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Clinton Street CSO Abatement Project Facility Plan (Nov 2005)
by Environmental Engineering Associates, LLP
for Onondaga County Department of Water Environment Protection
2. Clinton Street CSO treatment requirements
This section presents a description of the CSO
treatment requirements for the Clinton Street CSO
service area, as necessary to achieve compliance with
the ACJ. This section also includes a summary overview
of the various treatment technologies/approaches
available to meet these compliance objectives.
As presented in
Section 1.1,
pursuant to Paragraph 14
of the ACJ, Onondaga County is required to implement a
CSO control and upgrade program as stated below:
The County shall design, construct,
maintain, and modify and/or supplement, as necessary,
a CSO control and upgrade program in accordance
with DEC CSO guidance, as set forth in TOGS 1.6.3 (CSO
Control Strategy), which implements the "presumptive
approach" in EPA's control policy, as set forth in 59
F.R. 18688 (April 18, 1994). The County's program
shall achieve the following:
A. elimination or the capture for treatment of no less
than 85% by volume of the combined sewage collected in
the combined sewer system during precipitation events
on a system-wide annual average basis;
B. elimination or minimization of floating substances
in Onondaga Lake attributed to the County's CSOs; and
C. achievement of water quality standards for bacteria
for all portions of Onondaga Lake that are classified
as "Class B" pursuant to 6 NYCRR Part 895.
To achieve compliance with the above requirements, the
County shall complete the specific tasks by the
applicable milestone compliance date set forth in the
CSO Control and Upgrade Schedule that is attached as
Appendix B and hereby made an enforceable part of this
Amended Consent Judgment. All elements of the CSO
Control and Upgrade program shall be completed and in
full operation on or before January 1, 2012.
Consistent with the EPA's CSO Control Policy, the
County shall implement such additional upgrades and
other measures, subject to DEC's approval, as may be
necessary to ensure that the CSO discharges remaining
after implementation of the CSO Control and Upgrade
Program do not cause or contribute to conditions in
violation of water quality standards or impair the
designated best uses of the receiving waters.
In an effort to demonstrate compliance with the 85%
capture requirement, the County retained the services
of an independent sewer system modeling consultant
(Brown and Caldwell, formerly Moffa & Associates), and
developer of the County's computerized Storm Water
Management Model (SWMM), to execute the County's
calibrated SWMM model using the various CSO compliance
projects included in the ACJ. The SWMM model
executions were run using a long-term simulation
approach based upon over 30 years of precipitation
data for the Syracuse Metropolitan area. Due to the
ongoing facilities planning programs and the
implementation of facilities constructed in connection
with the various CSO compliance projects included in
the ACJ, the CSO volume capture tables prepared by
Brown and Caldwell for future conditions are
periodically updated to reflect the current
information available at the time of each facility
plan or design update. Tables 2-1, 2-2 and 2-3 are
the most current CSO volume capture tables as
presented in the draft report titled "Harbor Brook CSO
Abatement Facilities Plan", prepared by Brown and
Caldwell, dated December 29, 2004.
For existing conditions, without implementation of any
CSO abatement projects, the County Sanitary District
trunk and interceptor sewer system transports 74% of
the average annual volume of combined sewage collected
within the City of Syracuse collector and combined
sewer system to Metro for treatment. A summary of
this volume capture information is presented in Table
2-1.
Upon implementation of the ACJ CSO abatement projects,
as described above, the average annual volume of
combined sewage collected within the combined sewer
system for transport to Metro for treatment is
projected to be 89-91%. A summary of this volume
capture information is presented in Table 2-2. These
data indicate that the fully-implemented ACJ CSO
compliance projects will satisfy the 85% capture
requirements of the ACJ.
Table 2-2 indicates that in order for the County to
achieve the 89 to 91% capture of the average annual
combined sewage volume, the Clinton Street CSO
Abatement project must eliminate or capture for
treatment at least 107 million gallons (MG) of
combined sewage, on an average annual basis. This,
therefore, represents one of the CSO treatment
conditions for the Clinton Street CSO Abatement
project. Table 2-3 presents the CSO volume capture
information for the combined treatment of CSO both at
Metro and the proposed ACJ CSO compliance projects.
The project will achieve compliance with the ACJ
floatables requirement by providing adequate
facilities to either eliminate or minimize floating
substances from discharging to Onondaga Creek from the
Clinton Street CSOs.
