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/ ITEM: 15 <br /> SUBJECT: City of Manteca and City of Lathrop,Wastewater Quality Control Facility, San <br /> Joaquin County—Consideration of Revision of NPDES Permit <br /> DISCUSSION: The City of Manteca owns and operates the Wastewater Quality Control <br /> Facility that receives wastewater flows from the City of Manteca and some <br /> areas of the City of Lathrop. The City of Manteca currently discharges 5.3 mgd <br /> of treated domestic and industrial wastewater to 370 acres of farmland for <br /> irrigation adjacent to the treatment plant.Excess treated wastewater flows are <br /> discharged to the San Joaquin River. The City of Manteca has requested <br /> issuance of a revised NPDES permit which establishes a mixing zone for <br /> ammonia toxicity to resolve current biotoxicity violations at the treatment <br /> plant. <br /> Since the adoption of Order 97-115 in June 1997,the Discharger had violated <br /> effluent bioassay requirements. On 24 July 1998,the Regional Board adopted <br /> Cease and Desist Order No. 98-169 to investigate and implement methods to <br /> bring the discharge into compliance. The discharger began operating the plant <br /> in partial nitrification mode;however,modifications to plant operation alone <br /> did not produce continual compliance with Order 97-115. On 30 April 1999, <br /> the Regional Board adopted Cease and Desist Order No. 99-051 to identify and <br /> implement changes at the wastewater treatment plant to ensure reliable <br /> compliance. The Cease and Desist Order aclmowledged the City proposed to <br /> determine the extent of the current acute mixing zone for ammonia in their <br /> discharge. <br /> The City performed bioassay tests on receiving water collected immediately <br /> downstream from the point of discharge. These tests showed no ammonia <br /> toxicity from December 1998 through May 1999. On 9 July 1999,the City <br /> specifically requested a permit revision allowing dilution of the toxic effects of <br /> ammonia in their effluent with a mixing zone in the receiving water. The City <br /> claimed diluted samples more accurately represent the actual conditions in the <br /> receiving water. Using a computer model,the City predicted the acute and <br /> chronic mixing zones would be minimized with an effluent discharge diffuser <br /> and effluent storage basins. At worst case flow conditions,the acute and <br /> chronic mixing zones were predicted to be 2-3 %of the cross sectional area of <br /> the river at the maximum width of the mixing zone. <br /> The proposed revision to the NPDES Order requires the construction of a <br /> diffuser and includes discharge prohibitions during low river velocity periods <br /> to ensure aquatic life will be fully protected. Revisions to the effluent <br /> monitoring and reporting program are limited to acute effluent toxicity <br /> requirements,effluent ammonia limits and effluent monitoring. <br /> RECOMMENDATION: Receipt and review of comments on the proposed permit is not complete. A <br /> ���� / recommendation will be made at the Board meeting. <br /> Mgmt Review �h d <br /> Legal Review FLM <br />