HomeMy WebLinkAbout95-84 RESOLUTION-f
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RESOLUTION NO.
95-84
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A RESOLUTION AUTHORIZING THE MAYOR AND CITY
CLERK TO EXECUTE A REVISION TO THE CITY'S "201
FACILITY PLAN FOR WASTEWATER MANAGEMENT SYSTEM
IMPROVEMENTS" TO INCLUDE FACILITIES TO
DECHLORINATE THE SEWAGE EFFLUENT.
BE IT RESOLVED BY THE BOARD OF DIRECTORS OF THE CITY OF
FAYETTEVILLE, ARKANSAS:
That the Mayor and City Clerk are hereby authorized and
directed to execute a revision to the City's "201 Facility Plan
For Wastewater Management System Improvements" to include facili-
ties to dechlorinate the sewage effluent.
;rrPASSED AND APPROVED this 21st day of August , 1984.
RY
-4,
By
Clerk
APPROVED
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RESOLUTION NO. 95-84
RESOLUTION TO REVISE
201 FACILITY PLAN
FOR WASTEWATER MANAGEMENT
SYSTEM IMPROVEMENTS FOR
THE CITY OF FAYETTEVILLE, ARKANSAS
TO INCLUDE DECHLORINATION FACILITIES
WHEREAS, the City of Fayetteville, Arkansas has completed a 201
Facility Plan;
and
WHEREAS, the 201 Facility Plan was approved by the Arkansas Department
of Pollution Control and Ecology on May 31, 1984,
and
WHEREAS, since approval of the 201 Facility Plan, the ADPC&E has
required that an evaluation of effluent disinfection alternatives be
completed;
and
WHEREAS, said evaluation has been completed by CH2M Hill and has
concluded that chlorination/dechlorination is the preferred alternative for
disinfection;
and
WHEREAS, dechlorination capability is not
Facility Plan;
NOW, THEREFORE, BE IT RESOLVED BY THE BOARD
OF FAYETTEVILLE, ARKANSAS that said Board hereby.
Plan by inclusion of ,the CH2M Hill Technical
Alternative Analysis.dated July 9, 1984;
and
now included in the 201
OF DIRECTORS OF THE CITY
revises the 201 Facility
Memorandum Disinfection
FURTHERMORE, adopts said Revised 201 Facility Plan
Passed this 21st day of August , 1984.
ATTESTS,
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Lieu
Paul Nolan
Mayor
in its entirety.
itteeP
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TECHNICAL MEMORANDUM
FAYETTEVILLE WASTEWATER TREATMENT PLANT
SUBJECT: Disinfection Alternative Analysis
DATE: July 9, 1984
PREPARED BY: Stephen H. Riley
Thomas P. Walters
John M. Lannon
PROJECT: MG17335.E1
CH2M II HILL
EXECUTIVE SUMMARY
In June 1984, the City of Fayetteville was directed by the
State of Arkansas to provide disinfection (200 fecal
coliform/100 ml) at the new wastewater treatment plant and
provide an effluent with zero residual chlorine. To
accomplish this; four alternative methods of disinfection
were studied:
1. Chlorination/Dechlorination/Post Aeration
2. Ozonation/Post Aeration
3. Ultraviolet Irradiation/Post Aeration
4. Chlorination/Natural Dechlorination/Post Aeration
The chlorination/dechlorination/post-aeration process is the
most commonly used disinfection process in the United States
where effluent chlorine residuals are not allowed. The
process consists of adding chlorine for disinfection and
sulfur dioxide for destruction of residual chlorine. Post
aeration is then provided to raise the effluent dissolved
oxygen level to the required standard.
The ozonation process has received limited use in the United
States in wastewater disinfection applications. The process
entails convertion of air/oxygen to ozone for use as a
disinfectant. A small percentage of the air and oxygen is
actually converted to ozone. When the gas mixture is added
to the wastewater, an increase in dissolved oxygen is
achieved. However, post oxygenation is required to ensure
that the dissolved oxygen standards are met.
Ultraviolet (UV) irradiation is a relatively new technology
in the United States which provides disinfection by allowing
UV waves to penetrate and destroy the bacteria's cell
structure. The process is sensitive to effluent quality and
wastewater characteristics. Post aeration is required to
meet the effluent dissolved oxygen standards.
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Chlorination/natural dechlorination/post-aeration is used
where large storage areas are available to allow the chlo-
rine residual to naturally dissipate. The rates of decay
for chlorine residual are variable and site-specific. Post
aeration would be required to maintain oxygen levels.
Because of the variability of this process and the antic-
ipated difficulty in reliably producing a quality effluent,
a complete analysis of the system was not performed.
The present worth costs of the three remaining technologies
are summarized below:
Process Description
1. Chlorination/Dechlorination/Post-Aeration
2. Ozonation/Post-Aeration
3. Ultraviolet Irradiation/Post-Aeration
Present Worth
$2,092,000
3,543,000.