The project will achieve compliance with the ACJ
bacterial requirements based on recently completed
supplemental execution of the Annual Simulation Fecal
Coliform Model for Onondaga Lake (bacteria model),
assuming full implementation of the most recent
planned ACJ CSO compliance projects. The bacteria
model was executed assuming that each of the RTFs that
included disinfection prior to discharge to receiving
waters (i.e., Onondaga Creek and Ley Creek) would
achieve bacterial concentration reductions to less
than 200 colony forming units (cfu)/100 milliliters
(mL) for the 1-year design storm event. Appendix A
presents the updated bacterial model information for
Onondaga Lake.
The bacteria model information presented in Appendix A
indicates that if the current planned RTFs with
disinfection, including the Clinton Street RTF,
provide sufficient bacterial reduction to less than
200 cfu/100 mL for the 1-year design storm event prior
to any discharge (Scenario 1), the bacteria standard
for water quality requirements in Class B waters of
Onondaga Lake will be met or exceeded.
Click here for Appendix A. Bacterial model update for Onondaga Lake.
In summary, the following presents the minimum CSO
treatment requirements for the Clinton Street CSO
Abatement project to achieve ACJ compliance.
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Eliminate or capture for treatment at Metro at
least 107 MG of combined sewage on an average annual
basis;
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Provide adequate facilities to eliminate or
minimize floating substances (floatables) from
discharging to Onondaga Creek from Clinton Street
service area CSOs; and
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Provide sufficient facilities to reduce bacterial
concentrations to less than 200 cfu/100 mL for all
Clinton Street service area CSO discharges for the
1-year design storm event.
In addition, the Clinton Street CSO abatement
facilities must satisfy the requirements outlined on
Pages 7 and 8 of Appendix B of the ACJ, copies of
which are provided in Appendix B.
Click here for Appendix B. Pages 7 and 8 of ACJ Appendix B—Clinton CSO Project Description.
Based upon the specific CSO treatment requirements for
the Clinton Street CSO service area, as discussed in
Section 2.1, the following CSO treatment approaches
are determined to be capable of achieving these
requirements.
The separation of combined sewers into separate
sanitary and storm sewers is a historical method of
eliminating CSO discharges. Separation normally
requires the construction of a new sanitary sewer
system parallel to the existing combined sewer system.
The combined sewer system is then left in place to
serve as a storm sewer, and all sanitary connections
are switched over to the new sanitary sewer line. In
effect, sewer separation is an abatement method that
achieves a high degree of gross pollutant removal and
provides a system that requires low operation and
maintenance. While separation results in the
elimination of the sanitary sewage component of the
CSO discharge, the storm water component will continue
to discharge pollutants associated with urban runoff,
which may impact the program's ability to meet water
quality standards. Generally, sewer separation is most
cost-effective when applied to smaller CSO drainage
basins. Major sewer separation projects can be very
disruptive of urban neighborhoods, and separation of
large drainage basins may require the construction of
storm water treatment facilities to meet forthcoming
federal water quality regulations. Figures 2-1 and 2-2
depict graphical representations of a combined sewer
system and separate sewer system, respectively.
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Figure 2-1. Sewer separation (combined sewer shown)
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Figure 2-2. Sewer separation (separate sewers shown)
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If sewer separation were to be implemented, the City
of Syracuse would be responsible for providing
separate storm water treatment facilities to meet
potential future water quality requirements. In
addition, the City would assume ownership and
maintenance responsibility for the newly installed
separate sanitary and storm sewers and the existing
combined sewers that are converted to storm sewers.
Regional conveyance, in-line storage and treatment can
provide a cost-effective means of abatement for CSO
discharges from multiple overflow points. Piping is
required to divert overflows to a central or regional
treatment site. While land requirements may be sizable
for a regional facility, the routing of the conveyance
system may be designed to transport the overflows to a
suitable site. This provides increased flexibility in
site selection and reduces the number of neighboring
properties that may be impacted, which is often
difficult in urban areas. Regional facilities usually
provide cost advantages by reducing the number of
parcels of land to be acquired and consolidating
construction and operation and maintenance (O&M)
activities to a single site.
Although the ultimate performance of the facility is
dependent upon the treatment process utilized,
disinfection results may be improved through low
loading rates and greater contact times which can be
more readily accommodated by a larger facility.
Regional facilities also have the advantage of
flexibility in handling a wide range of flow rates
from different size storm events as well as
back-to-back storm events. The conveyance system may
also be oversized to attenuate flows and reduce the
size of the regional treatment facility.
Regional conveyance and storage would include the
installation of conveyance piping to collect and
divert overflows to a central or regional storage
site. The CSO volume generated during a wet-weather
event would be stored in a regional storage facility.
Following the wet-weather event, the stored CSO volume
would be discharged to a municipal wastewater
treatment plant (in this case, Metro) for at least
primary treatment prior to discharge.
