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Harbor Brook CSO Abatement Project Facility Plan (Aug 2005)
by Brown and Caldwell
for Onondaga County Dept of Water Environment Protection
3. Technology Evaluation
The purpose of this section is to review and assess
those technologies that are considered potentially
viable for the abatement of the Harbor Brook combined
sewer overflows (CSO). Although sewer separation is a
viable CSO abatement option, it is not considered a
technology and is covered in
Section 5.
An overview of the CSO treatment technologies that were evaluated is shown in Table 3.1.
Following initial assessment, those technologies still
considered applicable will be assembled into treatment
processes suitable for further evaluation with the
goal of meeting the requirements of the ACJ.
The various technologies considered are grouped in the following categories:
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Floatables control
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Storage/treatment
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Disinfection.
3.2.1.1 Net bags
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Figure 3.1. Typical net bags
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Net bags are frame-supported fabric nets that are
placed in the flow stream to capture floatables and
large solids. The bags typically have an effective
opening of ½-inch and can hold up to 25 cubic feet
of material per bag (Figure 3.1). The bags are placed
horizontally in the flow 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
½-inch, however, they are not capable of removing
significant amounts of conventional pollutants such as
TSS, BOD, TKN, or TP. Further, because the pollutant
removal rates are insignificant, additional provisions
may be required to remove these pollutants,
particularly TSS, in order to achieve target
disinfection limits.
The anticipated operation and maintenance costs are
moderate-to-high due to the intensive labor involved
in removing, disposing and replacing the net bags as
needed after storm events. There are no power
requirements associated with this technology; however,
specially designed hoisting equipment and adequate
facility access are required. Despite operation and
maintenance concerns, this technology does capture
floatable materials at a relatively low capital cost;
as such, it will be further evaluated for
applicability as a floatables control component of an
overall basin-wide CSO abatement program.
3.2.1.2 Trash racks and bar screens
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Figure 3.2. Mechanically cleaned bar screen
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These devices consist of vertical or inclined bars.
The bars for trash racks are typically spaced between
1½ and 4 inches apart, with 3/8- to 1½-inch bar
spacings for coarse bar screens, and less than
3/8-inch for fine bar screens. As flow travels
perpendicular to, and through the bars, material that
is too large to pass through the openings is trapped
on the bars and removed manually or by
mechanically driven rakes.
Trash racks are typically installed to protect
downstream equipment, such as fine bar screens or
pumps, from large debris and are typically manually
cleaned. Bar screens can be either manually or
mechanically cleaned. Screens placed in CSO
environments can be subject to rapid blinding,
therefore, mechanically cleaning is typically
recommended, particularly for remote, unmanned sites
that are expected to activate more frequently. For
manned sites, or sites where frequent activation is
not anticipated, manually cleaned screens may be
appropriate. Mechanically cleaned bar 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. Captured
material is pulled from the flow stream for separate
disposal. Coarse bar screens have been used for CSO
treatment at several installations in the United
States.
Operation and maintenance requirements for
intermittently operated bar screens can be medium to
high, depending on whether the screens have to be
raked or can be washed down with a hose. In certain
situations it may be possible to flush the trapped
material back into the sanitary flow, thus reducing
materials handling issues. Otherwise raking is
required, along with the associated material handling
and disposal costs. In either case, cleanup is
typically required at the conclusion of a storm event
to effectively control odors. Because TSS removal by a
coarse screen is insignificant, additional provisions
may be required to remove these pollutants in order to
achieve target disinfection limits. Due to its small
space requirements in relation to other CSO
technologies and proven efficiency in removing large
solids, trash racks and bar screens will be further
evaluated for applicability as a floatables control
component of this CSO abatement program.
3.2.1.3 Weir-mounted screens
![Figure 3.3. Weir-mounted screens [A: RSW vertical model, B: RSO horizontal (over screen). C: RSU: horizontal (under screen)]. Adapted from Hycor catalog.](gif/ol30303c.gif)
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Figure 3.3. Weir-mounted screens [A: RSW vertical model, B: RSO horizontal (over screen). C: RSU: horizontal (under screen)]. Adapted from Hycor catalog.