2,721,000
As shown above, the chlorination/dechlorination/post-aeration
alternative is the most cost-effective. This alternative
was also found to be the most commonly installed and easily
operated process. Based on these factors, the chlorination/
dechlorination/post-aeration process is recommended. The
plans and specifications should be revised to incorporate
this recommendation.
The estimated net increase in the cost of the new wastewater
treatment plant due to the construction of the new
chlorination/dechlorination/post-aeration facilities is:
Increase
Capital
Annual O&M
INTRODUCTION
Costs
$817,000
$ 37,000
In May 1984, the EPA mandated that the Fayetteville Wastewater
Treatment Plant Expansion incorporate a dechlorination
process or utilize an alternative disinfectant.
The purpose of this technical memorandum is to evaluate
disinfection alternatives for the Fayetteville WWTP effluent.
The four processes that were evaluated are:
1. Chlorination/Dechlorination/Post Aeration
2. Ozonation/Post Aeration
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1. Chlorination/Dechlorination/Post Aeration
2. Ozonation/Post Aeration
3. Ultraviolet (UV) Irradiation/Post Aeration
4. Chlorination/Natural Dechlorination/Post Aeration
Each evaluation includes a description of the process, a
review of operational experience, a discussion of design
criteria and equipment recommendations, and a present worth
cost analysis.
The effluent standards used in the analysis are:
o Fecal coliform cbunt = 200/100 ml
o Dissolved oxygen level = 10.2 mg/L (December -March)
7.8 mg/L (April -November)
CHLORINATION/DECHLORINATION/POST AERATION
PROCESS DESCRIPTION
The chlorination/dechlorination/post-aeration process has
been used successfully for many years and is a proven
technology. The chlorination/dechlorination/post aeration
alternative is a three step process designed to first
disinfect the effluent with chlorine; second, remove any
residual chlorine disinfectant with sulfur dioxide; and
third, raise the dissolved oxygen concentration to the
required levels. A simplified schematic of the process is
shown in Figure 1.
Disinfection is accomplished by mixing a high-strength
chlorine solution with the effluent and passing the chlo-
rinated wastewater through a contact basin. The; contact
basin is sized to detain the wastewater for a specific
period of time during which destruction of pathogenic
organisms and most bacteria is'accomplished. To achieve
this level of disinfection, an excess of chlorine must be
applied to the wastewater. The excess, or residual,
chlorine may be in the form of combined or free chlorine.
Dechlorination may be accomplished by a variety of methods
including sulfur dioxide injection, activated carbon adsorp-
tion, ion exchange, aeration and storage. Of these methods,
sulfur dioxide presents the most economical, efficient and
reliable means of wastewater dechlorination.
A sulfur dioxide, or sulfonation system, is virtually
identical to a chlorination system. A high-strength sulfur
dioxide solution is prepared by mixing sulfur dioxide gas
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with a small sidestream of treated wastewater. The solution
is injected into the process train immediately downstream of
the chlorine contact basin. The sulfite ion, SO2, formed
from the dissolution of sulfur dioxide in water, combines
with free and combined chlorine to form HCL. The reaction
occurs instantaneously and a contact basin is not needed.
In the process of reacting with the residual chlorine,
dissolved oxygen (DO) is removed from the wastewater. A
post aeration process must be employed to raise the DO
concentration to the required levels. The selected post
aeration process, as developed in the preliminary WWTP
design, consists of mechanical aeration followed by injection
of an oxygen saturated solution. .
There are several advantages of sulfur dioxide for effluent
dechlorination. These include: •
o Its reaction with chlorine residuals is
instantaneous.
o It is an established and reliable process for
removing residual chlorine.
The equipment required for the sulfur dioxide
system is basically identical to that for
chlorination.
The disadvantages of using sulfur dioxide include:
o It is a hazardous, highly corrosive and irritating
gas.
o It reduces the dissolved oxygen in the effluent.
DESIGN CRITERIA
The effluent chlorination system has been designed to
provide a chlorine dosage of 6 mg/L during a peak day design
flow of 22.8 mgd. This dosage'level is adequate to reduce
the fecal coliform count of a tertiary effluent to 200 per
100 ml; the anticipated State of Arkansas standard. The
peak chlorine demand is 1,140 pounds per day.
The chlorine contact basin must be capable of retaining the
peak hour flow (30 mgd) for a minimum period of 15 minutes
or the annual average flow (11.4 mgd) for 30 minutes;
whichever governs. For the Fayetteville WWTP, peak hour
flow conditions must be used to size the chlorine contact
basin.
Two separate contact cells will be provided, each sized for
the required volume or 41,800 cubic feet, not including
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freeboard. Each cell will consist of a channel with a
length -to -width ratio of approximately 40:1. The channel
will be divided into three passes by longitudinal baffling.
