Private-Side Inflow and Infiltration Reduction Success Utilizing a Public-Private Partnership (P3) Framework

By Edward Graham, M.A.Sc. Eng., P.Eng; Seth Monczka, B.Eng; Matthew Malone, M.Sc.; and Cassandra Lim, 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 thus 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 the Greater Toronto Area (GTA), have 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 inflow (i.e., a ‘net benefit’) in the public sewer system by only allocating a portion of the reduction.

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Selection of Sewersheds for I&I Investigations

The process of investigation, remediation and verification have been well tested by Civica. Once an I&I study area is defined within a municipality, typically through an innovative flow monitoring and data analysis process, specific sewer sheds are selected and prioritized for further investigations and sewer testing. The flow and rainfall analysis utilize a Smart City, GIS-Cloud data management system, DataCurrent, which connects field sensors in real-time to a cloud database that alarms and conducts I&I analysis. DataCurrent has the ability to separate dry-weather flow (DWF) from wet weather flow (WWF) and generate an I&I response for a sewer shed in near real-time during rainfall events. This is demonstrated in Figure 1.

This software system can quantify key performance indicators (KPIs)
and produce thematic maps of system responses for further comparison of areas, as shown in Figure 2. This leads to a smart selection of field investigation techniques aimed at identifying and achieving the fastest and most cost-effective I&I reductions.

Similar to other projects in North America, this program uses a number of industry-standard KPIs and thresholds to prioritize sewer sheds for I&I investigations. KPIs include Rainfall Derived I&I rates and volumes, Wet Weather Peaking Factor (PF), % of rainwater entering the sanitary system (CV%), and groundwater infiltration (GWI).

Benefits of the P3 Program

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

1) No program costs to municipalities
2) Mitigation of liabilities associated with the identification and remediation of I&I defects on private properties.
3) Improved handling of storm drainage and, in some cases, stormwater treatment through the retrofit of Low Impact
Development techniques.
4) Reduced operation and maintenance costs to the municipality as compared with adding more capacity by constructing more sewers, pumping stations, and treatment capacity.
5) Reduced overall costs to developer partner, due to the elimination of new sewers, pumping stations, treatment system design, and construction.
6) A faster timeline for achieving increased sewer capacity.
7) Increased understanding of residual capacity available for further growth planning and savings by allowing the pushback of planned new infrastructure.
8) Increased sanitary sewer system resilience and sustainability for addressing climate change risk and system aging.

In York Region alone, a 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 10,000 persons in new development.
After applying a 2:1 net benefit factor, demonstrated in Figure 3, additional capacity for another 10,000 population has been achieved that is retained by the Region, i.e., 20,000 population capacity created.

One of the key and greatest advantage of the P3 program has been the transfer of system improvement costs and liabilities within the private properties to the private partner. The private partner has assumed the responsibility and has developed the techniques to communicate, secure permissions, and undertake the works within the private properties efficiently and effectively. The private partner completes all the intrusive investigations and subsequent remediation works. The private sector then assumes the responsibility of construction works while the municipality holds security for a warranty period to ensure the quality and longevity of the works.

A robust customer-centric approach utilizing transparent and friendly communication ensures a high level of engagement and customer satisfaction. A dedicated, knowledgeable project coordinator is assigned to each private property project to reach an agreement with the homeowner and to facilitate construction works. The financial and physical facilitation of this program is seen as the defining factor in its success over other partially funded programs. In comparison to other incentive or enforcement-based programs, where homeowners undertook the work themselves, this program has achieved a near 100% success rate for stormwater disconnection from the sanitary sewers.

Status of Completion The program started in 2010 and is at varying stages within the GTA. To date, over 17,000 private lots have been investigated, using various techniques including specialty testing. Figure 4 depicts a typical example of a successful storm disconnection from a home with
all storm downspouts connected to the sanitary sewer.

Overall in this area, the testing resulted in the identification and remediation of over 250 stormwater defects, including over 150 residential downspouts, five commercial flat-roof leaders, 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, each stage of the program, from preliminary design to post-remediation warranty, has been completed and all necessary improvements have been 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 all storm connections remaining disconnected and properly connected to the storm drainage system.

Current investigations and remediation works are ongoing with municipalities in the GTA. Under the established framework, it is envisioned that the program will expand with demonstrable successes to include other areas. Opportunities to expand into various pilot projects are currently being planned in regional and local municipalities.


The success of this P3 program can be attributed to the specialized processes followed by the private consultant/contractor and the collaborative effort from all parties to meet their current needs, including identification and reduction of I&I from private properties. Municipalities have demonstrated significant I&I reductions resulting in successfully increasing sewer capacities that provide measurable flood protection and allow land developers accelerated servicing of sewage connections using sustainable practices. The primary focus of the program has been to produce ‘net benefits,’ while allowing new development flows after I&I reduction. The program has achieved tangible and quantifiable system fixes that can be relied on to keep the stormwater disconnected from the sanitary sewers. The P3 program offers an effective mechanism to increase the rate of improvement works and to challenge difficult fixes on private property that might be subject to ownership and legal constraints. The program’s flexible framework can be applied in other jurisdictions that are subject to the same growth and infrastructure drivers.


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

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