Combined Sewer Overflow Storage Strategy

Combined Sewer Overflows - Main and King CSO tank

Sewer infrastructure is an important and necessary component of any modern city and is fundamental to public health. Across Canada, younger cities have newer infrastructure and dedicated sewer systems. This means that one sewer system is dedicated to handle waste and the other is for storm water.

In the case of older cities like Hamilton, it has both types of systems and much of the downtown has a combined sewer system. This is where a single sewer system collects and handles both the sanitary sewage directly from households and storm water runoff. The City of Hamilton owns and operates one of the largest and most complex combined sewer systems on the Great Lakes and as a result has a number of combined sewer overflows that protect the system against surcharges and overflows during severe wet weather events. In fact, more than 600 km of combined sewers collect both sanitary and storm flows from an area of approximately 52 km. These flows are then directed to the Woodward Wastewater Treatment Plant for treatment and ultimately released into Hamilton Harbour through the Red Hill Creek.

During dry weather or a light rainfall, flows are conveyed through the combined sewer system before reaching the wastewater treatment plant on Woodward Avenue in the City’s east end. 

During a heavy rainstorms, sanitary and storm water inflows exceed the capacity of the system and the treatment plant and may overflow into the natural environment.

To respond to this, the City of Hamilton built a series of combined sewer overflow tanks beginning in the mid-1980s to prevent untreated wastewater from going directly into the 22 local receiving waters in and around Hamilton including Hamilton Harbour.

Royal Ave CSO constructionRoyal Ave CSO construction


These “CSO Tanks” hold the untreated wastewater until the Woodward Avenue treatment plant has capacity to treat it, possible several days after a wet weather event. The overflow tanks are also necessary to protect the treatment plant against hydraulic overloading that could upset the sewage treatment processes.

How does it relate to the Hamilton Harbour Remedial Action Plan?

Hamilton Harbour is among 43 Areas of Concern (AOC), and one of 17 Canadian AOCs, identified within the Great Lakes Basin by the International Joint Commission (IJC) for Canadian-American Boundary Waters in 1985. The Great Lakes Water Quality Agreement requires that Canadian and American governments develop Remedial Action Plans (RAPs) to address pollution problems and restore the beneficial use impairments identified in each AOC. Local governments are also charged with setting out a process for developing these plans.

Beneficial use impairments in Hamilton Harbour and Cootes Paradise, which are at least partially attributable to pollution from CSOs, include restrictions on dredging activities, eutrophication or undesirable algae, beach closings, degradation of aesthetics, added cost to industry for treatment of harbour water, degradation of phytoplankton and zooplankton and loss of fish and wildlife habitat. It is important to emphasize the role of nutrients in this regard and specifically phosphorous as a parameter of concern as a result of its role in the growth of algae.

Setting the goals for the Hamilton Harbour Remedial Action Plan

The Hamilton Harbour Remedial Action Plan (HHRAP) Stage 2 Report was completed in November 1992 and subsequently updated in 2002. It is a partnership through numerous stakeholders, including the Federal and Provincial governments (Environment Canada and the Ontario Ministry of Environment and Climate Change - MOECC), and the City of Hamilton (and the former Regional Municipality of Hamilton-Wentworth).

The report presented specific objectives for removing Hamilton Harbour from the list as an area of concern and recommended actions for achieving delisting. The report identified CSOs as a significant source of pollutants. It recommended that Hamilton undertake specific measures to eliminate or minimize pollution from these discharges.

The HHRAP set water quality goals for the Harbour and specific pollutant loading targets (initial and final) to achieve these goals for each of the significant pollutant discharges into Hamilton Harbour, including Hamilton’s CSOs. The HHRAP assigned the elimination or minimization of pollution from CSOs as its first priority and placed the highest priority on CSOs discharging into Cootes Paradise and the West Harbour.

Royal Ave CSO constructionRoyal Ave CSO construction

Hamilton's CSO Control Program 

Combined Sewer Overflows (CSOs) are the release of untreated sewage directly into the harbour. CSOs represent a potential hazard and can have adverse affects on aquatic life, recreation uses and water quality. CSOs in Ontario are governed by the Ministry.

MOECC procedures require municipalities with CSOs to prepare Pollution Prevention and Control Plans (PPCPs) that outline the nature, cause and extent of pollution problems, examine CSO control alternatives and propose remedial measures. Municipalities must then recommend an implementation program including costs and schedules.

Hamilton’s original PPCP was completed in 1991 which recommended a number of underground detention storage facilities to capture and temporarily store excess CSO volumes during wet weather. Once the storm had passed, the facilities would then slowly drain and convey their contents to the Woodward Avenue Wastewater Treatment Plant for full treatment during dry weather.

The original PPCP was updated in 2003 and in 2009 as part of the City of Hamilton’s new Integrated Wastewater Servicing Master Plan.

CSO Control Program progress

From 1989 to 2015, Hamilton has constructed nine CSO storage facilities, providing more than 314,000 m3 of total storage volume, which is equivalent to 125 Olympic-size swimming pools, to reduce CSOs to local receiving waters.

The cost to construct all the facilities over the years was approximately $89 million (or $163 million in today’s dollars). As recommended by the HHRAP, the City’s efforts have concentrated on reducing CSOs to Cootes Paradise, the Western Harbour Red Hill Creek and the Eastern Harbour through the construction of CSO tanks.

To reduce the cost of the program to ratepayers, the City of Hamilton sought and obtained funding of more than $20 million from a variety of government programs.