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Weir-mounted screens are similar to fine bar screens,
with the bars typically spaced between 4 and 10
millimeters (mm) apart and the bars being cleaned with
a mechanical raking device. The solids captured on a
weir-mounted screen are retained in the foul sewer
flow for subsequent separation and disposal at a
downstream location. This allows screening disposal
operations to be consolidated (Figure 3.3).
Although widely used in Europe, weir-mounted screens
are relatively new devices in the U.S. They are used
for removing floatables and solids of various sizes
down to 4 mm from a flow stream. These screens can be
situated either horizontally or vertically above an
overflow weir. Operation and maintenance requirements
can range from low to medium, depending on the
additional attention required at the conclusion of a
storm event. Because TSS removal by fine screens is
incidental, additional provisions may be required to
remove these pollutants in order to achieve target
disinfection limits.
Due to its small space requirements in relation to
other CSO technologies, low to medium operation and
maintenance concerns, and anticipated floatables and
large solids removal efficiencies, this technology
will be further evaluated for applicability as a
floatables control component of this CSO treatment
program. It should be noted that Onondaga County has
recently installed a weir-mounted mechanically
cleaned screen at a CSO that discharges to Teall Brook
3.2.1.4 Brush screens
A brush screen, also a weir-mounted screen, is a
relatively new innovation for floatables removal. The
screen consists of fine bristles that provide
effective solids removal down to about 4 mm in
diameter. The brush screen is mounted horizontally
above an overflow weir, on a shaft that rotates
countercurrent to the flow being treated. The brush is
driven by a water wheel that is driven by the energy
of the water falling over the weir. The rotating brush
is cleaned by a fixed comb, which collects the
captured solids in a trash storage area. The trash
storage area is emptied back into the sewer as the
water level recedes.
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. While this device does not
require electrical power, operation and maintenance
requirements are estimated to be fairly high, since
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 in the U.S.
with this device, this technology is not considered
appropriate for this project.
3.2.1.5 Rotary drums and sieves
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Figure 3.4. Internally fed rotary screen
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Rotary drums and sieves remove solids by passing flows
through a rotating screen. Flows can be introduced
from either the interior or the exterior of the
rotating drum, depending on the manufacturer (Figure
3.4). The screen surface of the drum can vary from
plastic plates with 5 mm circular perforations, to
stainless steel wedge wires with 0.25-2.5 mm spacing
between the wires, to a stainless steel cylinder with
2-4 mm wide slots.
Rotary drums which 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 flushed 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 solids to a disposal trough.
Rotary drums and sieves remove relatively small
diameter solids (5 mm); therefore, a coarse screening
device is typically provided upstream of the drum
screen in order to prevent larger solids from
collecting on and blinding the drum, which creates
excessive head loss. 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. A continuous high-pressure water
wash is required during a screening event, in order to
maintain a clean screening surface. Up to 20%
of influent TSS and BOD can be removed through use of
this technology due to the size of the perforations.
This technology is not considered appropriate for this
project due to requirements for a larger facility with
coarser screens upstream. Furthermore, operational
experience with this technology is limited in this
area.
3.2.2.1 Micro screens
Micro screens 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 backwashed by using pressurized filtered flow.
Screens are sized from 6-74 microns (0.00023-0.00289
inch) and mesh from 20-330 openings per lineal inch. A
coarse or fine screen would need to be installed
before the micro screen to prevent the micro screen
from rapidly blinding.
Due to their ability to remove minute particles from
the flow stream, micro screens are able to remove up
to 80% of settleable solids, 55% of TSS,
and 50% of BOD. However operation and
maintenance would be significant due to the large
amount of screenings that would be generated and the
anticipated blinding of the screen in a CSO
environment. Power usage is also anticipated to be
high due to the quantity of drums required and the
motor horsepower requirements. Because of the high
operation and maintenance concerns, this technology is
not considered appropriate for this project.
3.2.2.2 Continuous deflective separation (CDS)
CDS technology is essentially a static cylindrical
screen, originally developed to treat stormwater by
removal of litter and coarse sediments. The CDS
consists of a cylindrical tank and a cylindrical fine
screen (Figure 3.5).
Flow enters the CDS tank tangentially and along the
inside of the screen and must pass through the screen
before proceeding to the discharge. The continuous
swirling action within the cylinder flushes heavier
solids off the screen and allows them to settle to the
bottom.
