Chapter 21: Control and Treatment of Combined Sewer Overflows

This chapter provides guidance regarding combined sewer overflow (CSO) control as well as an overview of minimum treatment requirements. A discussion of source management and commonly used treatment technologies is also provided. Many of the control options presented in this chapter are operational measures which may serve to mitigate the impacts associated with combined sewer systems.

21.1 CSO Considerations

21.1.1 CSO Control Requirements

The designer should consider the requirements of the ministry Procedure F-5-5, Determination of Treatment Requirements for Municipal and Private Combined and Partially Separated Sewer Systems in the design or upgrading of sewage works associated with combined sewer systems. The minimum CSO controls in accordance with this Procedure should consist of the following:

  • Elimination of CSOs during dry-weather periods except under emergency conditions;
  • Establishment and implementation of Pollution Prevention programs that focus on pollutant reduction activities at source, as described in the Procedure;
  • Establishment and implementation of proper operation, regular inspection and maintenance programs for combined sewer systems;
  • Establishment and implementation of a floatables control program to control coarse solids and floatable materials;
  • Maximizing the use of the collection system for the storage of wet weather flows which are conveyed to the sewage treatment plant (STP) for treatment when capacity is available (e.g. by adjusting regulator settings);
  • Maximizing flow to the STP for the treatment of wet weather flows (e.g. by removing obstructions to flow), with the secondary treatment capacity at the STP being utilized to the maximum extent possible for treating these flows; and
  • Capture and treatment of all dry weather flow plus 90 percent of the volume resulting from wet weather flow that is above the dry weather flow for an average year, during a seven-month period commencing within 15 days of April 1, as specified in the Guideline.
21.1.1.1 Beach Protection

Additional controls above the minimum CSO controls described above are required for swimming and bathing beaches affected by CSOs. The designer should refer to Procedure F-5-5 for more information.

21.1.1.2 New Sanitary Connections to Combined Sewer Systems

New developments tributary to combined sewer systems should be avoided until the necessary upgrades to the combined sewer system can be completed. Conditions under which some development will be permitted are specified in Procedure F-5-5.

21.1.1.3 New Storm Connections to Combined Sewer Systems

As stated in Procedure F-5-5, “new storm drainage system should not connect to existing combined sewer systems if that increases the gross area serviced by the combined sewer system except where evaluations indicate that circumstances allow no other practical alternative. The evaluations should be documented as part of a Pollution Prevention and Control Plan.”

21.1.2 Treatment Levels

The treatment of wet weather flows from combined sewer systems may occur at the central STP or at other locations such as satellite treatment facilities. Minimum treatment requirements are described in Procedure F-5-5.

21.1.3 CSO Monitoring

The designer, in consultation with the municipality or operating authority of the combined sewer system, should consider the provision for monitoring equipment for sewage flows and overflows at various locations within the sewer system for the purposes of assessing upgrading requirements and determining compliance with ministry requirements.

21.1.4 CSO Modelling

The following subsections briefly summarize combined sewer system modelling objectives, model selection strategies and model development and application, including model calibration and validation and the different types of model simulations (e.g. long-term continuous versus storm event simulations). A more detailed discussion is provided in the ministry Stormwater Management Planning and Design Manual (2003) as well as the U.S. EPA guidance documents Combined Sewer Overflows Guidance for Long-Term Control Plan and Combined Sewer Overflows Guidance for Monitoring and Modeling.

21.1.4.1 Combined Sewer System Modelling Objectives

The primary objective of combined sewer system modelling is to understand the hydraulic response of the combined sewer system to a variety of precipitation and drainage area inputs. Combined sewer system modelling can also be used to predict pollutant loadings to receiving waters. Once the model is calibrated and verified, it can be used for numerous applications that support CSO planning efforts, including:

  • Predicting overflow occurrence, volume and in some cases, quality for rain events other than those which occurred during the monitoring phase. These can include a storm event of large magnitude (long recurrence period) or more frequent storm events over an extended period of time;
  • Predicting the performance of portions of the combined sewer system that have not been extensively monitored;
  • Developing CSO statistics, such as annual number of overflows and percent of combined sewage captured;
  • Optimizing combined sewer system performance. In particular, modelling can assist in locating storage opportunities and hydraulic bottlenecks and demonstrate that system storage and flow to the STP are maximized; and
  • Evaluating and optimizing control alternatives. The model can be used to evaluate the resulting reductions in CSO volume and frequency for both simple and complex control strategies.

