Expert Interview 

CSO/SSO expert, Carol Hufnagel, P.E.

Carol Hufnagel is Tetra Tech's National Practice Leader for Combined Sewer Overflows (CSOs). She has her Bachelor’s and Master’s degrees in Civil Engineering from the University of Michigan and has worked for Tetra Tech since graduating in 1984.

Her concentration in school was in hydraulics and hydrology, which led to work with sewer systems with wet weather issues, both combined sewer systems and sanitary sewer systems.

Ms. Hufnagel works on wet weather programs in the planning and implementation stages, including current projects in Ohio, Michigan, and Pennsylvania.

Q:  What are CSOs and SSOs?

A:  A CSO, or Combined Sewer Overflow, is a designed release point from a sewer system. Before 1900, sewer systems were constructed to carry wastewater away from homes and to the local river. As a result these sewers would carry both stormwater and wastewater flow. As public health and sanitation advanced, sewer systems were redesigned to carry the wastewater flow to a wastewater treatment plant, but the combined sewer overflows are remnants of those original sewer systems.

A Sanitary Sewer Overflow (SSO) occurs when a separate sanitary sewer system is overwhelmed—either by flows during wet weather or by a plug of the line or some damage to the line—and wastewater is released to the surface waters or to the ground.

Q:  What problems do combined sewers cause in communities?

A:  The major problem that combined sewers cause for communities is that they are no longer an acceptable way to manage wastewater and collection systems. The discharges from CSOs can degrade water quality and cause public health concerns. Communities with CSOs have requirements to address the amount of CSO that occurs and reduce the amount and impacts of discharges. From a community’s perspective, a CSO control program can be a very large capital program and difficult to afford.

Q:  What steps do communities normally take when they realize they have to address CSO issues?

A:  As part of the federal CSO policy and requirements of the Clean Water Act, communities with CSOs are required to develop long-term control plans to lay out their approach to cost-effectively address CSO discharges. Generally the expectation is not that they would be 100 percent eliminated—there will usually be some remnants of the original CSOs—but that the volume and frequency of discharge would be vastly reduced from pre-controlled conditions.

There are three primary ways that CSOs are controlled: sewer separation, which removes the vast majority of the stormwater from the sewer system, enabling all flow to reach the treatment plant; storage (in basins, tunnels, or oversized pipelines), where the excess flow volume is temporarily held until it can be transported to the treatment plant; and additional treatment, accomplished by expanding an existing wastewater treatment plant or through satellite treatment.

The majority of plans will use a combination of these approaches in order to maximize the benefit to the community at a reasonable cost.

Q:  And how does Tetra Tech help?

A:  Tetra Tech helps by providing the complete range of services that are needed to address CSO control needs, including planning, design, and construction-phase services. Each community has a unique situation, and Tetra Tech works to craft the best solution that meets the local needs.

Q:  Are there areas of the country that are more vulnerable than others?

A:  Combined sewer systems are generally more prevalent in areas of the country that were settled earlier in our country’s history, because they are remnants of the way sewer systems were originally constructed. As a result the areas most impacted include areas east of the Mississippi River, particularly the Northeast, and the states bordering Great Lakes. CSOs are also found in the Midwest and larger cities along the West Coast.

Q:  Tetra Tech is helping in all those places?

A:  Yes. In Boston we are currently involved with an extensive sewer separation program that involves difficult construction in congested neighborhoods. On the West Coast we’re working with Seattle on their CSO program and helping them to plan their future direction. Other planning efforts include Toledo, Ohio, and Allegheny County, Pennsylvania, where we are working to develop plans to carry them into the future. Tetra Tech has been working with communities in Michigan for over 20 years to plan and carry out CSO control programs and projects, and many of those programs are well into implementation.

Q:  What sorts of technologies are driving changes in the field?

A:  One of the primary concerns with the impacts of combined sewer overflows is the risk to public health. The disease potential of wastewater associated with human sewage is of greatest concern. In the last 20 years or so, we’ve become much more aware of the various disease-causing organisms in sewage. The U.S. Environmental Protection Agency (EPA) is concerned about the effectiveness of various CSO treatment technologies to actually control those organisms.

Q:  What gives Tetra Tech its expertise in addressing CSOs?

A:  Tetra Tech’s unique position with CSO control is in part that we provide the full gamut of services, including the understanding of the water body and the understanding of the methods to control the discharge from the infrastructure perspective. Tetra Tech has offices in areas of the country that were particularly advanced in addressing CSOs. Some areas of the country, for example the state of Michigan, began CSO control implementation approximately 20 years before other parts of the country. So working on these early projects has provided us with a wealth of experience in actually implementing controls.

Q:  Are there any success stories you can point to as an example of this?

A:  Well, it is important to be aware that major CSO programs generally require at least a 20-year time frame for planning through implementation. A number of the programs Tetra Tech is involved with are long-term programs of this type, and our involvement is ongoing for the duration of the project. With the City of Lansing, Michigan, which is one of our long-term CSO communities, we have been their community engineer since their first wastewater treatment plant was constructed. They began addressing wet weather issues in the 1980s and their CSO Plan was published in 1990. We have been working with the City of Lansing for the entire duration of their CSO program implementation, which recently celebrated the half-way milestone. Tetra Tech has adapted to the changing needs of the City over the years, incorporating stormwater bioinfiltration measures as a reflection of the City’s interest in low-impact and sustainable development.

Outside of Lansing, Tetra Tech has worked with several communities that have been recognized by EPA as having the top CSO control programs in the country. For example, in 2004, the City of Mount Clemens and the City of Port Huron, both in Michigan, were recognized by EPA by receiving the first- and second-place awards for CSO control. Both of those programs were managed and implemented by Tetra Tech.