Resolution No. 4793 - Laserfiche WebLink

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5. BASIS OF Blending Policy Blending or split flow refers to the practice of diverting flow around a treatment component (usually secondary treatment) during high flows; specifically, PWWFs. The WI>CF was designed to operate using blending when flow exceeds the secondary system capacity, currently estimated at 103 mgd. The practice is not acknowledged in the current NPDES permit and is currently under review by EPA. In late 2003 EPA issued for public comment a p~:oposed policy on blending that if adopted would clarify that blending is a legal practice, subject to the six principles outlined in the proposal. The proposal requires that all re-routed flows be combined before discharge. It is anticipated that the practice of blending will continue to be an acceptable approach for treating PWWFs. For the purposes of evaluating treatment technologies, it is recommended that the WPCF look at solutions that are flexible enough to implement either conventional primary effluent blending or equivalent secondary effluent blending. With the elimination of SSOs, blending will provide the most cost-effective opportunity for the WPCF to provide a treated effluent that meets secondary treatment standards in the most cost- effective manner. Using blended treatment would eliminate large capital investments for facilities that would be used very infrequently. In addition, treatment alternatives should attempt to maximize the capacity of the existing facility's primary and secondary treatment facilities, minimizing the frequency of blended treatment, and taking full advantage of MWMC's existing investment. Even under the best circumstances, significant investments and modifications to the facility will be required to convey and ~eat the projected I>WWF. Bacteria It is anticipated that the current bacteria limitations specified in the NPDES permit, which require a monthly geometric mean of 126 E. Colt per 100 mL and a maximum sample containing 406 E. Colt per 100 mL, will continue to be implemented in future NPDES permits. Although not required to meet anticipated bacterial limitations, alternative disinfection technologies should be evaluated to address operator and community safety issues associated with the current disinfection practice that uses gaseous chlorine and sttlfur dioxide. Turbidity The current turbidity standard is based on the Oregon State Water Quality Standard, which stipulates that the wastewater cannot increase the river turbidity by more than 10 percent at the edge of the mixing zone. This standard is currently under review by the state and it is possible that a more stringent numerical limit could be imposed in future NPDES permits. Treatment alternatives that include improved secondary treated effluent water quality and effluent filtration should be considered. Dissolved Oxygen The Willamette River in the Eugene area is not included in the 303(d) list for dissolved oxygen (DO). The current DO standard is based on the Oregon State Water Quality Standard; however, there is no current numerical limit in the NPDES permit. It is possible that a more stringent numerical limit could be imposed in future NFDES permits. The implementation MWI¢~C_50_REVS_¥KS.[)OC 5:11