Posts

Second in a series

The I/I Issue: What’s The Big Deal?

In our first post of this series, we defined inflow and infiltration, or I/I. In this post, we’ll explore the reasons it’s a huge problem in wastewater management.

The reason I/I presents huge challenges for municipalities stems from four basic issues:

  • Upstream – Pipeline Capacity: Conveyance system pipeline capacities being overwhelmed and causing sanitary sewer overflows (SSOs) during significant rain events
  • Upstream – Overflow Costs: Imposition of fines, consent decrees, and loss of funding opportunities dues to frequent SSOs
  • Downstream – Treatment Capacity: Huge amounts of clear water overwhelming the treatment capacity of wastewater treatment plants (WWTPs) and increasing pumping costs
  • Downstream – Treatment Costs: Dilution of sewage directly increasing costs of chlorination, ozonation, or ultraviolet disinfection

Let’s take a more detailed look at each of these problems.

Upstream Issues: Pipeline Capacity

As discussed in our previous post, I/I flows are referred to as “clear water.” This distinguishes it from sanitary sewage flows. This clear water gets treated right along with the sanitary flows, resulting in unnecessary costs for municipalities. The clear water also eats up valuable capacity inside a collection system. These systems are built and rated to accept only so much total flow. Most American cities’ collection systems are at least 75 years old. They were built to service far smaller populations than they must now accommodate. These systems are already working at maximum capacity.

Many communities are likely to experience some SSOs. However, older communities located downstream from overloaded city sewer systems will experience them most, due to their lower elevation in the watershed. The sanitary sewer systems in these older communities carry nor just their own wastewater and I/I; they also receive flows from their upstream neighbors’ sewer systems. When they experience higher volume created by stormwater leaking in from rain events, they must either divert flows into holding tanks, if available. If not, they suffer SSOs.

Upstream Issues: Overflow Costs

These SSOs are a health hazard. They can create serious flooding events that cause unsanitary water to back up into buildings. They can also create a situation in which the federal Environmental Protection Agency (EPA) issues a Consent Decree. This is an order for the city to fix the problem, at huge public cost. The city or agency may fail to meet federal or state guidelines for the integrity of its system. Excessive I/I can be an indicator of such failing systems. This situation can cause these municipalities to become ineligible for low-interest loans from grant or revolving fund opportunities. That makes this a significant and circular problem.

There are other costs associated with I/I overflows, including:

  • road and waterway cleanup
  • the potential for fines if the overflow problem is not corrected
  • litigation and potential liabilities for the responsible city or agency, resulting from sewer system backups into basements or households

Downstream Issues: Treatment Capacity

Clear water also takes up valuable capacity at WWTPs. According to Kirby Van Note, a water resources practice center leader at SEH (an engineering, architectural, environmental and planning company), a single leak from a joint in a manhole or pipe can generate 7,200 gallons of water daily. The cost of its treatment at the wastewater plant can translate into an annual cost of $6,500 for a city. Multiply this cost by the number of leaks across the sanitary sewer system, and you can imagine its hefty price tag for a municipality.

To accommodate massive flow influx from I/I, overwhelmed wastewater treatment plants must speed up their processes. As a result, poorly treated domestic and industrial wastewater is often discharged to the environment. Unremoved organics may be converted to disinfection byproducts through chemical disinfection prior to discharge, but not every plant has this capacity.

Downstream Issues: Treatment Costs

I/I costs water treatment facilities (and, indirectly, consumers) vast amounts of money in treatment operating expenses. All water entering a WWTP must be treated as wastewater. Dilution of sewage by clear water directly increases the cost of pumping the flows. Physical structures, including screens and pumps, must be enlarged to handle the peak flow. Primary clarifiers must also be enlarged to treat average flows, though primary treatment of peak flows may be accomplished in detention basins.

I/I also increases actual treatment costs. This includes chlorination, ozonation, or ultraviolet disinfection, depending on the type of treatment used by the plant.

What About Secondary Treatment?

Biological secondary treatment can only be effective while the concentration of soluble and colloidal pollutants remains high enough to sustain enough microorganisms to digest those pollutants. These pollutants are typically measured as biochemical oxygen demand, or BOD. Secondary treatment is expected to remove 85 percent of soluble and colloidal organic pollutants from sewage containing 200 mg/L BOD. But BOD removal by conventional biological secondary treatment becomes less effective with clear water dilution. It practically ceases when BOD concentrations entering the facility are diluted below about 20 mg/L.

In cases where this capacity overwhelm is happening, cities must often face difficult decisions. Do they invest dollars to reduce I/I, or limit new residential or economic development? New development can strain collection systems already operating at capacity. They sometimes even lead to the need for repairs. However, limiting residential or economic development opportunities stifles a city’s growth. It also sacrifices opportunities to capture tax revenue. Both can create a financial burden for the city.

You can easily see now why inflow and infiltration have become such enormous issues in any municipality’s wastewater management and public works operations and infrastructure budget. In our next post, we’ll take a look at how we arrived at this point, and how sources of I/I can be detected.

Series Post 1 header

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.