A Philadelphia Story: Minimizing CSO Overflows and Planning for the Future

As part of an innovative approach to the program, the consultant and the Water Department set up a project office inside the department's building. "There is a true synergy between the two groups," CDM Associate Ted Schlette reports. "The Water Department has done much of the groundwork to obtain and evaluate the data CDM needs for modeling the system. Given the staff's familiarity with the system, they bring unique insights and have been full partners in both setting objectives and solving problems." The NPDES permit application process required the CSO program team to achieve three significant milestones within the first year of the project: first, conduct a hydraulic and physical inventory and characterization report of the combined sewer system; second, characterize the hydrology of the tributary "sewersheds" and the hydraulic operation of the wastewater collection and transmission system; and third, document the implementation of nine minimum controls to alleviate the potential influence of CSO discharges A fourth regulatory milestone, the long-term control plan, had to be met by last September.

Minimizing Overflows
The engineering team analyzed modifications that can be made to the system to utilize efficiently the full capacity of the city's three wastewater treatment plants to provide increased capture of wet-weather flows. The team has successfully refined a detailed simulation of the interceptor sewer and regulator system, using the CDM-developed Extended Transport (EXTRAN) block of the U.S. Environmental Protection Agency's (EPA) Stormwater Management Model.

"The model indicated the interceptors didn't fill up as quickly as they could at the existing regulator settings, resulting in overflows to the river before the system reaches capacity," explains CDM project manager Jim Smullen. "Then, after modifying the regulator settings, we used the model to predict how the increased flows would impact the plants." Model simulations suggested the treatment plants could go to maximum capacity more often, reducing overflows to the receiving waters. The CSO project team's efforts have resulted in the capture and treatment of an additional one billion gallons per year of combined sewer flows that otherwise could have discharged into the Delaware and Schuylkill rivers.

In addition to the ongoing effort to optimize flows to the treatment plants, the team evaluated and submitted strategy proposals to implement the remaining nine minimum controls. These include: optimizing the operations and maintenance program; evaluating the pretreatment program; prohibiting dry-weather discharges; controlling solids and floatables; improving the inspection, monitoring, and reporting procedures; and developing a public participation, education, and pollution prevention program.

Public Involvement
The Water Department/CDM team also devised eight initiatives to educate the public and expand the understanding of and support for implementing the minimum CSO control technologies and potential capital improvements. The comprehensive educational package includes general information on Philadelphia's combined and separated sewer systems, maps of the system and the locations of CSOs, summaries of the EPA's CSO policy and the nine minimum controls, and a CSO newsletter. The team also will conduct workshops with citizen groups, elected officials, and the media to increase their awareness of CSOs, and to focus on the environmental improvements expected from the city's CSO abatement program.

Framework for the Future
"We've met with the regulators throughout the planning process, so that the plans we initiate on an interim basis do not compromise the long-term control plan," explained CDM's Schlette. "What we implement now will dictate what the Water Department may have to do over two or three decades to improve the system, so it is imperative that we have a consensus with federal and state regulators and do not design the system in a vacuum."

Consider the Costs
Because a major consideration in developing the long-term control plan is the financial impact on the public, the team performs affordability analyses on proposed modifications in conjunction with defining their benefits. Brian Marengo, the Water Department's CSO compliance program manager, had a final comment: "CDM is helping us to develop implementable, low-cost improvements to water quality while laying the groundwork for the future changes necessary to achieve affordable CSO compliance, and thus meet the requirements of our NPDES permits."

Editor's Note: This is an edited version of an article which appeared in a publication of CDM. For more information contact CDM in Cambridge, Massachusetts at: Tel. 617-621-8181; Fax. 617-577-7501.

Edited by Ian Lisk