For a specific system, the designer should consider whether analysis using complex computer models is needed. In simple systems, computation of hydraulic profiles using basic equations and spreadsheet programming might be sufficient for identifying areas where certain measures can be implemented and for evaluating their hydraulic effect. In complex combined sewer systems, that have looped networks or sections that surcharge, a hydraulic computer model is a useful tool.

Continuous simulation refers to the use of long-term rainfall records (e.g. from several months to several years) rather than rainfall records for individual storms (i.e., design storms). Continuous simulation has several advantages:

  • Simulations are based on a sequence of storms so that the additive effect of storms occurring close together can be examined;
  • Storms with a range of characteristics are included; and
  • Long-term simulations enable the development of performance criteria based on long-term averages, which are not readily determined from design storm simulations.
21.1.4.2 Combined Sewer System Model Selection

Combined sewer system modelling involves two distinct elements – hydraulics and water quality:

  • Hydraulic modelling consists of predicting flow characteristics in the combined sewer system. These characteristics include the different flow rate components (i.e., sanitary sewage, infiltration and stormwater runoff), the flow velocity and depth in the interceptors and sewers and the CSO flow rate and duration; and
  • Combined sewer system quality modelling consists of predicting the quality of the combined sewage in the system, particularly at CSO outfalls and at the treatment plant. Water quality is measured in terms of critical parameters, such as bacterial counts and concentrations of BOD5, suspended solids, nutrients and toxic contaminants.

Some models include both hydraulic and water quality components while others are limited to one or the other. The designer, in consultation with the system owner/operator, should determine which type of model is most appropriate.

21.1.4.3 Model Calibration and Verification

A general model should be adjusted to the characteristics of a specific site and situation. Modelers use model calibration and verification first to perform this adjustment and then to demonstrate the credibility of the model simulation results. Using an uncalibrated model (e.g. using typical industry values for model parameters) might be acceptable for screening purposes. Without supporting evidence, however, the uncalibrated result might not be accurate. To use model simulation results for evaluating control alternatives, the modeller should provide evidence demonstrating the model’s reliability.

21.2 Stormwater Management

The ministry Stormwater Management Planning and Design Manual provides technical and procedural guidance for the planning, design and review of stormwater management practices. Specifically, the manual provides guidance regarding:

  • Environmental planning;
  • Environmental design criteria;
  • Stormwater management plans and stormwater management practice (SWMP) design;
  • Infill development; and
  • Operation, maintenance and monitoring.

The designer should consult the ministry manual for guidance regarding the development of a stormwater management program and for the design of SWMPs, as part of an overall CSO control program.

21.3 Source Management and Control Technologies

Control measures can include technologies, operating strategies, public policies and regulations, or other measures that would contribute to some aspect of CSO control. Control measures can generally be classified under one of the following categories:

  • Source controls;
  • Collection system controls;
  • Storage technologies; and
  • Treatment technologies.

A brief description of some of the options available for each category is provided below.

21.3.1 Source Control

Source controls affect the quantity and/or quality of runoff that enters the collection system. Since source controls reduce the volumes, peak flows or pollutant loads entering the collection system, the size of more capital-intensive downstream control measures can be reduced or, in some cases, the need for downstream facilities can be eliminated. The source controls discussed below include both quantity and quality control measures. A more detailed discussion is provided in the U.S. EPA document Combined Sewer Overflows Guidance for Long-Term Control Plan.

21.3.1.1 Porous Pavements

Porous pavements reduce runoff by allowing stormwater to drain through the pavement to the underlying soil. Porous pavements, most commonly used in parking lots, require skill and care in installation and maintenance to ensure that the pores in the pavement do not become plugged.

21.3.1.2 Flow Detention

Detention ponds in upland areas and roof-top storage can store stormwater runoff temporarily, delaying its introduction into the collection system and thereby helping to attenuate peak wet weather flows in the collection system. The detention facilities drain back to the collection system when peak wet weather flows subside.

21.3.1.3 Area Drain and Roof Leader Disconnection

In highly developed areas with relatively little open, pervious space, roof leaders and area drains are commonly connected directly to the combined sewage collection system. Rerouting of these connections to separate storm drains or available pervious areas can help reduce peak wet weather flows and volumes.