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Figure 3.5. CDS Unit (Source: CDS Technologies)
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This technology is capable of removing floatables and
relatively small solids, as well as TSS (reportedly up
to 10%), due to screen openings of less than
1/6-inch. Operation and maintenance of this system
includes disposal of the collected solids at the
conclusion of a storm event. This can be accomplished
by underflow pumping, using a clamshell bucket, or a
vacuum truck. Though there are no power requirements
associated with this technology, the labor-intensive
methods of removing solids from the system after a
storm event may result in moderate operational
requirements.
There are several CDS installations in the US that
treat CSOs., Based on operations in Louisville, KY,
the CDS units have some ability to remove TSS and BOD.
CDS technology is not considered to be appropriate for
this project, due to limited full-scale experience and
operation and maintenance concerns.
3.2.2.3 Vortex separators
Vortex units have the primary advantage of operating
at surface-overflow rates ranging from 5,000 gallons
per day per square foot (gpd/sf) to 100,000 gpd/sf.
This technology removes floatables and settleable
solids by directing the flow tangentially into a
cylindrical tank, creating a vortex (Figure 3.6). The
vortexing action tends to concentrate settleable
solids towards 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 high surface loading
rates. It has been reported that vortex separators are
capable of removing 90% of the settleable
solids, 35% of TSS and BOD, and some incidental
removals of TKN and TP (between 5 and 15%).
Power is not required for operation of the unit,
although influent, effluent and underflow pumping may
be required depending on the hydraulics of the
specific installation. Operation and maintenance
requirements are low since the majority of the
captured settleable solids and floatables are flushed
into the foul sewer during the storm event. Some
periodic manual cleaning may be required to
effectively control odors caused by the buildup of
grit.
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Figure 3.6 - EPA swirl concentrator
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Vortex separator technology will be further evaluated
for applicability to this CSO abatement program, due
to its solids removal efficiency, low operation and
maintenance concerns, and proven performance
experience in CSO applications.
3.2.2.4 Ballasted flocculation
Ballasted flocculation is a high-rate sedimentation
process that introduces coagulation and flocculation
agents during high speed mixing to promote settlement
and enhance solids removal (Figure 3.7). In the
process, flow enters the first zone of the facility
where the coagulating agent is added and mixed with
diffused air. The coagulating agents are typically
metal salts and/or polymer. The flow then enters the
second zone where the flocculating agent together with
a flocculating aid, either recirculated sludge or
sand, is added. In this area, gentle 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 a
lamellar settling zone to remove residual floc
particles and the final effluent is discharged. The
concentrated sludge is either recycled back to the
second zone or wasted. Sludge from technologies
utilizing sand as a flocculating aid are conveyed
through a separation process whereby the sand is
separated from the waste sludge and recycled back into
the process or stored for future flow events.
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Figure 3.7. Schematic of ballasted flocculation system
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Ballasted flocculation has been reported to be capable
of removing nearly 100% of settable solids, up
to 84% of TSS, 54% of BOD, 25% of
TKN, and 90% of TP in CSO applications.
However, it has been reported that the process
requires approximately 10 to 30 minutes of startup
time in order to stabilize before it is able to
accomplish the above-stated pollutant removal
efficiencies. In addition, preliminary screening is
required before the flow is treated with ballasted
flocculation. The operation and maintenance concerns
associated with the system are high, considering the
requirements for screenings disposal, chemical
addition, sludge processing and disposal and the
relatively high amount of power consumption.
The advantage of ballasted flocculation system is that
it provides a high degree of treatment. One
disadvantage of this process is the physical size of
the facility required. Ballasted flocculation
facilities would cover approximately twice the surface
area of vortex facilities. Other disadvantages are the
time required to stabilize the system and high
operation and maintenance concerns. As such, this
technology is not considered feasible for remote,
unmanned CSO treatment sites.
3.2.2.5 Wetlands treatment
The use of manmade wetlands for the treatment of
stormwater is part of best management practices. The
possibility of using manmade wetland facilities for
the treatment of CSO has been investigated. If
designed and operated properly, this type of facility
would effectively reduce BOD, COD, TSS, nitrogen and
phosphorous in the CSO. It is also more cost effective
to operate and maintain in comparison to some of the
other viable technologies.
