How Does I/I Reduction Compare? The Story of Two Utilities

[b]Background[/b] Both the Madison Metropolitan Sewerage District (District) and the NEW Water, the brand name of the Green Bay Metropolitan Sewerage District, have started to implement a Regional Infiltration and Inflow (I/I) management program. Both serve communities that are experiencing rapid growth alongside communities with infrastructure that is more than 100 years old. Separately each utility hired Brown and Caldwell (BC) to perform an evaluation to identify the impact of I/I on their respective collection systems and wastewater treatment plants, to identify how the cost of I/I reduction compares to other options for handing wet weather flows. Different scenarios were analyzed for how wet weather flows impact these systems and what regional infrastructure would require upgrades if no I/I reduction were performed. The cost of reducing I/I was then compared to conventional capacity improvements to satisfy established levels of service for the regional system. This paper explores the similarities and differences between the two evaluations. In general, I/I reduction is most cost-effective in areas that currently have high I/I.

[b]Approach[/b] Both utilities followed the same evaluation process. Note that the process requires a calibrated sewer system model that represents the regional system, which both utilities had to varying degrees prior to starting the evaluation. BC simulated a variety of flow scenarios for the District's and NEW Water's evaluation. Both evaluations looked at one scenario based on the Baseline condition and one based on flows increasing from future development projections with no change in I/I rates. The District also explored the impact of additional I/I that could result from system degradation if a regional I/I program were not established. This provided the District with a four point perspective on the extent to which I/I reduction was cost-effective compared to building capacity for current and future conditions. For simulations of wet weather, the District evaluation was based on two historical rain events that occurred within their service area: one with a 10-year flow recurrence and one with a 25-year flow recurrence. The NEW Water simulations were also based on historical rain events from within their service area, but the events were reported as 10% and 2% probability events and aligned with the evaluation events used in their 2018 Interceptor Master Plan. After evaluating the different flow scenarios, BC explored the following options to address the identified system capacity deficiencies for each scenario, including developing planning-level construction costs for each option:
• Capacity Improvements Only: replacement of capacity-deficient wastewater conveyance and treatment systems (wastewater systems).
• I/I Reduction Only: reducing the I/I to the level where there are no longer any wastewater systems that are deficient.
• Combination: combination of replacing capacity-deficient wastewater systems along with some level of I/I reduction. Estimated costs for capacity and treatment were based on previous utility projects and studies. BC's approach for I/I reduction costs considers the I/I reduction unit cost that can be used for planning-level analysis and is based on the evaluation of dozens of I/I reduction projects across the United States. In both the District's and NEW Water's evaluations, the I/I reduction was assumed to be completed in a prioritized fashion where those areas with the highest I/I rates are reduced first to achieve the systemwide flow reduction. [b]Results[/b] For each utility, BC prepared graphs to show how the cost of I/I reduction compares to the cost of traditional upgrades to the wastewater system for the most extreme flow scenario. In these graphs, I/I reduction is most cost-effective at the minimum value of the total cost curve; this point is circled in each of the figures. For the District, this is when there is 40 million gallons per day (mgd) of systemwide peak I/I flow removed. For NEW Water, it is when 8% of the I/I is reduced from the wastewater system. The cost of I/I reduction is lower for NEW Water than for the District because NEW Water has more existing I/I in their system and the more I/I there is to begin with, the more cost-effective it is to remove. In addition, the construction costs used for the NEW Water evaluation were different than those used for the District, since both utilities had different planning-level costs from their respective Facilities Plans.

[b]Conclusions[/b] A general commonality between the two evaluations was that both showed the lowest cost alternative to be a package of projects that include conventional capacity improvements and a limited degree of I/I reduction that targets areas with high I/I to begin. With these projects, the utilities can accommodate future growth, strengthen aging collection systems, and have the resiliency to weather larger storm events from climate variations.