Sewer backup: Blaming infrastructure for the wrong reason

In a study, the City of Hamilton found that a submerged sanitary manhole cover with two pick axe holes can allow up to two litres of stormwater per second into the sanitary sewer system.

When sanitary sewers back up into basements, the knee-jerk reaction by homeowners (and many insurers) is to blame the state of the local public infrastructure. Homeowners will almost always point the finger at their local government, even before they know the real cause of the backup.

But while it may not be wrong to blame public infrastructure for sanitary sewer surcharge (assuming that the backup wasn’t due to a problem on the private lot, as it so often is), it is commonplace to misattribute the cause as being one of pipes that are too small to handle the load (though, in a manner of speaking, this is partly right).

Critics often call for larger sanitary pipes, but this is largely a red herring.

It is not that the pipe is too small (chances are it was sized properly for the amount of sanitary sewage that was expected to go through it), the problem is that the pipe wasn’t designed to handle sewage plus the large amount of extraneous storm and ground water that makes its way into it from various sources.

The issue is one of Inflow and Infiltration, what those in the trade call ‘I&I’.

According to Dr. Ted Kesik, Professor, John H. Daniels School of Architecture, University of Toronto, in the ICLR study ‘Best practices guide: Management of inflow and infiltration in new urban developments’: “In the simplest terms, inflow involves the entry of storm water from rainfall and snowmelt events entering the sanitary sewer system directly from the surface or indirectly from storm water drainage system connections to sanitary sewers. Infiltration involves the entry of groundwater into the buried sanitary sewer system.”

On the inflow side, extraneous water entering into the sanitary system can be sourced from downspouts, foundation drains and sump systems that are connected directly into the sanitary system (practices that are largely no longer allowed in Canada); leaky manhole covers or covers with grated tops or pick axe holes; and accidental and intentional cross connections between storm sewers and sanitary sewers, to name a few.

Pooling storm water can get into the sanitary sewer (here at the top left) and overload the system flooding basements pic.twitter.com/Z9mKCTAQ0t

— Glenn McGillivray (@ICLRCanada) May 15, 2014

Infiltration into the sanitary occurs when water enters via defective pipes; leaky pipe joints; poor connections between sewer system components; and damaged, deteriorated or defective maintenance holes.

Sanitary sewer systems just aren’t designed to handle the extra load caused by extraneous water entering the system, and designing them to do so would add significant costs from at least two fronts. First, there would be the cost of installing higher capacity pipes and second, there would be the cost of processing the extra amount of sanitary water entering water treatment plants (which, over the long run, would likely be the bigger number of the two, as extra processing capacity would likely have to be added to these facilities).

Most underground storm sewer systems in Canada were designed to handle 2 year, 5 year and, sometimes, 10 year events (i.e. events whose probability of occurring are 1 in 2, 1 in 5 and 1 in 10 in any given year, respectively). In some (fairly rare) cases, a few municipalities are replacing old underground systems with new infrastructure that can handle 1 in 100 year events (Toronto is a prime example).

Sanitary sewer systems, on the other hand, are not built with a return period in mind but, instead, are designed according to actual or projected numbers of users connected into the system as well as other considerations, including a small amount of I&I.

The trouble is, many sanitary sewer systems are experiencing I&I that is well above this allowable limit.

Says Kesik: “Ideally, a sanitary sewer system would only convey sewage from connected laterals to the sewage treatment facilities and no external sources of water would inflow or infiltrate the wastewater conveyance network. In reality, a certain amount of I&I is unavoidable due to factors such as: 1) local climate, soil and groundwater conditions; 2) imperfect design, materials and workmanship; 3) the settlement and deterioration of piping, connections and maintenance holes; and 4) the connection of storm water and foundation drainage sources to the sanitary system, unintentional and otherwise. Put simply, it is not practically possible to maintain a perfectly watertight sanitary sewer system over its life cycle.”

The good news is that “many jurisdictions have demonstrated it is possible to cost effectively manage I&I within acceptable limits.”

One of the key issues (and something that ICLR is working diligently to change) is the lack of best practices for I&I management in new developments. With such best practices in place, “municipalities will be able to direct scarce resources toward the remediation of I&I in older existing developments to better manage assets.”

To this end, ICLR commissioned the above-noted study, which was published in February 2015. Later this year or early next, the latest ICLR commissioned research will be completed that will look deeper into the area of reducing the risk of urban flooding through application of measures at the time of construction of urban subdivisions. Part one of this work will identify practical measures municipalities and developers can apply to reduce the risk of I&I over the service life of subdivisions, and part two will identify progressive measures that can be applied to manage storm water in urban subdivisions.

Parallel to these efforts, ICLR will continue to work with its insurance industry members (through the Institute’s Insurance Advisory Committee) and municipal government contacts to inform homeowners of the actions they can take to 1) reduce the amount of extraneous water they are directing into the sanitary sewer system via connected downspouts, foundation drains and sump systems (this is key because, as Kesik notes, it is commonly reported that most I&I problems originate from the private side of the sewer system), and 2) protect their homes from sewer back up and basement flooding through the implementation of risk reduction measures.

I&I in sanitary sewer systems is large scale problem that is costing Canadian homeowners, municipalities and insurers a great deal of money. Alongside the overt, widely experienced problem of sanitary sewer backup into homes comes the expense associated with the processing of extra sanitary sewer flows in water treatment facilities (a large cost being absorbed by ratepayers).

As noted by Kesik: “In many ways, inflow and infiltration in new sanitary sewer systems are a barometer of the quality, care and stewardship underlying the municipality, its system of governance, the community’s planning vision and its infrastructure engineering excellence. What can be said about a 21^st century civilization that cannot properly design, construct and sustain its vital infrastructure? Hopefully, it is a question that should not have to be answered by future generations of Canadians.”

The problem of I&I must be understood by all it touches, and addressed.