Work underway on two major City of MLT construction projects

Decant facility construction, Feb. 7 017
The site of the stormwater treatment facility.

Two significant city construction projects are continuing to change the landscape around Mountlake Terrace.

City residents can’t escape noticing the huge pile of fill dirt forming on the site of the former city hall at 232nd Street Southwest and 58th Avenue West. The dirt is being excavated from a work site just north of the Mountlake Terrace Library where a new city stormwater treatment plant will be built.

Following completion of the $1 million project, some of the dirt will be used as fill at a city service road near the Recreation Pavilion at 52nd Avenue West. City officials have approved the dirt transfer plan, determining that moving dirt to the service road will have minimal impact on nearby Terrace Creek.

Interested spectators just outside the MLT Library.
Interested spectators just outside the MLT Library.

Once completed, the stormwater treatment plant will be covered with a new parking lot for the library. The entire project is expected to be completed this summer.

The site of the decant facility and a portion of 215th Street Southwest that will experience some lane closures this week.
The site of the decant facility and a portion of 215th Street Southwest that will experience some lane closures this week.

Construction also continues on a new decant facility next to the city’s operation facility at 6204-215th St. S.W. The $800,000 project, once completed, will be used to settle wet sediments collected during maintenance work on water, sewer and stormwater utility systems.

Work at the decant construction site will require some lane closures of 215th Street Southwest between 61st Place West and 66th Avenue West this week. The project is expected to be completed this spring.

— Story and photos by Doug Petrowski

  1. Is the Terrace big dig a storm water treatment facility is the surface water just contained? I thought it was a massive storm water retention vault. Maybe you can elaborate on its purpose.

    1. Good question. It’s an infiltration facility made up of large diameter buried pipes. During large storms the pipes will fill up with (detain) stormwater while the stormwater is being infiltrated into the ground. The act of infiltrating the stormwater into the ground is removing pollutants (treating).

  2. Thanks, Dustin. Another explanation the article could benefit from is the purpose and use of a decant facility, how these two projects are related in a coordinated stormwater management program (if they are), and the impetus for these construction projects. Google can answer some of these questions, but you have proven to be a much more succinct explainer of these things in our local context.

    1. Thanks, James. I wouldn’t say the two projects are directly related though they both do deal with stormwater. During maintenance of the City’s storm, water, and sewer systems there is often a need to dispose of water or wastewater with a high percentage of solids in it. For example during pipe cleaning, catch basin cleaning, or street sweeping. If this material would be directly dumped in to the sewer system it would possibly clog pipes or put a big toll on the wastewater treatment plant and pumps. What the decant facility does is simply separate out much of the solids from the liquids. The liquids are discharged back into the sewer system and the solids could have a multitude of fates, most likely going to a landfill.

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