Storage is divided into two categories; either in-line
or off-line. In-line storage is one of the more
cost-effective approaches to achieving CSO volume
reductions by utilizing the conveyance capacity of the
combined sewers to attenuate flow. Because combined
sewers are typically sized to carry the maximum flow
from the design storm event, during most storms there
is considerable unused capacity in the sewers. By
controlling the conveyance of flow, water levels in
the sewers can be caused to back up in the existing
sewers, thus utilizing the available capacity. In-line
storage can also be designed into new conveyance
pipelines. The disadvantages to using in-line storage
in existing sewer systems may include: the increased
risk of basement or street flooding; increased
opportunity for sediment deposition, and higher costs
associated with increased maintenance of the flow
control devices (i.e., weirs, orifices, gates, vortex
valves)
Off-line storage consists of tankage constructed to
store flows diverted from the combined sewer system.
There are two approaches to sizing the volume of the
off-line storage; using the volume of a specific
design storm and using long-term simulations to
address the impacts of back-to-back storm events. A
disadvantage of sizing off-line storage according to a
design-storm volume is the potential inability of the
facility to capture a follow-on storm. A follow-on
storm could occur before the storage facility and/or
retained active CSO is dewatered from a preceding
storm event, resulting in a potential untreated
overflow discharge from the storage facility and water
quality violations during the follow-on storm. The use
of the long-term simulation approach results in a
larger storm volume that better addresses the
back-to-back storm situation.
Once the storage capacity is reached, additional flow
is discharged through or bypassed upstream of the
storage facility. For large storm events, little or no
treatment would be provided once the storage facility
was filled. Due to the large volume of CSO to be
stored, regional storage facilities may need to be
larger in size. Siting of these facilities in urban
areas may be difficult because of lack of land on
which to construct them.
Storage facilities may be appropriate for the control
of specific pollutants and can be beneficial in the
reduction of annual CSO discharge volume. Recent
storage applications incorporate tipping buckets or
other flushing mechanisms for the removal of solids
from the tank bottom following dewatering. Care must
be taken to site storage facilities only in those
locations where downstream interceptor or trunk sewers
are not subject to sedimentation. Manual cleaning of
the storage tanks will also be required, however, on a
regular basis to minimize odor causing conditions
within the tanks.
It should be noted that centralized collection and
conveyance of CSOs to the Metro plant for storage and
subsequent treatment at Metro was evaluated as part of
the County's 1991 CSO Facilities Plan, and re-assessed
as part of the 2001 CSO Evaluation Report. The 1991
CSO Facilities Plan and 2001 CSO Evaluation Report
concluded that centralized conveyance, storage, and
treatment of CSOs at the Metro plant is not
cost-effective and may not be physically feasible.
In conjunction with the above-noted CSO treatment
approaches, a “long-list” of CSO treatment
technologies was evaluated to determine those most
appropriate for further evaluation when considered as
part of a Regional Conveyance and Treatment approach.
These alternative CSO treatment technologies and their
ability to provide floatables, settleable solids, and,
in some cases enhanced pollutant removals for
biochemical oxygen demand (BOD), total suspended
solids (TSS), total kjeldahl nitrogen (TKN), and total
phosphorous (TP), are described below. A preliminary
screening of the feasibility of these technologies for
further evaluation is also presented below.
A. Vortex separators
Vortex separators remove floatables and settleable
solids by directing the flow tangentially into a
cylindrical tank, creating a vortex. The vortexing
action tends to concentrate settleable solids toward
the center of the tank and removes the concentrated
solids through a foul sewer outlet located at the
bottom of the tank. The influent flow travels under a
scum plate that captures floatables and spills over a
circular weir located in the center of the tank. The
vortex separator has no moving parts and is designed
to operate under extremely high flow conditions.
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Figure 2-3. Vortex separator schematics
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It has been reported that vortex separators are
capable of removing up to 90% of settleable solids, up
to 35% of TSS and BOD; some nominal removals of TKN
and TP (between 5 and 15%) have also been reported. In
some applications, no power is required for operation
of the unit as the influent and underflows may be
conveyed by gravity through the vortex separator.
Depending on the available hydraulic head, pumping of
the vortex influent flows or the underflows may be
required. If influent flows require pumping, large
capacity pumps are required. Operation and maintenance
requirements are low since the majority of the
captured settleable solids and floatables are
discharged into the foul sewer during and immediately
following the storm event. However, if large capacity
(horsepower) pumps and odor control systems are
employed, they may significantly increase the energy
costs associated with the treatment technology.
Due to its solids and floatables removal efficiency,
generally low operation and maintenance requirements,
and proven performance in CSO applications, the vortex
separator technology is considered to be appropriate
for further evaluation. Schematics of the three types
of vortex separators are shown on Figure 2-3.
