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The I/I Issue: The Basics of Inflow and Infiltration

Welcome to the first in our I/I series! Because it’s a huge problem in wastewater management, our industry talks a lot about I/I, or inflow and infiltration. But not everyone knows the exact definitions of these terms. So let’s take a closer look at exactly what these things are.

I/I Defined

Inflow and infiltration are terms defined against the definition of sanitary wastewater flows. Those flows consist of wastewater from sanitary fixtures inside houses, places of business, and other buildings. Sanitary fixtures include toilets, lavatories, sinks, bathtubs, and showers. We can think of these flows as created to be intentionally conveyed through wastewater collection systems consisting of underground pipes laid beneath streets and rights-of-way to a wastewater treatment plant (WWTP).

I/I, on the other hand, are considered unintentional flows; i.e., they find their way into these sanitary collection systems, but they don’t belong there. Inflow and Infiltration water is referred to as “clear water,” though it may be dirty, to distinguish it from normal sanitary sewage water in the collection system.

Overflowing manhole imageInflow: The Renegade Stream

Inflow is stormwater runoff that enters sanitary sewer systems at direct connection points in these systems. Many sources may contribute to inflow, including:

  • manhole covers
  • footing/foundation drains
  • roof drains or leaders
  • downspouts
  • window well drains
  • exterior basement stairwells
  • driveway drains
  • sump pumps
  • streams and other waterways
  • cross-connection to a stormwater system
  • yard drains where urban features prevent surface runoff, and storm drains are not conveniently accessible or identifiable

Typically, these sources are, at least, improperly or, at worst, illegally connected to sanitary sewer systems. This happens either through direct connections, or discharge into sinks or tubs that are directly connected to the sewer system. This type of improper connection allows water from sources other than sanitary fixtures and drains to enter sanitary sewers. In a proper scenario, this clear water should be entering the stormwater sewer system, or be allowed to soak into the ground. It should never enter the sanitary sewer system.

Improper connections may be made in residential homes, businesses or public buildings. This can happen because the person hooking up the connection is not a professional plumber and doesn’t know any better, or it can happen because that person is trying to avoid the expense of having to do it properly.

Either way, such connections can and do contribute a significant amount of water to sanitary sewer systems. Why is this a problem? Because 8-inch sewer pipes can adequately move the domestic wastewater flow from up to 200 homes, but only eight sump pumps operating at full capacity—or six homes with downspouts connected to the sanitary sewer pipe—will overload the capacity of the same eight-inch sewer pipes. A single sump pump can contribute over 7,000 gallons of water to sanitary sewer systems in 24 hours. This is the equivalent of the average daily flow from 26 homes.

Peak flows caused by inflow may generate a foul flush of accumulated biofilm (a collection of microorganisms on the water’s surface) and sanitary solids, scoured from the perimeter of oversized sewers during peak flow turbulence.

Infiltration: The Sneaky Flow

Infiltration is groundwater that enters sanitary sewer systems through defects in the sanitary sewer pipes. These defects may include:

  • Leaky manhole joints and pipe penetrations
  • Defective mainline pipe joints
  • Defective or leaking house lateral connections to the mainline
  • Cracked or collapsed pipe or manholes
  • Damaged and broken sewer cleanouts

Old cracked brick pipe with tree roots intruding

These defects may be caused by:

  • age-related deterioration
  • loose joints from ground shifting, hydraulic or seismic loads
  • poor design or installation
  • maintenance errors or neglect
  • damage from differential ground movement, heavy vehicular traffic on roadways overhead, or careless construction practices in nearby trenches
  • tree or plant root penetration

These cracks or leaks provide a ready entryway for groundwater wherever sanitary sewer systems lie below water tables, or where soil above the pipes becomes saturated. Sewer pipes are often installed beneath creeks or streams, because these locations are naturally the lowest point in the area. It’s more expensive to install pipe systems beneath roadways, which would be the next easiest location.

Because of their close proximity to such waterways, these sewer pipes are especially susceptible to infiltration when they crack or break. It is not unheard of for them to drain entire streams into sanitary sewer systems.

A Lurking Menace

The average designed service life of most sewer pipes is about 20-50 years, depending on the type of material used in their manufacture. However, repairing and replacing them is an arduous and expensive process, leading many municipalities to kick that job down the road. As a result, America’s sanitary sewer system pipes, along with the lateral pipes attached to households and businesses, have often gone much longer without inspection or repair. As a result, they are likely to be cracked or damaged, inviting infiltration during the next rain event.

In our next post, we’ll examine why inflow and infiltration are a serious problem for American towns and cities.

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