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In order to meet the Environmental Protection Agency's and the Colorado Department of Health's water quality standards when the Cash mine is dewatered, the Gold Hill Mining Company has commissioned the construction of a water treatment plant. This water treatment plant was designed by the Environmental Services Division of ENV Incorporated, 716 West 14th Street, Long Beach, California. Using water samples of the Cash mine water, their staff has designed a water treatment plant that will meet the Environmental Protection Agency's and the State of Colorado's limits for discharging mine water into the en- vironment. An application for a National Pollutant Discharge Elimination System Permit to Discharge the Cash mine water will be filed concurrently with thr Colorado Mined Land Reclamation Board Mining and Reclamation Permit. The water will be pumped out of the Cash mine by using four air-operated diaphragm pumps in series. These pumps will de- water the mine workings at a rate of 30 gallons per minute of flow or 43,200 gallons per day over 24 hours. A flow re- stricting valve on the inlet to the first treatment tank will assure that the flow does not exceed 30 gallons per minute and upset the water treatment system. The first tank of the treatment system is designed to precipi- - tate the metals contained in the mine water as insoluble hy- droxides and sulfides. Automatically controlled caustic addi- tion will keep the pH around 8.0 to ~.5. A pH probe in this tank will sense the hydrogen ion (H ) concentration in the tank, and add caustic (NaOH) until the concentration corres- ponds to a pH of 9.5. The metals will fall out as insoluble hydroxides due to the addition of this caustic. Since simple caustic addition is insufficient to remove enough metals to meet the Federal Environmental Protection Agency's and the Water Quality Control Division of the Colorado Depart- ment of Health's water quality standards, sodium hydroxide and sodium sulfide will be added to precipitate the remaining met- als as insoluble sulfides. The addition of this sulfide"pre- cipitation unit will allow this water treatment plant to eas- ily meet the Environmental Protection Agency's and the Colorado Department of Health's limits with at Least a 10 fold safety factor. The sulfide addition will be automatically con- trolled much like the caustic addition. The sulfide probe will sense the sulfide concentration and the unit will add sodium sulfide until the sulfide concentration becomes appre- ciable. A concentration of 0.5 mg/1 will mean that all of the metal sulfides have precipitated, because the previously added sulfides have combined with the metal ions and precipitated them. An increasing sulfide concentration will indicate that there are no more metal ions to precipitate and unused sulfide has built up in solution. • In addition, Dearborn 421 Anionic Polymer and seed sludge will • be added to aid in the formation of large metal hydroxide- . sulfide sludge particles. This will allow the sludges to set- tle more easily, and result in a clearer effluent. A three-compartment settling tank has been designed to give a 45 minute retention time, which will allow the sludge to set- tle to the bottom of. the tank with a 15 minute safety factor. The tank will have hopper bottoms with clear water flowing from one tank to the next through a hole in the baffles, The sludge will be drawn off the bottom of the settling tanks and the clarified water passed to the final tank. In this final tank, another pH control unit using hydrochlorir_ acid will lower the pll to the 6.5-7.0 range. This will lower the pll to within the Environmental Protection Agency's limits for discharge to 5.0-9.D and cause any remaining sulfide to become hydrogen sulfide gas which buhbles out of solution. Tire treated water will go to the, environment and the hydrogen -8-