The primary disadvantage of wetland treatment of CSO
is that it requires large amounts of land surface
area. The latest information has demonstrated land
requirements of between 20 to 60 acres per million
gallons of CSO. It has been estimated that at a
minimum, a 204-acre site would be required to treat
all 18 Harbor Brook CSO in accordance with the ACJ
requirements. Since the Harbor Brook combined
sewershed is primarily developed urban land,
constructing a wetland treatment facility would only
be possible for CSO 018 and 078. The 1-year design
storm generates a total CSO volume from outfalls 018
and 078 of 1.7 million gallons (MG). he area of
wetlands required to treat this volume of CSO would be
34 acres. The existing detention basin is
approximately 35 acres. Additionally, the existing
detention basin near the outfall for CSO 018 and 078
will not be able to be used for CSO treatment if its
function for stormwater detention would be
compromised.
Another disadvantage of wetlands treatment of CSO is
that it provides minimal treatment of fecal coliform
and other bacterial species. The climate of a
particular area will also impact the treatment
efficiency of the facility. Colder temperatures create
a lower rate for these processes. If wetlands
treatment were to be applied to CSOs 018 and 078,
there would also be a requirement for floatables
removal and disinfection before the discharge to open
wetlands. This would decrease the cost-to-benefit
ratio of this type of facility.
Due to the considerable land requirements, and other
issues noted above, wetlands treatment is not
considered a viable alternative for the treatment of
CSO in dense urban areas like the west side of the
City of Syracuse along Harbor Brook. However, the
County is investigating the possibility of a pilot
wetlands program in the detention basin, which could
be tied to local research efforts and educational
programs for nearby schools.
3.2.2.6 Storage
Another approach to CSO abatement is to store the
excess flow generated during a wet- weather event and
to then release the stored volume to the interceptor
sewer and eventually to the wastewater treatment plant
when capacity in the system becomes available. Storage
is divided into two categories, either in-line or
off-line.
In-line storage is one of the more cost-effective
approaches in achieving CSO volume reductions by
utilizing the conveyance capacity of the pipes in the
collection system 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 volume in the conduits. By
controlling the passage of flow, water levels in the
sewer can be caused to backup in the existing sewer,
thus utilizing the available volume. In-line storage
can also be designed into new conveyance lines. The
disadvantages of using in-line storage in existing
sewer systems may include: the increased risk of
basement or street flooding, increased opportunity of
sediment deposition, and higher costs associated with
increased maintenance of the flow restrictions (i.e.,
weirs, orifices, gates, vortex valves).
Off-line storage consists of tankage constructed to
store flows diverted from the collection system. There
are two approaches to sizing the volume of 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 the design-storm volume
is the potential inability of the facility to capture
a follow-on storm. A follow-on storm could occur
before the tank is drained, resulting in an overflow
condition and leading to water quality violations
during that second storm. The use of the long-term
approach results in a larger storage volume that
better addresses the back-to-back situation.
Once the storage volume is filled, flow is discharged
through or bypassed upstream of the tank. For larger
storms, little or no treatment would be provided once
the tank is filled. The stored volume remaining at the
conclusion of a storm event would be bled back to the
sewer system for treatment at a municipal wastewater
treatment plant, once capacity in the sewer system is
available. The captured flow conveyed to a municipal
wastewater treatment plant would be treated to the
higher removal efficiencies typically available at
such facilities. Operation and maintenance
requirements associated with off-line storage include
cleaning and flushing of solids at the conclusion of a
storm event; and depending on the hydraulics of the
specific installation, power use for influent and/or
pump-back pumps.
Both in-line and off-line storage will be further
evaluated for application to this project, due to
their ability to capture flows for enhanced treatment
and proven performance experience in CSO applications.
3.2.2.7 Overflow retention facilities
Overflow retention facilities (ORF) are a form of
off-line retention or storage that can provide
preliminary/primary treatment for storms beyond the
design storage volume (Figure 3.8). For smaller
storms, the tank would act as storage. For larger
storms, the storage volume is exceeded and the tank
would act as a clarifier. The volume remaining in the
tank at the conclusion of a storm event is bled back
to the sewer system when capacity in the system is
available for eventual treatment.
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Figure 3.8. Plan of typical overflow retention facility Without Disinfection
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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. The primary
tanks for an ORF are usually designed with multiple
rectangular basins, which allow common-wall
construction and the compartmentalization of the
storage volume. Flow is often distributed sequentially
to the compartments to minimize maintenance and
cleanup following small storm events. Baffles are used
to dissipate the energy of influent flows, reduce
short-circuiting and trap scum and floatables.
