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Frequently Asked Questions

  Why is this Class Environmental Assessment (EA) being undertaken?

This Class EA is being undertaken to identify the preferred strategy to address future capacity limitations of the Duffin Creek Water Pollution Control Plant (WPCP) outfall.


What is the Duffin Creek Water Pollution Control Plant (WPCP) outfall?

The outfall conveys treated effluent from the Duffin Creek WPCP to Lake Ontario. The outfall consists of the following two main elements:

  • A three-metre diameter main pipe that extends from the Duffin Creek WPCP to approximately 1.1 kilometres into Lake Ontario. This pipe lies below the lake bed surface.
  • Sixty-three (63) smaller pipes that extend vertically from the main pipe into the lake along the last approximately 180 meters of the main pipe. Of these 63 pipes, currently 38 are open. Each of these is fitted with an elbow and outlet cone called a “diffuser port,” which directs flow into the lake, parallel to the lake bed. The diffuser ports are at an average depth of nine metres below the water surface of Lake Ontario. The ports act similar to a Jacuzzi nozzle in a bathtub and are designed to facilitate rapid mixing of the effluent in the Lake.

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Does the outfall discharge raw sewage into Lake Ontario?

No, the outfall does not discharge any untreated sewage into Lake Ontario. The outfall discharges effluent, which is the liquid remaining following sewage treatment. Unlike some sewage treatment plants in the Greater Toronto Area, the Duffin Creek WPCP does not have the capability to by-pass raw sewage into Lake Ontario. Even during high precipitation events, all incoming sewage must pass through the treatment processes at the Duffin Creek WPCP.


What effect does the Duffin Creek WPCP and existing outfall have on Lake Ontario water quality?

The Duffin Creek WPCP produces one of the highest quality effluents along the Greater Toronto shoreline. Recent Lake Ontario water quality sampling supports this as results document very low bacteria and nutrient levels in the vicinity of the outfall diffuser. Links to figures documenting Lake Ontario water quality sampling results can be found below.

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Are the Duffin Creek WPCP and existing outfall responsible for beach postings in Pickering and Ajax?

There is no evidence that the Duffin Creek WPCP and the existing outfall are responsible for local beach postings. In Ontario, beaches are posted with warnings when E. coli (bacteria) levels in the water are found to exceed 100 counts per 100 millilitres (mL). Actual water quality testing undertaken in 2008 and 2009 (see links above) found E. coli levels in the vicinity of the Duffin Creek WPCP outfall, where treated effluent is discharged approximately one kilometre off-shore, ranging from one count to 31 counts per 100 mL. The same 2008 and 2009 sampling found near-shore levels of E. coli as high as 1,700 counts per 100 mL. Significant contributors to near-shore E. coli levels along the Ajax waterfront are local stormwater discharges, surface water runoff and creek discharges.

It should also be noted, in a separate study, bacteria collected from the beach at Rotary Park in Ajax was subjected to DNA testing in 2006, and no evidence of human E. coli was found. Subsequent studies concluded that the likely source of the E. coli along the Ajax waterfront is the large populations of Canada Geese and other waterfowl frequenting the area. Canada Geese are attracted to the area by the large expanses of turf grass upon which they graze.

The 2005 Ajax Waterfront Management Plan identified birds as the cause of beach postings and the 2011 Stormwater Management Retrofit Study also concluded that "Stormwater runoff is contributing to observed elevated levels of Total Suspended Solids, Total Phosphorous, and E.Coli., which are creating higher levels of turbidity, promoting algae growth, and contributing to beach postings along the Ajax shoreline" .

The Town of Ajax has taken and is currently taking significant steps to improve local water quality in the area. The 2005 Ajax Waterfront Management Plan provided a framework for future improvements, recommended policies and management strategies, along with an outline for projects and actions for specific locations along the waterfront. This framework is being utilized to address issues identified in the Study noted above. In addition, in 2011 the Town of Ajax endorsed a Shoreline Improvement Strategy, which encourages new recreational, cultural and educational experiences while at the same time enhances the ecological integrity of the shoreline. Based on these established and new initiatives, it is expected that water quality in the area will increase significantly.

What is Cladophora and what impact do the Duffin Creek WPCP and existing outfall have on its growth?

Cladophora is a type of green algae found along the Great Lakes coastlines and typically grows underwater on hard surfaces such as rocks and logs. Cladophora needs both phosphorus and sunlight to grow; wind and wave action can cause it to wash up on the shore where it can decay, releasing a pungent odour. A University of Waterloo report completed in October of 2009 found that the Duffin Creek WPCP had a relatively small influence on the conditions for Cladophora growth. Water quality sampling undertaken by the Toronto and Region Conservation Authority in 2008 and 2009 (links above) supports this view as total phosphorus levels in the vicinity of the outfall are quite low in comparison to near-shore levels. The following factors may have resulted in the increased instances of Cladophora in recent years:

    1. Increased prevalence of zebra mussels. Zebra mussels filter suspended material increasing water clarity, which in turn increases the penetration of sunlight. This, as well as the concentrated nutrient-rich waste zebra mussels deposit on the bottom of the lake, provides a favourable environment for Cladophora growth.
    2. Warmer water temperatures – Cladophora thrives at temperatures between 50 and 70 degrees Fahrenheit. Water warming trends in Lake Ontario may contribute to increased growth.
    3. Human activity – beginning in the 1970s federal and provincial water quality regulations have dramatically reduced the amount of phosphorus entering the Great Lakes from wastewater treatment plants. However phosphorus and other nutrients continue to be deposited into the Great Lakes by way of other sources such as overland storm run-off, storm sewer discharge, and agricultural run-off.

