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.