Minimizing cost while maximizing achievement of an identified level of control or water quality objective.

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1 7. Screening of Combined Sewer Overflow Control Technologies A wide range of technologies exist for CSO control, and the available technologies were screened for application within the BSA s CSS. This section summarizes the technology screening that was completed for the 2004 LTCP. The results of this screening were applied in this LTCP summarized in later sections of this report. 7.1 Technology Screening Criteria The factors that drive the selection of a preferred technology are case-specific, and vary between regulators and within the three planning Districts in the BSA s system. As a result, multiple factors were considered during the initial screening process to identify technologies to be used for CSO control. Factors considered included: Controlling an individual regulator locally with a single identified level of control objective, versus controlling multiple regulators at a single downstream CSO location with multiple control levels. In the controlling multiple regulators scenario, it may be desirable and/or cost-effective to obtain a higher level of control at the consolidated facility than preliminary levels of control identified for individual regulators. Minimizing cost while maximizing achievement of an identified level of control or water quality objective. Minimizing O&M requirements. Constructability. Meeting overall control objectives identified in the District-specific technical memoranda and in the water quality based approach. For example, at a regulator/cso where aesthetic control is desired, a storage technology that captures flows may be more desirable than a flow-through facility, even if they both achieve the same level of control. Inter-district control technologies. Possibility that further evaluation may be required before a single preferred technology can be selected. REVISED JANUARY

2 7.2 Control Methodologies Matrix of Technologies A range of technologies was screened using the above criteria in the preparation of the 2004 LTCP. The technologies screened included: Source controls; Collection system controls; Storage; Treatment; Floatables control; and Non-CSO source alternatives. Preliminary screening of improvement technologies was performed considering the BSA s goals and known characteristics of the collection system. Advantages and disadvantages of the technologies were identified. A discussion of the preliminary screening of each alternative technology is presented in this section Source Controls Source controls are methods of reducing overflow volumes, floatables and/or BOD and suspended solids loads by controlling wet weather flows and loadings at their source. Source control methods include: Catch basin cleaning This measure typically involves cleaning of catch basins by maintenance crews using a vacuum truck. Street cleaning This measure involves cleaning of street litter by mechanical street cleaning. The USEPA recommends that street cleaning should be done as often as once or twice per week and after each storm. However, street sweeping performed at that frequency may not be feasible due to O&M costs incurred and logistical difficulties in large urban areas. Trash receptacles This measure involves the provision of standard trash receptacles throughout major public areas within the system. REVISED JANUARY

3 Public education programs This measure involves the implementation of programs to educate the public on initiatives such as litter control (with information regarding associated fines and penalties), illegal disposal, and the link between litter and CSO impacts. Public notification typically includes postings in public places, radio and television advertisements, and letter notification to residents and commercial entities. The BSA currently incorporates such programs in implementation of the BMPs, as noted in Section 5. The primary advantage of the use of source controls is low capital cost. The primary disadvantage of this technology is its inability to meet WQS for DO, suspended solids, and fecal coliform. Additional disadvantages include increased O&M costs required for cleaning streets and inlets and potential for street and yard flooding associated with local stormwater storage technologies. Due to the nature of source controls, numerical estimation of their effects on collection system and receiving water body responses is not possible. Also, source control methods are typically considered to be independently insufficient for total CSO control. Due to their inability to meet WQS, source controls are not considered as an alternative for complete CSO control. Source controls typically are recommended in conjunction with other selected alternatives at discrete locations. These kinds of controls may be implemented as part of an overall program to address CSOs based upon community and regulatory perceptions, capabilities, and goals Green Infrastructure The term green infrastructure covers a broad range of source control technologies offering the potential of reducing peak storm overflow rates, as well as the volume of stormwater generated by a site. Green infrastructure can be used to store, infiltrate, evaporate, and/or detain runoff. Common green infrastructure technologies include: Rain gardens/vegetated swales: Rain gardens and vegetated swales are shallow depressions, typically planted with native plants to collect, infiltrate and filter rain that falls on hard surfaces like roofs, driveways, alleys, vacant properties, or streets to reduce the flow entering the sewer system and to minimize negative impacts of excessive runoff from these surfaces on receiving water bodies. Downspout disconnections/rain barrels: Within CSSs, downspouts from residential and commercial properties are typically directly connected to the sewer system. Disconnection of the downspouts and redirection of the roof runoff can eliminate a major source of storm water into the combined system. In conjunction with downspout disconnection, rain barrels are often placed at the end of disconnected roof downspouts to capture and hold runoff from roofs. The water in the barrel can then be put to beneficial use to water vegetation. The barrel top typically has a protective screen to inhibit mosquitoes. REVISED JANUARY

