Combined Sewer Overflow Controls 

Each year more than 850 billion gallons of untreated sewage is released in combined sewer overflows (CSOs) nationwide. The U.S. Environmental Protection Agency (EPA) is stepping up enforcement in response to CSO discharges in order to bring communities into compliance with the Clean Water Act (CWA) and federal CSO policy.

Tetra Tech's planning and infrastructure experts help communities develop and implement programs to address CSO control needs and meet EPA requirements. As Engineering News-Record's (ENR) number one water design firm in 2008—for the fifth year in a row—Tetra Tech has a unique perspective on the techniques that can support cost-effective and innovative management of CSO programs.

During wet weather, sanitary waste and runoff water can overwhelm aging sewer
systems. Tetra Tech is helping to upgrade water infrastructure systems where
the need is greatest.

Wet Weather Problems
In both CSOs and sanitary system overflows (SSOs), untreated sewage and stormwater overflows the system and ends up in streams, rivers, or other bodies of water. Currently, about one third of the nation's waters are impaired by wet weather discharges.

CSO control steps include evaluation of the existing system and receiving water, developing engineering alternatives to address the control of flow and pollutant discharges, design of facilities, and other infrastructure improvements followed by construction of the improvements.

During the implementation of CSO controls, coordination with the regulatory agency, public, stakeholders, and financial planning team is required. Issues of facility siting must be addressed. The benefits of the control facilities must be assessed and integrated into any adjustments to the plan as it moves forward.

Alternatives for improvements fall into broad categories, which may involve additional treatment capacity (either at a local wastewater treatment facility or a remote site), reducing quantities of flow (sewer separation or control of inflow and infiltration), or temporary storage for later treatment (basins/ tunnels).

A Long History of Solving Water-related Problems
Tetra Tech was founded in 1966 as a water resources engineering firm. Tetra Tech has long-established credentials in watershed management and related CWA support services. More recently, our water engineering expertise has grown considerably through the organic expansion of our capabilities and acquisitions.

All programs have sought to maximize the existing infrastructure, address the pollution control need, and provide for long-term infrastructure sustainability. Tetra Tech clients in Mt. Clemens and Port Huron, Michigan, won first and second place honors in EPA's 2004 CWA CSO Awards.

Tetra Tech's wet weather services include planning, design, construction, and renovation of wastewater systems, as well as watershed management and stream restoration. Over the past 20 years, Tetra Tech has provided these services to key municipal clients including Lansing, Michigan; Detroit, Michigan; Boston, Massachusetts; Seattle/King County, Washington; Milwaukee, Wisconsin; Louisville, Kentucky; Huntsville, Alabama; Toledo, Ohio; and Cincinnati, Ohio.

The Watershed Approach
Tetra Tech's approach is to support clients early in the process—at the watershed planning level—to provide an integrated and effective approach to planning and implementation of the upgrades.

Baby Creek and Disinfection Facility; Detroit, Michigan

Tetra Tech designed a CSO screening and disinfection facility that will disinfect CSO discharges before they reach the Rogue River.

Tetra Tech's current support for the Milwaukee Metropolitan Sewerage District is one of the first examples of a truly integrated, watershed-based strategy for evaluating CSO, SSO, and long-term water quality solutions. In this project, a team of CSO, watershed, and modeling experts are developing watershed models that encompass more than 1,100 square miles. The models project future conditions to 2020 and provide the basis for alternatives analysis, so water quality-based, cost-effective solutions can be systematically ranked and prioritized.

The alternatives include both conventional structural solutions and "green" alternatives that emphasize land preservation/wetland restoration and innovative Best Management Practices (BMPs). The initial results of the study were presented to over 400 stakeholders at the annual Watershed Initiative Conference in March 2006.

North Creek Storage Facility; King County, Washington

Tetra Tech designed this state-of-the-art facility consisting of an underground tank and hydraulics. It protects Lake Washington from CSOs.

The watershed approach is increasingly being linked to CSO mitigation. An October 2005 consent decree for Northern Kentucky's Sanitation District #1 defined an $880 million improvement program that includes $476,400 in civil penalties for CSO/SSO violations. This consent decree is unique because it specifically calls for an "adaptive watershed management approach" in the design and execution of the remedy program.

Meeting the Need
Grassroots organizations like the Natural Resources Defense Council, Friends of the Earth, Sierra Club, and others are now using litigation to compel sanitation utilities to control CSOs and SSOs; likewise, EPA is directing considerable enforcement attention to the issue and has issued several consent decrees in CSO communities. EPA estimates that it will cost taxpayers about $50.6 billion to control CSOs and $88 billion to control SSOs over the next 20 years.

Boston Water and Sewer Commission (BSWC) CSO Programs; Massachusetts

For more than 15 years, Tetra Tech Rizzo has helped BSWC address CSOs affecting Boston Harbor and tributaries.

Needs are continually emerging for planning, cost evaluations, and design solutions in cities throughout the country. Tetra Tech provides a full range of integrated services to support best-value solutions for our clients.

Tetra Tech leads the nation in developing and implementing innovative and effective solutions to address water quality problems. For more information, please email This e-mail address is being protected from spambots. You need JavaScript enabled to view it .