Along with the many upgrades at the Woodward Avenue Wastewater Treatment Plant, the CSO tanks have helped to significantly reduce the amount of raw sewage and surface debris from entering local receiving waters.

This allowed for the re-opening of local beaches, achieved the HHRAP’s initial pollutant loading targets and made significant strides towards meeting HHRAP final targets.

The remaining components of Hamilton’s CSO Control Program include the current implementation of a Construction of Greenhill CSO tank computerized Real Time Control (RTC) system to optimize the operation of the CSS and maximize the volume of CSO sconveyed to and treated by the wastewater plant. This also includes the expansion of the Woodward Wastewater Treatment Plant to treat more CSOs during wet weather.

Construction of the CSO tanks

The City of Hamilton has successfully reduced the volume of CSOs to receiving waters by half. Currently, this equates to two billion litres of wastewater and storm water not going into receiving waters because of the construction of the numerous CSO storage facilities.

The frequency and magnitude of storms means that receiving waters including Hamilton Harbour, nearby creeks, beaches and other recreational watercourses would be impacted by CSOs every few days. This would mean that the recreational areas would be off-limits for use and would be negatively environmentally impacted.

The first tank, known as HC001 (83,500 m3), or the original Greenhill CSO Tank, was constructed in 1988 at a cost of approximately $5 Million. This tank reduces the frequency and volume of CSOs to Red Hill Creek but was designed and constructed prior to the 2002 provincial guidelines, which stipulates 90% volumetric control of all wet weather flows in an average year.

The first tank did not meet this level of control but did greatly reduce the frequency and volume of CSOs resulting from storms greater than 15mm of rain. This facility was later integrated and improved with a sister facility (HCS06 or Greenhill 2).

HCS02 (21,000 m3) at Bayfront Park was the second CSO tank to be constructed at a cost of $9.3 Million. This tank was completed in 1993 and was designed to reduce the frequency of CSOs to the West Harbour from 13 times a year down to once a year. It also controls more than 90% of the wet weather flow tributary to it.

Construction of Greenhill CSO tankConstruction of Greenhill CSO tank


The construction of this facility has greatly improved water quality and allowed for the West Harbour to be open to the public. Washrooms and an observation platform overlooking the harbour were integrated into this facility.

HCS03 (1400 tank + 1800 inline storage) is known as the James Street CSO tank. This facility was built in 1993 for $1.5 Million and is designed to reduce the frequency of CSOs from 24 times a year to once a year and reduce the volume of CSOs entering the West Harbour at the foot of James Street near the Royal Hamilton Yacht Club and other in-water recreational facilities.

HCS04 is the Main-King CSO tank (approximately 77,000 m3) and was completed in 1997 for $23 Million. This CSO tank is tributary to an environmentally sensitive area known as Cootes Paradise. It is designed to reduce the frequency of CSOs from 29 times a year to 3 times a year and reduce the volume of CSOs. It also controls 90% of the tributary wet weather flow. A park was created above the facility that also contains a cricket pitch and a dog park.

HCS05 is the Eastwood Park CSO tank and was completed in 1997 at a cost of $8.5 Million. It controls CSOs to the eastern-most portion of the West Harbour area and reduces frequency of CSOs from 27 times a year down to two or three times a year. It also controls 90% of wet weather flow.

HCS06 is located adjacent to the original Greenhill CSO tank and was constructed in 2003 at a cost of $16 Million and was designed to work in series with the original Greenhill CSO tank to improve performance at Red Hill Creek and achieve modern performance standards of 90% control.

The Royal Avenue CSO Tank was constructed in 2007 at a cost of $9M. Its volume is ~ 15,000 m3 and was designed to provide 90% volumetric control of all wet weather flows in an average year for its tributary system to Chedoke Creek and, ultimately, Cootes Paradise.

The McMaster CSO tank was constructed in 2010. It is 5,935 m3 and controls 90% of tributary wet weather flow.

The Red Hill Superpipe is the most recent CSO facility to be constructed and was coordinated with the massive construction of the Red Hill Creek Expressway. This facility consists of nearly 1.7 km of oversized (2250 and 3000 mm diameter) sewer pipes and provides approximately 15,000 m3 of storage volume.

This facility operates via motorized and remotely controlled sluice gates which retain sewage within the pipe and manholes during a storm.

It then slowly releases this sewage for treatment at the Woodward Avenue WWTP following the storm event. This prevents capacity overloads of the wastewater treatment plant to process and treat the water.

CSO Red Hill SuperpipeCSO Red Hill Superpipe
Red Hill Valley CSO overflowRed Hill Valley CSO overflow

How does it relate to the Clean Harbour Program?

The Clean Harbour Program was branded in 2013 and represents a series of projects undertaken by the City of Hamilton aimed at reversing the environmental damage to Hamilton Harbour and in support of the HHRAP.

A number of projects that represent significant investment by the City of Hamilton have been completed over the years in support of the HHRAP and the CSO Control program is foundational in this regard. Hamilton will be revisiting its Water and Wastewater Master Plan in 2016 to begin updating the plan with a fresh review of the CSO Control program to explore what further benefits that this very effective program may provide for the future.

What is the return-on-investment for the residents of Hamilton?

The harbour is protected as the series of combined sewer overflow prevents untreated wastewater from going directly into local receiving waters in and around Hamilton including Hamilton Harbour. It’s a sound, environmental solution to protect the harbour from contamination.