4 Infiltration trenches: Excavated trenches backfilled with stone to create subsurface basins that provides storage for water and allow infiltration into soil rather than the collection system. This can be implemented either on occupied parcels, or in a City like Buffalo, there are a number of vacant properties owned by the City that can be used for the implementation of infiltration trenches, reducing the total amount of water entering the collection system. Blue roofs: Blue roofs are designed to collect and retain a portion of the precipitation (typically 1-inch or less) that falls on flat roofs. The collected water is then allowed to evaporate over time during dry weather. Rainfall in excess of the retained amount is allowed to discharge from the roof via the building downspouts. Green roofs: The practice of constructing pre-cultivated vegetation mats on rooftops to capture rainfall, reducing runoff entering the combined system. Permeable pavement: A type of surface material that reduces runoff to the CSS by allowing precipitation to infiltrate through the paving material and into the earth. Storage chambers/perforated pipes: At some parking lots, small storage areas consisting of chambers or pipes are located directly under the parking surface to collect overland flow and detain the flow during the wet weather event. Following the event, the storage area is dewatered to the CSS for conveyance to, and treatment at, the WWTP. Constructed wetlands: Constructed wetlands act as a combination of vegetated swales and detention ponds to reduce the amount of flow that enters the combined sewer system. In general, green infrastructure technologies are applied over a relatively large area in order to achieve a significant reduction in runoff volume and/or flow rate to the CSS. This reduction can often be achieved at a relatively low capital cost per gallon of storm water removed, especially when coupled with other municipally funded capital projects such as street reconstruction. Green infrastructure techniques typically become even more cost-effective as part of property transfer or redevelopment activities, allowing implementation when sites have already been excavated, allowing substantial construction cost savings. In the case of rain gardens and rain barrels, significant participation and cooperation of business and private property owners is required. In some cases, implementation of green infrastructure requires revisions to the applicable building code, which can be a lengthy process. However, the City of Buffalo has already completed the draft of their Green Code that will become a component of the City s Building Code. The provisions of the Green Code will be used to promote the inclusion of green infrastructure in future redevelopment efforts within the City of Buffalo. The BSA anticipates that the Green Code will be adopted by the City in Finally, through the BSA s review of development within the City, various green infrastructure techniques are already being implemented on a case-by-case basis. Because of their potential to achieve significant reductions of storm REVISED JANUARY

5 water flows entering the BSA s CSS and therefore reducing the CSOs, green infrastructure technologies were considered for further evaluation. Further discussion on green infrastructure technologies considered for Buffalo is provided in the BSA s Green Infrastructure Master Plan presented in detail in Section Collection System Controls Collection system controls are methods of reducing overflow volume and frequency by optimizing system conveyance and/or storage or increasing system capacity. Methods of collection system control include pump station modifications, regulator modifications, sewer separation, express sewers, flow diversion, and other conveyance options. The primary advantage of the use of collection system controls is the potential for significant control of wet weather flows using in large measure, existing infrastructure. These technologies can lead to significant improvement in terms of level of control, and ultimately, attainment of WQS. Further, some of the technologies, such as flow diversions or regulator modifications, can achieve significant wet weather control for relatively little capital investment. Some of these control technologies, such as sewer separation or express sewers, have high capital costs when compared to source control technologies. Additional disadvantages include higher O&M costs for pump stations, potential for disruption during construction, and potential for street and yard flooding associated with regulator modifications. Because collection system controls may, in whole or in part, provide the BSA with the ability to achieve significant improvement and comply with WQS, collection system controls were considered for further evaluation Pump Station Modifications Reduction in volume and frequency of overflow can be accomplished by modification of the existing hydraulic capacities and control features of pump stations, (i.e., increased pump capacity, control of wet well operating elevations, etc.). These types of modifications are pump station-specific and feasible only if existing excess downstream capacity is available and the resulting hydraulic gradient upstream of the pump station can be reestablished at a safe elevation to prevent flooding or the increase of other overflows. This type of collection system control can be advantageous for pump stations with high overflow frequencies, but low volumes of discharge. Pump station modifications were not considered for further analysis, as the pump stations contained within the BSA s collection system do not have high overflow frequencies. REVISED JANUARY

