration. Four probes have been taken back by the manufacturer to determine
the cause of the problem.
The DO probes in the operating oxidation tanks are cleaned and calibrated once a week and
recharged about once every 8 months. Cleaning and calibration of 12 probes normally requires 11A
man-hours. Recharging 12 probes normally requires 2 man-hours.
B. Palo Alto, Calif., Water Quality Control Plant
The Palo Alto Water Quality Control Plant2'4'5 is an activated sludge facility with a current
average dry weather flow capacity of 34 mgd and an average wet weather flow capacity of 53 mgd.
Four oxidation tanks are provided with piping arranged for plug flow or reaeration modes of op-
eration. Air is supplied by three 6,400-scfm, positive displacement air blowers, and delivered to each
oxidation tank through a sparge ring. A 50-hp, fixed-speed mechanical mixer in each tank is used to
mix the rising air bubbles with the mixed liquor.
Each of the three air blowers are 550-r/min, motor-driven units designed to deliver 6,400 scfm
at 8 psig. The blowers are positive-displacement lobe-type units installed in 1972; each unit is driven
by a 300-hp, wound rotor motor. Two saturated core reactor, variable-speed drive units are used to
vary the speed of the three air blower motors. One drive unit is dedicated to one blower, while the
other drive unit can be switched between the remaining two blowers using transfer contactors. The
blowers discharge into a common manifold that delivers air to each oxidation tank through separate
14-inch risers.
A DO probe is installed in each oxidation tank and is located halfway between the mechanical
aerator units and the tank dividing wall. A portable DO probe is also available to measure DO con-
centrations in the tanks. An instrumentation and control diagram of the DO control system is
shown in figure 11-34. Components include the following:
• Three positive displacement air blowers with saturated core reactor, variable-speed drive
• Four manual flow control stations for each tank, including orifice plate, flow transmitter,
square root extractor, flow indicator, motor-operated butterfly valve, and a remote, man-
ually operated valve position controller
• One flow recorder for total flow delivered to all four oxidation tanks
• Four DO probes, agitators, and analyzers
• Four fixed-speed mechanical mixers, one in each oxidation tank
53
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50 HP
MECHANICAL
MIXER (Typ)
SPARGE
RING (Typ)
300 HP BLOWER
(Typ)
-SATURATED REACTOR CORE
VARIABLE SPEED DRIVE (Typ)
Figure 11-34. Palo Alto DO control system.
® One single-channel DO recorder with a manually operated selector switch
Operation-Remote-Manual. Normally, the Palo Alto plant is operated in the conventional
activated sludge mode. Each oxidation tank simultaneously receives primary effluent and discharges
its effluent to an associated final tank. DO concentration in each oxidation tank is indicated on a
control panel in the plant operations building. A single-channel recorder may be switched from tank
to tank to record the DO level.
Based on DO concentration in each tank, the operator modulates the blower motor speed on
the motor-operated butterfly valve in the corresponding tank air feed header. Normally, only two
blowers are operating. Primary control of DO in the tanks is achieved with blower speed modula-
tion, while secondary control is made with the remote-operated header feed valves. Blower speed is
typically altered three times a day while the header valve positions are changed two or three times a
day. The valve in the tank header farthest from the blowers is normally left fully open. DO con-
centration in the oxidation tanks is usually maintained at 0.5-1.0 mg/1.
Operation-Remote-Semiautomatic. In 1973, Systems Control, Inc., the city of Palo Alto
(under a grant by EPA), and the California State Water Resources Control Board conducted a study
to compare manual versus digital computer control of the DO control system.4-5 Following 4 weeks
of monitoring normal control procedures in the manual mode, a digital computer system was in-
tegrated into the DO control system with a programed DO control algorithm. The computer
received a 4-20-mA DO signal from each probe analyzer, computed any changes required in blower
54
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flow rates, and typed out such changes on a teletype. A change in computed airflow rates of more
than 100 scfm was required before a control change message was typed. Because the operator was
still required to perform the blower speed change requested, this type of control is most accurately
designated as computer-assisted open-loop control.
The DO control algorithm used in the test was a proportional plus integral process control
program designed to compute the required change in airflow rate. On occasion, such change
commands required the operator to place blowers in or out of service.
The testing program at Palo Alto was divided into three stages, summarized in table 11-12.