Each pass will be approximately 140 feet in length with a
width of 10 feet and a water depth of 10 feet at peak hour
flow.
The EPA is requiring the allowable residual chlorine concen-
tration in the White River to be 0.0083 mg/L or less. To
achieve this standard, the Fayetteville WWTP will have to
eliminate all residual chlorine on a continuous, year-round
basis. The sulfonation system will be sized to reduce the
residual chlorine concentrations to 0.0 mg/L. For every
1.0 mg/L of residual chlorine to be removed, approximately
1.1 mg/L of sulfur dioxide must be applied.
The sulfonation system will be sized to deliver a dosage up
to 5 mg/L. Based on a 5 mg/L dosage, the peak day (22.8 mgd)
sulfur dioxide demand would be 951 pounds and the average
day (11.4 mgd) demand would be 476 pounds.
Like chlorine, sulfur dioxide is delivered in one -ton
containers. Sulfur dioxide has a much lower vapor pressure
than chlorine and it must be kept at a temperature between
70 degrees F and 100 degrees F to permit gas -phase withdrawal
from the cylinders. The maximum recommended gas withdrawal
rate from ton containers is 250 pounds per day at 70°F. At
higher rates, reliquification of the gas may occur. Based
on this recommendation, 4 one -ton cylinders of sulfur
dioxide must be on-line to satisfy the 951 -pound per day
peak day demand. To assure continuous dechlorination of the
wastewater, an automatic switchover system will be provided
with four full cylinders en standby As with the chlorine
supply, adequate sulfur dioxide will be kept in storage to
satisfy the annual average demand for a 30 -day period.
Thus, storage space will be supplied for 8 -ton containers.
It is recommended that the on-line cylinders be kept in an
enclosed, heated area.
A single 1,900 -pound per day1 sulfonator and 1,000 pounds
per day injector would be used to dissolve the sulfur
dioxide gas into solution. Redundancy will be provided by
connecting the sulfur dioxide gas piping, instrumentation,
and controls to the spare (RAS) chlorinator which will serve
as a standby sulfonator. Treated effluent would be used for
the injector water supply. The solution will be piped to a
perforated pipe diffuser located at the effluent chlorine
contact basin. Mixing of sulfur dioxide will be provided by
mixers located near the diffuser.
1Sulfonators are available in 475 and 1,900 pounds per
day capacities.
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The third treatment unit process is post aeration. The
Fayetteville WWTP is required to maintain a DO concentration
of 10.2 mg/L between the months of December and March and
7.8 mg/L between the months of April and November. To meet
these standards, mechanical aeration followed by injection
of an oxygen saturated solution would be utilized. Effluent
would be discharged to the effluent pump station.
Table 1 presents a list of the required major facilities for
the chlorination/dechlorination/post aeration unit
processes.
OPERATIONAL EXPERIENCE
The Redding, California, advanced wastewater treatment
plant, with a design capacity of 8.8 mgd, utilizes a sulfona-
tion system for effluent dechlorination.. The chlorine
residual analyzers are utilized, one to measure residual
chlorine at the head of the chlorine contact basin and the
other to measure chlorine residual at the tail end of the
basin, immediately upstream of the sulfur dioxide injection
point. The compound loop control method is utilized to
control the chlorine gas feed rate. The signal from the
first analyzer is combined with a flowmeter signal to
maintain the optimum residual at the head of the contact
basin. Likewise, the signal from the second analyzer is
combined with the flow signal to control the gas feed rate
on the sulfonator.
The plant superintendent reports that the system works well
and has not presented any major or continuing maintenance
problems. Daily checking and monitoring of the system
requires 1/2 to 2 hours.
PRESENT WORTH EVALUATION
Table 2 presents a present worth evaluation of the chlo-
rination/dechlorination/post aeration system. The present
worth value of the annual O&M costs is based on an interest.
rate of 10 percent for a period of 20 years.
The estimated construction cost is $1,326,000, the estimated
annual cost is $90,000, and the present worth value is
$2,092,000.
OZONATION/POST AERATION
PROCESS DESCRIPTION
Ozonation has had limited use as a wastewater disinfectant
in the United States and is considered an emerging
technology. Ozonation systems for disinfection of
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Table 1
CHLORINATION/DECHLORINATION/POST AERATION
DESIGN CRITERIA AND EQUIPMENT RECOMMENDATIONS
Facility
Chlorination System
Chlorination Systems
Effluent Chlorination
Contact Basin
Sulfonation System
Post Aeration System
Mechanical Aeration
LOX Storage Tank
LOX Vaporizers
Design Criteria
Peak day
dose = 6 mg/L
Withdrawal rate =
160 lbs/cylinder/day @ 20°F
Provide 15 -minutes
detention at peak
hour flow.
