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The Settling Tank was sized to hold the sludge generated during the life of the mine and <br />still maintain at least one foot of water over the top of the sludge. The calculated life of <br />mine sludge volume required is 508 CuFt. which is about half the available volume in the <br />Settling Tank. Theoretically, this would result in 2 feet of water and 3 inches of <br />freeboard remaining above the precipitated sludge at the end of the mine life. <br />The Settling Tank volume was also checked to verify that there was adequate retention <br />time to promote settling. Based on bench scale tests of similar mine water effluent, most <br />of the settling occurred within 20 minutes without adding flocculent. To be safe, two <br />hours of retention is recommended. This is equivalent to 2,400 gallons or 320 CuFt of <br />tank capacity at a 20 GPM treatment rate. The Settling Tank has about three times this <br />volume at the start of treatment and is projected to still have about 500 CuFt of capacity <br />after 10 years. <br />Table 3.3 - Settling Tani; xetentnon <br />Retention Time in Settling Tank <br />508.0 CuFt Available Area for Settling <br />3800.0 Gallons in Available Area <br />170-190 Minutes of Retention <br />3.0 Hours of Rentention <br />The Settling Tank is sized to hold all the calculated sludge for the projected 10-year mine <br />life. If the settling time remains sufficient, removal of sludge may not be required until <br />the end of the mine life. However, if the sludge were to build up to the point where the <br />retention time is too short, the sludge would need to be periodically removed and <br />disposed of in accordance with applicable regulations. An inadequate retention time can <br />be identified by the precipitation of sludge in the Polish Tank. <br /> <br />11