21.3.1.4 Foundation Drains

Foundation drains may be connected to storm sewers in some areas. Sump pumps may be used to discharge foundation drainage to the surface or to soakaway pits. In areas where the seasonal high water table is within 1 m (3 ft) of the foundation drain, sump pumps should not be used in order to prevent the formation of a looped pumping operation and excessive pump operation. Where sump pumps are not feasible, a “third pipe” may be used to convey foundation drainage to a suitable discharge point.

21.3.1.5 Use of Pervious Areas for Infiltration

Detention of storm flow in pervious areas not only helps attenuate peak wet weather flow in the collection system but also reduces runoff volume through infiltration into the soil. Grassed swales, infiltration basins and subsurface leaching facilities can be used to promote infiltration of runoff. Infiltration sumps can be used in areas with well draining soils. The designer should consider the impact of cold climate conditions. This type of control might be more appropriate as a requirement for future development or re-development and could be implemented through sewer use by-laws and through strict review of proposed development plans.

21.3.1.6 Solid Waste Management

Although littering is generally prohibited, it is a common problem in many communities. Street litter typically includes metallic, glass and paper containers, cigarettes, newspapers and food wrappers. If not removed from the street surfaces by cleaning equipment, some of these items often end up in combined sewer overflows (Section 8.5.6.4 - Bypasses and Overflows at Sewage Works Serving Combined Sewers), creating visible pollution due to their floatable nature.

Enforcement of anti-litter by-laws, public education programs, and conveniently placed waste disposal containers might be effective, low-cost alternatives, especially in urban business areas. The proper disposal of leaves, grass clippings, crankcase oil, paints, chemicals and other such wastes can be addressed in a public education program. Because the results of such a program depend on voluntary cooperation, the level of effectiveness can be difficult to predict.

21.3.1.7 Street Sweeping

Frequent street sweeping can prevent the accumulation of dirt, debris and associated pollutants, which may wash off streets and other tributary areas to a combined collection system during a storm event. Current sweeping practices can be analyzed to determine whether more frequent cleaning will yield CSO control benefits. The overall effectiveness of street sweeping as a CSO control measure depends on a number of factors, including frequency of sweeping, size of particles captured by sweeping, street parking regulations and climatic conditions, such as rainfall frequency and season.

21.3.1.8 Fertilizer and Pesticide Control

Fertilizers and pesticides washed off the ground during storms contribute to the pollutant loads in stormwater runoff. It is important that all users follow proper handling and application procedures. The use of less toxic formulations should also be encouraged. Because most of the problems associated with these chemicals are a result of improper or excessive usage, a public education program may be beneficial.

21.3.1.9 Snow Removal and De-Icing Control

This abatement measure involves limiting the use of chemicals for snow and ice control to the minimum necessary for public safety. This, in turn, would limit the amount of chemicals (i.e., normally salt) and sand washed into the collection system and ultimately contained in CSOs. Proper storage and handling measures for these materials might also reduce the impacts of runoff from material storage sites.

21.3.1.10 Soil Erosion Control

Controlling soil erosion is important in relation to CSOs and water quality for a number of reasons:

  • Soil particles carry nutrients, metals, and other toxics which may be released into the receiving water, contributing to algal blooms, potential toxic effects and bioaccumulation of toxics; and
  • Eroded soil can contribute to sedimentation problems in the collection system, potentially reducing hydraulic capacity.

Properly vegetated and/or stabilized soils are not as susceptible to erosion and thus will not be washed off into combined sewers during wet weather. Like for fertilizer and pesticide control, an educational program may be useful in controlling soil erosion. Implementation and enforcement of erosion control regulations at construction sites can also be effective.

21.3.1.11 Commercial/Industrial Runoff Control

Commercial and industrial lands, including gasoline stations, railroad yards, freight loadings areas and parking lots contribute grit, oils, grease and other pollutants to combined sewer systems. Such contaminants can run off into receiving waters. Installing and maintaining oil/grit separators in catchbasins and area drains can help control runoff from these areas, while pretreatment requirements can be identified as part of the municipality’s sewer use by-laws.

21.3.1.12 Animal Waste Removal

This measure refers to removing animal excrement from areas tributary to combined sewer systems. The impact of this control measure is difficult to quantify; however, it might be possible to achieve a minor reduction in bacterial load and oxygen demand. This best management practice (BMP) can be addressed by a public information program and “pooper-scooper” by-laws.