Channel length -to -width
ratio of 40:1
Peak day dose = 5 mg/L
Withdrawal rate =
250 lb/cylinder/day
at 70°F.
Peak week flow; standard
oxygen transfer rate =
2.5 lb/hp-hr; achieve
minimum DO = 6.4 mg/L
during winter.
Provide minimum
3 -month supply
during wintertime
conditions = 6,500 gal.
Peak week flow; provide
8,580 of/day capacity
during wintertime
conditions.
gnTM8/02 -8-
Recommended
Capacity or size
8 cylinders
4 on-line, 4 on standby.
2 contract cells each,
w/41,840 cubic feet
water volume.
Each cell with 3
passes; each 140 feet
in length. Chanel
width = 10 feet. Water
depth at peak hour
flow = 10 feet.
8 cylinders --
4 on-line,
4 on standby.
Two 20 -hp floating
mechanical surface
aerators.
Use existing
11,000 -gallon LOX
storage tank.
Use three existing
36,000-cf/day
vaporizers; total
capacity =
108,000 cf/day.
Table 2
CHLORINATION/DECHLORINATION/POST AERATION
PRESENT WORTH EVALUATION
Item
Construction
Present Worth
Estimate Value
Mobilization/Demobilization $ 42,000
Sitework 53,000
Chlorine Contact Basin 467,000
Chlorination & Sulfonation
Systems
Chemical Building
Post Aeration Basin
167,000
203,000
224,000
Finishes 21,000
Electrical 85,000
Yard Piping 64,000
Subtotal --Construction $1,326,000 $1,326,000
Annual O&M
Chemicals
Chlorine $ 28,000
Sulfur Dioxide 16,000
Oxygen 23,000
Power 11,000
Labor 12,000
Subtotal --O&M $ 90,000 766,OOOa
TOTAL PRESENT WORTH $21092,000a
aEqual to present worth of annual cost at interest rate of 10 percent
for period of 20 years. (Equals Annual Cost x 8.514).
gnTMS -9-
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wastewater include three separate operations --gas
preparation, ozone generation, and ozone dissolution. The
gas involved in ozone generation can be either air,
oxygen -enriched air, or high -purity oxygen. Ozonation
systems fed with oxygen -enriched air or high -purity oxygen
are generally not cost competitive with air feed systems
unless other unit processes require the use of high -purity
oxygen. Therefore, only air -fed ozonators will be
considered for the Fayetteville WWTP.
Ozone (0 1) is generated by passing air through a high-energy
electrical discharge called a corona. In this process,
diatomic oxygen in the air is disassociated into single
oxygen atoms, which then combine with molecular oxygen to
form ozone. To obtain a true corona discharge, there must
be at least one layer of insulating material, called a
dielectric, such as glass. In addition, the feed gas to the
ozonator must be very dry to maintain the highest possible
ozone yield and energy efficiency. The feed gas (air)
should be dried to a dew point of minus 60 degrees F or
less. This step is accomplished by the use of compressors
and refrigerant and desiccant dryers. Because of the
release of most of the energy as heat, a cooling medium such
as water or air is required. Water-cooled systems are
generally used on larger ozonators (greater than 100 pounds,
per day of 03).
Another important step is the dissolution of the ozone in
the wastewater. The most commonly used transfer device for
dissolution of ozone into wastewater is the porous ceramic
tube. The most common interferences in the dissolution of
ozone into wastewater are chemical oxygen demand (COD) and
total suspended solids (TSS) concentration. Efforts should
be taken to minimize the COD and TSS concentrations prior to
ozonation.
The final step in the ozonation process is the collection of
the off -gases from the ozone,contactor and passing them
through an ozone destruction device to reduce the amount of
ozone vented to the atmosphere: A process schematic diagram
of an air -fed ozonation system for the Fayetteville WWTP is
shown in Figure 2.
There are advantages to the use of ozone for effluent
disinfection.' These include:
o Ozone is a powerful oxidizing agent.
o Ozone has a faster reaction time than
therefore requiring a smaller contact
o Ozonation will increase the -dissolved
the wastewater.
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o Ozonated effluents are not believed to be toxic to
aquatic life.
The disadvantages of ozone include:
o Ozone, being unstable, cannot be stored and
therefore must be produced onsite.
o Ozone production requires high power consumption.
o The required ozone dose is dependent upon suspended
solids or turbidity levels in the effluent.
o The process should be piloted prior to selection
to verify the dosages required and effectiveness
of disinfection.
o Equipment maintenance and operation can be very
intensive.
o A small chlorination system would•be required for
RAS chlorination and filter chlorination.
OPERATIONAL EXPERIENCE
The use of ozone for wastewater disinfection has had limited
use in the United States. As part of this evaluation, three
WWTP's using ozone were contacted: (1) Springfield,
Missouri/35 mgd, (2) Murfreesboro, Tennessee/8 mgd, and
(3) Cotter, Arkansas/0.3 mgd.
