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.
Hillsboro Sewage Treatment Plant (STP): Final NPDES Wastewater Permit Related Documents
Permitted Mixing Zone Size for Toxicity:
Waterbody: Tualatin River at about River Mile (RM) 43. Wastewater discharges are not allowed during specified low flow conditions in the Tualatin River.
The NPDES Fact Sheet (p. 13) describes the primary Hillsboro Outfalls as follows:
Acute Mixing Zone Radius: 10 feet in all directions from the two (2) outfall pipes.
Acute Mixing Zone Surface Area: Estimated SA is 628 square feet total for the two primary outfall pipes.
Chronic Mixing Zone Radius: 100 feet in all directions from each of the two diffusers (at least this is what the permit language allows despite the channel width being unquantified).
Chronic Mixing Zone Surface Area: Estimated SA is 5,000 square feet based on assumed MZ widths of 25 feet for each of the two primary outfall pipes (i.e., SA is about 25 by 100 feet if only the downstream direction is considered, for each of the two outfalls, and the MZ width is assumed in lieu of DEQ providing it).
Chronic Mixing Zone Volume: DEQ does not provide Tualatin River MZ depth in the fact sheet so MZ volume is not yet determinable.
"3.3. Hillsboro WTF
The Hillsboro WTF discharged to Jackson Slough several hundred yards from the main river channel until outfall improvements were completed in about 1995. In 1994 a mixing zone study was completed for the Jackson Slough discharge. The study found Jackson Slough provided inadequate dilution and poor mixing for the effluent at its confluence with the river. The study recommended that effluent be piped to the Tualatin River. All flow is now piped from the plant to a splitter box at the effluent storage pond. The engineering resulted in two separate discharge pipes to the river out of the splitter box. Outfalls are about a third of a mile apart on the Tualatin River and are designated 001A at river mile 42.9 and 001B at river mile 43.3. A third pipe connection can send effluent to storage. The flow control structure determines the final destination of the treated effluent. This facility will continue to discharge to the river on a seasonal basis under the same stream flow restrictions as described previously for Forest Grove WTF."
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