Sevenson presented USACE NE District’s Bunker Hill Award

 

Presented To: Aponowich, Driscoll & Associates, Inc. with Sevenson Environmental Services, Inc. (Contractor/Operator)
Presented By: American Academy of Environmental Engineers  Scientists

ADA and Sevenson Environmental Services (contractor) have been working on the NW Natural Gas site in Portland Oregon since 2008 on an extensive remediation project, removing pollutants from the site being discharged into the Willamette River via groundwater.

In 2020 and 2021, it was determined that the groundwater flow patterns had changed over 8 years of operation. Additional measures were necessary to prevent this groundwater source from reaching the river. This groundwater showed much higher pollutant levels than existing flows. A new pretreatment system was required to collect and treat the so-called “Interceptor Trench” groundwater. The new system would then be tied into the existing Pretreatment Plant and ultimately pumped to the Main Treatment Plant for further treatment and final discharge.

Key Takeaways from the project are:

  • Alternative air stripping methods were not deemed to be feasible due to the iron fouling which would occur.
  • While diffused-air type air strippers are relatively rare and their performance not extensively documented, the Interceptor Trench unit has shown that, properly designed, a diffused air system can produce remarkable organics removal results, even with high solids in the wastewater.
  • The iron in the groundwater was oxidized effectively and used as a no-cost method of removing heavy metals.
  • The higher-than predicted removal of semi-volatile PAHs in the diffused-air unit has doubled the life of the liquid phase activated carbon system downstream.
  • Since the Interceptor Trench project was commissioned, it has intercepted and successfully treated over 150 million gallons of polluted groundwater.
  • The plant discharge to the Willamette River has continually met the NPDES standards, many of which are below drinking water standards. The NPDES permit limits are summarized in a following section.
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