During the first stage of approximately 4 weeks, the plant was operated in the remote-manual mode
as previously described. During stages 1 and 2, the header feed control valves were left in a fixed
position, and the blower speed was changed only twice daily.
During the next stage of the study, data were collected on plant performance for both a
3V2-week "nonintensive" and a 3-day "intensive" collection phase. The intensive data collection
period encompassed the extreme operating conditions of the plant as well as an average condition.
During the intensive period, all relevant data were collected every 2 hours.
During the final stage of the study, the semiautomatic DO control system, as described earlier,
was activated. After a 4-week period of process stabilization relative to the new control mode,
nonintensive and intensive studies of process performance were made. Data collection phase
durations for the final study step were identical to those of the previous step.
Performance. Average values of plant operating variables during the first and second phases of
test stages 1 and 2 are shown in table 11-13.
Figures 11-35 and 11-36 illustrate the performance difference between the manual and
semiautomatic DO control systems in maintaining a 1.0 mg/1 DO set point during the intensive
Table 11-12.—Manual and semiautomatic DO control testing program, Palo Alto, 1973
Stage
1 . .
2
3 . ....
Phase
1
2
1
2
3
Duration
4 weeks
3'/2 weeks
3 days
4 weeks
31/2 weeks
3 days
Remarks
Remote manual mode of
operation
Infrequent data collection
under remote manual mode
Frequent data collection
under remote manual mode
for average and extreme
operating conditions
Process stabilization
under semiautomatic mode
Infrequent data collection
under semiautomatic mode
Frequent data collection
under semiautomatic mode
for average and extreme
operating conditions
55
-------
Table 11-13.— Perform ance comparison of manual and remote semiautomatic DO control,
Palo Alto Water Quality Contol Plant
Parameter
BOD-removal efficiency,
percent
SS-removal efficiency,
percent
TOC removal efficiency,
percent
COD-removal efficiency,
percent .
Air supplied:
ftVgal influent
ft3/lb BOD removed0 . .
BOD removed Ib/kWh
Manual3
83.9
46.3
53.1
63.1
.447
525
2.94
Semiautomatic13
84.2
52.8
59.8
63.6
.450
448
3.44
Percentage
improvement
0.3
14.0
10.8
.8
14.7
14.5
aAverage daily flow—24.0 mgd; average BOD applied to oxidation tanks—24.4 lb/1,000 ft /day.
bAverage daily flow—23.6 mgd; average BOD applied to oxidation tanks—28.2 lb/1,000 ft3/day.
cComputed from total air supplied over testing period, average BOD in primary effluent, and reported BOD-removal efficiency.
Note.—Operating mode for manual and semiautomatic was contact stabilization. Test duration was 4 weeks for each control
mode.
E
o
12,000 -
10,000 -
8,000 -
6,000 -
4,000 -
2,000 -
-
—
x
X
/
/
I
/
/
;
/
/
/
l
1
l
1
1
l
l
I
1
1
\
\
\
\
\
\
\
\
\
\
\
\
\
\
V ,A
* / \
\ / \ .
V Vx
>\
! V
; v*
/ \
x- DO
/ \
/
/
/
/
/
/
/
/
i
-J
_
_
0
i
6
1 1 1
12 18 0
Sunday, 8/5/73
i
6
\
\
\
—
\
\ ,— AIR FLOW
» /
l /
1
1
1
I
i
i
i
1
1
1
I
1
1
1
\ /\
v-^A
\
1 l l
12 18 0
Monday, 8/6/73
s
(
1
1
1
1
I
1
1
1
1
1
4
1
6
\
\
\
v —
\
\
\
\
\ /
\ / -
\ /-'
1 1
o.u
4.0
E
Q.
Q.
„ . z
3.0 O
h-
<
rr
h-
•z.
uu
o
2.0 z
O
o
0
Q
1.0
n
12 18 0
Tuesday, 8/7/73
DATE AND TIME OF DAY
Figure II-35. DO and air flow at Palo Alto, manual DO control.
56
-------
12,000
10,000 -
8,000
E
o
O 6,000 -
QC
<
4,000 -
2,000 -
E
Q.
a
<
cc
LU
o
z
o
o
o
Q
6 12 18
Sunday, 9/9/73
0
6 12 18
Monday, 9/10/73
DATE AND TIME OF DAY
0
6 12 18
Tuesday, 9/11/73
0.0
Figure II-36. DO and air flow at Palo Alto, semiautomatic DO control.
testing periods. Under manual control, wide excursions in DO concentration were experienced as
shown in figure 11-35. Figure 11-36 shows that under semiautomatic control, the controller
maintained the DO at or near set point, although substantial changes in airflow occurred because of
considerable loading variation.
