Technologies and Costs for Removal of Arsenic From Drinking Water
Technologies and Costs for Removal of Arsenic From Drinking Water
Technologies and Costs for Removal of Arsenic From Drinking Water
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The solids content <strong>of</strong> the backwash from coagulation assisted micr<strong>of</strong>iltration processes were foundto be less than 0.5 percent in one study (Clif<strong>for</strong>d et al, 1997).Selection <strong>of</strong> H<strong>and</strong>ling <strong>and</strong> Disposal OptionsThe wastes from coagulation assisted micr<strong>of</strong>iltration processes will consist <strong>of</strong> a very diluteslurry. Gravity thickening may be used as to increase the solids content <strong>of</strong> the sludge prior to the use<strong>of</strong> other mechanical or non-mechanical dewatering options. Filter presses <strong>and</strong> centrifuges areappropriate methods <strong>of</strong> residuals h<strong>and</strong>ling. However, these methods are capital intensive <strong>and</strong> maynot be appropriate <strong>for</strong> extremely large systems. Evaporation ponds <strong>and</strong> storage lagoons are alsoappropriate means <strong>of</strong> h<strong>and</strong>ling the residuals generated by coagulation assisted micr<strong>of</strong>iltration. Bothrequire little oversight <strong>and</strong> maintenance, but are l<strong>and</strong> intensive. As such, they may not be appropriate<strong>for</strong> large systems. A thorough comparison <strong>of</strong> h<strong>and</strong>ling options should be conducted to select the mostcost effective method.L<strong>and</strong> application may be a suitable disposal method <strong>for</strong> sludges from coagulation assistedmicr<strong>of</strong>iltration processes. As discussed in section 4.3.2, total arsenic cannot exceed 41 mg/kg ifsludges are to be applied with no restrictions. Sludges with arsenic concentrations between 41 <strong>and</strong>75 mg/kg may be l<strong>and</strong> applied provided that the total loading does not exceed 41 kg per hectare.All coagulation assisted micr<strong>of</strong>iltration sludges must be dewatered prior to l<strong>and</strong>fill disposal.If the residuals pass the TCLP test they may be disposed in a sanitary l<strong>and</strong>fill. Otherwise, residualsmust be disposed in a hazardous waste l<strong>and</strong>fill. However, tests conducted by Clif<strong>for</strong>d (Clif<strong>for</strong>d,1997) <strong>and</strong> the University <strong>of</strong> Colorado (AWWARF, 1998) indicate that the sludges from this treatmentprocess will pass the TCLP test <strong>for</strong> arsenic toxicity by a considerable margin, making it unlikely thathazardous waste disposal will be necessary. Hazardous waste l<strong>and</strong>fill disposal should only be usedas a last resort if waste fails the TCLP test.4.4.5 Lime S<strong>of</strong>teningThe quantity <strong>of</strong> residuals produced at LS facilities is typically much greater than the quantityproduced by C/F plants (AWWARF, 1998). The quantity <strong>of</strong> sludges produced is a function <strong>of</strong> waterhardness. LS <strong>for</strong> carbonate hardness removal produces approximately twice the amount <strong>of</strong> solids perpound <strong>of</strong> hardness removed than the use <strong>of</strong> LS <strong>for</strong> the removal <strong>of</strong> non-carbonate hardness.LS plants typically produce 1,000 to 8,000 pounds <strong>of</strong> solids per million gallons <strong>of</strong> watertreated depending on the hardness <strong>of</strong> the influent water (AWWARF, 1998). <strong>Arsenic</strong> concentrations4-15
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