Creating Wastewater System Capacity to Support Housing: Private-Side Inflow and Infiltration Reduction Success

Murari Raghavan (, B.Tech, M.Sc., Civica Infrastructure Inc.
Matthew Malone (, M.Sc., MBA, Civica Infrastructure Inc.


The impacts of reduced sewer conveyance and treatment capacity in wastewater systems caused by Inflow and Infiltration (I&I), bring about problems including increased risk of basement flooding, restricted new urban development, increased wastewater operation and maintenance costs, and increased potential for environmental spills and overflows to receiving waters. Various studies have shown that I&I generated from private property can represent over 60% of the total I&I in a wastewater system (York Region, 2012), and presents unique challenges for municipalities. This number can be significantly higher in older systems. These challenges have been outlined by Belanger et. al., (2015) and can be summarized as follows:

  • Policy and Legal Issues;
  • Funding;
  • Public Outreach, and;
  • Implementation.

Therefore, a significant reduction of I&I originating from private properties has been difficult to achieve in many municipalities in Ontario. However, Civica Infrastructure Inc. (Civica), in partnership with several municipalities in Ontario and the Greater Toronto Area (GTA), has successfully reduced the amount of private-side I&I through a Public-Private-Partnership (P3).  The specific P3 framework allows municipalities to pass most, if not all, of the liability, cost, and risk of the investigations, remediations, and verifications to the private partner (i.e. land developer), while ensuring proper communication, coordination, and warranties are in place. In return, the private partner receives a portion of the sanitary servicing capacity created through the sewer flow reduction. This process produces a net reduction in flow (i.e. a ‘net benefit’) in the public sewer system by only allocating a portion of the reduction.

Benefits of the P3 Program

The benefits of reducing significant I&I gives obvious advantages over adding more conveyance and treatment capacity in a system. Additional direct benefits of a P3 include:

  • No program costs to municipalities: The burden of program costs is shifted to the private partner (developer), relieving municipalities of financial obligations.
  • Liability mitigation: The identification and remediation of I&I defects on private properties become the responsibility of the private partner, reducing liabilities for municipalities.
  • Improved storm drainage handling: Through the retrofit of Low Impact Development techniques, storm drainage capabilities can be enhanced, and in some cases, stormwater treatment can be improved.
  • Reduced operation and maintenance costs: Compared to constructing additional sewers, pumping stations, and treatment facilities, I&I reduction leads to decreased long-term operation and maintenance expenses for municipalities.
  • Lower costs for developers: The elimination of new sewer construction, pumping stations, and treatment system design and construction reduces overall costs for private developers.
  • Faster timeline for increased sewer capacity: By focusing on I&I reduction, the program achieves increased sewer capacity more quickly than traditional infrastructure expansion.
  • Residual capacity for future growth: By understanding the available residual capacity, municipalities can defer planned new infrastructure, leading to potential savings.
  • Enhanced system resilience: The program contributes to the sanitary sewer system’s resilience and sustainability, helping address climate change risks and system aging.

In York Region alone, significant addition of capacity in the sewers has been realized through I&I reduction allowing for more service areas and populations serviced by the same sewer system. More specifically, capacity has been created for over 12,000 persons in new development. After applying a 2:1 net benefit factor, demonstrated in Figure 1, an additional capacity for another 12,000 population has been achieved that is retained by the Region, i.e. 24,000 population capacity created.

Figure 1: Inflow and Infiltration Reduction using a 2:1 net benefit approach.
Figure 1: Inflow and Infiltration Reduction using a 2:1 net benefit approach.

The successful transfer of improvement costs and liabilities to the private partner has been a key advantage of the P3 program. The private partner takes on the responsibility for intrusive investigations, remediation works, and construction, while the municipality holds a security for a warranty period to ensure the quality and longevity of the works.

A customer-centric approach based on transparent and friendly communication has contributed to the program’s success. Each private property project is assigned a dedicated and knowledgeable project coordinator who facilitates agreements with homeowners and oversees construction works. The financial and physical facilitation of the program has played a vital role in achieving near-perfect success rates for stormwater disconnection from sanitary sewers. This success surpasses other programs relying on homeowner-led initiatives or enforcement-based approaches.

Status of Completion

The program commenced in 2010 and is at different stages of implementation throughout the GTA. To date, over 25,000 private lots have undergone investigations using various techniques, including specialty testing. Figure 2 depicts a typical example of a successful storm disconnection from a home with all storm downspouts connected to the sanitary sewer.

smoke testing for housing property
Figure 2: Smoke is blown through the sanitary system to identify any potential I&I sources and is shown on a residential private property.

Overall, testing has resulted in the identification and remediation of over 250 stormwater defects, including over 200 residential downspouts, 25 commercial flat-roof drains, five catch basins (over 7,000 m2) disconnected from sanitary, four reverse sloped driveway drains, the entire roof area of a hotel, and other open storm access hatches draining to sanitary sewers.

Across the GTA, the program has completed each stage, from preliminary design to post-remediation warranty, with all necessary improvements made. The last stage of the program is the warranty period. The warranty mechanism has worked well, with little to no follow-up repairs required and ensuring all storm connections remain disconnected and properly connected to the storm drainage system.

Ongoing investigations and remediation works are taking place in multiple GTA municipalities. Based on the established framework’s demonstrable successes, the program is expected to expand to include other areas in Ontario facing similar growth and capacity pressures. The completed projects have already created sewer capacity equivalent to serving a population of nearly 50,000, similar to the size of a city like North Bay (see Table 1). 

Table 1: Population accommodated with the elimination of wet weather flow in wastewater systems in Ontario


Project Start Date​

Capacity Assigned to Date (Units/Persons)​

City of Markham​



City of Vaughan​



Town of Aurora​



Town of Richmond Hill​



Town of Newmarket​

2015 (2 projects)​


City of Toronto​

2016 (11 projects)​


City of Kawartha Lakes



Total​ (With 2:1 Net Benefit)


Total​ (Without 2:1 Net Benefit)



The success of P3 projects can be attributed to the specialized processes followed by private consultants/contractors and the collaborative efforts of all involved parties. Municipalities have achieved significant I&I reductions, resulting in increased sewer capacities that offer flood protection and enable land developers to expedite sewage connections using sustainable practices.

The primary focus of the program has been to create “net benefits” by reducing I&I and allowing new development flows. The program has achieved tangible and quantifiable fixes that ensure stormwater remains disconnected from sanitary sewers. The P3 program provides an effective mechanism to accelerate improvement works and address challenging fixes on private properties that may be subject to ownership and legal constraints. This flexible framework can be applied in other jurisdictions facing growth and infrastructure challenges.

In summary, the P3 program’s successful reduction of private-side I&I has provided municipalities in the GTA with increased wastewater system capacity. The program’s benefits extend to cost savings, liability mitigation, improved storm drainage, and enhanced system resilience. With ongoing expansion and the potential to replicate the program in other regions, the P3 approach holds promise for addressing I&I challenges and supporting sustainable growth in wastewater systems. 


Belanger, F., Hubbard, P., Lehman, R., Lukas, A., Swarner, B., Vallabhaneni, S., Zipkin, J. (2015). Private Property Infiltration and Inflow. Water Environment Federation

Inflow and Infiltration Reduction Strategy. (2012). Regional Municipality of York

Learn more at – 

Developer – Led Inflow and Infiltration Reduction Program

Clair Gordon Sewershed Sanitary Sewer Capacity Assessment

Hamilton Storm Sewer Capacity Assessment

Private-Side Inflow and Infiltration Reduction