ORF’s are not typically provided with mechanical
sludge collection equipment due to the intermittent
operation of CSO facilities. Tank floors are sloped to
drain to a collection sump and flushing systems are
used to direct solids to the sump. It is important to
keep tanks clean between storm events in order to
minimize odor buildup and maintain tank capacity.
There are four types of automatic flushing systems
currently used, namely: header-mounted spray nozzles,
turret-mounted high-pressure spray nozzles, tipping
weirs and flushing gates. These automatic systems can
be augmented with manual flushing using yard hydrants
and hoses. Dewatering systems can use centrifugal
pumps for initial tank drawdown operations and solids
handling pumps when draining the lower portion of the
tank.
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 at approximately 1,000 gpd/sf
up to 90 percent of floatables, 80% of settleable
solids, 50% of TSS, and 35% of BOD can be removed
through this technology. ORFs can also be fitted with
baffles and weirs for floatables control. Operation
and maintenance requirements associated with this
technology include: cleaning and flushing of the basin
at the conclusion of a storm event, and depending on
the hydraulics of the specific installation, power use
for influent, effluent and/or pump back pumps.
Because ORFs achieve moderate gross pollutant
removals, this technology will be further evaluated
for this CSO treatment program.
3.2.3 Disinfection
In general there are two means of destroying bacteria
and viruses in wastewater: chemical or physical.
Common forms of chemical disinfection are chlorine,
chlorine dioxide and ozone. Physical destruction of
bacteria is generally accomplished by exposing the
bacteria to energy waves, such as that used in
ultraviolet disinfection. The most common wavelength
is 252 nanometers (nm), which is an ultraviolet
light wavelength.
Chlorine is usually administered in the form of
chlorine gas or liquid sodium hypochlorite. Recent
studies (Spring Creek 1997 and 2000, and Newell Street
1999) have compared the effectiveness of several
disinfectants; namely sodium hypochlorite, chlorine
dioxide, ozone and ultraviolet light. On the basis of
a four-log reduction of fecal coliform and fecal
coliform effluent concentrations less than 1000
colony forming units/100 mL, required doses of sodium
hypochlorite (as chlorine), chlorine dioxide (as
chlorine), ozone and for ultraviolet light, were found
to be 20-28 mg/L, 8-10 mg/L, 24 mg/L and 60-80
mWs/cm², respectively. The current industry standard
for disinfection is chlorine and because of safety
concerns, sodium hypochlorite is typically used for
CSO applications.
During the 1997 Spring Creek and 1999 Newell Street
disinfection studies, conceptual costs were developed
to compare the relative costs of the aforementioned
disinfectants. Sodium hypochlorite and chlorine
dioxide were determined to be the most cost effective
technologies. These experiences showed that given the
intermittent nature of CSO events and the high peak
flows involved, the high capital cost of ultraviolet
light makes it cost prohibitive.
Dechlorination can be described as the practice of
removing all or a specified fraction of the total
chlorine residual. Dechlorination is generally
required when disinfecting CSO with chlorine because
of the relatively high concentration of sodium
hypochlorite that is applied.
When dechlorinating with sodium bisulfite each part of
residual chlorine removed requires 1.46 parts of
sodium bisulfite. Sodium bisulfite storage and feed
systems are similar to sodium hypochlorite systems,
and have proven to be reliable for intermittent CSO
applications.
For the purposes of this discussion, it has been
assumed that induction type mixers will be used to
disperse both the sodium hypochlorite and the sodium
bisulfite throughout the flow. Further evaluation of
alternative mixer designs will be left for the design
phase of the project, as appropriate.
The purpose of this section is to assemble applicable
technologies into treatment systems that are necessary
to meet the requirements of the ACJ. The following
technologies were identified as being feasible during
the preliminary screening evaluation:
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Floatables Control
Screening technologies (net bags, trash racks, bar screens, weir-mounted screens)
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Storage/Treatment
Vortex separation
Overflow retention facilities (ORF)
In-line storage
Off-line storage
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Disinfection
High-rate disinfection
The treatment technologies were compared on the basis
of size, operation and maintenance considerations,
ability to meet ACJ objectives and performance. The
performance criteria include floatables removal,
settleable solids removal, suspended solids removal
and effect on percent volume capture. Examples of
where the various technologies have been applied are
summarized for each technology to develop an
appreciation for other project experiences.