University of Waterloo Report


What impact do pharmaceuticals and emerging contaminants have on my drinking water and my health?

Many scientific studies have been undertaken in North America and Europe on the fate of pharmaceuticals and emerging contaminants during wastewater treatment and their potential effects on the environment. None of the scientific studies to date have effectively linked the extremely low levels of pharmaceuticals and emerging contaminants in wastewater effluent to adverse health effects in humans.

The Ministry of the Environment (MOE) recently completed a survey of pharmaceuticals and emerging contaminants to determine levels in Ontario source water and treated drinking water. The compounds that were detected were found in extremely low levels, and pose no risk to human health. Based on the levels detected, an individual would have to drink thousands of glasses of drinking water a day to match even a single dose of the compound. The full study can be found at:


What does the term 20:1 dilution ratio mean?

The 20:1 dilution ratio means that the discharge from the outfall must be diluted at one part treated effluent to 20 parts lake water within the immediate area of the diffuser under calm lake water conditions.


What is the capacity of the Duffin Creek WPCP Outfall?

The Duffin Creek WPCP Outfall has a physical capacity to meet an average day flow of 630 million litres per day (ML/d).  Note that the physical capacity (actual amount of sewage flow) is independent of regulatory (parameter) limits.  Through modelling undertaken during the Stage 3 Class EA, it was determined that the Outfall meets the MOE (Ministry of the Environment) near-field dilution requirement of 20:1 at an average daily flow of 560 ML/d.  The MOE has limited the operation of the outfall to an average daily flow of 520 ML/d with provision to operate at 540 ML/d provided that certain criteria are met. 


What is an Environmental Compliance Approval (ECA)?

An Environmental Compliance Approval (ECA) is essentially an operating license issued by the MOE (Ministry of the Environment) to the owner of water and sewage works. By law, any facility releasing emissions to the atmosphere, discharging contaminants to ground or surface water, or storing, transporting or disposing of waste, must have an ECA.  The ECA details the individual components comprising the facility, may indicate approved upgrades, provides definitions, states terms and conditions the owner is subject to, states operating objectives and sets effluent limits including both treatment quality and volume of treated effluent to be discharged to the lake. 


Does the Duffin Creek WPCP have any additional certifications?

In July of 2008, the Duffin Creek WPCP obtained ISO 14001:2004 Environmental Management Standard certification. Since this date, the plant has been operated as an ISO 14001 certified plant.


What is the treatment capacity of the Duffin Creek WPCP?

The Duffin Creek WPCP has recently been expanded to treat an average daily flow of 630 million litres per day (ML/d). However, due to limitations placed on the outfall in the C of A, the plant is currently approved to treat and discharge up to an average daily flow of 520 ML/d.


What treatment processes are employed at the Duffin Creek WPCP?

The following treatment processes are utilized at the Duffin Creek WPCP:

    • Mechanical Screening: This initial step removes large solids such as branches, rags, cans and large rocks from the flow.
    • Grit Removal: This step removes materials such as sand, silt, coffee grounds and other inert material.
    • Primary Settling: The next step is a physical process where the flow rate is reduced to allow much of the suspended organic solids to settle out in large rectangular tanks and be removed from the flow.
    • Aeration: Air is introduced to the flow to provide the oxygen that bacteria need to break down the sewage and to assist in keeping the solids from settling prior to secondary clarification. The bacteria levels are monitored and kept at optimum levels.
    • Secondary Clarification: Is a further settling process which removes additional solids following the biological treatment processes.
    • Phosphorus Removal: Ferrous chloride is added to the flow to reduce the amount of phosphorus in the effluent.
    • Disinfection: Chlorine is added to the effluent to remove remaining pathogens.
    • Dechlorination: Sodium bisulphate is added to the effluent to remove the chlorine prior to discharge to Lake Ontario.
    • Discharge: Effluent is discharged to Lake Ontario through the diffuser ports of the outfall.

The above describes the treatment of the liquid prior to being discharged to Lake Ontario. In addition to the above-mentioned processes, sludge (settled solids) digestion, sludge dewatering, incineration and energy recovery also takes place on the solids removed from the flow at the Duffin Creek WPCP.


Are the facilities comprising the Duffin Creek WPCP inspected and maintained?