Four measures of pollutant-removal efficiency monitored over the manual and automatic
testing periods are compared in table 11-13. Although the improvements for BOD and COD removal
under the semiautomatic modes were relatively slight, SS and total organic carbon (TOG)
percentage removal under semiautomatic DO control show a marked improvement compared to
manual control.
During both stage 1 and stage 2 tests, airflow applied to the oxidation tanks was totalized and
BOD in the primary effluent was monitored. Samples were taken from the primary effluent at
4-hour intervals and combined for composite BOD analysis every 24 hours.
Assuming a linear relationship between the blower output and power consumption and a
power cost of 1 cent per kWh, power costs corresponding to the air required for each mode were
computed, as shown in table 11-13. The computed annual cost saving for the automatic DO control
system was $5,400.
Maintenance. According to the chief operator at the Palo Alto plant, DO probe calibration is
typically performed once every month. During the course of the study, it was determined that
preventive maintenance was required every 2 weeks; 1 man-hour was required to check all probes.
57
-------
Approximately once every 3 months, about 4 man-hours were required to thoroughly check, clean,
and recalibrate all probes. Recharging requires about 2 man-hours per year for all four probes.
The DO analyzers were reported to produce an electrically noisy signal with inherent
oscillating variation about a specific value. Electronic filtering was employed to correct this
problem.
Although no problems with computer downtime were reported during the stage 3 test, the
computer did fail at other times during other phases of the testing program. Over a period of about
13 months, approximately 20 computer failures were reported, of which about 50 percent would
have resulted in loss of the DO controller. If the DO contoller had been operating, the data loss
affecting DO control would have been 49 hours. Because the computer operated continuously for
about 14 months, the computer was capable of effecting automatic DO control 99.5 percent of this
period.
Computer system failures occurred predominantly with the peripheral devices. The disk mem-
ory accounted for 70 percent of the total computer downtime.
C. San Jose-Santa Clara, Calif., Water Pollution Control Plant
The San Jose-Santa Clara Water Pollution Control Plant2-6'7 began operation in 1964 with a
design average dry weather flow capacity of 94 mgd. In 1973 the plant was expanded to a dry
weather flow capacity of 160 mgd, and the plant is currently treating an average flow of 90 mgd.
The plant is an activated sludge treatment facility that employs the Kraus Nitrified Sludge Inter-
change Process. Two tank batteries are provided, each composed of six two-pass oxidation tanks
and two two-pass nitrification tanks. However, piping for each battery is arranged to permit con-
version of two more oxidation tanks to nitrification tanks if desired. Wastewater may be treated
using conventional, step-feed, or tapered aeration activated sludge operating modes. Normally, the
oxidation and nitrification tanks are operated in the two-pass mode.
The Kraus process involves mixing a portion of the activated sludge with supernatant and
digested sludge from the anaerobic digesters and aerating the combination for about 24 hours in the
nitrification tanks. The mixture is then pumped to the oxidation tanks for further aeration with the
primary effluent.
Two air systems are provided to deliver air to the secondary process at different pressures. The
high-pressure, or diffused air, system delivers 8 psig air to all 12 oxidation and 4 nitrification tanks
at a level 2 feet above the tank bottom. Air is introduced from one side of each tank pass through
fine bubble diffusers. The low-pressure, or distributed air, system delivers 4 psig air to the 12 oxida-
tion tanks at a level 5 feet below the tank surface. This system introduces air into each tank pass
opposite the fine bubble diffusers and produces a much larger diameter bubble.
Six engine-driven, single-stage, centrifugal blowers supply air for the high- and low-pressure air
systems. Four blowers are furnished for the high-pressure system and two for the low-pressure
system. The diffused, or high-pressure, system engine-driven blowers are each designed to deliver
60,000 scfm of air at a pressure of 8 psig and are driven by 2,400-hp engines. The distributed, or
low-pressure, system engine-driven blowers are each designed to deliver 85,000 scfm at 4 psig and
are driven by 1,850-hp engines. The engines are trifuel units that can operate on
58
-------
• A blend of digester gas and natural gas
• A blend of digester gas, natural gas, and diesel fuel
• Diesel fuel
In addition, four motor-driven rotary-lobe-type, positive displacement blowers were installed in
1970 to augment the high-pressure air system. These blowers are driven by 400-hp motors and are
designed to each deliver 10,000 scfm at 8 psig.