Currently, there is not a great deal of information
regarding disinfection of CSOs. However, based on the
limited database, high TSS and nutrient concentrations
reduce chlorine disinfection efficiency. These
constituents can chemically react with the
disinfectant and reduce its effectiveness as a
bactericide. Additionally, constituents such as TSS
can harbor the bacteria, thereby limiting their
exposure to the disinfectant. The Water Environment
Research Foundation (WERF) is sponsoring a wet-weather
disinfection demonstration project in Columbus,
Georgia, known as the Advance Demonstration Facility
(ADF). Findings from the ADF suggest that there is an
inverse relationship between chlorine disinfection
effectiveness and influent TSS concentration (WWETCO,
1997). In other words, as TSS concentration increases,
the effectiveness of chlorine as a disinfectant
decreases. A similar, but somewhat weaker relationship
was observed in a recent disinfection study for
chlorine and chlorine dioxide for the New York City
Department of Environmental Protection (Spring Creek
Disinfection Studies, 1998 and 2000).
Alternatives for minimizing the effects of high TSS and nutrient concentrations on
chlorine disinfection performance include reducing TSS concentrations through
treatment, increasing chlorine dose and increasing
chlorine contact time. However, those
measures present disadvantages such as increased capital and operation and maintenance
costs. Increasing chlorine dose will result in an increase in the generation of disinfection
by-products (DBPs) and the required amount of
dechlorination agent. In light of these
disadvantages it is preferable to reduce TSS and nutrient concentrations before
disinfection.
In a high-rate disinfection study (EPA, 1975), bench-scale tests were conducted using
microscreening followed by high-rate disinfection of
City of Syracuse CSOs. City of
Syracuse CSOs exhibited a great variability in chemical and bacterial composition and
therefore, a comparison of disinfection to screened and unscreened CSO showed little or
no predictable effects. It was generally concluded that screening alone does not enhance
disinfection.
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 limit of no greater
than 1,000 colonies/100 milliliter (ml) - less stringent than ACJ requirements - with
influent fecal coliform concentrations ranging from 40 colonies/100 mL to 110,000
colonies/mL. In addition, TSS concentrations average
300 mg/L. According to the
facility operations personnel, due to the variability in influent TSS and fecal coliform
concentrations, achieving adequate disinfection has been difficult since the screens do not
remove any appreciable TSS.
In summary, the screening technologies typically applicable to CSO treatment:
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Do not remove significant amounts of TSS
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Result in relatively higher chlorine dosages causing concerns regarding increased
effluent toxicity
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Result in higher operation and maintenance costs.
As such, screening immediately followed by high-rate
disinfection does not appear to be a viable CSO
treatment system. However, screening technologies do
provide effective floatables control and may be used
either where disinfection is not required, or in
conjunction with other treatment technologies and
high-rate disinfection.
There are three different vortex designs: the EPA
swirl concentrator and the Fluidsep vortex separator
that can be designed for a Surface Overflow Rate (SOR)
of 67,000 gpd/sf and can operate as high as 100,000
gpd/sf. The Storm King hydrodynamic separator has been
operated at 5,000 gpd/sf.
The EPA swirl concentrator is the only vortex
technology that has design information that is not
proprietary. As such, the vortex technology discussed
here will refer to the EPA design. Further
evaluation of the alternative vortex designs will be
left for the design phase of the project, as
appropriate.
Vortex technology was introduced by EPA (1971) as a
wet-weather control and treatment device because of
its ability to handle a wide range of flows without
any moving parts.
Such treatment was initially envisioned as not being a
full equivalent to primary treatment, but being able
to sufficiently remove floatables and heavier
settleable solids before disinfection. However, at
surface loading rates below the design rate, treatment
equivalent to primary may be obtained. During the
1970s, vortexes proved to be an effective preliminary
device before disinfection (Syracuse, NY, EPA, 1979;
Rochester, NY, EPA, 1979). Ongoing studies in New York
City indicate that vortexes are effective in removing
almost 100% of “true” floatable material and between
50% and 90% of neutrally buoyant material, depending
on the unit manufacturer (NYC, Corona Ave.). These
results were achieved at the Corona Ave. Vortex
Facility, where a full-scale pilot study using three
different vortex designs is currently being performed.