The Springfield, Missouri advanced wastewater treatment
plant utilizes a high purity oxygen feed system with the
spent oxygen gas vented from the enclosed contact chamber.
and recycled to the high -purity oxygen activated sludge
treatment tank. Maintenance problems have been encountered
with the air-cooled ozone generators. The dielectrics
within the units last about 18 months before replacement.
Installation of new dielectrics is reported to be difficult.
Personnel are evaluating water-cooled generators as -an
alternative.
The Murfreesboro, Tennessee, advanced wastewater treatment
plant utilizes a high -purity oxygen feed system with the
spent gas recycled to the generators. Personnel report that
the ozone generators work satisfactorily but that the ozone
destruction unit on the contact chamber requires frequent
maintenance. The plant is not able to meet disinfection
standards during periods in which the effluent contains high
carbonaceous oxygen demand (COD) and high total suspended
solids (TSS) concentrations.
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The Cotter, Arkansas, installation is a small package -type
treatment plant with air -fed, water-cocled ozone generators.
Personnel reported no operating problems.
DESIGN CRITERIA AND EQUIPMENT RECOMMENDATIONS
The design criteria and equipment recommendations for a
ozonation disinfection system are given in Table 3. The
design criteria are based on actual operating criteria of
existing ozonation system and ozonating equipment manufac-
turers' recommendations. Due to limited ozonation process
experience in wastewater disinfection, it is recommended
that a pilot study be performed prior to final design.
The proposed ozonation system is sized for an ozone dosage
of 10 mg/L at the peak week flow of 17.0 mgd. This represents
a required ozonation capacity of approximately 1,400 pounds
of ozone per day. Three 700 -pound -per -day ozonators (two
are necessary to meet the ozonation capacity requirements
and one is provided for standby capacity) are recommended.
The proposed ozone contactor is sized for a contact time of
15 minutes at the peak hour flow of 30.0 mgd. The covered
contactor will have two trains with four compartments per
train. Each compartment will be separated by a baffle which
will promote an over -and -under flow pattern through the
contactor. Post aeration of the effluent is required even
with ozonation to assure that dissolved oxygen standards are
met. The last compartment of the ozone contactor will be
used to add supplemental pure oxygen from the existing
oxygenation facilities at times when ozonation alone cannot
bring the DO concentration to the required levels. An
additional building to house the air preparation and ozone
generating equipment will be required.
PRESENT WORTH EVALUATION
Table 4 presents a present worth
ozonation/post aeration system.
the annual O&M costs is based on
cent for a period of 20 years.
evaluation of the
The present worth value of
an interest rate of 10 per -
The estimated construction cost is $2,079,000, the estimated
annual cost is $172,000, and the present worth value is
$3,543,000.
ULTRAVIOLET IRRADIATION/POST AERATION
PROCESS DESCRIPTION
Ultraviolet irradiation (UV) has received limited use for
disinfection in the municipal wastewater market in the
United States. To destroy microorganisms, the electro-
magnetic waves of the ultraviolet irradiation must penetrate
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Table 3
OZONATION/POST AERATION
DESIGN CRITERIA AND EQUIPMENT RECOMMENDATIONS
Facility
Ozonation System
Contact Basin
Post Aeration
System
•
Design Criteria
Peak Week dose =
10 mg/L
Provide 15 minutes
detention at peak
hour flow .
Provide supplemental
oxygen to raise DO
to required levels.
gnTM8/04 -14-
Recommended
Capacity or Size
3-700 lb/day
ozone generators;
2 -on-line
1 standby
2 contact cells
each with
4 chambers
separated by
overflow or
underflow baffles
Single cell volume
= 20,900 cubic feet
LOX storage tank,
vaporizers and
eduction system
as outlined in
Table 1
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Table 4
OZONATION/POST AERATION
PRESENT WORTH EVALUATION
Item
Present Worth
Estimate Value
Construction
Mobilization/Demobilization $ 67,000
Sitework 83,000
Ozone Contact Basin 196,000
Ozonation System 1,224,000
Ozonation Building 144,000
RAS Chlorination System
and Building 100,000
Finishes 33,000
Yard Piping 99,000
Electrical 133,000
Subtotal Construction $2,079,000
Annual O&M
Chemicals
Oxygen $ 9,000
Power 139,000
Labor 24,000
Subtotal --O&M $ 172,000
$2,079,000
1,464,000a
TOTAL PRESENT WORTH VALUE $3,543,000a
aEqual to present worth of annual cost at interest rate of 10 percent
for period of 20 years. (Equals Annual Cost x 8.514).
gnTM8/05 -15-
the organism. The germicidal effect of UV energy is
associated with its absorption and destruction of organic
molecular components essential to the functioning of cells.
