As part of efforts to comply with its recent consent decree, Washington’s King County will consider adding vegetated roofs such as this one and other “green” infrastructure to reduce the volume of storm-water runoff entering its combined sewer system. King County Wastewater Treatment Division
As part of separate consent decrees with environmental agencies, the City of Seattle and King County, Washington, could spend upwards of $1 billion to reduce combined sewer overflows.
July 9, 2013—As part of separate consent decrees recently entered into with the U.S. Environmental Protection Agency (EPA) and the Washington State Department of Ecology, the City of Seattle and King County will make improvements to their sewer systems totaling nearly $1.5 billion (in 2010 dollars). Actions taken by the city and the county are expected to achieve significant reductions in combined sewer overflows (CSOs) to Puget Sound and other area waterways. The consent decrees also contain provisions intended to encourage the use of “green” infrastructure in meeting these goals.
Announced in mid-April, the decrees seek to build on long-term efforts by the city and the county to reduce CSOs from their conveyance systems. CSOs discharged from outfalls managed by the city or the county totaled 30 billion gal in 1970. By 2012, such overflows had been reduced to approximately 1 billion gal—a significant achievement—but further improvements are still required.
Serving an 88 sq mi area, the conveyance system overseen by Seattle Public Utilities (SPU) includes 968 mi of combined sewers, accounting for roughly a third of the city’s overall collection system. Rather than treat the wastewater itself, SPU conveys its sewage to King County’s collection system. However, SPU manages 87 CSO outfalls, 35 of which are uncontrolled, meaning that overflows from them occur on average more than once a year. From 2007 through 2010, the city discharged approximately 200 million gal of CSOs each year, according to the EPA. Upon completing the projects stipulated in the consent decrees, Seattle and King County will have reduced the remaining CSO volumes by 99 percent.
Under its consent decree, the City of Seattle must develop a long-term control plan for addressing CSOs by December 31, 2025. Implementing the measures required by the decree is expected to cost approximately $600 million, according to the EPA.
Of this $600 million, the city expects to spend half before the consent decree even takes effect in 2015, says Susan Stultzfus, a spokesperson for SPU. This $300 million will go toward CSO reduction projects called for by the city’s current National Pollutant Discharge Elimination System permit. The projects, Stultzfus says, are intended to “jump-start” efforts to reduce overflows to Lake Washington, a priority waterway along Seattle’s eastern edge. Construction has begun on three underground storage tanks that are to be completed in 2014, and two more are planned for completion in 2018. These “early action projects” will help the city address a “big portion” of its CSOs, Stultzfus says. Additional improvements include retrofits of the existing combined sewer system to increase storage capacity.
SPU is evaluating other options for reducing sewage overflows as part of its long-term control plan. Such alternatives include constructing multiple underground storage facilities throughout the city, some possibly in conjunction with King County, as well as implementing sewer improvements to boost storage capacity. The city is also considering partnering with the county to construct a large storage tunnel along the east or west end of the Lake Washington Ship Canal.
In June 2012 the EPA finalized an integrated planning approach that provides a legal framework through which local governments can set priorities for investments in wastewater and storm-water infrastructure so as to comply with Clean Water Act requirements as cost-effectively as possible (see “Mayors Call on Congress, White House to Reform Clean Water Act,” Civil Engineering, July/August 2012). Under its consent decree, Seattle has the option of developing an integrated plan that would seek to reduce the volume of polluted storm-water runoff entering local waterways and to decrease CSOs. “If we choose an integrated plan alternative, we can prioritize some storm-water projects before CSO projects,” Stultzfus explains. “We’re able to make that decision if we can show our regulators that doing storm-water projects in certain locations would provide a greater benefit to a waterway than CSO [reductions] alone,” she says. SPU plans to release drafts of the city’s integrated plan and long-term control plan in the spring of 2014.
Serving an area of roughly 420 sq mi, King County’s conveyance system encompasses approximately 389 mi of sewers, of which about 118 mi are combined sewers. The county manages 38 outfalls that overflow to local waterways when portions of its combined system exceed their capacities. From 2006 through 2010 county CSOs amounted to roughly 900 million gal a year, according to the EPA.
The consent decree with the EPA is the culmination of decades of efforts by King County to address its CSOs, says Pam Elardo, P.E., the director of the county’s wastewater treatment division. In the 1970s the county began taking steps to decrease the volume of untreated discharges and uncontrolled CSOs from its system by more than 90 percent. In October 2012 it amended its long-term plan for controlling CSOs, and this plan forms the basis of the consent decree. Upon completion of the long-term control plan, all of the county’s outfalls will comply with what is known as the control standard, meaning that on average there is no more than one overflow per outfall each year.
King County will have to meet all of the requirements of the consent decree by December 31, 2030. Between now and then, it expects to spend approximately $860 million on improvements and upgrades. Of this amount, nearly $711 million will go toward nine projects that are explicitly called for in the consent decree. These projects will address the county’s 14 remaining CSOs that do not meet the control standard. Seven of the nine projects will entail increased conveyance and storage capacity in the form of new storage tanks and pipes. The projects will provide peak CSO storage capacity ranging from 320,000 gal to nearly 7.9 million gal. The cost for the individual storage facilities will range between $14.8 million and $95.4 million.
On the basis of preliminary analyses, four of the seven projects have “some potential” for incorporating green infrastructure, Elardo says. In fact, King County may be able to substitute green infrastructure entirely for so-called gray infrastructure at two of the sites. “Those are the ones we’re focusing our energy on right now,” Elardo says. In particular, one of the sites is currently slated to receive a 320,000 gal storage pipe, while the other would be outfitted with 3,200 ft of 84 in. diameter pipeline. As an alternative, green infrastructure approaches at these sites might take the form of subsidies to property owners who install rain gardens. The county also intends to investigate the use of bioswales and similar approaches that rely on infiltration, Elardo says.
The consent decree also calls on the county to construct two high-rate clarification facilities to treat CSOs before discharge, one a $139.7-million, 66 mgd plant to be completed by the end of 2022 and the other a $270.8-million, 151 mgd treatment facility to be completed by the end of 2030. Although the treatment process to be employed at the facilities has not yet been determined, space limitations will probably lead King County to choose chemically enhanced primary treatment, Elardo says. “We also want to get away from using chlorine for disinfection,” she notes.