• Despite space restrictions, a creative solution will enable CDM Smith and the Dublin City Council to upgrade and expand the Ringsend wastewater treatment works to meet demands and preserve the waters of Dublin Bay.

file under: Water, Design-Build, Europe, Government

A Tunnel to Clean Waters

Dublin Protects its Treasured Bay

The city of Dublin wraps itself around Dublin Bay, where the River Liffey expands into the Irish Sea. More than the largest shipping port in Ireland, the bay is a destination for residents and tourists, swimmers and sun bathers. It is the industrial and recreational heart of the city— a treasure the people of Dublin are committed to protecting.

By taking a holistic approach to the challenge, we were able to identify the optimal solution and protect all water quality standards. 

Working for Water Quality
By the 1990s, it was clear that the bay was deteriorating. The Dublin City Council (DCC) developed the Ringsend wastewater treatment works to improve water quality and prevent further pollution. Put online in 2003, Ringsend successfully restored the bay’s waters and contributed to two of Greater Dublin’s beaches receiving Blue Flag status—indicating water quality of the highest standard, based on the European Union Bathing Waters Directive.

Today, CDM Smith is helping DCC upgrade and expand Ringsend to maintain quality, meet demands of a growing population, and comply with new regulations. According to Bob Gaudes, CDM Smith senior vice president, the firm was familiar with the treatment works, having suggested odor control measures in 2005. “We used sampling and modeling to recommend an increase in odor control capacity. In approximately 2 years, the $45 million project reduced the facility’s odor by 75 percent.” The current upgrade will include the remainder of the odor control improvements.

Daunting Limitations
The design-build project will increase Ringsend’s capacity from 5.7 cubic meters per second (m3/s) to 7.0 m3/s [in Europe, capacity is generally expressed in population equivalent (PE); the plant will be increased from 1.6 million to 2.4 million PE]. However, as designer, CDM Smith was faced with significant challenges beyond the expansion. The Urban Waste Water Treatment Directive has declared the Liffey River Estuary a “sensitive” water body, requiring nitrogen and phosphorus removal in water discharged to the estuary.

With its current configuration, Ringsend has limited ability to meet the nutrient removal requirements, so the team evaluated several new treatment scenarios. The most daunting problem was the plant’s restricted space, which immediately eliminated biological nutrient control as a practical solution. Membrane bioreactors were also deemed to be financially unfeasible. Chemically enhanced primary treatment combined with deep shaft aeration was an option, but would produce sludge in excess of the sludge stream capacity, requiring further expansion.  

A New Perspective
“We had to take a step back and look at alternatives beyond nutrient removal. A long sea outfall discharging beyond the sensitive waters would eliminate the nitrogen and phosphorus reduction requirement,” explains Gaudes. Discharging at greater depths and farther out in the bay where currents are stronger makes the outfall a safe and viable solution.

The team assessed other similar tunnel projects, including the CDM Smith-designed sea outfall in Boston. Serious consideration was also given to the effects the project would have on the area’s beaches and marshes. “Everything checked out—a long sea outfall discharging at secondary treatment standards was feasible from technical, financial and environmental perspectives,” notes Gaudes. In addition, this alternative would have lower risk than others. It would consume less energy and chemicals, and produce less sludge and greenhouse gases, supporting Dublin’s sustainability goals. It would also be simpler to operate and maintain.

“By taking a holistic approach to the challenge, we were able to identify the optimal solution and protect all water quality standards,” says Dermot Molphy, DCC project engineer. “Ringsend is an important part of the Greater Dublin Strategic Drainage Study, which aims to create sustainable development in Dublin and meet demands created by recent growth. It is vital that we implement a project that advances the region’s compliance with Irish and European Union legislation and prepares us for the future.”  

Public Assurance
CDM Smith and DCC are partnering to ensure that the public is comfortable with the Ringsend upgrade and expansion, which is scheduled to be complete by 2015. Community outreach efforts include focus groups, a website and a brochure to assist with education. “We developed a list of frequently asked questions and a glossary of terms to help residents understand what we are doing and how it will affect them,” explains Gaudes.

Throughout the winter of 2011, the team will conduct “Open Days,” all-day events held at public venues, such as schools and community centers, where members of the community can learn about the project and ask questions. Dynamic displays will help people to visualize the project, and similar successful projects—including Barcelona, Lisbon, Miami, Los Angeles and Boston—will be used as examples to let residents know that using a long sea outfall is not a new technique. Molphy emphasizes, “The environment is extremely important to the people of Dublin. We are responsible for assuring them that we are making the best decisions for the preservation of Dublin Bay.”