Resolution No. 4793 - Laserfiche WebLink

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BASIS OF PLANNING Discharge from the facility is to a stream that is water quality-li~rdted; Heat.is.con~buted to the stream above a water quality-lhnited stream segment; Reasonable potential exists for the discharge to have a measurable impact outside of the assigned n~xing zone; and · Temperature-sensitive endangered fish may be present The Temperature Management Plan for the facility was developed and approved by the DEQ prior to permit renewal. The current NPDES permit was issued with a thermal load limitation that was based on the dry weather design average flow. The NPDES permit specifies that the fac~ty must meet this thermal load limit for the maximum week during the dry season. Because of the way that the excess thermal load limit is calculated, the facLlity has the potential to surpass the permitted excess thermal load limit of 3.1 billion BTUs during peak week flows. Since the Temperature Management Plan was approved by DEQ, Oregon's water quality standard for determining thermal load limitations has been somewhat in flux. After a period of uncertainty, the regulatory envkonment for temperature d~charges has become dearer. In June 2003 DEQ published guidance that specifies that the maximum weekly design flow should be used to calculate the excess thermal load. In early March 2004, EPA approved Oregon's new water quality standards for temperature. The DEQ is developing a temperature TMDL for the Willamette River based on the new Oregon standard. The details of the T~DL are s~-~ll to be worked out, but could result in a revised thermal load limJtation for the facility once the permit is up for renewal. In the absence of the new TIv[DL standards, the WPCF will continue planning for facilities using the current thermal load limitation and will continue to implement the approved Temperature Management Plan. A detailed thermal load analysis was performed for this facilities plan to estimate the projected dry season thermal load through 2025. Thermal loads discharged to the river are the product of wastewater flows and their corresponding temperatures. The analysis evaluated 12 years of historical flow and temperature data to develop average and peak week dry season thermal loads From this data, a peak week thermal load peaking factor was calculated and applied to fiature average thermal loads developed from projected flows and historical average temperatures. The resulting projected peak week thermal load is Shown in Figure 5.1.3-5. This figure also shows the flow rate that would be required to be removed from the river to meet the current peak week thermal load l~rdt specified in the NPDES permit. Sanitary Sewer Ovedlows (SSOs) Perhaps the most significant impact to potential future treatment tedmologies lies in the changing regulations for SSO requirements. Currently, untreated emergency SSOs have specific limits on the seasonal t/ming and storm event conditions that create circumstances such that these discharges are unavoidable and allowable under Oregon state law. Oregon's current SSO rules are embedded in the bacteria water quality standard, wtdch prohibits overflows from less than a 5-year 24-hour winter storm, and from a less than lO-year 24-hour summer storm. Proposed federal rule changes for SSO requirements are currently on hold for MWMC_50_R£VB_VKS DOC 5-9