Finding and fixing sewer inflow and infiltration problems

By Barbara Robinson, Dwayne Grondin and Antonietta Giofu

In 2021, the Standards Council of Canada published a new national sewer guideline, “Developing an Efficient and Cost-Effective Inflow and Infiltration (I/I) Program” (Norton & ICLR, 2021). It conceptually covered many things that need to be considered to address I/I in an existing system.

Since then, Norton Engineering has fleshed out many of the concepts into specific actions to be taken by municipalities to find and fix I/I, now. This article describes some of these actions, none of which require special by-laws or any other changes to be implemented. They have been designed to be actionable immediately, as we already have all the tools needed to reduce I/I in existing systems.

An important concept to understand about addressing I/I is that it is not just an engineering issue. To implement a longterm I/I program, the entire municipality needs to be involved. The work must be supported by the CAO and engineering, development, building, by-law, and planning departments need to participate. Since work with residents is involved, political support is necessary. Engineering staff should take responsibility for demonstrating to senior management the absolute necessity of a long-term I/I program. Cost and risk alone should suffice to convince them.

Although the guideline covers many topics, this article will focus on private side I/I, since this needs to be addressed now.

Experts across North America agree that private side I/I generates 50% – 60% of all the I/I we see at wastewater treatment plants (WWTPs). If we are to reduce I/I and therefore flooding, risk and costs, in the face of more extreme events, we must tackle private side I/I. If we can remove even 30% of I/I, we introduce capacity into our sewers. This means they can convey higher wet weather events and therefore reduce flooding.

This can be achieved without any special tools. But, one of the reasons we have not tackled private side I/I until now, is that municipalities believe their residents will resist these changes. However, if we provide residents with the information that they need to understand why these changes are essential to reduce flood risk and costs to all, we believe there will be substantially more buy-in.

Norton Engineering has been presenting widely to service club groups over the past few years (Probus, Zonta, Rotary), and finds that residents are very keen to help solve these flooding risks, once they understand them.

Private side I/I is illegal in Ontario, since all sewer use by-laws in the province make it an offense to discharge rainwater or groundwater to sanitary sewers. It is important to understand that once I/I (from public or private side) enters the sanitary sewer, it becomes sewage. It cannot be costed dif- ferently to sewage and should not be.

Our industry needs to start assigning the cost of I/I as the amount we charge to residents on their utility bill. This amount covers all the costs involved, including treatment, staff salaries, benefits and pensions, insurance, costs of flood, etc. Deferral of WWTP expansion is an enormous cost savings that must also be considered.

Here are some simple strategies that we must start implementing, now:

Roof Leader Disconnection

Obviously, disconnection of roof leaders is the simplest and least expensive of all the private side I/I fixes, and yet few municipalities have tackled it. Two that implemented such a program are Halton Region and the City of Toronto.

The cost of an average connected roof in Kitchener is $2,000 per house per year (average rainfall x average roof area). This cost is paid for by all existing residents on their utility bill. Furthermore, connected roof leaders obviously contribute substantial peaking of the sewage flow, so contribute directly to flooding when rain events occur.

If even one house is connected, it increases the risk of sanitary sewer backup to all residents, particularly to the neighbours on either side and downstream (depending on hydraulics of course).

Imagine the sanitary sewer is beginning to back up and is just at the threshold of flood. Add a roof leader, with the substantial and peaky flow it discharges and flooding occurs. If we explain this to residents clearly, we may find we get a lot more interest. Indeed, we need to make it socially unacceptable for residents to have connected roof leaders. We can encourage them to check with neighbours to ensure they disconnect.

In a combined system, every roof leader, even those in separated areas, eventually discharges into a combined interceptor at the downstream end of the system before it flows into the WWTP. Combined systems obviously experience more frequent combined sewer overflows (CSOs). Every drop of roof water contributes directly to these CSOs and, in places prone to chronic flooding, to these floods. While we spend trillions on storage, etc. to avoid CSOs, it would be infinitely cheaper and easier to disconnect all roof leaders first.

As stated earlier, municipalities do not need a new by-law, since the sewer use by-law already makes it illegal to discharge rainwater into a sanitary sewer. They simply need a program to identify and enforce connected roof leaders, which can be implemented relatively inexpensively. Or, a contractor can be hired to do the disconnection.

Connected roof leaders should be made illegal across Canada, through provincial or federal legislation, to strengthen the message to the public, and assist municipalities in making these changes. Organizations like the Association of Municipalities of Ontario (AMO) could take ownership of this. Let’s educate all residents about the increased flood risk, increased overflows/CSOs, and increased costs that these roof leaders engender, and get them removed, now.

Connected Foundation Drains

Obviously, connected foundation drains are another substantial source of I/I on the private side. There are two kinds of foundation drain connections, and they differ markedly:

1. Foundation drains connected at construction, in most older homes constructed before about 1980 when in Ontario, the Ministry of the Environment published the model sewer use by-law that prohibited them. These can be considered legal, non-conforming. That is, they were legal when constructed,