The facilities and equipment comprising the Duffin Creek WPCP are inspected and maintained on a frequent and regular basis in order that the plant can function properly and meet all regulatory and legislated requirements. For example, the Duffin Creek WPCP outfall is inspected by divers annually and during this time, any required maintenance activities are carried out.


What communities are currently serviced by the York-Durham Sewage System (YDSS)?

The urban areas currently serviced by the YDSS have been listed below:

The Regional Municipality of Durham

City of Pickering – 100%
Town of Ajax – 100%

The Regional Municipality of York

City of Vaughan ~70%
Township of King ~20%
Town of Aurora – 100%
Town of Markham – 100%
Town of Newmarket – 100%
Town of Whitchurch Stouffville ~70%
Town of Richmond Hill – 100%
Town of East Gwillimbury ~10%

It should be noted that a number of rural homes and farm areas in the above cities and towns are serviced by private septic systems.


How will the Growth in these communities be serviced in the future?

The YDSS system has undergone a number of capacity upgrades over the past decade to accommodate the planned growth in the communities serviced by the YDSS system. Further Environmental Assessments are currently underway to determine the preferred solutions for servicing the planned growth in Town of East Gwillimbury and to address the future capacity limitations of the existing outfall at the Duffin Creek Water Pollution Control Plant. Further information can be found on the progress of these EA’s at: and


Will the County of Simcoe be connected to the YDSS in the future?

York Region has no plans to extend service outside of its municipal boundary to the County of Simcoe. Furthermore, YDSS infrastructure has not been designed to accommodate additional flow from the County of Simcoe.


What does average daily sewage flow mean?

Average daily sewage flow is a single flow value that averages variable flow patterns that occur over time. For example, the average daily sewage flow for a particular year is calculated by dividing the total sewage received at the plant during the year by the number of days in the year. The 2010 average daily sewage flow to the Duffin Creek WPCP was:

Total Sewage to Duffin Creek WPCP in 2010 =

119,017 ML

Days in 2010 =


Duffin Creek WPCP 2010 Average Daily Sewage Flow =
(119,017 ML ÷ 365 days)

326 ML per day


What does peak sewage flow mean?

Peak sewage flow is the highest flow that has occurred over a specified period. Peak sewage flows at the Duffin Creek WPCP occur during periods of high water usage but are usually more closely related to high levels of inflow and infiltration, which can occur during rain storms and when groundwater levels are particularly high.


What is Inflow and Infiltration (I&I)?

Infiltration occurs when groundwater enters the sanitary sewage system through cracks and leaking joints in sanitary sewage infrastructure (such as in pipes and manholes). Inflow occurs when storm or drainage water enters the sanitary sewage system through the direct connections of storm water systems and other drainage systems to sanitary sewage infrastructure (such the connection of foundation drains and storm drains to sanitary sewer pipes.)


What is the Canadian Environmental Assessment Act (CEAA) and how will it affect this Provincial Municipal Class EA?

The original version of the Canadian Environmental Assessment Act, 1992 (CEAA 1992), was recently repealed and replaced with the new Canadian Environmental Assessment Act, 2012 (CEAA 2012), which came into effect on July 6, 2012.

In the CEAA 1992, the federal environmental assessment process was applied whenever a federal authority had a specified decision-making responsibility in relation to a project, also known as a “trigger” for an environmental assessment.

In the CEAA 2012, a project may require environmental assessment by the Agency based on the following:

  • If a proposed project has physical activities that fall under the Regulations Designating Physical Activities, the proponent submits a project description to the Canadian Environmental Assessment Agency (the Agency) for review to decide if a federal environmental assessment is required.
  • When there is the potential for adverse environmental effects that are within federal jurisdiction.

Also, fostering cooperation and coordinated action between federal and provincial governments is one of the principles of CEAA 2012. The goal is to prevent costly duplication while ensuring high-quality environmental assessments and protection of the environment. To this end, the Agency may delegate an environmental assessment to a province. If the Minister of the Environment is satisfied that the substantive requirements of CEAA 2012 can be met by a provincial process and if that province requests it, he or she must allow for the substitution of the federal environmental assessment process by the provincial process.

The project activities associated with each of the short-listed alternative solutions of this Class Environmental Assessment to address Outfall Capacity Limitations at the Duffin Creek Water Pollution Control Plant do not fall under the Regulations Designating Physical Activities. In addition, the evaluation criteria and process for selecting among alternatives utilized in the Schedule C Class EA to address outfall capacity limitations at the Duffin Creek WPCP allows for consideration of environmental effects that are within federal jurisdiction, along with the identification of mitigation measures to address the effects. The Regions have also developed and are implementing a consultation program that exceeds the requirements for Schedule C projects as specified in the Municipal Engineers’ Class Environmental Assessment. Since the current Schedule C Class EA can meet CEAA 2012 requirements, it is likely that a federal environmental assessment would not be required. The project team is in the process of confirming this with the Canadian Environmental Assessment Agency.

View detailed information regarding the Canadian Environmental Assessment Act.

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