Greater Infrastructure Investments Needed to Reduce Combined Sewer Overflows

By Michael Mezzacapo, IJC

cso graphic combined sewer overflows
Figure 1: A graphic depicting a CSO event in extreme weather. Credit: Michael Mezzacapo

Many older sewer systems in Canada and the United States mix stormwater runoff with raw or partially treated sewage and discharge the excess into the Great Lakes during periods of heavy precipitation. These discharges are known as combined sewer overflow events (CSOs). CSO systems can handle typical rain events (Figure 1) but during more intense rain events the capacity of the treatment plant and its connecting systems is exceeded, causing the excess water to be discharged to a nearby lake or river (Figure 2).

CSO runoff can contain contaminants, including pathogens like E.coli and chemicals and nutrients like phosphorus and nitrogen, which impact the drinkability, swimability and fishability of Great Lakes waters. Outflows from CSOs also have health and economic impacts, resulting in drinking water supplies requiring greater and more costly treatment and beach closures in order to protect human health. The IJC recommends zero discharge of inadequately treated or untreated sewage into the Great Lakes and connecting waters in its recently released First Triennial Assessment of Progress (TAP) under the Great Lakes Water Quality Agreement.

The TAP report notes that in just one year, 20 Great Lakes cities in Canada and the US released a combined total of 92 billion gallons of untreated sewage and stormwater to the Great Lakes, mostly via CSOs. That’s roughly equal to 147,000 Olympic size swimming pools. Between Canada and the US, there are about 291 cities in the Great Lakes basin with antiquated sewer systems which release CSOs, 109 in Canada and 182 in the United States. The map below shows the discharge in millions of gallons per year between 2005-2008 for 49 cities in the US and Canada.

csos map great lakes basin
Map showing the discharge volume of CSOs for 49 cities between 2005-2008 within the Great Lakes basin. Credit: GLEAM

The release of wastewater just above Niagara Falls this summer by the Niagara Falls Water Board sparked public outrage and government fines. The July discharge was highly visible, occurring during the peak tourist season. Citizens may be unaware of how frequently CSO events occur around the Great Lakes basin. In 2014, a US Environmental Protection Agency report cited 1,482 untreated CSO events in US states within the Great Lakes basin. Although the province of Ontario issues guidance to municipalities on CSOs, there are no comprehensive reports detailing these events. Releases may intensify as aging sewer systems are impacted by a changing climate due to precipitation pattern shifts and population increases.

An Aug. 15 discharge of sewage into the Niagara River
An Aug. 15 discharge of sewage into the Niagara River. Credit: Christine Hess

Citizens may not notice impacts from CSO discharges. The old adage of “out-of-sight and out-of-mind” often comes into play. Those that live directly on the shoreline and frequent beaches are most likely to notice the foul smells and discolored waters, while others who aren’t adjacent to CSO releases may not even be aware they are occurring, although many states require public notice of such events.  Ontario does not require public notice of such events, though some cities do notify the public.

More than 35 million people rely on the Great Lakes for drinking water, recreation and employment. CSOs have the potential to cause human illness over large sections of the population, not just those who recreate in the contaminated water. A study published in the Journal of Environmental Health Perspectives detailed a 13 percent increase in emergency room visits related to gastrointestinal illness in Massachusetts following extreme precipitation events in areas with sewers that discharged CSOs into drinking water sources. Another study co-authored by IJC Health Professional Advisory Board member Dr. Tim Takaro noted that drinking water systems can prevent illness by developing planning tools and building resilience and capacity into infrastructure for future events; the study was done in Vancouver, British Columbia.

The IJC has consistently expressed concern about the need to increase the governments’ attention to water quality and human health. The IJC recommends in its recent TAP report that older sewer systems that contribute CSOs to the Great Lakes be upgraded to separated sewer systems which do not combine stormwater and sewage. In its 14th Biennial Report in 2009, the IJC highlighted the safety risk to human health by exposure to contaminants from CSOs through fish consumption, drinking water and swimming.

To protect human health and reduce exposure to untreated and inadequately treated sewage, the IJC recommends in its new TAP report that Canada and the US determine an accelerated and fixed period of time by which zero discharge of inadequately treated or untreated sewage into the Great Lakes will be virtually achieved. Given the importance of public health and lake recreation to the Great Lakes public and local economies, the IJC recommends that sufficient resources be dedicated to proactively and systematically improve the capacity of city sewer systems to respond to extreme storm events, especially as related to combined sewer overflows, in the areas of planning, zoning and adaptation.

The Canadian and US governments have slowed needed investment on infrastructure since the 1970s and 1980s, due to increasing demands placed on municipal budgets in other areas. The need for increases in funding was highlighted in a recent US Clean Watersheds Needs Survey, which found that over the next 20 years, six of the eight Great Lakes states (Minnesota, Wisconsin, Illinois, Indiana, Michigan and Ohio), will need an estimated US$77.5 billion to upgrade wastewater and stormwater infrastructure, by separating stormwater and sewage with adequate treatment capacity.

Tackling CSOs will require a concerted and calculated effort between local, provincial, state and federal governments. Engaged citizens can voice their concern over CSO releases and the need to increase spending to upgrade aging infrastructure by writing their local, state, provincial or federal governments. Citizens should inform themselves of CSO events in their area and obey signage for beach closures and fish consumption advisories. Finally, you can also have your voice be heard by submitting comments through

Michael Mezzacapo is the 2017-2018 Michigan Sea Grant Fellow at the IJC’s Great Lakes Regional Office in Windsor, Ontario.

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