6 Regulator Modifications Passive Reduction in volume and frequency of overflow at specific regulators can be accomplished by modification of the existing hydraulic control features of the regulator (i.e., raising the elevation of weirs, modifications to orifice area, etc.). These types of modifications are regulator-specific and feasible only if existing excess interceptor capacity is available and the resulting hydraulic gradient upstream of the regulator can be reestablished at a safe elevation to prevent flooding or the increase of overflows at upstream locations. This type of collection system control can be advantageous for regulators with high frequencies and low discharge volumes Regulator Modifications With Real Time Control (RTC) In certain cases, the regulator modification technology can take the form of a dynamic ( real-time ) regulator control (e.g., adjustable weir) combined with a control system to maximize capture and minimize CSOs, street and basement flooding, and inflow peaks to the WWTP. This kind of real-time control adjusts the regulator control equipment (gates, pumps, and valves) in response to real-time system conditions. The equipment is controlled to use in-line and off-line storage assets to equalize and dampen peak flows, allowing the downstream collection system to convey the optimal amount of combined flows to the treatment facility. In addition, conveyance optimization is achieved by ensuring the collection system is fully utilized before CSOs occur upstream. Generally, regulator modifications by themselves are not sufficient for complete CSO control. However, because they are often relatively inexpensive, regulator modifications (both passive and with real time control) were considered for further evaluation as components of system-wide alternatives for CSO control Sewer Separation Sewer separation involves the installation of an additional conduit, typically to convey storm water, alongside the existing CSS. The existing lines would be left in place to convey sanitary sewage to the treatment plant, since sanitary laterals are already attached and the existing pipe goes directly to the plant. Separation can be an effective method of removing storm water flows from the sanitary sewer systems, reducing CSO volumes, and increasing equipment life and capacity at the WWTP. There are two different degrees of sewer separation: full separation and partial separation, in the form of storm water inflow removal. Full Separation Full separation involves the separation of the all sources of runoff from the combined sewer area tributary to any overflow point or regulator. The removal of storm water leaves the existing system with enough capacity to carry sanitary flow and reduces overflowing of the sanitary system. Only areas that have both sanitary REVISED JANUARY

7 and storm flows in a single pipe require the installation of an additional conduit. Full separation would require the installation of new storm sewers in all combined sewer areas, and the uncoupling of any storm water connections to the present combined system. Full separation was considered for further analysis as a potential system-wide alternative for CSO control, but is a very capital-intensive solution. Because of the high capital cost involved, developing cost estimates for full sewer separation applied system-wide can establish an upper cost limit for total CSO control that serves as a benchmark against which other more feasible solutions can be compared. Another full separation approach would be constructing new separate sanitary sewers while disconnecting the existing combined sewers from sanitary service and maintaining them for stormwater drainage. This approach, however, is typically more expensive and difficult to implement than constructing new storm sewers, and therefore, was not considered further. Partial Separation (Storm Water Inflow Removal) Storm water inflow removal, or partial separation, is accomplished by installing new storm sewers in local, discrete areas within combined sewer subbasins to reduce direct storm water input to the existing CSS. Inflow removal is considered viable and cost-effective in areas where gravity discharge of collected storm water could be accomplished through relatively short outfalls to the receiving water or to a storm sewer with excess capacity. As part of the BSA s on-going infrastructure improvement program, several partial separation projects have been completed as part of Phase I projects. For this reason, partial separation was considered for further evaluation as a potential component of alternatives for CSO control. For this reason, partial separation was considered for further evaluation as a potential component of the 2004 preferred for CSO control alternative carried forward for further evaluations. Due to new stormwater regulations and greater regulatory emphasis on green infrastructure technologies, for new alternatives developed under this LTCP revision effort, green infrastructure was considered in lieu of new partial separation projects Sanitary Express Sewers New separate sanitary express sewers can be provided to convey flow from existing separate sanitary sewer areas in outlying, or tributary, communities directly to the treatment plant. The express sewers bypass areas of combined sewers and permit preferential treatment of separate sanitary sewer flows at the treatment plant, while removing flow from the combined parts of the system. Express sewers are typically feasible as a collection system control only if excess treatment plant capacity exists and is underutilized because of a lack of conveyance capacity within the combined portions of the collection system. REVISED JANUARY

8 Sanitary express sewers were not considered for further evaluation as an alternative for CSO control as capacity of the existing treatment plant is not underutilized Flow Diversion Flow diversion from existing overloaded parts of the collection system to parts of the system with existing excess capacity can be accomplished by the construction of new relief sewers or pump stations. These modifications are area-specific and feasible only if existing excess capacity is available elsewhere within a reasonable distance and the resulting hydraulic gradients in the receiving part of the system can be reestablished at safe elevations to prevent flooding or an increase of overflows at upstream locations. By itself, flow diversion is generally not sufficient for complete CSO control. However, flow diversion in portions of the BSA s collection system, where appropriate, was considered for further evaluation for CSO control Storage Total storage volume within the system (e.g., in-line, tunnel, or storage/treatment basins) is typically limited by the ability of the treatment plant to accept and treat the stored flow as the storage facilities are dewatered. Generally, this stored flow would have to be treated to secondary standards. In order to avoid septicity within the storage basins and increase the likelihood that storage is available when needed, target dewatering periods are assumed to be 24 hours. The primary advantage of the use of storage controls is the potential for significant control of wet weather flows. These technologies can lead to significant improvement at the benefit-effective level of control. As with collection system controls, the primary disadvantage of this technology is its high capital cost when compared to source control technologies. Additional disadvantages include increased O&M costs for pumping (if necessary) and cleaning, potential for disruption during construction, and siting requirements. Because storage control may, in whole or in part, provide the BSA with the ability to achieve significant improvement and comply with WQS, storage control methods were considered for further evaluation. Storage control methods evaluated include in-line storage with real time control, satellite storage facilities, and deep rock tunnels In-Line Storage with Real Time Control In-line storage can be provided in existing large diameter pipes having excess hydraulic capacity. In-line storage is typically induced by the construction of gates or inflatable dams within the existing pipe, along with a suitable control system that activates the storage and release of flow exceeding storage volume. REVISED JANUARY

