Utility Conquers Combined System Problems

The Cambridge Department of Public Works is counting on an engineered wetland to help reduce CSOs.
Utility Conquers Combined System Problems
Tom Molinari of consulting and engineering firm Kleinfelder takes a site survey for the Cambridge Department of Public Works Sewer Separation Project. The City has been working on separating its combined sewer system since the 1990s.

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As the home of Harvard and the Massachusetts Institute of Technology, the City of Cambridge, Massachusetts, has never been short on big ideas. The Cambridge Department of Public Works (CDPW) is currently basking in the success of its own big idea — the establishment of the Alewife Reservation Stormwater Wetland to help control combined sewer overflows

Much of the early history of Cambridge was driven by economic development, often in direct competition with the City of Boston to the south. Lowlands were raised, marshes and swamps were drained and the population expanded. The city’s first sewers were built in the 1850s in a typical combined system design. As with many other cities, the control of CSOs has currently become an environmental and civic priority.

The city of 110,000 people is served by the Massachusetts Water Resources Authority (MWRA), a state agency providing wholesale water and sewer services to 2.5 million people in 61 metropolitan communities around Boston.

The CDPW provides a host of services, including sewer maintenance, stormwater management, engineering, parks, street and sidewalk maintenance, and urban forestry. The Cambridge Water Department is a separate municipally owned and operated water utility.

Fighting CSOs

“We are a very old city, and much of our original infrastructure was designed and built as a combined system,” says Catherine Woodbury, engineering project coordinator with the CDPW. “While the city is making strides to reduce CSOs, the extreme costs of separating the systems often exceed the immediate benefits.”

The MRWA has operated under an enforcement action by the U.S. Environmental Protection Agency since 1985 as a result of a remedial framework established during the Boston Harbor Case, which focused on wastewater pollution of Boston Harbor and its tributaries. To date, MWRA has completed 29 of 35 area projects under the resulting CSO Control Plan and has begun construction on an additional four projects.

Projects directly affecting Cambridge have included eliminating CSO outfall points, expansion of sewer pipe volume and sewage diversion.

The city operates under a National Pollutant Discharge Elimination System permit under the Clean Water Act. There are seven permitted CSOs on the Charles River — two owned by the MWRA and five by Cambridge. There are eight permitted CSOs on the Alewife Brook — one owned by the MWRA, six by Cambridge and one by the neighboring City of Somerville.

“We’ve already made significant progress on reducing CSOs to both these waterways,” Woodbury says. “Since 1988, we’ve reduced CSO discharges to the Charles River by 98 percent, and since 1987, we’ve reduced discharges to the Alewife Brook by almost half. We began a series of sewer separation projects in the 1990s and the system is now 40 percent separated.”

A century system

Parts of the wastewater and stormwater system date back more than a century. The stormwater system is primarily made of concrete pipe as large as 72 inches in diameter. The oldest sewer pipes are made of clay. Outside crews are contracted to perform new construction with reinforced concrete pipe generally used for larger installations and PVC used for smaller diameters.

Combined system manholes are being isolated and replaced, with additional piping installed to allow for manhole relocation. Common trench installations, in which sewer and stormwater pipes were placed in the same excavation, are also being reversed.

“The sewer line would be located at the bottom of the trench and a separation plate would be installed to isolate the two systems,” Woodbury says. “However, over time the plates were damaged or removed. During rain events and with damaged pipes, some of these trenches would function as a combined system.”

Effective pipe and infrastructure maintenance also form an important role in CSO control. The CDPW’s Sewer Division inspects and clears obstructed sewer lines, cleans catch basins, repairs broken lines, and inspects and approves private connections to the public system.

The department owns a Vactor combo unit and splits cleaning duties between in-house crews and outside contractors.

“The flatness of our topography means that some of the pipes going out to water bodies are subject to backflow from the rivers,” Woodbury says. “Keeping pipes clean of sediment is always a challenge.”

Closed-circuit television inspections are carried out by contract with outside suppliers.

The department also expends significant effort educating the public about proper connections to the city’s sewer system, while identifying and isolating illegal connections.

Engineering a wetland

However, the city’s most novel approach to reducing CSOs has been the creation of the Alewife Reservation Stormwater Wetland, an engineered marsh.

“When the city was developing, swamps were often filled in to control mosquitoes or to level the land,” Woodbury says. “Other wetlands were filled with invasive, low-quality plant species. The Alewife Brook was channelized.”

The engineered wetland project began with the creation of the city’s first hydraulic stormwater model, completed by engineering firm MWH using Innovyze InfoWorks software. The analysis identified the potential for a 3.5-acre detention pond to be located in the Alewife Brook Reservation conservation area. The caveat: The reservation is a state park managed by the Massachusetts Department of Conservation and Recreation.

“The only way to implement the plan was to create a detention pond and place detention equipment on state lands,” Woodbury says. “We had to approach the state from the position that the plan was an enhancement of the reservation and the wetlands.”

The CDPW worked closely with the state to create a design that supported the conservation area’s master plan. A detention pond constructed in the form of a wetland would be enhanced by the planting of original native plant species and the addition of walking paths through the new wetland.

“We also had to demonstrate that the design of the detention pond wouldn’t increase the peak flow downstream along the Alewife Brook,” Woodbury says.

Upstream controls

A series of passive stormwater management devices, such as flaps and throttles, were located upstream to regulate water levels. The upstream pipelines also feature catch basins by BMP Supplies Inc., flushing systems, and sump manholes to capture grit and floatables.

The wetland border provides stormwater storage via a 6-foot earthen berm designed to retain a 10-year storm event.

The wetland itself was designed as a multistage ecological filter. In the first stage, fine sediment settles in an articulated concrete block forebay where it’s later trucked out for disposal. Stormwater then slowly passes through a vegetated swale before entering the main basin.

The city partnered with consultants and engineers to plan the placement of 120,000 plugs of wetland species and 400 upland trees and shrubs.

Polishing the effluent

“The wetland is designed to take advantage of its topography to force the water to take the longest route through the system,” says Woodbury. “The vegetation not only acts as a sponge to slow the water down, it also naturally treats and removes biological nutrients and other pollutants. They polish the water to the point that it can be released to the Little River and downstream to the Alewife Brook.”

The facility began accepting water in April 2013 and was opened to the public in October of that year.

The $26 million project has not only earned the accolades of park users and educators, it earned a 2014 National Recognition Award from the American Council of Engineering Companies and a 2014 Environmental Project of the Year award from the American Public Works Association.

“Right now, we’re waiting for the vegetation to establish itself, but we’re on the way to developing a very healthy ecosystem including snapping turtles, muskrats, deer, swans and geese,” says Woodbury. “Once we achieve a healthy base flow, the wetland will be capable of accepting the full complement of water from upstream, perhaps by the end of 2015.”



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