Because vortex separators in conjunction with
high-rate disinfection have demonstrated satisfactory
bacterial reductions consistent with ACJ requirements,
this is considered a viable treatment system. In
addition, the CSO transmission lines required to
convey overflow volumes to the RTF can be used as
in-line storage. At the end of the event, the volume
still residing in the conveyances, vortex and the
disinfection tank can be pumped back to the
interceptor sewer for subsequent treatment at Metro.
Similar to Section 3.3.3, this treatment system
includes vortex separators and high-rate disinfection
and includes additional upstream storage to reduce the
vortex and disinfection facilities by about one-half
of what would otherwise be required. The conveyances,
upstream storage, vortex and disinfection tank would
be pumped back to the interceptor sewer for subsequent
treatment at Metro.
The removal efficiencies for an ORF are dependent upon
flow variability and settleability of the solids in
question. It is expected that ORFs operated
appropriately will provide removal efficiencies
similar to high-rate primary sedimentation tanks, and
generally greater than those observed at vortex
facilities. The enhanced removals achieved in an ORF
may result in a lower chlorine demand during
disinfection, allowing lower dosages of chlorination
and dechlorination chemicals. In addition, the
required CSO transmission lines that convey overflow
volumes to the ORF can be used as in-line storage. At
the end of the event, the volume still residing in the
conveyance and the ORF can also be pumped back to the
interceptor sewer for subsequent treatment at Metro.
The use of either vortex or ORF technologies, followed
by disinfection, both offer distinct advantages and
disadvantages. In general, vortex and ORF technologies
offer roughly equivalent floatables and settleable
solids removals. ORF offers enhanced TSS and BOD
removals and also enhances the percent volume capture
on an average annual basis. For the purposes of this
report, the ORF is sized based on the volume to
completely capture the 1-year, 2-hour design storm. In
addition, a high-rate disinfection contact tank will
be required for either facility. The size of the
facility will be of particular significance when
considering tight urban sites, public acceptance and
neighborhood impacts. Size will also directly impact
construction and operation and maintenance costs.
ORF is considered to be a viable technology for this
project, if higher levels of treatment are desired at
this time, more stringent regulatory requirements are
anticipated in the future; space is currently
available to accommodate the larger footprint and/or
if adequate funds are available.
A storage system would consist of in-line storage
capacity of the CSO transmission lines and the
off-line storage capacity of an above or below-ground
tank. The basis for design would be the 1-year
recurrence interval volume determined by long-term
simulation. This would determine the required storage
volume that would address back-to-back storms as well
as eliminate or minimize discharge-permit violations.
Once the storage volume is filled, additional flow
would be discharged through or bypassed upstream of
the tank. The bypass could be equipped with a
screening device to capture floatables. The stored
volume remaining at the conclusion of a storm event
would be returned to the sewer system for treatment at
Metro, once capacity in the sewer system is available.
The volume of storage required depends on the area of
the collection system that is served. The volume
required implies a “footprint” at least as large as an
ORF sized on the same basis. The size of the storage
facility will result in construction and operation and
maintenance costs greater than an ORF. Alternatively,
consideration could be given to multiple smaller
storage facilities. However, the disadvantage of this
approach would include:
-
Difficulty of acquiring multiple suitable sites in a dense urban area
-
Greater number of areas impacted during construction
-
Higher project and operation and maintenance costs.
As such, off-line storage is considered to be a viable
technology for this project, if higher levels of
treatment are desired, more stringent regulatory
requirements are anticipated in the future; space is
currently available to accommodate the larger
footprint and/or if adequate funds are available.
Based upon the assessments presented above, the
following treatment processes have been retained for
further evaluation and comparison to sewer separation:
-
Screening, where disinfection is not required
-
ORF with in-line storage and high-rate disinfection
-
Vortex separators with in-line storage and high-rate disinfection
-
Offline-upstream storage with vortex separators and high-rate disinfection
-
Off-line storage at the Metro Tertiaries
-
Regional Off-line storage.
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3. Technologies
Susan Miller, Project Deputy Director
Onondaga County Dept of Water Environment Protection