B. Enhanced vortex separators
Enhanced vortex separators include the use of
dissolved air flotation or physical-chemical
flocculation additives to enhance the operation of the
vortex separators. To date, this technology has only
been demonstration-tested in one location
(Scarborough, Canada) at low operating/loading rates.
Results of this testing have demonstrated slightly
enhanced removal efficiency in the vortex separator.
However, these results occurred at low
operating/loading rates and it was reported that a
lengthy start-up time was required to stabilize the
operation prior to achieving the enhanced removals.
Due to the fact that all CSO treatment facilities will
be required to operate at fairly high loading rates
due to the peak flow characteristics of the storm
events, and that a period of time is required to
stabilize the system, this technology is not
considered appropriate for further evaluation at this
time. However, if this technology becomes further
advanced and water quality considerations require an
enhanced removal efficiency that may be available from
this technology, additional consideration can be
applied to this technology.
C. Continuous deflective separation (CDS)
CDS is a variation of the vortex separator technology.
The CDS consists of a cylindrical tank which utilizes
a physical barrier, typically, a fine screen, between
the influent flow and outlet discharge. Flows enter
the CDS tank tangentially and are deflected from the
discharge by entering a deep sump. Flows are conveyed
into the center of the sump and must pass through a
screen before proceeding to the discharge. The
continuous swirling action in the sump causes heavier
solids to fall to the bottom and keeps them away from
the screen, thereby eliminating the need for a
cleaning mechanism. However, solids accumulated in the
bottom of the sump must be removed at the conclusion
of a storm event. The CDS manufacturer also reports
that periodic removal of solids from the sump during
storm events may be required to prevent these solids
from accumulating too densely and blocking the
discharge screen.
Since the screen provides openings of less than
1/6-inch, this technology is capable of removing small
solids and floatables, as well as TSS (reported to be
up to 10%). Operation and maintenance of this system
includes disposal of the collected solids at the
conclusion of (and possibly during) the storm event.
This can be accomplished by installing sump-pumping
facilities, using a clamshell bucket, or a vacuum
truck. Operation and maintenance requirements also
include properly cleaning the screen following the
storm event. Figure 2-4 provides a schematic of this
treatment technology.
Due to its capability to remove floatables and smaller
solids, this technology is considered to be
appropriate for further evaluation.
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Figure 2-4. Continuous deflective separation (CDS) schematic
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D. Ballasted flocculation
Ballasted flocculation is a high-rate coagulation and
sedimentation treatment process that introduces
flocculation and coagulation agents during high-speed
mixing to promote settlement and enhance solids
removal. In the process, flow enters the first zone of
the facility where a coagulating agent is added and
mixed with diffused air. The coagulating agent is
typically a metal salt or polymer. The flow then
enters the second zone where a flocculating agent
together with a flocculating aid, either recirculated
sludge or microsand (i.e., ballast), is added. In this
area, intense mixing occurs to promote the formation
of suspended floc particles. The flow then enters the
settlement zone where the dense flocs settle out and
are concentrated at the bottom of the basin. Clarified
effluent passes through an inclined plate or tube
settlers to remove residual floc particles and the
final effluent is discharged. The concentrated solids
are either recycled back to the second zone or wasted.
Concentrated solids from technologies utilizing sand
as a flocculating aid are conveyed through a
separation process where the sand is separated from
the waste solids and recycled back into the process or
stored for future flow events. A schematic of a
typical ballasted flocculation (high-rate flocculated
settling) system is presented on Figure 2-5.
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Figure 2-5. Typical ballasted flocculation schematic
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Ballasted flocculation has been reported to be capable
of removing nearly 100% of settleable solids, up to
84% of TSS, 54% of BOD, 25% of TKN, and 90% of TP in
CSO applications. However, it is reported that the
system requires approximately 10 to 30 minutes startup
time in order to stabilize before it is able to
accomplish the above-stated pollutant removal
efficiencies. In addition, preliminary screening of
solids greater than 1/6-inch-diameter is required
before the flow is treated with ballasted
flocculation. The operation and maintenance concerns
associated with the technology are high, considering
the requirements for large quantities of chemicals,
solids processing and recycling, and high energy
consumption during a storm event. In addition,
significant land area is required to accommodate the
necessary multiple treatment processes. The ballasted
flocculation system provides a high degree of
treatment, but due to the time required to stabilize
the system and its high operation and maintenance
requirements, this technology is not considered
appropriate for further evaluation.