The UV treatment does not alter water chemically; however, a
slight temperature rise will occur due to the energy applied
to the water. The efficiency of the UV disinfection process
is dependent upon effluent quality. To be effective, the
water must be free of particles that would shield the
organisms and prevent penetration of the radiation.
The system will be ccmprised of a new pump station
discharging to a series of above ground pressurized tanks
containing tubular mercury vapor lamps. The wastewater
would enter the tank and flow through the nest of lamps
mounted axial to the direction of flow. The lamps are
spaced such that each will irradiate a radial area one -inch
from the outer surface of the lamp. After a detention
period within the tank, the wastewater is discharged and
flows to the post aeration unit process as outlined in the
chlorination/dechlorination alternative. A schematic
diagram of the treatment process is shown in Figure 3.
There are advantages of using UV wastewater disinfection.
These include the following:
o UV irradiation is the only physical disinfection
process developed to date.
o Since there is no gas-liquid interface to
penetrate, UV irradiation requires no diffuser
systems.
o UV systems are simple to operate and require no
chemical feed and handling systems.
o UV systems do not require the transportation or
onsite storage of hazardous chemicals.
The disadvantages of UV irradiation include:
o Reliability of UV disinfection is dependent upon
effluent quality. Some of the factors that may
affect penetration of UV energy into wastewater
are solids, turbidity, and color. This results in
formation of a coating or film on the lamp jackets,
which will adversely affect disinfection efficiency.
o The determination of UV dosages is complicated and
calibration techniques vary. This variability
leads to a lack of consistency in dose -response
data. A UV system should be piloted prior to
installation.
o The process is energy intensive.
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station:
OPERATIONAL EXPERIENCE
The Albert Lea, Minnesota WWTP, with an 18-mgd capacity,
utilizes an enclosed UV disinfection system. The system
routinely reduces fecal coliform counts from 10,000 to 200
per 100 ml or less with a dosage of 16,000 pW-sec/cm2.
Operators report no operational problems with the equipment.
However, the cost of bulb replacement is high.
UV systems piloted for wastewater di2sinfection have utiliz2ed
dosage ranges from 16,000 pW-sec/cm to 250,000 jW-sec/cm .
Reduction of fecal coliform bacteria counts to less than
200/mL have been accomplished with dosages in the range of
20,000 to 35,000 uW-sec/em . However, there are indications
that certain types of bacteria and viruses may become
reactivated if exposed to energy wavelengths in the visible
light range. This phenomenon, know as photoreactivation,
was the cause of a one -log reacivation of fecal coliform
subjected to a 35,000 pW-sec/cm dosage in one study.
DESIGN CRITERIA AND EQUIPMENT RECOMMENDATIONS
The design criteria and equipment recommendations for the UV
disinfection and post aeration system is presented in
Table 5. The designcriteria is based on pilot studies and
equipment manufacturer's recommendations. It is recommended
that a pilot study be performed pricr to final design.
For the purpose of this evaluation, it has been assumed that
a dosage of 16,000 iW-sec/cm will provide a permanent fecal
coliform reduction to a value less than the required
200 count/100 mL. The detention time within the irradiation
tanks is based upon 10 seconds at the peak hour flow of
30 mgd. The UV lamp requirement is based on 12 gpm/lamp.
Based on this criteria, a system of five irradiation tanks,
each with a 6 mgd treatment capacity has been developed.
Each tank will house approximately 350 lamps. The tanks
would be approximately 5 feet in diameter and 12 feet long.
The actual dosage should be determined through piloting.
The estimated head loss through the treatment tanks is 2 to
3 feet. Because of this additional head loss and the
ground -level location of the tanks, a low lift pump station
would be needed upstream of the units to provide sufficient
head to discharge during high river stages.
-18-
r'.
Table 5
ULTRAVIOLET IRRADIATION/POST AERATION
DESIGN CRITERIA AND EQUIPMENT RECOMMENDATIONS
Facility Design Criteria
UV System Peak hour dose =
16,000 mWs/cm2
1 lamp/12 gpm
at peak hour flow
Detention time =
10 seconds at peak
hour flow
Recommended
Capacity or Size
5-6 mgd treatment
tanks each con-
taining 350 lamps;
cylindrical tanks -
5 ft diameter,
12 ft long
In -Plant Pump Peak hour flow 5-25 hp pumps
Station with wet well
Post Aeration Provide air and
System-- supplemental oxygen
to raise DO to
required levels
gnTM8/06 -19-
Mechanical aerator,
LOX storage tank,
vapirozers and
eduction system as
outlined in Table 1
The post aeration system will
and oxygenation as outlined in
alternative.
