2018-11-05_PERMIT FILE - C1981044 (12)

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• COLORADO DEPARTMENT OF HEALTH Water Quality Control Division Rationale - Page 9 Permit No. CO -0034142 APPLICABLE RULES AND REGULATIONS: pH: (Continued) The second part of the mixed pH determination is to find the total carbonate carbon of the mix from a graph relating total alkalinity, pH, and total carbonate carbon was read from this graph for the respective upstream pH (8.4 s.u.) and alkalinity, and the maximum assumed effluent pH (9.7 s.u.) and alkalinity values. After determination of the upstream and effluent discharge total carbonate values, the following mass balance equation was used to determine the total carbonate carbon value of the mixture: (CTu)(Qu) + (CTd)(Qd) - CT mix Qu + Qd The final portion of determining the pH mixture value is by using the above discussed graph, and finding the pH mix from total alkalinity mix • and total carbonate carbon mix values. In all cases (i.e., with the upstream alkalinity value of 70.3 mg/l as CaCO3), and with both assumed WWTF alkalinity values of 500 and 700 mg/l as CaC031 the pH mixture was below 9.0 s.u., somewhere in the range of 8.2-8.5 s.u. Thus this indicates that the stream pH value is not affected by the facility's discharge pH contribution, since 8.42 was the median upstream pH value. Thus, an effluent limitation for pH of 6.0 to 9.7 s.u. can be assigned while still maintaining stream standards. Effluent pH limitations of 6.0 to 9.7 s.u. will thus allow protection of the existing uses of the Williams Fork River. Since no chemical treatment processes are employed for this domestic facility (chlorination is excluded from this category), a deviation from the secondary treatment standards of 6.0 to 9.0 s.u. for pH is allowable, provided that the water quality standards are not violated. (This rationale was drafted by Don Holmer in late 1983 and 1984). References 1. U.S. Environmental Protection Agency. 1982. Wasteload Allocation Draft Guidance on Performing Ammonia Toxicity Analyses. Washington, D.C. •