NPDES Permits Potentially Affecting Water Quality
Final NPDES Wastewater Permits - Issued by Oregon Department of Environmental Quality
This list consists of permit documents* (see end) made available by Oregon DEQ and is not all inclusive. All documents are PDF except where indicated.
Durham Sewage Treatment Plant (STP): Final NPDES Wastewater Permit Related Documents
Permitted Mixing Zone Size for Toxicity:
Waterbody: Tualatin River at about River Mile (RM) 9.2 just upstream of the confluence with Fanno Creek.
The NPDES Fact Sheet (p. 13) describes the primary Durham Outfall Diffuser as follows:
Acute Mixing Zone Radius: 10 feet in all directions from the diffuser.
Acute Mixing Zone Surface Area: Estimated SA is 2,234 square feet based on a diffuser length of about 96 feet.
Chronic Mixing Zone Radius: 100 feet in all directions from diffuser (at least this is what the permit language allows despite the channel being only 200 feet wide).
Chronic Mixing Zone Surface Area: Estimated SA is 9,600 square feet based on a diffuser length of about 96 feet (i.e., SA is about 96 by 100 feet if only the downstream direction is considered).
Chronic Mixing Zone Volume: DEQ does not provide Tualatin River MZ depth in the fact sheet so MZ volume is not yet determinable.
"3.1. Durham AWTF
The Durham AWTF Outfall (D001) discharges to the Tualatin River at river mile 9.2, just upstream from the Fanno Creek tributary. The river channel at this point is described as being approximately 200 feet wide. Effluent is discharged through a 96-foot wide multi-port diffuser that was laid on the bottom of the river nearly in the center of the channel. The diffuser has twelve evenly spaced ports."
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