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Staff Report • • - 8- <br /> City <br /> 8-City of Manteca and City of Lathrop <br /> Wastewater Quality Control Facility <br /> renewals in the Bay Protection Hot Spot Policy, adopted. The Policy states: "For new and <br /> expanded discharges, staff will recommend on a case-by-case basis stringent effluent <br /> limits to ensure no increase in oxygen demand to the South Delta.... However, load <br /> reductions from existing dischargers will not be required...unless it is clear...that the <br /> specific load reduction would be required." Manteca is embarking on a major expansion <br /> of its wastewater treatment plant,which will serve the growth of the community for many <br /> years. The current NPDES Permit does not require any significant improvement in effluent <br /> quality related to the discharge of oxygen demanding substances. If Manteca is going to <br /> significantly upgrade the quality of its effluent, the time for such an upgrade is during a <br /> major plant expansion. Manteca obviously contributes to the mass of oxygen demanding <br /> substances in the Delta, although the significance of Manteca's current and future discharge <br /> is not known. If the mixing zone is granted, Manteca will be allowed to continue to <br /> discharge high concentrations of ammonia and that mass loading of ammonia to the Delta <br /> will increase as Manteca's flows increase. If the mixing zone is not granted, Manteca will <br /> need to proceed with another means of achieving compliance with the NPDES Permit and <br /> C&D Order, most likely the construction of nitrification facilities as part of the treatment <br /> plant expansion. Construction of nitrification facilities would reduce Manteca's discharge <br /> of ammonia, thus reducing the impact on Delta dissolved oxygen. The impact of Manteca's <br /> discharge on the oxygen concentration in the Delta is, however,not the subject of this <br /> hearing. The proposed Permit modifications only concern Manteca's compliance with <br /> effluent biotoxicity standards. <br /> • Biotoxicity Monitoring. By allowing the high concentrations of ammonia in the effluent, <br /> the normal effluent acute toxicity test procedures will need to be modified. There are <br /> several possible modifications possible, such as by diluting the effluent and then running the <br /> test, or removing the ammonia and then running the test. All methods of dealing with the <br /> ammonia toxicity will reduce the ability of routine biotoxicity monitoring to detect the <br /> presence of other toxic substances which maybe present in the effluent. <br /> SUMMARY <br /> Manteca currently discharges ammonia in concentrations that violate the biotoxicity <br /> requirements in the NPDES Permit and probably create toxic conditions in the river, at least <br /> under low-river flow conditions. <br /> The proposed project to construct an effluent storage basin and diffuser will allow Manteca to <br /> reduce the existing receiving water impacts without construction of nitrification facilities to <br /> reduce effluent ammonia concentrations, saving the City approximately$9.4 million. <br /> An acute mixing zone would be established which,under worse case conditions, would allow 12 <br /> plumes extending 40 feet downriver from the diffuser ports, and collectively impact 1.4 percent <br /> of the river cross section. This will allow 98.6 percent of the river cross section to be fully <br /> protected, providing a zone of passage for aquatic organisms around the mixing zones. The time <br /> of passage of an organism drifting through the mixing zone is sufficiently short that no lethality <br />