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Attachment C Page 4 of B <br /> SIGMA CIRCUITS Inc . , CITATION DIVISION <br /> FIELD SITE <br /> TREATMENT UNIT OPERATION <br /> UNIT 1 ( Continued ) <br /> Process: <br /> A) Wastewater from the process is collected in the WRT. <br /> In addition , the WRT also receives metal-bearing <br /> filtrate from the filter press in this system and <br /> clarified waste for rework . <br /> Sodium hydroxide ( caustic soda) is added to raise the <br /> pH to about 9. A special metal complexant / <br /> precipitant is also added to this tank . Small amounts <br /> of sodium sulfide are also added to aid in metal <br /> removal from solution . <br /> Initial precipitation of the dissolved metals of <br /> concern in the wastewater, notably copper, lead , and <br /> nickel , occurs in this tank . Retention time is <br /> normally 15-20 minutes, depending on the flow rate into <br /> the tank . <br /> B) The slurry in the WRT, containing some precipitated <br /> metals, is then pumped to the second treatment tank , <br /> termed the Metal Mixing Tank (MMT) . A liquid cationic <br /> polymer is added to this tank . During the residence <br /> time in the MMT, about 45-50 minutes at 20 gpm, this <br /> polymer acts as a coagulant and leads to "pin floc" <br /> formation , or small , BB-sized agglomerates in this <br /> tank . <br /> C) The slurry from the MMT then flows out the side of the <br /> tank to the first flocculation tank at the top where <br /> additional mixing of the slurry occurs. Flocculent is <br /> added to the top of this tank . Residence time is about <br /> 30-35 minutes, depending on the flow. <br />