21.3.1.13 Catchbasin Cleaning

The regular cleaning of catchbasins can remove accumulated sediment and debris that could ultimately be discharged in CSOs. In many communities, catchbasin cleaning is targeted more toward maintaining proper drainage system performance than pollution control.

21.3.2 Combined Sewer System Control

Combined sewer system controls and modifications affect CSO flows and loads for stormwater runoff that has entered the collection system. This category of control measures can reduce CSO volume and frequency by maximizing the volume of flow stored in the collection system, or maximizing the capacity of the system to convey flow to a STP and includes the control alternatives described in the following subsections. A more detailed discussion is provided in the U.S. EPA guidance document Combined Sewer Overflows Guidance for Long-Term Control Plan.

21.3.2.1 Sewer Line Flushing

Sediments that accumulate in sewers during dry weather can be a source of CSO contaminants during storm events. Periodically flushing sewers during dry weather conditions will convey settled materials to the STP. The cost effectiveness of such a program, however, depends on treatment, labour costs, physical sewer characteristics and productivity.

Sewer cleaning usually requires the use of a hydraulic, mechanical or manual device to re-suspend solids into the sewage flow and carry them out of the collection system. This practice might be more effective for sewers with very flat slopes. Cleaning costs increase substantially for larger interceptors due to occasional accumulations of thick sludge blankets in inverts.

Grit management should be considered and precautions taken to avoid overloading the STP during the cleaning process. Consideration should also be given to disposal of debris dislodged during flushing. Additional information is provided in the ministry Stormwater Management Planning and Design Manual (2003).

21.3.2.2 Maximizing Use of Existing System

This control measure involves maximizing the quantity of flow collected and treated, thereby minimizing CSOs It involves ongoing maintenance and inspection of the collection system, particularly flow regulators and tidegates. In addition, minor modifications or repairs can sometimes result in significant increases in the volume of storm flow retained in the system. Strict adherence to a well-planned preventive maintenance program can be a key factor in controlling dry and wet weather overflows.

21.3.2.3 Sewer Separation

Separation is the conversion of a combined sewer system into separate stormwater and sanitary sewage collection systems. Sewer separation is a positive means of eliminating CSOs and preventing sanitary flow from entering the receiving water during wet weather periods and may be applicable and cost-effective on a site-specific basis. The benefits of separation should be evaluated, with consideration given to a potential increase in the loading of stormwater runoff pollutants (e.g. sediment, bacteria, metals, oils) being discharged to the receiving water, cost (it is relatively expensive) and the potential disruption of traffic and other community activities during construction.

21.3.2.4 Infiltration/Inflow Control

Excessive infiltration and inflow (I/I) can increase operation and maintenance costs and can consume hydraulic capacity, both in the collection system and at the STP. In combined sewer systems, surface drainage is by design the primary source of inflow. Sources of inflow in combined sewer systems should be controlled, including roof leaders, sump pumps, and tidal inflow (i.e., through leaking or missing tidegates, where applicable).

Infiltration is groundwater that enters the collection system through defective pipe joints, cracked or broken pipes, manholes, footing drains and other similar sources. Infiltration flow tends to be more constant than inflow. Significant lengths of sewers usually need to be rehabilitated to effectively reduce infiltration and the rehabilitation effort should include house laterals.

Implementation of an effective maintenance and inspection program, consisting of closed-circuit television (CCTV) inspections, manhole and lateral assessments, is necessary to control infiltration.

21.3.2.5 Regulating Devices and Backwater Gates

Flow regulating devices have been used for many years in combined sewer systems to direct dry weather flow to interceptors and to divert wet weather combined flows in excess of interceptor capacity to receiving waters (i.e., as CSOs).

In general, regulators fall into two categories: static and mechanical. Static regulators have no moving parts and, once set, are usually not readily adjustable. They include side weirs, transverse weirs, restricted outlets, swirl concentrators (i.e., flow regulators/solids concentrators) and vortex valves. Mechanical regulators are adjustable and might respond to variations in local flow conditions or be controlled through a remote telemetry system. They include inflatable dams, tilting plate regulators, reverse-tainter gates, float-controlled gates and motor-operated or hydraulic gates.

The designer should also take into account maintenance issues (i.e., frequency and complexity) associated with the particular type of regulator being considered.