9 Typically, to develop a significant amount of in-line storage, pipe diameter and length must be the equivalent of the major intercepting sewer. In-line storage with real time control was considered for further evaluation as an alternative for CSO control in those portions of the collection system containing trunk sewers of sufficient diameter and length Satellite Storage Facilities Satellite storage facilities are typically constructed between the existing regulator and the receiving water body. Storage basins are sized to provide the storage volume associated with the selected level of control. Flows in excess of this volume are routed around the storage basin for direct discharge to the receiving water body. This discharge is considered a CSO event in the new system. The basin and settled solids are dewatered to the interceptor. As described in Section 7.2.4, it is assumed that dewatering would be accomplished with pumps capable of dewatering the basin within 24 to 48 hours, in order to avoid septicity and to increase the likelihood that storage will be available when needed. Storage basins capture all the volume associated with overflow events up to the selected level of control, and the first flush of larger events. Detention storage basins were considered for further evaluation as a potential alternative for CSO control Deep Rock Tunnels Storage is sometimes provided by the mining of storage tunnels below grade, and if possible, in bedrock. The tunnels are sized to store overflows from all captured regulators up to the selected level of control. Flows in excess of this level would be bypassed directly to the receiving water body. There are three areas in which deep rock tunnels are evaluated as an alternative for CSO control: Black Rock Canal Tunnel (also known as the North-South Tunnel) Scajaquada Tunnel (also known as the East-West Tunnel) Buffalo River Tunnel An initial review of available geological information for the City of Buffalo collection system area did not identify any potential problems that would preclude the construction of deep rock storage tunnels. Therefore, deep rock storage tunnels were considered for further evaluation as an alternative for complete CSO control. REVISED JANUARY

10 7.2.5 Treatment Treatment control is a method of reducing untreated overflow volume and frequency by increasing a system s treatment capacity. Treatment typically involves some form of solids (and associated BOD) removal and/or disinfection. Treatment control methods evaluated include treatment detention basins, vortex separators with disinfection, and enhanced high rate treatment (EHRT) with disinfection. The primary advantage of the use of treatment controls is the potential for significant control of wet weather flows. These technologies can lead to significant improvement at the benefit-effective level of control. As with collection system and storage controls, the primary disadvantage of this technology is the high capital cost when compared to source control technologies. Additional disadvantages include increased O&M costs for new mechanical equipment as well as settling and disinfection chemicals, potential for disruption during construction, and siting requirements. Because treatment control may, in whole or in part, provide the BSA with the ability to achieve significant improvement and comply with WQS, treatment control methods were considered for further analysis Treatment Basins Treatment basins are constructed between the current regulator and the receiving water. The basins are typically sized to provide 30 minutes of detention time at the peak flow rate associated with the selected level of control. When the regulator activates, flow rates up to the peak overflow rate are routed to the basin, detained for at least 30 minutes, disinfected, and then discharged to the receiving water. Flow rates above this level bypass the basin and are discharged directly to the receiving water body. The bypassed discharge would be considered a CSO event in the new system. The basin and settled solids are dewatered to the interceptor and conveyed to the treatment facility for treatment at the end of the wet weather event. It is assumed that dewatering would be accomplished with pumps capable of dewatering the basin within 24 hours. Treatment basins would treat all flow associated with overflow events up to the selected level of control, and a portion of the flow throughout the duration of larger events. Treatment basins were considered for further evaluation as an alternative for controlling CSOs Vortex Separators with Disinfection Vortex separators are typically constructed between the current regulator and the receiving water. The regulators are sized to provide 15 minutes of disinfection contact time at the selected level of control peak flow rate, or to provide a maximum loading rate of 5 gallons per minute per square foot (gpm/sq. ft.) at the selected level of control peak flow rate. Vortex separators can be covered and odor control facilities REVISED JANUARY

