Lessons Learned from Separating Stormwater from Combined Sewers in Lancaster

Background
The City of Lancaster, Pennsylvania comprises approximately 1916.6 hectares (7.4 square miles) with an estimated population of nearly 60,000. As with many historic urban town centers, the City is highly impervious and approximately 50% of the City is served by combined sewers. During wet weather events, combined sewage flows exceed the capacity of the Advanced Wastewater Treatment Plant and untreated combined sewage is discharged directly to the Conestoga River, causing pollution, harming aquatic ecosystems, and increasing public health risks.

The Conestoga River is a tributary of the Susquehanna River which discharges to the Chesapeake Bay. The bay is a high priority for pollutant load reductions required by total maximum daily loads (TMDLs) and a Watershed Implementation Plan (WIP). In addition, the City is updating its long-term control plan to reduce the frequency and volume of combined sewer overflows (CSOs) and also addressing its stormwater discharges. The City has developed an integrated approach to reduce the impacts of these pollutant sources using green infrastructure (GI) and is achieving cost savings and high levels of implementation by integrating GI programs with its core public works practices. Since 2011, the City has implemented 84 projects through the GI program which collectively manage around 82 acres of impervious area.

Approach and Methodology
In addition to decentralized GI project implementation, the City has turned to stormwater separation projects as a critical strategy to addressing Lancaster's CSO challenge. Stormwater separation mitigates CSOs by diverting stormwater into dedicated drainage systems, preventing it from entering the combined sewer network. This reduces the hydraulic load on wastewater treatment facilities and minimizes overflow events. Furthermore, stormwater separation supports climate resilience by addressing the increasing frequency and intensity of precipitation events. By improving water quality, enhancing urban flood management, and supporting compliance with regulatory requirements, stormwater separation projects offer a sustainable, long-term solution for cities facing CSO challenges.

In 2023 and 2024, the City of Lancaster advanced designs for two stormwater separations (Water Street and Duke Street) and is planning for a third separation project (Broad Street) to be started in 2025. The projects will reduce stormwater flows in three of their four combined sewer basins that discharge into the Conestoga River. The new storm sewers were designed to maximize flow separation by designing collection and conveyance systems to capture runoff from the right-of-way as well as select private ground level and building impervious where it is feasible to connect to the new separate storm sewers. The new piping networks were sized to provide capacity to carry the peak rate from the 25-year, 24-hour design storm and were integrated into the City's existing SWMM model to evaluate the benefits to localized flooding and stormwater capacity as well as the reduction in frequency and volume of combined sewer overflows in the average City year.

Anticipated Results
The Water Street and Duke Street Stormwater Separation Projects collectively include around 1524 linear meters (5,000 linear feet) of new stormwater collection trunk lines and branches that are being designed in coordination with other major City multimodal transportation improvement projects along the Water Street and Duke Street right-of-way corridors. The results of the SWMM modeling indicate that these projects will provide the following benefits:

Water Street Separation in the Engleside Combined Sewer Basin (Figure 1)
- Separation of 14.9 hectares (36.8 acres) of impervious surface from the combined sewer system under Phase 1 and 32.4 hectares (80.6 acres) under full implementation (Phases 1-3)
- A reduction in CSO volume of 90.8 million liters (24 million gallons) in an average year
- A 45% reduction in flooding in the Engleside Sewer Basin (1.9 MG)
- Cost of implementation for Phase 1 is $114,300 per impervious acre separated and $124,400 per acre for Phases 1-3.

Duke Street Separation in the Susquehanna Combined Sewer Basin (Figure 2)
- Separation of 7.9 hectares (19.5 acres) of impervious surface from the combined sewer system
- A reduction in CSO volume of 2.6 million liters (0.7 million gallons) in an average year
- A 75% reduction in CSO events from 12 to 3
- Cost of implementation is $83,500 per impervious acre separated.

This presentation will focus on the innovative solutions and insights gained in planning and designing these stormwater separations in highly urban and congested rights-of-way. These case studies which are currently being put out to bid for construction will highlight the design process from concept to bidding, including alternatives evaluations and field investigations, multi-agency coordination and permitting, as well as detailed design challenges encountered in developing horizontal and vertical pipe alignments that navigate around existing utilities and grade changes while providing required flow capacities. While stormwater separations are often considered expensive and difficult to implement, the case studies will discuss strategies for cost efficiencies through project phasing and integration with other City capital improvement projects, such as bikeway planning and other streetscape improvements.