E. Coarse screening
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Figure 2-6. Typical coarse screening devices
Source: John Meunier, Inc., US Filter
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A coarse screening device consists of vertical or
inclined bars, typically spaced greater than 1-inch,
which remove floatables, rags, sticks and solids
greater than the bar opening by capturing them on the
bars. Influent flow travels perpendicular to and
through the bars. Debris that is too large to pass
through the openings is retained on the bars and
removed by manual or mechanical raking arms. Screens
placed in CSO applications are subject to rapid
blinding; therefore, mechanically cleaned bar screens
are necessary for effective operation. Typical coarse
screening devices are shown on Figure 2-6.
Coarse screens are used with proven results at raw
sewage pumping stations and headworks of wastewater
treatment plants to prevent large objects and stringy
materials from damaging downstream pumps and process
equipment. Several CSO coarse screening installations
exist in the United States; however, most are used as
preliminary treatment devices to protect downstream
processes. Operation and maintenance requirements for
intermittently operated coarse screening devices are
fairly high and, typically, the coarse screening
facility will require additional attention at the
conclusion of a storm event. Since minimal BOD and TSS
removal is accomplished by the screens, additional
provisions may be required to remove these pollutants
in order to achieve target bacterial reduction limits,
as both BOD and TSS impart their own disinfectant
demand in wastewater.
Due to its small space requirements in relation to
other CSO technologies and proven floatables and
solids removal efficiency, this technology is
considered to be appropriate for further evaluation.
F. Fine screening
Fine screens are similar to coarse screening devices
with the exception that they remove smaller size
solids by capturing them on a bar screen with openings
typically less than 1-inch and greater than 1/6-inch.
Fine screens are continuously cleaned by a mechanical
raking device, and captured solids are either removed
or directed to a foul sewer for disposal.
Similar to coarse screens, fine screens are proven
devices in removing floatables and solids from a flow
stream. The screens can be situated either
horizontally or vertically, depending on the
manufacturer. Operation and maintenance requirements
can be significant, as the facility may need
additional attention for equipment
maintenance-cleaning at the conclusion of a storm
event. Since minimal BOD and TSS removal is
accomplished by fine screens, additional provisions
may be required to remove these pollutants in order to
achieve target bacterial reduction limits, similar to
coarse screening. Typical fine-screening devices are
shown on Figure 2-7.
Due to its small space requirements in relation to
other CSO technologies and proven floatables and
solids removal efficiency, this technology is
considered to be appropriate for further evaluation.
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Figure 2-7. Typical fine screening devices
Source: Left: Waste Tech, Inc., Right: Parkson Corporation
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G. Brush screens
A brush screen is a relatively new innovation for
solids removal. The screen consists of fine bristles
which provide effective solids removal down to 4
millimeters (1/6 inch) in diameter. The brush screen
is mounted horizontally on a center shaft that rotates
countercurrent to the flow being treated. The rotating
brush is cleaned by a fixed comb that directs captured
solids into a collection trough. Figure 2-8 depicts a
typical brush screen.
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Figure 2-8. Typical brush screen
Source: Grande Novac & Associates, Inc.
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Brush screens have been applied to CSOs in Europe and
are reported to be somewhat effective at removing
floatables and solids from the waste stream. There are
currently no operating installations in the United
States treating CSOs. Operation and maintenance
requirements are fairly high, however, as the brush
screen is reported to have a tendency to capture and
retain stringy materials that ultimately wrap around
the shaft making cleaning difficult.
Due to the limited operating experience of brush
screens for CSO treatment and the tendency to
accumulate stringy materials and create a potentially
significant maintenance issue, this technology is not
considered feasible for further evaluation.
H. Rotary drum screens/sieves
Rotary drums and sieves remove solids by passing flows
through a rotating screen. Flows can be introduced
from either the interior or exterior of the rotating
drum, depending on the manufacturer. Rotary drums that
receive flows from the exterior, screen the flow as it
passes through a perforated drum or sieve, retaining
captured solids on the outside of the rotating drum.
The screened flow is then discharged and the retained
solids are either scraped or scoured off into a
collection trough. Rotary drums that receive flow from
the interior of the drum, screen the flow as it passes
through a perforated drum or sieve, leaving captured
solids on the interior of the rotating drum. The drums
are typically inclined to promote migration of the
captured solids to a disposal trough.
Rotary drums and sieves remove solids greater than 2
millimeters in diameter; therefore, a coarse screening
device is required to precede the rotary drum in order
to prevent larger solids from collecting on and
blinding the drum. Some rotary drums have experienced
rapid blinding due to hair pinning. Hair pinning
occurs from fibrous material becoming interwoven with
the drum perforations or wire sieves creating
maintenance problems. In order to maintain a clean
screening surface, a continuous high-pressure water
wash is required during a screening event. Due to the
rotary drums' small openings, it has been reported
that up to 20% of influent TSS and BOD can be removed
through use of this technology. An example of a rotary
drum screen is depicted on Figure 2-9.