PRESENT WORTH EVALUATION
consist of mechanical aeration
the chlorination/dechlorination
Table 6 presents a present worth evaluation of the UV/post
aeration system. The present worth value of the annual O&M
costs is based on an interest rate of 10 percent for a
period of 20 years.
The estimated construction cost is $1,819,000, the estimated
annual cost is $106,000 and the present worth value is
$2,721,000.
CHLORINATION/NATURAL DECHLORINATION/POST AERATION
Prolonged storage of chlorinated wastewater will result in a
gradual (natural) dissipation of residual chlorine. Under
controlled conditions, the residual decreases in a linear
relationship with time. However, field conditions,
including the presence of sunlight and wind, tend to
increase the rate of dissipation. Research on residual
chlorine dissipation in natural waters is -limited. Data
presented in an EPA document entitled "Impacts of Wastewater
Disinfection Practices on Coldwater Fisheries," dated
July 1982, indicated decay rate values ranging over three
orders of magnitude.
There are advantages to the use of storage as a means of
dechlorination. These include:
o It is the least costly dechlorination alternative
in terms of power and chemical costs.,
The disadvantages of storage include:
o Sufficient operational data does not exist by
which required storage time and storage volume
could be established -for the design.
o The residual dissipation rate is sensitive to
environmental conditions such as wind and sunlight.
The chlorine residual concentration in the storage
pond could be expected to be highly variable based
on weather conditions.
o Storage of effluent within an open pond could
subject the effluent to increases in suspended
solids from algal growths and vegetation, increases
in coliform count and BOD from wildlife, and
decreases in the near saturation dissolved oxygen
levels attained in the post aeration unit process.
-20-
Table 6
ULTRAVIOLET IRRADIATION/POST AERATION
PRESENT WORTH EVALUATION
Present Worth
Item
Estimate Value
Construction
Mobilization/Demobilization
$ 58,000
Sitework
73,000
S
UV System
888,000
UV Building
144,000
In -Plant Pump Station
200,000
Post Aeration Basin
224,000
Finishes
29,000
Yard Piping
87,000
Electrical
116,000
Subtotal --Construction
$1,819,000 $1,819,000
Chemicals --
Oxygen 23,000
Acid (for lamp cleaning) 1,000
Power 31,000
Labor 18,000
Replacement Lamps 33,000
Subtotal --O&M $ 106,000
TOTAL PRESENT WORTH VALUE
$ 902,000ra
$2,721,000
Equal to present worth of annual cost at interest rate of 10 percent
for period of 20 years. (Equals Annual Cost x 8.514).
gnTM8/07 -21-
o To comply
ponds wou
with, for
residual.
it is not
achieved.
with discharge standards, the storage
Id have to reliably produce an effluent
all practical purposes, zero chlorine
Due to an absence of operational data,
known if this can consistently be
Due to the significant disadvantages of pond storage
dechlorination, this alternative has been eliminated from
further consideration.
PROCESS COMPARISONS
The comparison of the three remaining alternatives is
summarized in Table 7. The table illustrates the
chlorination/dechlorination alternative to have the highest
ranking.
This ranking is, primarily the result of the chlorination/
dechlorination system having a lower capital and operation
and maintenance cost. The chlorination/dechlorination
system is also less susceptible to the effects of energy
escalation. The ozone and ultraviolet irradiation process
are both high energy users. A rise in energy costs could
result in an additional operating expense.
In addition, the operation of the chlorination/dechlorination
system may not be required on a continuous basis if the
flows in the White River are great enough and testing
proves chlorine is immediately taken up by the organic
material in the river. A testing program could be initiated
with the approval of the State of Arkansas after the plant
comes online.
The chlorination/dechlorination process is a less complex
system to operate as the equipment used can be interchanged
in the event a mechanical failure occurs. The ozone and UV
systems have several additional pieces of mechanical
equipment which would require periodic maintenance and
adjustment.
The chlorination/dechlorination system has been used
throughout the United States and would not require a piloting
program to verify its effectiveness and reliability. The
ozone and UV systems are relatively new technologies and
would require piloting to develop design criteria to assure
sufficient disinfection.
-22-
11
Table 7
COMPARATIVE RANKING OF DISINFECTION ALTERNATIVES
Pointsa
Chlorination/
Ultraviolet
Assigned
Dechlorination
Ozonation
Irradiation
Present Worth
30
30
10
20
Process Reliability
20
15
8
8
Process Experience
20
15
10
8
Complexity of Operation
20
15
10
15
Land and Safety
Requirements
10
5
9
7
TOTAL
100
80
47
57
a Higher values indicate more positive ranking.
gnTM8/08 -23-
RECOMMENDATIONS
The chlorination/dechlorination system is recommended
because of its lower costs, easier operation, and process
reliability. A revised site plan is shown in Figure 4 and
Table 8 illustrate the recommended design criteria to be
used.
