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
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
4.l.2 Methods <strong>for</strong> Estimating Residuals H<strong>and</strong>ling <strong>and</strong> Disposal <strong>Costs</strong>Residuals h<strong>and</strong>ling <strong>and</strong> disposal costs can be difficult to estimate. There are a number <strong>of</strong>factors which affect capital <strong>and</strong> O&M costs, <strong>and</strong> disposal costs can be largely regional. EPA haspublished two manuals <strong>for</strong> estimating residuals h<strong>and</strong>ling <strong>and</strong> disposal costs; Small <strong>Water</strong> SystemByproducts Treatment <strong>and</strong> Disposal Cost Document (DPRA, 1993a), <strong>and</strong> <strong>Water</strong> System ByproductsTreatment <strong>and</strong> Disposal Cost Document (DPRA, 1993b). Both present a variety <strong>of</strong> h<strong>and</strong>ling <strong>and</strong>disposal options, applications <strong>and</strong> limitations <strong>of</strong> those technologies, <strong>and</strong> capital <strong>and</strong> O&M costequations.4.2 RESIDUALS HANDLING OPTIONS4.2.1 Gravity ThickeningGravity thickening increases the solids content <strong>of</strong> filter backwash, sedimentation basins, <strong>and</strong>treatment process sludges. It is generally used as a first step in residuals processing, precedingmechanical dewatering processes, evaporation ponds, <strong>and</strong>/or storage lagoons.Filter backwash streams are high volume, low solids slurries generated during the cleaning<strong>of</strong> granular filter media. Backwash volume depends on the number <strong>of</strong> filters used by a system <strong>and</strong> thefrequency with which they are cleaned. Typical backwash volumes range from 0.5 to 5 percent <strong>of</strong> theprocessed water flow with larger plants creating less backwash per million gallons produced thansmall systems due to increased plant efficiency (DPRA, 1993a). Backwash waters have an averagesolids concentration <strong>of</strong> 0.8 percent, compared to coagulation sludges which are typically characterizedby an average solids concentration <strong>of</strong> 0.5 to 2.0 percent (DPRA, 1993a).When possible, backwash waters are recycled within the treatment process. In gravitythickening, backwash waters are fed to a tank where settling occurs naturally. The sludges resultingfrom gravity thickening are discharged <strong>and</strong> further treated <strong>for</strong> ultimate disposal, <strong>and</strong> the decant iseither recycled or discharged to a surface water or to a publicly-owned treatment works (POTW).Gravity thickening reduces the quantity <strong>of</strong> water sent to waste due to backwashing, as well as the totalquantity <strong>of</strong> sludge generated (DPRA, 1993a). When recycling is not feasible, backwash waters maybe discharged to a surface water or a POTW, or may be treated by other mechanical or non-4-2
Change language