Significant I/I and SSO Volume Reduction Achieved by Unique GIM Dynamic Models and Optimized Capacity Improvement Projects Under Consent Decree

This paper describes the use of a unique Groundwater Infiltration Model (GIM) to actively guide the complex challenges of I/I and Sanitary Sanitary Sewer Overflow (SSO) reduction under a Consent Decree (CD) for the DeKalb County sewage collection system. Hydraulic modeling solutions and applications discussed in this paper include advanced GIM improved dynamic models, optimization of capacity improvement alternatives using the model, identification of capacity related SSO Priority Fix List (PFL) projects, trunk sewer rehabilitation projects, and CD design-build (DB) packages under DeKalb's Priority Area Sewer Rehabilitation Program (PASARP). DeKalb's system-wide sewer hydraulic model became a critical tool to meet increasing demand from population growth and at the same time reduce sewage spills in such a large and complicated system. For the entire DeKalb's collection system, advanced GIM improved dynamic models including a total of seven independent sewershed/basin models, are used to support the County Sewer Master Plan and other Capacity, Management, Operation, and Maintenance (CMOM) Programs under the CD. The models incorporated data from a system-wide flow monitoring program with approximately 270 meters and 41 rain gages to consider the changing flow conditions that result from the active system-wide PASARP activities. Due to the challenge of SSO management, hydrologic model solutions offered in InfoWorks ICM were applied to address the impact from antecedent moisture conditions for Rainfall Derived Infiltration/Inflow (RDII) simulation. The InfoWorks ICM GIM solution was implemented as a hybrid model solution with empirical modeling assumptions. Compared to the traditional approaches, this unique approach not only improved model calibration accuracy, but also reduced the calibration parameters to only key parameters by taking advantage of the 'bucket' storage concept to account for the inter-storm events impact (Figure 1). The completed dynamic models have confirmed the effectiveness of this approach in 2019 by much improved calibration results .With completed dynamic models, the collection system was evaluated under a 2-year 24-hour design storm to identify the capacity restriction areas and optimize the capacity improvement alternatives. Combined with condition assessment through field inspection data review, a total of 1,095,557 LF of pipe was identified for rehabilitation or replacement. Three design-build packages (DBs) of 33 individual projects contained small diameter capacity relief projects under the PASARP and projects involving simple rehab of CIPP and point repairs under Ongoing Sewer Assessment and Rehabilitation Program (OSARP) or other contracts. Overall, most of the projects are 50-80% completed with over 64% of the pipes rehabbed with cured-in-place pipe (CIPP) and approximately 10% upsized. The total estimated costs are close to $300M. Trunk sewer rehabilitation projects were also identified based on dynamic modeling of capacity restricted trunk sewers with pipe sizes between 18'—72'. These projects were grouped into six contracts. The projects include replacement and upsizing, as well as parallel relief pipes. The trunk sewer projects are all under the design phase with an estimated cost of $400M. A summary of the PASARP DBs and Trunk Sewer projects is listed in Table 1. A PFL was added into the MCD and consists of repeat SSO locations. The PFL currently includes a total of 200 sites which are the focus of ongoing rehabilitation, replacement and maintenance activities with the goal of achieving adequately fixing all PFL sites by 2027. PFL rehabilitation projects are covered by the PASARP DBs, the trunk sewer projects, and operation and maintenance contracts. So far, 134 of the PFL sites, or about 67%, have been determined to be adequately fixed. The PASARP DB projects, trunk sewer projects, as well as PFL sites are shown in Figure 2. A pilot study was conducted in 2021 for a small metershed with 28,260 LF pipe (Figure 3), which observed high levels of I/I with wet weather flow 20 times greater than typical dry weather peak flow. The rehab work completed in this metershed included full lining of 19,400 LF mains (new or PVC mains were not lined), full lining of 95 manholes, replacing all vented manhole lids, sealing of 210 lateral connections to main (including replacement of defective cleanouts/missing caps and lining of all laterals from main to within 5 ft in the lateral)). The total cost for this pilot area was approximately $2.6M. The pilot study used dynamic models to simulate pre- and post- rehab conditions within the metershed, which showed that the completed comprehensive rehab resulted in a 25% I/I reduction in peak flow and 50% reduction in volume. Additional programs such as future private property I/I reduction will further reduce I/I flow and volume. The success of the completed rehabilitation and capacity relief projects implemented to date is illustrated by the significant SSO volume reductions observed in the past three years (Table 2 & Figure 4). Even though the results could partially be impacted by weather variation, the completion of PASARP capacity relief projects is believed to be the main factor for the significant I/I and SSO reduction. The pilot study and observed SSO volume reductions demonstrated the effectiveness of GIM dynamic models for optimizing capacity improvement projects and SSO reduction for DeKalb County under CD.