Spokane’s Solution to Combined Sewage Overflows

A few weeks ago, we had the privilege of touring a Combined Sewage Overflow (CSO) tank currently being constructed underneath Pettit Drive (better known as Doomsday Hill). The City’s investment in multiple tanks similar to the one pictured below is preventing millions of gallons of combined sewage and stormwater overflow from ending up in our river.  This project and ones like it are great news for the health of the Spokane River.

jule-in-cso-tank
Riverkeeper technical lead, Jule Schultz, stands in the CSO tank at the top of Doomsday Hill.

So what are CSOs, and why do they exist in the first place? When Spokane’s sewer system was initially installed in the late 1800s, sewage and stormwater were removed via a singular pipe system, and all pipes drained directly into the Spokane River or Hangman Creek (!). A Wastewater Treatment Plant (WWTP) was installed by 1958 with a capacity of 50 million gallons a day (mgd). Sewage and wastewater that were treated discharged into the river. However, on days when flow exceeds the WWTP’s maximum capacity– typically a result of precipitation or rapid snowmelt – runoff overwhelms the ability of the WWTTP to handle this combined sewage and stormwater. On these days, the excess combined pollution flows into the River before reaching the sewage treatment plant, hence the term Combined Sewage Overflow (see figure below).

cso-diagram
Figure 1. A typical CSO which sends sewage to be treated at the Wastewater Treatment Plant during dry weather, but diverts sewage to the river during wet weather. (Image courtesy of Wikipedia).

Though our sewage treatment plant’s capacity has increased to a present-day maximum capacity of 150 million gallons per day, CSOs continue to occur, albeit at significantly reduced numbers. The City’s 2015 annual report found that 54 million gallons of untreated combined sewage and stormwater was discharged into the river.

cso-tank-diagram
Figure 2. A typical CSO storage tank adds capacity to a sewer system and reduces sewage flowing to the river. (Image courtesy of King County).

To further combat CSOs, in 2005 the City of Spokane initiated a plan calling for the addition of CSO control facilities to the city’s sewer system. These control facilities are large, underground concrete tanks. In the event of an excessive flow, CSO tanks temporarily store combined sewage and stormwater until flow has decreased to the point where the treatment facility has the capacity to process the stored overflow. The tank we toured under Doomsday will have a capacity of 694,000 gallons.

Currently, 11 CSO tanks are operating with a total combined volume of 3,598,300 gallons, but during the duration of a storm, they can redirect much more combined sewage than that (Figure 3 below). For example, a December 2015 four tanks diverted 4.2 million gallons! With the completion of all tanks under construction and in design, there will ultimately be 24 tanks with a combined capacity of 14,579,300 gallons. This should essentially eliminate CSO outfalls into the river, ensuring cleaner water for humans and wildlife. For more information on CSOs, see the City’s CSO Overview/Monthly Reports, 2015 CSO Annual Report, 2005 CSO Plan Amendment, and Integrated Clean Water Plan.

three-days-in-a-cso-tank

A huge thanks to Rachel Fricke, our intern, for writing this blog.

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