The plans and specifications would be revised to reflect:
1. A new chlorine contact basin
2. A new post aeration basin/liquid oxygen system
3. A new hydraulic profile to account for the new
basins, piping layouts, -and miscellaneous
revisions
4. A revised road, drainage, and grading plan for the
affected area.
5. Electrical system revisions to revise the chlorina-
tion system.
6. A new dechlorination instrumentation system with
chlorine residual analyzers.
7. An enlarged chlorine and sulfur dioxide area in a
new Chlorination/Dechlorination Building. This
area would require heating and ventilation.
8. The existing drawing and specifications will
require updating for piping, HVAC, electrical,
instrumentation and control, structural, and civil
disciplines.
A preliminary review of the hydraulics and site layout
indicates the chlorination/dechlorination/post aeration
process can be located approximately at the same location
the original post aeration basin was to be located. Minimal
impact on the hydraulic profile can be achieved by
redesigning the yardpiping downstream of the filters,
relocating the parshall flume, redesigning the entrance and
exit structures of the basin to minimize energy losses and
relocating the high purity oxygen system. The 60 -inch pipe
to the effluent pump station will remain with minor
rerouting. The smaller conduits (pipes, electrical, storm
drains, etc.) will be revised to reflect the new structures.
The chlorination building and storage area will be
redesigned to provide an enlarged area and a heated space
for the rew sulfur dioxide facilities. The chlorination
equipment and instrumentation will be revised to provide a
residual analyzer/feed back control system to control the
-24-
Table 8
DESIGN CRITERIA --SUM ARY AND RECONJ
I
I
Process Design Criteria
Chlorination System --
Chlorine Cylinders
RAS Chlorination
Reserve
Chlorinators
Injectors
Injector Water Supply
Parshall Flume
Contact Basin
Sulfonation System --
Sulfur Dioxide Cylinders
Reserve
Sulfonators
Injectors
Post Aeration System --
Mechanical Aeration
gnTM8/09
Peak day dose = 6 mg/L
Withdrawal rate =
160 lbs/cylinder/day
4 lb C1/1,000 lbs
MISS; 5.9 mg aeration
tank volume with
withdrawal rate =
160 lbs/cylinder/dap
3,500 mg/L MLSS
annual average flow
Annual Average Flow;
30 -day supply
Peak Day
Peak Day
60 gpm per injector
total --120 gpm
Peak hour
Provide 15 -minutes
detention at peak
hour flow
Channel length -to -
width ratio of 40:1
Peak day dose = 5 mg/L
Withdrawal rate =
250 lb/cylinder/day
at 70"F
Annual Average Flow;
30 -day supply
Peak Day
Peak Day
Peak week flow; standard
oxygen transfer rate =
2.5 lb/hp-br; achieve
minimum DO = 6.4 mg/L
during winter
Recommendations
8 cylinders --
4 on-line, 4 on standby
5 cylinders
• 9 cylinders
Two 2,000 lb/day
chlorinators
Two 2,000 lb/day
injectors
W-3 water supply
w/connection to
existing W-2 system
for emergency
30.0 mgd
capacity Parshall Flume
2 contact cells each
w/41,840 cubic feet
water volume
Each cell with 3
passes; each 140 feet
in length. Chanel
width = 10 feet.
Water depth at peak
hour flow = 10 feet
8 cylinders --
4 on-line,
4 on standby
8 cylinders
One 2,000 lb/day
sulfonator
One 2,000 lb/day
injector
Two 20 -hp floating
mechanical surface
aerators
,
Table 8
(continued)
Process
Design Criteria
Recommendations
LOX Storage Tank
Provide minimum
Existing
3 -month supply
11,00G -gallon LOX
during wintertime
storage tank
conditions = 6,500 gal
LOX Vaporizers
Peak week flow; provide
Existing
8,580 cf/day capacity
36,000-cf/day
during wintertime
vaporizers; total
conditions
capacity =
108,000 cf/day
Sidestream Piping
Provide 15 seconds contact
185 Linear feet of
time at Q = 270 gpm
3 -inch -diameter pipe
Eductors
Provide 270 gpm capacity
Eight new 3/4 -inch
eductors
-27-
chlorination/dechlorination system. The existing drawings
will be revised to reflect the structural, mechanical,
electrical, and instrumentation changes. These changes will
• impact some areas outside the chlorination/dechlorination/
post aeration system because of the interconnecting/
interface relationship of the plant.
The anticipated schedule for accomplishing the revisions
would be:
o Receive verbal approval from the City of
Fayetteville and the State of Arkansas by July 13,
1984, to begin redesign effort.
o Complete redesign effort 8 weeks after authoriza-
tion to process (September 7, 1984)..
o Print project for advertisement by the end of
September 1984.
o Advertise for bids in October and November.
o Receive bids first week of December 1984.
gnTM8/01
-28-
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