11 provided. Foul flow pumps would convey the concentrated underflow to the interceptor. Hatches are provided around the perimeter for washdown with hoses. A sodium hypochlorite system could be used for disinfection. Vortex separators with disinfection were considered for further evaluation as an alternative for controlling CSOs Enhanced High Rate Treatment (EHRT) with Disinfection EHRT facilities are constructed between the current regulator and the receiving water body. EHRT facilities flocculate and settle suspended solids to primary removal efficiencies so that treated CSO flows may be subsequently disinfected for the peak flow rate associated with the selected level of control. When the regulator activates, flow rates up to the peak overflow rate are routed to the EHRT, disinfected, and discharged to the receiving water body. Flow rates above this level bypass the EHRT and are discharged directly to the receiving water body. The bypassed discharge would be considered a CSO event in the new system. The settled solids are pumped to the interceptor for conveyance to the treatment plant at the end of the wet weather event. EHRT facilities would be sized to treat all flow associated with overflow events up to the level of control, and a portion of the flow throughout the duration of larger events. EHRT facilities were considered for further evaluation as an alternative for controlling CSOs. While there is no official definition of high-rate disinfection (HRD), wet-weather practitioners have used the term to define disinfection that occurs in a shortened period of time using a high dose of disinfection agent with intense mixing. The most common chemicals used with HRD are liquid sodium hypochlorite for disinfection and liquid sodium bisulfite as a dechlorination chemical. Other possible disinfection chemicals available include gaseous chlorine and gaseous sodium dioxide for disinfection and dechlorination, respectively. While contact times vary, five minutes is typically used for disinfection and one minute for dechlorination. HRD was considered after EHRT, before discharge of the EHRT effluent into the receiving water body. With the use of EHRT TSS, removal rates are high enough to allow alternate disinfection systems, such as ultraviolet (UV) disinfection. UV disinfection is the most common disinfection alternative to chlorine-based chemicals with approximately 20 percent of wastewater treatment plants using this mode of disinfection. The popularity of UV disinfection is primarily due to the safety and health benefits it provides over chemical disinfectants, as UV light is a physical disinfecting agent that utilizes specific wavelengths of electromagnetic radiation. UV systems are power-intensive with medium pressure technology generally being associated with largest power requirements. Because of this, sodium hypochlorite disinfection was considered instead of UV disinfection, for sites with EHRT facilities. REVISED JANUARY

12 7.2.6 Floatables Control Street litter and floatables, such as plastic bottles, cups, leaves, cans, and rags, typically enter a CSS either by surface runoff generated during wet weather events or by deliberate dumping of trash into catch basins or sewers. Floatables cause aesthetic and odor problems in populated areas and contribute to the CBOD load of affected waterways. Floatables control is a means of preventing visible debris from entering waterways. The primary advantage of the use of floatables control is the ability to improve stream aesthetics at a relatively low capital cost, as compared to other control technologies. The disadvantage of floatables controls is the inability to meet WQS for DO, suspended solids, and fecal coliform, without additional controls. Floatables control technologies screened as part of the development of the BSA s LTCP include: Catch basin modification; Underflow baffles; Screening devices; Vortex-type separators; and Netting systems. This section provides information on the configuration and operational characteristics of various floatables control technologies and approaches Catch Basin Modification One method of floatables control is to prevent the floatables and solids from entering the combined sewers (i.e., floatables source control), through the use of a preliminary separation system. The simplest form of pre-separation that can be provided on a CSS involves catch basin modifications, such as: Replacement of existing castings with new castings, including coarse screens to catch larger solids and floatables. Installation of a catch basin trap consisting of a hood and a hanger plate. The catch basin trap is installed around the existing outflow pipe to prevent floating debris from entering the combined system. REVISED JANUARY

13 Due to the magnitude of catch basin modifications required to effectively achieve floatables control, catch basin modifications were not considered for further evaluation in the LTCP development Underflow Baffles Larger solids and floatables can be captured within the collection system with underflow baffles. Underflow baffles consist of stainless steel or aluminum plates installed in existing regulator structures. The effectiveness of the underflow baffles depends on the specific design of the diversion points for the overflows. Underflow baffles generally have lower capital and O&M costs than other solids and floatables removal devices such as screens and netting. Removal effectiveness of underflow baffles is likely to be lower, however, because of turbulence in the flow stream, which tends to entrain solids and floatables, especially those that are relatively close to neutral buoyancy. The advantages of underflow baffles include: The technology is well-known and understood, as noted in USEPA reports. Underflow baffles have been recommended by the USEPA as an effective means of floatables control. Ease of operation compared to other screening alternatives. Ease of construction without interfering with the current operation of existing CSOs. The disadvantages of underflow baffles include: CSO regulators do not completely flush themselves clean, but instead may become clogged with solids and floatables over time. The addition of baffles are likely to make the solids and floatable accumulation problem worse, requiring more frequent cleaning of the CSO regulator. Performance and reliability factors are unknown, as only laboratory studies have been conducted. The Massachusetts Water Resource Authority (MWRA) conducted a study on the use of underflow baffles for CSO floatables control. Although the laboratory studies provided promising results, there is insufficient field data to prove acceptable reliability or performance. Therefore, field performance can be difficult to predict. Lower solids and floatables removal efficiencies are likely as compared to other technologies. Potential exists for clogging of interceptors with solids/floatables after wet weather events. Due to the disadvantages summarized in this section, underflow baffles were not considered for further evaluation in LTCP development. REVISED JANUARY