21.3.2.6 Real-Time Control

System-wide real-time control (RTC) programs can provide integrated control of regulators, outfall gates and pump station operations based on anticipated flows from individual rainfall events, with feed-back control adjustments based on actual flow conditions within the system. Computer models associated with the RTC system allow an evaluation of expected system response to control commands before execution. Localized RTC might also be provided to individual dynamic regulators, based on feedback control from upstream and/or downstream flow monitoring elements. As with any plan for improving in-line storage, to take the greatest advantage of RTC, a combined sewer system should have relatively flat upstream slopes and sufficient upstream storage and downstream interceptor capacity.

21.3.2.7 Flow Diversion

Flow diversion is the diversion or relocation of dry weather flow, wet weather flow, or both from one drainage basin to another through new or existing drainage basin interconnections. Flow diversion can relieve an overloaded regulator or interceptor reach, resulting in a more optimized operation of the collection system. Flow diversion can also be used to relocate combined sewer flow from an outfall located in a more sensitive receiving water area to an outfall located in a less sensitive one.

21.3.3 Storage

Wet weather flows can be stored for subsequent treatment at the STP once treatment and conveyance capacity have been restored. Specific design guidance for storage facilities is provided in the Environment Canada CSO Treatment Technologies Manual.

21.3.3.1 In-Line Storage

In-line storage is storage in series with the sewer. In-line storage can be developed in two ways:

  • Construction of new tanks or oversized conduits to provide storage capacity; or
  • Construction of a flow regulator to optimize storage capacity in existing conduits.

The new tanks or oversized conduits are designed to allow dry weather flow to pass through, while flows above a design peak is restricted, causing the tank or oversized conduit to fill. A flow regulator on an existing conduit functions under the same principle, with the existing conduit providing the storage volume. Developing in-line storage in existing conduits is typically less costly than other, more capital-intensive technologies, such as off-line storage/sedimentation and is attractive because it provides the most effective utilization of existing facilities. The applicability of in-line storage, particularly the use of existing conduits for storage, is very site-specific, depending on existing conduit sizes and the risk of flooding due to an elevated hydraulic grade line.

21.3.3.2 Off-Line Near Surface Storage

This technology reduces overflow quantity and frequency by storing all or a portion of diverted wet weather combined flows in off-line storage tanks. The storage arrangement is considered to be parallel with the sewer. Stored flows are returned to the interceptor for conveyance to the STP once system capacity is available. In some cases, flows are conveyed to a CSO treatment facility.

21.3.3.3 Deep Tunnel Storage

This technology provides storage and conveyance of storm flows in large tunnels constructed well below the ground surface. Tunnels can provide large storage volumes with relatively minimal disturbance to the ground surface, which can be very beneficial in congested urban areas. Flows are introduced into the tunnels through dropshafts and pumping facilities are usually required at the downstream ends for dewatering.

21.3.4 Treatment Technology Options

The design of CSO treatment facilities involves all the normal considerations associated with any STP design plus the rather unique aspect of an intermittent and highly variable influent. In addition, special attention should be paid to siting issues and the potential operating impacts such as odours that may be associated with a satellite facility.

Most CSO treatment facilities are comprised of a number of unit processes tied together in a train. Multiple processes are employed because of the specialized function of each unit (e.g. solids removal, disinfection). Some processes such as screening and degritting are employed primarily to protect downstream process equipment. Even so, screens and degritting serve also to remove gross solids and heavy grit particles, which enhance the quality of the treated effluent. Some unit processes, such as UV disinfection, require more extensive pretreatment to be viable. UV disinfection design guidelines are provided in Section 14.4 - Ultraviolet Irradiation. A complete CSO treatment process train should address both liquid and solids treatment and residue disposal.

The location of the proposed CSO treatment facility should also be considered in finalizing the process train. Satellite facilities should incorporate all liquid train unit processes on site. Depending upon circumstances it may not be desirable to utilize a process requiring chemical storage at some satellite locations. Community sensitivity may in this case dictate a different process train selection.

Other design considerations may also influence process train selection. For example, the facility footprint may be a major factor where limited siting opportunities are available. In turn, this may favour very high rate treatment, integrating a number of unit processes within a single facility. Other processes are complex to operate and require skilled attention. These may not be well suited to satellite locations. Finally, both capital and operating costs play a major role in technology selection and should be carefully weighed during process train evaluation.

