On 6/16/2021 Midway Sewer District Wastewater Treatment Plant (WWTP) experienced a utility power upset from Puget Sound Energy.   This dip in service power caused multiple pieces of equipment around the treatment plant to trip out and halted their operations. Two of those pieces of equipment were the Trickling Filter feed pumps, which handle all Primary Effluent flow that is headed to the Secondary Treatment side. When those pumps are not operating, the wet well level will increase until capacity is met.  Once the wet well is full, the flow navigates through a flow diversion structure as a fail-safe, sending it into the outfall line. The outfall line goes out into Puget sound, 2,200 feet offshore from the beach. None of the overflow went into Des Moines Creek.  The estimated overflow discharge was calculated at 99,888 total gallons of primary treated wastewater, without secondary treatment or UV disinfection. The event occurred on 6/16/2021 at 7:49pm until 8:20pm for a duration of 31 minutes, with an average flow of 4.64 MGD (Million Gallons per Day) for that time period.

Summarized list of events:

  • The power grid experienced an instability event that coincides with the time of the event at the plant.
  • Generators at the WWTP did not start due to the power was not out long enough to initiate this action.
  • All Eaton VFDs (Variable Frequency Drives) in the electrical building shutdown.
  • Our SCADA system provided notifications of the alarm conditions; we drove to the WWTP as soon as we realized we were not able to start the VFDs via the online SCADA system.
  • When we arrived 31 minutes later, we reviewed the VFDs and noted the controls to start or reset the VFD were not functioning.
  • Shutting down the power to each drive and then restarting allowed the VFDs to return to operation.

We have contacted the Department of Ecology, Washington State Department of Health, King County Department of Health, and the City of Des Moines of this overflow event.

We have contacted the vendors involved and re-programming is in the works. Midway is also working with our SCADA technician and amongst our Operators to formulate a plan to prevent an occurrence of this nature from happening again.

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