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Response to Written Comments -32- 24 April 2007 <br /> City of Tracy WWTP <br /> San Joaquin County <br /> STATE WATER CONTRACTORS (SWC) <br /> SWC — NPDES COMMENT #1: In June 2006, the SWC provided comments on the <br /> May 2006 tentative Order and we are encouraged that some of the issues we raised <br /> have been addressed in the revised tentative Order. In particular, we support salinity <br /> options 2 and 3, which will help ensure salinity objectives for the South Delta are met. <br /> We would also like to echo the comments that one or our member agencies, <br /> Metropolitan Water District of Southern California. We share MWD's concern with the <br /> apparent incorrect specification of the interim maximum daily effluent limitations for <br /> ammonia. <br /> RESPONSE: See response to MWD-NPDES COMMENT #1, below. <br /> METROPOLITAN WATER DISTRICT OF SOUTHERN CALIFORNIA (MWD) <br /> MWD — NPDES COMMENT #1: The tentative Order includes interim maximum daily <br /> effluent limitations for ammonia that exceed the historical mean plus 3.3 standard <br /> deviations. In addition, there is neither an average monthly nor average weekly interim <br /> effluent limit for ammonia, and we ask that such limits be included. <br /> RESPONSE: When developing performance-based interim effluent limitations it <br /> is necessary to consider effluent variability. In developing the interim limitation, <br /> where there are ten sampling data points or more, sampling and laboratory <br /> variability is accounted for by establishing interim limits that are based on <br /> normally distributed data where 99.9% of the data points will lie within 3.3 <br /> standard deviations of the mean (Basic Statistical Methods for Engineers and <br /> Scientists, Kennedy and Neville, Harper and Row). However, for ammonia in the <br /> City's discharge, the observed maximum effluent concentration (MEC) was 42 <br /> mg/L. In cases where the observed MEC exceeds the 99.9%, the MEC is used <br /> for the interim limit. The proposed Order has been updated to make this clearer. <br /> The interim effluent limit is not intended to be protective of beneficial uses. The <br /> purpose of the interim limit is to put a ceiling on the discharge. Therefore, it is <br /> not appropriate to include average weekly or average monthly effluent limitations. <br /> Regional Water Quality Control Board, Central Valley Region <br /> Board Meeting — 3/4 May 2007 <br /> Item#17 <br />