The low-pressure air system blowers are throttled by varying engine speed through a flow
control loop that senses blower manifold discharge flow. The high-pressure air system blowers are
throttled by varying engine speed through a pressure control loop that senses blower manifold dis-
charge pressure. The set point for each 4-psig and 8-psig header is currently manually derived from
plant flow and experience, but will soon originate from a DO or oxidation-reduction potential
(ORP) probe located near the second pass end of each tank. The set point for the main 4-psig
header air flow controller is set by operating experience. Each tank header control valve is throttled
by a cascade flow control loop to maintain 4 psig in the low-pressure manifold.
During the 1970-73 plant expansion, the plant's diffused and distributed air systems were
placed under direct digital control using a dual computer system. The pneumatic control systems
installed earlier remain intact and functional, but the computer was interfaced directly with the
primary and final control elements. Control functions previously accomplished by pneumatic analog
systems are now affected by either plant computer, using suitable control algorithms analogous to
the pneumatic analog control functions. Four nitrification tanks were added to the original 12 tanks
during the expansion. The new tanks have all-electric instrumentation, thereby eliminating the need
for pressure/current and current/pressure converters. An instrumentation and control diagram of
the DO control system is shown in figure 11-37. Components include the following:
« Six single-stage, centrifugal, engine-driven blowers with flow-regulated surge control system,
current transmitter, and high- and low-speed alarms
• Four rotary-lobe-type, positive displacement, motor-driven blowers
* One low-pressure blower discharge flow control system with pitot tube, square root ex-
tractor, flow transmitter, and flow controller
• One high-pressure blower discharge manifold pressure control system with pressure sensor,
pressure transmitter, and pressure controller
• 14 low-pressure header butterfly throttling valves and flow control systems
« 12 high-pressure header butterfly throttling valves and flow control systems
• Five DO probes with analyzer/transmitter (plus five in the future)
• 10 ORP probes with analyzer/transmitter (in the future)
• Computer
59
-------
»• To other 13 F I C s
«• To other II OXIDATION TANKS
OTHER I860 hp ENG INE
DRIVEN CENTRIF BLOWER
I ^
8 PS. g
4psig
PASS 1
/-~~\oo
(AE) PASS 2
^
^
OX I DAT I ON TANK
(Typ of 12 )
SOFTWARE
ALGORITHM (Typ )
OXIDATION TANKS
PASS
PASS 2
-D
1 OTHER 3 MOTOR-
j DRIVEN,POSITIVE
I 0 ISP L AC EMEWT
I BLOWERS
To other 3
N ITRIF ICATION
TANKS
NOTE ,
N I TR I F ICAT ION TANK
(Typ of 4 )
Blower speed control loops and
ox i dot ion lank header f low control
loops can presently bs computer
con trolled
Figure 11-37. Automatic DO control system, San Jose-Santa Clara Water Pollution Control Plant.
Operation. All six blowers at the San Jose-Santa Clara plant are currently computer controlled
to deliver a specific flow rate for the low-pressure system and a specific pressure for the high-
pressure system. However, blowers must be started and stopped manually from local control panels.
The computer receives a linearized flow signal from the flow-indicating transmitter (FIT) connected
to the square root extractor (FY) for the pitot tube (FE) in the low-pressure discharge manifold.
Using a proportional and integral flow control algorithm, the computer develops an output signal
that controls the fuel supply to the blower engine drives. The high-pressure blower speed is con-
trolled in a similar manner with the exception that the computer receives a pressure signal from the
pressure-indicating transmitter (PIT) connected to a pressure sensor in the high-pressure discharge
manifold, and a pressure control algorithm is used.
The low-pressure, or distributed air, supply headers are flow controlled by the computer. The
computer receives a signal from the PIT connnected to the header flow tube, inputs it to a flow
control algorithm with a preset set point and outputs a control signal directly to the butterfly posi-
tioner in the header.