14 Screening Devices Screening devices are used to prevent floatables from being discharged from CSOs to receiving water bodies during wet weather events. Screening of CSOs can be challenging because the quantities and loading rates of floatables and solids vary widely during the course of a wet weather event, from first flush at the initiation of the event to more dilute conditions towards the end of the event. If a period of drought is followed by a significant storm event, the quantity of floatables and solids discharged from CSOs will likely be high. However, if two storm events occur on consecutive days, the quantity of floatables and solids discharge from the CSOs from the second day s storm would be reduced. Selected screening systems for CSO control must be designed with sufficient flexibility to adapt to the fluctuations in floatables and solids loading conditions. Screening systems for floatables control in combined systems are typically installed in regulator chambers to prevent solids from being discharged from CSO outfalls. Screening devices that were included in the technology screening process include: Static bar screens; Vertical mechanical bar screens; Horizontal mechanical bar screens; and Rotary drum screens. Vertical, horizontal, and rotary drum screens are considered as high-level floatables control. Screening devices are typically independently insufficient for total CSO control; however, where bacteria are the only pollutant of concern, they can be coupled with chemical disinfection facilities to provide sufficient solids removal for effective disinfection. Due to the simplicity of operation, in addition to the other advantages further detailed in this section, screening devices were considered for further evaluated in LTCP development. Static Bar Screens Static bar screens are one of the least expensive forms of screening technologies available. A static bar screen consists of sturdy bars, aligned parallel to one another. The screens are fixed in place, trapping solids and floatable material. Static bar screens are manual, stand-alone systems without any mechanical moving parts or any automated cleaning mechanisms. However, static bar screens have the following disadvantages: REVISED JANUARY

15 Periodic manual cleaning of solids and floatables from the screen is required. Maintenance crews are generally required to visit each screen during and after each storm event to ensure that screens do not become clogged, restricting flow. Regular visitation of bar screens increases the frequency of confined space entry by maintenance personnel. Static bar screens typically require significant space for installation, which potentially could limit access to the manholes in which they are installed. Static bar screens have the potential of clogging with solids and floatables, which may adversely affect flow patterns to CSO outfalls. Flow restrictions to the outfall pipe can also surcharge trunk sewers, leading to further problems such as basement backups and overflowing catch basins and chambers. Because of flow restriction limitations, use of static bar screens sometimes requires the installation of new screening chambers, which add significant costs to this approach. Vertical Mechanical Bar Screens Vertical mechanical bar screens are typically equipped with a vertical, inclined, static bar screen rack which remains submerged below the water surface, and a mechanical rake arm which remains above the water surface. When the bar rack requires cleaning, the mechanism periodically drives the rake arm down below the water surface and onto the bar rack, raking the bars clean. The rake arm continues to rake upward on the screen to a discharge chute, where the solids and floatables are dumped into a storage container. The advantages of vertical mechanical bar screens include: The technology is well-known, understood, and reliable and has been used in wastewater treatment for decades. The rake arm mechanism prevents the bar screen from clogging and may be programmed to activate when high water levels are detected in a chamber. Bar screens consist of thick, heavy-duty bars, which are more structurally sturdy during storm events when compared to other wire mesh-type screens. Addition of flushing water systems is possible to flush solids and floatables back to the interceptor. REVISED JANUARY

16 Mechanical bar screens are effective for removal of solids and floatables of 0.5 inches and greater in size depending on the bar spacing. The disadvantages of vertical mechanical bar screens include: The mechanical and electrical components have more O&M requirements than other non-mechanical screening options. High height clearances are involved, which may present a problem at some overflow locations. Additional concrete or other structures are typically required to house the screening facilities, resulting in higher capital costs. Horizontal Mechanical Bar Screens Horizontal mechanical bar screens are a relatively new technology utilized in the United States to screen solids and floatables, though the screens have been utilized for a longer period in Europe for CSO control. The screens are rigid, weir-mounted, and constructed of narrow, corrosion resistant stainless steel bars with evenly spaced openings. The screening bars are designed in continuous runs with no intermediate supports to collect solids. The screen is activated automatically by a level sensor as storm water rises sufficiently to overflow the weir of the screen. When the screen requires cleaning, a hydraulically-driven rake assembly travels back and forth across the screen, combing away solids trapped on the screen. The combing tines carry the solids to one end of the screen for disposal back into the wastewater channel. The advantages of horizontal mechanical bar screens include: The rake arm assembly prevents the bar screen from clogging and may be programmed to activate when high water levels are detected in the chamber. Bar screens consist of thick, heavy-duty bars, which are more structurally sturdy during high storm flows than other wire mesh-type screens. Solids and floatables are pushed back into the wastewater channel to be handled at the treatment plant. Therefore, there are minimal maintenance personnel costs for screenings pickup and transportation. Horizontal mechanical bar screens are effective for removal of solids and floatables of 0.5 inches and greater in size depending on bar spacing. The disadvantages of horizontal mechanical bar screens include: REVISED JANUARY

