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,” to distinguish it from sanitary sewage flows. This clear water gets treated right along with the sanitary flows, and this results in unnecessary costs for municipalities. The clear water also eats up valuable capacity inside a collection system, which is built and rated to accept only so much total flow. Most American cities’ collection systems are at least 75 years old, built to service far smaller populations than they must now accommodate. They are already working at maximum capacity.
Many communities are likely to experience some SSOs, but older communities located downstream from overloaded city sewer systems will experience them most, due to their low elevation in the watershed. The sanitary sewer systems in these older communities not only carry their own wastewater and I/I, they also receive the flows from their upstream neighbors’ sewer systems. When they experience higher flows 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
Not only are these SSOs 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, ordering the city to fix the problem at huge public cost. If the city or agency fails to meet federal or state guidelines, it can cause them to become ineligible for low-interest loans from grant or revolving fund opportunities
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 are potentially converted to disinfection byproducts by 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, although primary treatment of peak flows may be accomplished in detention basins.
I/I also increases actual treatment costs, including those for chlorination, ozonation, or ultraviolet disinfection, depending on the type of treatment used by the plant.
Biological secondary treatment can only be effective while the concentration of soluble and colloidal pollutants (typically measured as biochemical oxygen demand, or BOD) remains high enough to sustain a population of microorganisms digesting those pollutants. 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 as BOD concentrations entering the treatment facility are diluted below about 20 mg/L.
In cases where this type of capacity overwhelm is happening, often cities must decide between investing dollars to reduce I/I, or limiting new residential or economic development. New development can strain collection systems already operating at capacity, sometimes leading to the need for repairs. However, limiting residential or economic development opportunities stifles a city’s growth, and sacrifices opportunities to capture tax revenue. Both can create a financial burden for the city.
It’s easy to 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.