The high-pressure, or diffused air, supply to each oxidation tank and nitrification tank is also
flow controlled by the computer. Currently, the computer receives a flow input from the FIT in
each high-pressure air feed header, compares it to a set point using a proportional and integral con-
60
-------
trol algorithm, and outputs valve position changes as required to the butterfly valve positioner in
each air header.
At this time, the plant is installing DO probes in the effluent end of five of the oxidation tanks
in one battery. At the computer console, the operator will be able to select DO, plant flow, or other
variables as a control reference for computer computation of a flow control set point for the flow
control algorithm in each high-pressure air feed header.
Performance. In October 1975, a DO control study test was run at the San Jose-Santa Clara
plant. A DO probe was installed in the effluent end of four oxidation tanks in battery B and the DO
output wired to the computer. A program called for a printout of each DO probe reading on 15-
minute intervals. Testing commenced on October 21, 1975, and ran for a total of 8 days. Each ox-
idation tank was operated under manual DO control October 21, 23, 25, and 27, and under auto-
matic DO control on October 22, 24, 26, and 28.
Under manual DO control, the air header feed valve on the 8-psig header to each tank was
manually modulated approximately every 4 hours. The amount of valve position change required
was estimated, based on the computer printout of DO in the respective tank.
Under automatic DO control, the computer modulated the air header feed valves as required to
maintain a DO set point of 2.5 mgA in each tank. The control algorithm included proportional and
integral control modes. Results of the performance tests are shown in table 11-14.
Table 11-14 shows a general improvement in almost all performance parameters under auto-
matic DO control. In particular, the air supplied per unit quantity of BOD removed improved over
12 percent. Improvement of this parameter would have been about 16 percent if data obtained on
October 26 had been neglected. The air supplied per amount of BOD removed was inexplicably high
on this day.
Table 11-14.— Performance comparison of manual and automatic DO control, San Jose-Santa Clara
Water Pollution Control Plant, October 21-28, 1975
Parameter
BOD-removal efficiency,
percent
SS-removal efficiency,
percent
Sludge volume index, mg/l
Air supplied:
ft3 /gal influent
ft3/lb BOD removed0
Manual3
84.8
86.0
102
.89
595
Automatic13
85.2
85.8
101
.80
522
Percentage
improvement
0.5
.2
1.0
10.3
124
aAverage daily flow—45.0 mgd; average BOD applied to oxidation tanks-54.3 pounds per 1,000 ft3/day.
bAverage daily flow—45.9 mgd; average BOD applied to oxidation tanks—56.1 pounds per 1,000 ft3/day.
°Computed from total air supplied over testing period and 24-hour composites of primary effluent BOD minus secondary
effluent BOD.
61
-------
REFERENCES
1Stenquist, R. J., D. S. Parker, and T. J. Dosh, "Carbon Oxygen-Nitrification in Synthetic
Media Trickling Filters," J. Water Pollut. Cont. Fed., 46., 2327, 1974.
2Brown and Caldwell, Design Procedures for Optimal Dissolved Oxygen Control of Activated
Sludge Systems, U.S. Environmental Protection Agency, Municipal Environmental Research Labora-
tory, Contract No. 68-03-2130.
3Roesler, J. F., "Plant Performance Using Dissolved Oxygen Control," J. Environ. Eng. Din.,
ASCE, 100 (EE5), Oct. 1974.
4Systems Control Inc., Palo Alto, Calif., personal communication.
5U.S. Environmental Protection Agency, Advanced Automatic Control Strategies for the
Activated Sludge Treatment Process, Environmental Protection Technology Series, EPA
670/2-75-039, May 1975.
6Belick, M., and F. N. Van Kirk, "California Plant Gets Straight A's in Computer Control,"
Water Wastes Eng., Mar. 1975.
7 "San Jose-Santa Clara Expand Water Pollution Control Plant," Power/Compression News,
Spring 1965.
-------
METRIC CONVERSION TABLES
Recommended Units
Description
Length
Area
Volume
Mass
Force
Moment or
torque
Flow (volumetric)
Unit
meter
kilometer
millimeter
micrometer or
micron
square meter
square kilometer
square millimeter
hectare
cubic meter
litre
kilogram
gram
milligram
tonne
-""- •
newton
newton meter
cubic meter
per second
liter per second
Symbol
m
*
km
mm
urn QT n
m2
km2
mm2
ha
m3
1
kg
g
mg
t
N
N-m
m3/s
l/s
Comments
Basic SI unit
The hectare (10,000
m2) is a recognized
multiple unit and will
remain in interna-
tional use.