17 The technology is relatively new in the United States. The mechanical and electrical components have more O&M requirements than other non-mechanical screening systems. Rotary Drum Screens Rotary drum screens are used in wastewater treatment facilities for a variety of applications including municipal and industrial wastewater, food processing and pulp and paper industries, and CSOs and SSOs. The screens consist of wedge wire, which is wrapped around to form a drum screen that is open on both ends. The drum screen is mounted on a carriage of mechanical rollers, rotating around a horizontal axis parallel to the sewage flow. The screening action takes place inside the drum. Combined sewage enters through the one end of the unit and is screened as it drops through the wall of the drum. The screenings are moved to the other end of the rotating drum by a set of spiraled conveying vanes fixed on the interior wall of the drum. The screenings are discharged from the opposite end of the unit and screenings are collected in a container. The advantages of rotary drum screens include: The technology is well-known and well-understood. The rotating action and an internal spray cleaning system prevent the drum screen from clogging. Drum screens are effective for removal of solids and floatables of 0.5 inches and greater in size. Drum screens have crossbars across the wedge wire, which create smaller slot openings than mechanical bar screens. Drum screens have lower height clearances than bar screens. The disadvantages of rotating drum screens include: The mechanical, electrical, and water spray components have more O&M requirements than mechanical bar screens and other non-mechanical screening systems. Additional concrete or other structures are typically required to house the screening facilities, resulting in increased capital costs. REVISED JANUARY

18 The wedge wires for the drums are not constructed of thick, heavy duty bars (unlike) bar screens, raising the concern of whether or not the wedge wire construction can withstand the force from the repeated high flows generated by CSOs. The number of mechanical components associated with this technology is greater than other screening devices, and therefore, the potential for failure of this type of device is greater. Maintenance personnel costs are increased due to required screenings pickup, transportation, and disposal Vortex-Type Separators A vortex separator is a cylindrical unit, which uses the hydrodynamics of swirling or vortex velocities to concentrate and remove solids and grit. The unit has no moving parts. Storm flows enter the unit tangential to the cylindrical chamber to create a swirling vortex that imparts velocities beneficial to separating solids out of liquids. Vortex separation occurs when the circulating suspended solids are drawn to the center of the swirl and are directed down toward the center of the unit where the solids concentrate. This mixture of concentrated solids and wastewater is then removed from the bottom of the unit by a foul sewer pipe, which directs the solids flow back to the interceptor conveying flow to the treatment plant. The clarified effluent exits the top of the unit and is discharged to the receiving outfall through an outfall pipe from the vortex separator unit. Currently, there are several types of vortex separators in use in the United States; despite variations among the different types, the principles of operation are essentially the same among the various units. The advantages of vortex separators include: Vortex separators are a viable CSO control technology that has been installed in several locations in the United States, Great Britain, Germany, Japan, and other countries. Depending on the type of vortex separator, it may be possible to pump the floatables and solids collected by the vortex separator into the interceptor with a cleanout pump, thus minimizing mechanical cleaning. The disadvantages of vortex separators include: Vortex separator units for large urbanized areas may require a large footprint area for installation. In general, the spatial requirements are higher than those required for screening or netting technologies. REVISED JANUARY

19 More extensive construction is needed for vortex separator systems. Typical vortex separator units approach an average depth of 30 ft, which is more than three times the typical depth required for concrete chambers for screening or netting technologies. Performance of larger vortex separator systems has not yet been confirmed. Overall performance results of vortex separators are scattered. Depending on the type of vortex separator, removal of solids from the vortex units may or may not require mechanical cleaning, which would incur additional O&M costs. A vortex separator system with a cleanout pump included in the design would also incur additional O&M costs associated with pump operation and maintenance. Vortex separators without cleanout pumps would incur additional costs associated with manual cleaning and maintenance. Vortex separators are independently insufficient for total CSO control. However, due to the advantages summarized in this section, vortex separators were considered for further evaluation in LTCP development Netting Systems Two types of netting systems were identified during the development of the control alternatives: End-of-pipe; and In-line. Because of their simplicity in capturing floatables, netting systems were considered for further evaluation in LTCP development. End-of-Pipe Netting Systems End-of-pipe netting systems are designed to catch floatable materials shortly after being discharged by CSOs. Most applications consist of simple components, such as pontoons, support columns, nylon netting, polyvinyl chloride (PVC) sheet baffles or curtains, wood beams, and concrete anchors. The standard endof-pipe netting system consists of a floating pontoon structure that can accommodate nylon mesh bags that are positioned at a given distance into the water facing the end of the outfall pipe. The end of the outfall pipe is channeled into the mesh bags, which are each sized to capture a given volume of floatable material. The floating pontoon structure is held in place during excessive storm events by fixed and firmly anchored roller columns. REVISED JANUARY