Specific design parameters, such as flow rate, hydraulic loading and solids loading for a chosen technology will depend on the treatment objectives and the characteristics of the specific unit selected. These parameters are often established through piloting and in consultation with the manufacturer. Additional information is also provided in Environment Canada’s CSO Treatment Technologies Manual.

21.3.4.1 Screening Devices

Screening technologies can be applied in a CSO treatment train in one of two modes:

  • Part of an overall CSO treatment process train. This application is associated with CSO treatment facilities capable of providing “primary equivalent treatment”. The screening device can be employed either for pretreatment or for effluent polishing; and
  • Stand-alone where the screen is the main treatment device although not generally able to provide “primary equivalent treatment”. This application is used to address gross solids including floatable materials in the remaining overflows discharging without “primary equivalent treatment” or CSO treatment facility bypass streams.

There are a wide variety of screens and screening devices in all manner of configurations, aperture sizes and applications. Therefore, the designer should consult the manufacturer for specific design guidance.

21.3.4.2 Floatables Control

Several technologies are available for floatables control, including screens, fabric nets, rotary sieves, or by using systems operating on the vortex principle, traps in catchbasins and simple underflow baffles in the overflow path.

21.3.4.3 Ballasted Clarifiers

Ballasted clarification is a term applied to proprietary technologies that employ ballasted coagulation-assisted settling. The main advantage of these processes is the very high rate of treatment possible that allows a reasonably small footprint. Because of the typically high coagulant and coagulant aid dosages employed, these technologies may also yield a greater degree of pollutant removal.

These technologies may be employed at satellite locations or as part of a stand alone or integrated facility at a STP. The designer should consult with the manufacturer regarding specific design requirements. Pilot testing is recommended for site-specific applications. For additional information on ballasted clarifiers see Section 15.5 – High Rate Clarification.

21.3.4.4 Retention Treatment Basins (Assisted and Unassisted)

Retention treatment basins (RTB) are intended to provide removal of CSO pollutants through sedimentation, removal of floatable materials (i.e., through integral baffling and screening) and full or partial capture of CSO discharges.

RTBs typically consist of several compartments to allow smaller overflow events to be captured and/or treated without the utilization of the entire facility. Dividing the storage volume of the RTB into separate compartments also allows different portions of the tank to be used for other unit operations, such as disinfection. Positively buoyant materials are usually removed by some type of baffle or skimmer arrangement or by installation of screens. Smaller storm events and portions of larger events are captured within the storage volume afforded by an RTB.

Specific guidance for the design of RTB is provided in the Environment Canada CSO Treatment Technologies Manual.

21.3.4.5 Inclined Plate Settlers

Installation of inclined plate separators may increase the effective settling area and allowable surface overflow rate of a sedimentation unit. This technology should be combined with chemical treatment if a significant increase in suspended solids removal is needed. Plate settlers are generally incorporated into other process units, such as RTB or ballasted clarifiers.

21.3.4.6 Flow Balance Methods

The flow balance technique is a retention treatment system using weighted plastic curtains hung from pontoons in shore locations in natural receiving water bodies. During operation, extraneous fluid (stormwater/CSO) flows into a pontooned multi-celled system while displacing currently stored fluid (lake water). This method relies on stratification of natural water and CSOs because of their different specific gravities. In freshwater bodies, multiple cells can be used in place of a bottom outlet.

After the storm event, the reverse pattern is created by pumping contents from the first or influent cell of the storage facility back to the sewer for subsequent treatment. Deposited solids can also be retrieved by pumping. For a small runoff event, only one or more of the cells are used. For a large event, all cells are filled and the system can be overflowed. Relative efficiency in transferring flows through plug-flow operation is determined by the number of cells and relative placement.

A feed pump and the discharge point are connected to the first compartment. Excess stormwater flow is diverted through large openings in intermediate baffles. Openings are placed alternatively at the bottom and top to prevent stratification caused by water temperature differences. The baffles, made of plastic cloth, hang from pontoons. The cloth is attached to the bottom with weights. There is no demand for absolute tightness against the bottom, as the sole function of the baffles is to create plug-flow conditions.

Additional information is provided in Water Environment Federation, WEF Manual of Practice FD-17 Prevention and Control of Sewer System Overflows. The designer should consult with the manufacturer regarding specific design requirements.