Basic SI unit
1 tonne = 1,000 kg
The newton is that
force that produces
an acceleration of
1 m/s2 in a mass
of 1 kg.
The meter is mea-
sured perpendicular
to the line of action
of the force N.
Not a joule.
Customary
Equivalents*
39.37m -3.281 ft =
1.094yd
0.6214 mi
0.03937 in
3.937 X 10-5 in = 1 X 104 A
10.76 sq ft • 1.196sqyd
0.3861 sq mi = 247.1 acres
0.001 550 sq in
2.471 acres
35.31 cuft = 1.308 cu yd
1.057 qt = 0.2642 gal =
0.8107 X 104 acre ft
2.205 Ib
0.03527 02=1 6.43 gr
0.01543 gr
0.9842 ton (long) -
1.102 ton (short)
0.2248 Ib
= 7.233 poundals
0.7375 Ib-ft
23.73 poundal-ft
15. 850 gpm =
2,1!9cfm
15.85 gpm
Description
Velocity
linear
angular
Viscosity
Pressure or
stress
Temperature
Work, energy.
quantity of heat
Power
Application of Units
Description
Precipitation,
run-off.
evaporation
Flow
Discharges or
abstractions.
yields
Usage of water
Unit
millimeter
cubic meter
per second
liter per second
cubic meter
per day
cubic meter
per year
liter per person
per day
Symbol
mm
m3/s
l/s
m3/d
m3/year
I/person/
day
l
Comments
For meteorological
purposes, it may be
convenient to meas-
sure precipitation in
terms of mass/unit
area (kg/m2).
1 mm of rain =
1 kg/m2
1 l/s = 86.4 m3/d
Customary
Equivalents*
36.31 cfs
15.85 gpm
0.1835 gpm
264.2 gal/year
0.2642 gcpd
Description
Density
Concentration
BOD loading
Hydraulic load
per unit area,
e.g., filtration
rates
Air supply
Optical units
Recommended Units
.Unit
meter per
second
millimeter
per second
kilometers
per second
radians per
second
pascal second
centipoise
newton per
square meter
or pascal
kilonewton per
square meter
or kilopascal
bar
Celsius (centigrade)
Kelvin labs.)
joule
kilojoule
watt
kilowatt
joule per second
Symbol
m/s
mm/s
km/s
rad/s
Pa-s
1
N/m2
or
Pa
kN/m2
or
kPa
bar
°C
°K
J
kj
W
kW
J/s
Comments
1 joule = 1 N-m
where meters are
measured along
the line of action
of force N.
1 watt - 1 J/s
Customary
Equivalents*
3.281 fps
0.003281 fps
2,237 mph
9.549 rpm
0.6722 poundal(s)/sq ft
1.450 X tO'7 Reyn ((j)
0.0001450 Ib/sq in
0.14507 Ib/sq in
14.50 Ib/sq in
l°F-32)/1.8
°C + 273.2
2.778 X10'7
kw-hr •
3.725 X 10'7
hp-hi = 0.7376
ft-lb = 9.478 X '
10-"Btu
2.778 X 10'4 kw-hr
44.25 ft-lbs/min
1.341 hp
3.412 Btu/hr
Application of Unitf
Unit
kilogram per
cubic meter
milligram per
liter (water)
kilogram per
cubic meter
per day
cubic meter
per square meter
per day
cubic meter or
liter of free air
per second
lumen per
square meter
Symbol
kg/m3
mg/l
kg/m3/d
m3/m2/d
m3/s
l/s
lumen/m2
Comments
The density of water
under standard
conditions is 1.000
kg/m3 or 1,000 g/l
or 1 g/ml.
If this is converted
to a velocity, it
should be expressed
in mm/s (Imm/s =
86.4 m3/m2/day).
Customary .
Equivalents*
0.06242 Ib/cu ft
1 ppm
0.06242 Ib/cu ft/day
3.28 tcuft/sq ft/day
0.09294 ft candle/sq ft
"Miles are U.S. statute, qt and gal are U.S. liquid, and oz and Ib are avoirdupois.
ft U. S. GOVERNMENT PRINTING OFFICE: 1977-757-056/5536 Region No. 5-11
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