Effective Mitigation of Inflow and Infiltration Requires Careful Analysis

Modify your cost-effective analysis elements to increase I&I mitigation efficiency

Effective Mitigation of Inflow and Infiltration Requires Careful Analysis

Photo courtesy of Western Lake Superior Sanitary District

Finding the most efficient method for inflow and infiltration mitigation and removal is simply a matter of rethinking your analysis, according to a white paper written by Rick Nelson, director of conveyance technology at CH2M Hill, and presented at a North American Society for Trenchless Technology No-Dig Show.

I&I is one of the most important issues for stormwater utilities, and cost-effective analyses, or CEAs, have long been used to aid in that venture. NASTT provides recommendations for improving your CEA to ensure the best I&I removal strategy.

CEAs are typically fragmented into three areas of concentration. The paper suggests minor alterations to each portion can significantly impact your strategy.

“A CEA is used to determine the percentage of I&I that is cost-effective to remove and the associated rehabilitation, treatment, storage and relief costs,” according to the paper. “These improvements include more detailed and accurate system modeling, genetic algorithms, enhanced sewer system evaluation procedures and wet-weather treatment technologies.”

It’s not a new concept; CEAs have been used to assess stormwater infrastructure since at least 1991. But for the most part, the mechanics have not changed in that time. The NASTT paper suggests three essential changes to improve efficiency.

Nonuniform percent I&I removal

Though the description may not excite the imagination, in bare-bones form it means instead of calculating the value of I&I removal by type across the entire system, a more accurate method is to predict regionally.

There are inherent issues when trying to extrapolate across an entire system. The variability between basins makes it nearly impossible to accurately quantify the repair cost of a singular method on a broad scale.

Assessing a system regionally, ideally on a basin-by-basin level, allows for a more refined approach.

So essentially, instead of picking the cheapest method and applying it to the entire system, municipalities can get much more bang for their buck by finding the basins with the highest rates of I&I, which would be the most cost-effective to address.

Optimization using genetic algorithms

Computer-aided analyses have made great strides, becoming more available to the consumer market. One such technology is data modeling. Again, the idea of using data to predict I&I removal isn’t new, but the paper’s writers suggest an upgrade on what many are used to.

They describe genetic algorithms, or GAs, as an enhancement on traditional master planning techniques, which essentially use trial-and-error modeling to evaluate a small number of possible solutions. GAs are a step up in that they can compare nearly every conceivable combination of practices to find the most effective strategy.

The idea of GA optimization is that the model will proceed through various "generations" of simulation, altering the scenario every time until it finds the combination of methods with the lowest total cost.

It can also vary the hydrological profile, for example generating the costs for a spectrum of I&I removal percentages. It can calculate the cost of 15, 20 and 30 percent in one area, and 30, 40 and 50 percent in another.

Optimizations can consider traditional considerations like storage, rehabilitation and treatment, as well as less-considered efforts like best management practices.

“Each simulation represents a different combination of system improvement options and for each simulation the total project cost is calculated and the hydraulic performance is evaluated,” according to the paper. “In each solution, different combinations of capital improvements and operational settings are evaluated.”

GA optimizations can increase cost-efficiency by evaluating a far greater number of possible investments, thus ensuring that the best option is on the table.

Consideration of future value

Lastly, an element that seems simple but can frequently be overlooked is the idea of incorporating the future value of I&I removal into CEA and strategic planning.

“It is recognized that continual aging of sewers will result in increased I&I in the future, unless this is properly managed through long-term rehabilitation. Eventually, sewers will need rehabilitation and replacement,” according to the paper. “In order to account for this future needed renewal, the CEA can include an estimate of when and how much rehabilitation would be required through one or more life cycles, the costs expressed as present worth and incorporated into the CEA.”

So instead of looking at I&I as a one-time system improvement plan, it is fiscally sensible to consider the long-term costs of future system degradation, taking into account not only the cost of the initial rehabilitation investments, but the accrual of costs and cost savings over time with each potential option under consideration.

Big possibilities

As I&I becomes an ever-growing concern and system age continues to surpass new infrastructure, finding the most efficient and cost-effective plan is essential. Fine-tuning your CEAs is a small step that could yield big results in achieving desired levels of system performance.

“Tools and approaches to develop the individual cost curves across the range of I&I removal have improved dramatically,” the paper states. “The incorporation of refinements into the CEA will enhance the quality of the analysis and provide more confidence in the results.”



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