In consideration of the removal efficiencies for
smaller sized solids and the ability to remove gross
pollutants, this technology is considered to be
appropriate for further evaluation.
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Figure 2-9. Typical rotary drum screen
Source: John Meunier, Inc.-US Filter
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I. Microscreens
Microscreens are used to remove fine particles from a
flow stream. They consist of a rotating horizontal
drum with a cylindrical surface made of a fine screen
or a fabric mesh. The flow enters from inside of the
drum and flows outward in a radial direction. Screens
are cleaned by using pressurized filtered flow to
backwash the screens. Screens are sized from 6 to 74
microns (0.00023 to 0.00289 inch) and mesh from
20 to 330 openings per linear inch. A coarse or fine
screen would need to be installed prior to the
microscreen to prevent the microscreen from rapidly
blinding.
Due to its ability to remove extremely small particles
from the flow stream, the microscreen is able to
remove up to 80% of settleable solids, 55% of TSS, and
50% of BOD. However, operation and maintenance of the
microscreen would be significant due to the large
quantity of screenings that would be generated and the
expected blinding of the screen in a CSO application.
Energy consumption is also expected to be high due to
the number of drums that would be required and the
motor horsepower requirements.
Since microscreens are not manufactured for use in CSO
treatment and have high operation and maintenance
concerns, this technology is not considered
appropriate for further evaluation.
J. Net bags
Net bags are fabric nets that are placed in the flow
stream to capture floatables and larger solids. The
bags typically have an opening of 1/2-inch and can
contain up to 25 cubic feet of material (per bag). The
bags are placed horizontally in the channel and can be
stacked in both the horizontal and vertical directions
to accommodate large flow requirements.
Net bags are capable of removing solids greater than
1/2-inch; however, they are not capable of removing
significant gross pollutants, such as TSS, BOD, TKN,
or TP. Since BOD and TSS removal by net bags is
generally considered to be minimal, additional
provisions may be required to remove these pollutants
in order to achieve target bacterial reduction limits,
similar to coarse and fine screening.
Operation and maintenance requirements for net bags,
as experienced by the County at three existing
installations, are high due to the labor required to
remove, dispose, and replace the net bags after each
storm event. There are no power requirements
associated with this technology; however, specially
designed hoisting equipment and adequate facility
access for net bag removal are required.
Due to the highly intensive labor requirements
necessary to remove, dispose, and replace the net bags
from the facility and the anticipated need to provide
additional treatment technology to achieve target
bacterial reduction limits, this technology is not
considered appropriate for further evaluation.
K. Overflow retention facility (ORF)
An ORF can act as both a storage tank and a high-rate
sedimentation tank depending on water quality
objectives. An ORF is generally sized to retain a
volume equal to a specific storm event. Once that
volume is exceeded, the tank will act as a high-rate
primary sedimentation facility with detention time to
provide effective solids removal, floatables removal,
and possible disinfection. The volume remaining in the
tank at the conclusion of a storm event would be
conveyed back to the municipal wastewater treatment
plant (in this case Metro) for treatment. Figure 2-10
depicts a typical ORF.
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Figure 2-10. Typical overflow retention facility (ORF)—plan view without disinfection
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Captured flow that is subsequently conveyed back to
the municipal wastewater treatment plant (i.e., Metro)
will have a high level of gross pollutant removal
efficiencies. When operating as a high-rate
sedimentation facility, up to 90% of floatables, 80%
of settleable solids, 50% of TSS, and 35% of BOD can
be removed through this technology. Operation and
maintenance requirements associated with this
technology include cleaning and flushing of the basin
at the conclusion of a storm event, and moderate power
use from pumps to return flow to the sewer system.
Since the ORF has generally moderate operation and
maintenance requirements and has the ability to
achieve effective floatables, solids, and gross
pollutant removals, as well as the potential for
disinfection, it is considered to be appropriate for
further evaluation.
A summary overview of the alternative CSO treatment technologies/approaches evaluated is presented in Table 2-4.
Based on the above preliminary screening of
alternative CSO treatment technologies/approaches, the
following are determined to be feasible for further
evaluation to achieve the Clinton Street CSO service
area ACJ treatment requirements.
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Sewer separation
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Regional conveyance and storage
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Regional conveyance, in-line storage and treatment (including disinfection) using:
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Vortex separators
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CDS
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Coarse/fine/rotary drum screens (screening technologies)
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ORF
A secondary screening of the preliminary-screened
treatment technologies and approaches was conducted to
further assess the capabilities of these technologies
and approaches to meet the specific requirements of
the ACJ for the Clinton Street CSO service area.