21.3.4.7 Vortex Separators (Assisted and Unassisted)

Vortex separators can be applied in satellite facilities or as part of an integrated or stand-alone facility at a STP. The separators are as versatile as RTB and can be employed with the following configurations:

  • In-vessel coagulant addition;
  • In-vessel chemical disinfection;
  • Integral fine screens; and
  • Add-on disinfection (chemical or UV).

Separators do not necessarily require pretreatment although they are most often preceded by coarse screens. Fine screens (screens can be add-on or integral) can also be added if removal of neutral density floatables is required. Separators produce significant quantities of underflow which need to be transported and treated at a STP. Additional information on Vortex separators, as they are applied to preliminary treatment, can be found in Section 10.3.3.4 - Vortex Grit Removal.

The designer should consult with the manufacturer regarding specific design requirements. Pilot testing is recommended for site-specific applications.

21.3.4.8 Compressible Filter Media

A compressible filter media system is a proprietary technology that provides a high rate of solids removal through the use of synthetic fibre spheres. These processes are typically used as a polishing step in a more complex CSO treatment train, which may include other physical separation technologies upstream and UV disinfection downstream of the unit. Influent to the filters usually requires pretreatment to remove heavy solids and coarse floatable materials.

In theory, these units could be employed at satellite locations. However, until more experience is gained with automated operation, it is suggested that they be applied either as an integrated or stand-alone CSO treatment at STP locations. The designer should consult the manufacturer for specific design guidance. Pilot testing is recommended for site-specific applications.

21.4 Handling and Disposal of CSO Solids Residuals

Residue management is a principal concern when using screening and degritting to treat CSOs. The handling of collected materials on trash racks and manually cleaned bar screens involves removing larger debris that may be collected, since the spacing will allow most of the floatables to pass through the openings. As the screen aperture decreases, there will be an increase in the amount of collected materials requiring removal. In addition, material collected immediately upstream and downstream of the screen may require periodic removal.

Typical volumes of residue removed by screens range from 3.5 to 84 L per 1,000 m3 (0.47 to 11.2 ft3/(million US gal)) of influent. The actual amount of residue to be disposed of will vary depending on the following factors:

  • Drainage system configuration;
  • Time of year;
  • Storm intensity and inter-event time;
  • Velocity of flow through the screen; and
  • Screen aperture.

The disposal of screenings may be provided by any of the following:

  • Removal by hauling to disposal sites (landfills);
  • Incineration, either alone or in combination with sludge and grit;
  • Disposal with municipal solid wastes; and
  • Return to sewage, either directly or via grinders and macerators.

To minimize handling and disposal costs, it is preferable to have screenings and grit returned to the sewer. Alternatively, screenings can be discharged to a container for ultimate disposal at both satellite and STP locations. Screening residues can also be returned to an interceptor sewer at satellite locations. In this case, the screenings will be once more removed at the STP and then taken to disposal.

Residuals management for screening and degritting operations is addressed in the Environment Canada CSO Treatment Technologies Manual as well as in Section 10.6 - Screenings, Grit Handling and Disposal.

21.5 Disinfection

Effluent disinfection is required where CSO affects swimming and bathing beaches and other areas where there are public health concerns. The interim effluent quality criterion for disinfected combined sewage during wet weather is a monthly geometric mean not exceeding 1000 E. coli organisms per 100 mL. This criterion may be modified by the Regional staff of the ministry on a case-by-case basis due to site-specific conditions.

All overflows and bypasses at the STP should be subjected to the disinfection process where available in order to reduce the bacterial loadings at discharge.

Specific guidance regarding the design of disinfection systems is provided in Chapter 14 - Disinfection as well as in the Environment Canada CSO Treatment Technologies Manual.

21.5.1 Chlorination and Dechlorination

In cases where chlorination is used as the disinfection process, subsequent dechlorination of the sewage works effluents should be used to minimize the adverse effects of chlorine residuals on public health and the aquatic environment where necessary.

21.5.2 Ultraviolet Irradiation

Ultraviolet (UV) irradiation for the disinfection of CSO should be used in conjunction with other unit processes capable of meeting primary-equivalent treatment requirements which should address floatables, TSS and CBOD5. UV irradiation should follow pretreatment and solids separation processes. UV irradiation can be applied in satellite or STP integrated or stand-alone facilities.