20 When the mesh bags are full, they are winched to shore and lifted by crane to an on-land location or picked up in the water by skimmer boats. The waste materials are usually landfilled and clean nets are replaced on the system. The advantages of this system include: Construction of an on-land concrete chamber to hold screening equipment is not required. The system can be constructed without interfering with current operation of existing CSOs. End-of-pipe netting is effective for removal of solids and floatables of 0.5 inches and greater in size. The mesh bags provide more screening surface area per unit flow area than any other screening alternative. The system may be easily expanded with additional mesh bags for only minimal design and construction effort relative to other alternatives where expansion may not be economically feasible. The disadvantages of this system include: Operation and personnel costs will increase due to required localized screenings pickup, transportation, and disposal, and to install new nets. A mobile hoisting crane is required to retrieve and remove the full nets from the water. Access to the nets may be difficult in some areas. The technology is not applicable to outfalls with shallow water depth or where outfalls do not enter the water surface with adequate submergence. Wetlands and stream coastal encroachment permitting will be required. In-Line Netting Systems In-line netting can be installed where end-of-pipe installations are not technically feasible. This system operates on the same principle as the end-of-pipe nets but consists of a concrete chamber to hold the mesh bag netting, net support guides, and access hatches, and a mesh bag net insert. REVISED JANUARY

21 This system allows for the netting, floatables, and solids to be removed from the chamber by hoisting the nets out of the chamber with a crane, which may then be loaded on a truck for disposal. In addition to the advantages mentioned for the end-of-pipe netting system, advantages for this alternative include: Wetland and coastal encroachment permits may not be required. Personnel and equipment will be more accessible for removal and disposal of the nets than the end-ofpipe netting alternative. Disadvantages of the in-line netting system include: Operation and personnel costs will increase because screenings pickup, transportation, and disposal will be required with this alternative for the manual disposal of the solids and floatables captured in the netting and for installing new nets. A mobile hoisting crane is required to retrieve and remove the full nets from the water Non-CSO Source Alternatives In addition to express sewers discussed in Section , non-cso source alternatives may include other actions to reduce wet weather inflows to the CSS from tributary separate sanitary systems, such as inflow and infiltration (I/I) removal. The primary advantage of non-cso source alternatives is the relatively low O&M costs of certain technologies (i.e., I/I removal will typically result in a decrease in O&M). The primary disadvantages are the high capital costs associated with I/I identification and removal. Furthermore, some construction associated with I/I removal would be disruptive to local residents. In the case of the BSA service area, however, the vast majority of separate sewer systems are owned and maintained by tributary municipalities where the BSA has little control. Non-CSO source technologies are typically used in site-specific applications. However, these methods are generally considered to be independently insufficient for total CSO control. Due to their high capital costs, non-cso source alternatives were not evaluated as an alternative for complete control, but were considered in conjunction with other alternatives. REVISED JANUARY

22 7.3 Level of Control Curves by Combined Sewer Overflow Developed During the 2004 LTCP In order to develop the 2004 LTCP, CSO control levels were selected to define the performance targets for abatement alternatives. To facilitate this, level of control curves were generated for each CSO within the BSA collection system for both a generic storage and a generic treatment option Design Storm Development The first step in the process to develop the level of control curves was to develop a set of design storms to be used to define the different levels of control. For this purpose, 6-hour duration design storms were developed for the following return periods: 1-month, 2-months, 3-months, 4-months, 6-months, and 1-year. During Phase I, Stage 1, design storms with return periods of 1-month, 2-months, 3-months, 6-months, and 12-months were developed based on historical rainfall. However, during Stage 2, it was determined that these design storms may generate higher CSO volumes than would typically be expected at the designated return periods due to the varying durations of the design storms. Also, the Phase I, Stage 1 and 2 evaluations did not include a 4-month design storm. Therefore, a new set of design storms was developed for Phase I, Stage 3. The new set of design storms contains 6-hour duration storms, eliminating the effect of variable storm duration on generated CSO volume. A 4-month design storm was developed as an additional point of system-evaluation, as well. The rainfall volumes for the Phase I, Stage 3 design storms were determined by extrapolating from the 6- hour duration rainfall volumes for Buffalo, New York, for return periods of 2, 5, 10, 25, 50, and 100 years as specified in the Northeast Regional Climate Center Atlas of Precipitation Extremes for the Northeastern United States and Southeastern Canada. In the absence of data specific to Buffalo, the rainfall volumes for the lower return periods were extrapolated by assuming that their values relative to the values for the higher return periods were similar to the relationship between rainfall volumes for different return period 6-hour storms in Cleveland, Ohio, as specified in the Midwest Rainfall Frequency Atlas. These rainfall volumes were distributed using a first quartile Huff distribution. This distribution is appropriate for storms with durations of six hours or less. The resulting storm hyetographs for the six design storms are shown in Figures 7-1 to 7-6. Table 7-1 compares the Phase I, Stage 1 and Stage 3 design storm parameters. All model simulations conducted under Phase I, Stage 3, use the Stage 3-developed design storms. REVISED JANUARY