A. Sewer separation
Because sewer separation essentially eliminates or
removes CSO discharges in the service area being
separated, this CSO abatement approach remains a
viable approach for achieving compliance with the
Clinton Street CSO service area ACJ treatment
requirements.
B. Regional conveyance and storage
Regional conveyance and storage will achieve
compliance with the Clinton Street CSO service area
ACJ treatment requirements, if designed to achieve
compliance with ACJ bacterial requirements. For the
Clinton Street CSO service area, this requires that
the regional storage facility provide sufficient
storage to permit achievement of water quality
standards for bacteria for all portions of Onondaga
Lake that are classified as "Class B" pursuant to 6
NYCRR Part 895. As presented in the "Bacteria Model
Update for Onondaga Lake" memorandum presented in
Appendix B, a bacteria violation of the ACJ standard
is defined as Onondaga Lake fecal coliform bacteria
concentrations exceeding 200 cfu/100 mL on an
instantaneous basis in Cell #2 through Cell #11 of
Onondaga Lake. [Note: cell boundaries are described
in the aforementioned memorandum and are depicted on
Figure 1 of the memorandum.]
As this CSO abatement approach will achieve compliance
with Clinton Street CSO service area ACJ treatment
requirements when sized to prevent bacterial
violations as described above, the approach remains
viable for further evaluation.
C. Regional conveyance, in-line storage and treatment
Because this alternative approach includes in-line
storage plus CSO treatment technologies to remove
floatables and post-treatment disinfection to achieve
adequate bacterial reductions prior to discharge, this
CSO abatement approach remains a viable approach for
achieving compliance with the Clinton Street CSO
service area ACJ treatment requirements.
The alternative CSO treatment technologies to be used
with the regional conveyance, in-line storage, and
treatment approach were then compared on the basis of
size, operation and maintenance considerations,
ability to meet ACJ objectives, and performance.
Performance criteria included floatables removal,
settleable solids removal, TSS removal, BOD removal,
and effect on CSO volume capture.
In order to satisfy ACJ bacteriological requirements,
high-rate disinfection will be a required component of
any of the selected cso treatment technologies to be
used with the regional conveyance, in-line storage,
and treatment approach. Based upon the performance
data available regarding the disinfection of CSOs, TSS
and nutrients have been demonstrated to have an effect
on the disinfection of CSOs. These constituents can
chemically react with the disinfectant and reduce its
effectiveness as a bactericide. Additionally,
constituents such as TSS can limit the exposure of
bacteria to the disinfectant by harboring the bacteria
within the solids.
Liquid sodium hypochlorite continues to be used
extensively for disinfection of CSOs. When followed
by dechlorination using sodium metabisulfite or other
dechlorination agents prior to discharge, chlorination
residual levels and disinfection by-products in the
discharge are substantially reduced.
In response to the citizen concerns regarding the
potential toxicity of chlorinated CSO and wet-weather
discharges, the USEPA has funded recent studies to
assess these impacts. In a 2001 USEPA peer-reviewed
CSO demonstration project, concentrations of
chlorination residuals and by-products in treated
discharges were found to be low. For example, treated
discharge concentrations of chloroform and
trichloroacetic acid were both found to be less than
20 micrograms/liter (µg/L). Based upon USEPA's human
and aquatic toxicity data bases, which assume the most
conservative exposure pathways, concentrations of
chloroform and trichloroacetic acid less than 20 µg/L
would not be considered a chemical of potential
concern. The demonstration project referenced herein
used chemical doses in the order of 25 milligrams per
liter (mg/L). Sodium hypochlorite doses expected to
be used in connection with a Clinton Street CSO
regional treatment facility would be 14 mg/L. Based
on these data, it is expected that the chlorination
residuals and by-products associated with the Clinton
Street CSO disinfected discharges would not be
considered to be of potential concern.
In addition, pursuant to current NYSDEC requirements,
chlorinated discharges must be dechlorinated to a
level of not more than 0.1 mg/L of free chlorine
residual prior to discharge. As a comparison, New
York State regulations for public water supply systems
require that, for systems using chlorine, the free
chlorine disinfection concentration in the water
entering the disinfection system cannot be less than
0.2 mg/L for more than 4 hours.
A. Vortex separators
There are presently three different vortex separator
design configurations: the USEPA swirl concentrator,
the Fluidsep® vortex separator, and the Storm
King® hydrodynamic separator.
Click here for Appendix C. Summary listing of vortex separator installations in the U.S. and Canada.
