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General Minimal Discharge Permit Rationale <br />Pernut No. COG600000 <br />Page 11 <br />V. TERMS AND CONDITIONS OF PERMIT (wm.) <br />B. Water Quality Standard Based Efl9uent Limitations <br />1. Total Residual Chlorine: The mass balance equation is: <br />M= _ <br />MaQr ' MiQ~ <br />Qz <br />A'here: Q, =Chronic low flow (30E3), MGD or Acute low flow IE3, MGD. <br />Q~ = 30 day average or daily maximum e~Iuent flow, MGD. <br />Q, =Combined downstream fiow (Q, + Q~. <br />M, = 7RC, Bromine, (mg/P) or Fecal Coliform (q/IOOm[) ambient instream <br />concentration. <br />M7 = TRC, Bromine, (mg/P) or Fecal Coliform (it/100 ml) effluent <br />limitation. <br />Mj =Water Quality Standard for TRC, Bromine, (mg/P) or Fecal Colifotm <br />(Ji/100 ml). See rationale Pan V.B.I. (lRC), Part V.D.3. (Bromine), <br />or Part V.B.2. (F.Coli.). <br />For discharges to waters not classified as Use Protected (waters subiect to anriderradation). <br />the chlorine limits, set in the method described above, are set at the maximum level ar which <br />further review is not necessary. The ejjluem limitation necessary to ensure compliance with <br />the instream level of 0.0054 mg/P (ague), and 0.0042 mg/P (rronic), is determined from the <br />previous mass balance equation using the following numbers for M~ : WQS for 1RC, 0.0054 <br />mg/P (acute), 0.0042 mg/P (chronic). <br />The TRC limitations have been based on the following assumption. In order for the discharge <br />to be "considered nor to result in signi, figW degradation', the amount of assimilative <br />capacity allocated to the discharge must be limited through segion 3.1.8. (3) (c) (iii) of "The <br />Basic Standards" (Ref. A). This section sped, fees that the increased loading consumed, after <br />mixing with the receiving stream, must be less than 15'% of the available inaemenr between <br />the !ow flow pollutant concentrations and the relevant standards. Assuming the low flow <br />pollutant concentrations are currently equal to the previous stream standard of 0.003 mg/P, <br />as allowed in the general permit, IS% of the increment between 0.003 mg/P and the new <br />(acute) standard of 0.019 mg/P is equal to 0.15 ^ (0.019-0.003) = 0.0024. Therefore, the <br />new ague (daily maximum) permit limitation will be calgdated using the mass balance <br />equation and a maximum allowable instream concentration of 0.003+0.0024 = 0.0054 mg/P. <br />For chronic conditions, the corresponding maximum allowable instream concentration is <br />0.0042 mg/P (30-day Average), using the same method of calculation as the ague. <br />