Although there are a number of vortex separator
installations in the United States for CSO treatment,
there is limited performance data for the Fluidsep®
vortex separator and Storm King® hydrodynamic
separator. Because the USEPA swirl concentrator was
the subject of numerous research and performance
studies as part of the agency's Research and
Development Program in the mid- to late 1970s and
early 1980s, significant performance data are
available for the swirl concentrators. These data
demonstrate the USEPA swirl concentrator to be an
effective preliminary treatment device prior to the
high-rate disinfection of CSOs (Syracuse, NY, USEPA,
1979; Rochester, NY, USEPA 1979).
Due to the established ability of vortex separators,
and in particular the USEPA swirl concentrator, to
provide effective preliminary treatment for high-rate
disinfection, vortex separators remain a viable
technology for compliance with the Clinton Street ACJ
CSO treatment requirements.
B. Continuous deflective separation (CDS)
The CDS technology was developed to treat storm water
for the removal of litter and coarse sediments. There
are several CDS installations in the U.S. that treat
CSOs. Based on operations in Louisville, KY, the CDS
units have demonstrated some ability to remove TSS and
BOD. However, there is no further supporting
documentation for CSO treatment using the CDS
technology. Due to limited operating experience of
CDS for treating CSOs, this technology, combined with
high-rate disinfection, is not considered feasible to
meet the Clinton Street ACJ CSO treatment
requirements.
C. Overflow retention facility (ORF):
An ORF is equivalent to a primary sedimentation tank,
with the exception that an ORF operates intermittently
and, therefore, does not require automatic solids
removal. As described
previously, ORFs are generally sized to retain a
volume equal to a specific storm event.
Click here for Appendix D. Summary listing of overflow retention facilities (used for CSO treatment).
Because the ORF essentially operates as a retention
basin and a high-rate primary sedimentation process
for flow conditions exceeding the design storm volume,
pollutant removal efficiencies for the high-rate
sedimentation operation are generally expected to be
at the low end of typical primary sedimentation
removal efficiencies for domestic sewage (i.e., 50%
TSS removal, 35% BOD removal, and 80% settleable
solids removal). Baffles are also used in the ORF to
dissipate the energy of influent flows, reduce
short-circuiting, and trap floatables. Due to the fact
that an ORF provides storage for the design storm
event and sufficient preliminary treatment prior to
disinfection for flow conditions greater than the
design storm volume, this technology remains a viable
technology for compliance with the Clinton Street ACJ
CSO treatment requirements.
D. Screening technologies
In 1979, the USEPA Research and Development report
entitled "Disinfection/Treatment of Combined Sewer
Overflows" was issued detailing the results from
bench-scale high-rate disinfection studies conducted
on City of Syracuse CSOs that received microscreen
treatment. The City of Syracuse CSOs exhibited a
great variability in chemical and bacterial
composition and, therefore, a comparison of
disinfection effectiveness to screened and unscreened
CSOs showed little or no predictable effects. It was
generally concluded from this study that screening
does not enhance the disinfection of CSOs.
A full-scale CSO screening facility in the City of
Atlanta has been operating for several years. This
facility consists of coarse mechanical screening
followed by rotary drum screens and high-rate
disinfection. The facility has a fecal coliform
discharge limit of no greater than 1,000
cfu/100 mL—less stringent than the Clinton Street CSO treatment
requirement of 200 cfu/100 mL—with influent fecal
coliform concentrations ranging from 40 cfu/100 mL to
110,000 cfu/100 mL. In addition, influent TSS
concentrations average 300 mg/L. According to the
facility operations personnel, adequate disinfection
has been difficult to achieve since the screens do not
remove any appreciable TSS due to the variability in
influent TSS and fecal coliform concentrations.
Since screening technology typically used in CSO
applications does not remove appreciable levels of
TSS, bacterial reduction following screening treatment
alone is extremely difficult to achieve due to the TSS
interference with the disinfection process. For this
reason, screening immediately followed by high-rate
disinfection is not considered feasible to meet the
Clinton Street ACJ CSO treatment requirements for
bacterial reduction.
Based upon the results of the preliminary and
secondary screening evaluations, as described in
Sections 2-2 and 2-3, the following CSO treatment
technologies/approaches are determined to be the most
feasible for achieving compliance with the Clinton
Street CSO service area ACJ treatment requirements.
A. Sewer separation
B. Regional conveyance and storage
C. Regional conveyance, in-line storage, and treatment
(with disinfection), using
-
Vortex Separators; or
-
ORF
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2. Treatment
Susan Miller, Project Deputy Director
Onondaga County Dept of Water Environment Protection