Technologies and Costs for Removal of Arsenic From Drinking Water

mechanical dewatering processes. When backwash slurries cannot be recycled or discharged to asurface water or POTW, they must be treated and disposed.4.2.2 Mechanical DewateringMechanical dewatering processes include centrifuges, vacuum-assisted dewatering beds, beltfilter presses, and plate and frame filter presses (DPRA, 1993a). Such processes generally have highcapital and high O&M costs, compared to non-mechanical dewatering processes that can handlesimilar waste volumes, e.g., storage lagoons. Due to their high costs, mechanical dewateringprocesses are generally not suitable for application at very small water systems.Filter presses have been used in industrial processes for decades, and their use has beenincreasing in the water treatment industry over the past several years. These devices have beensuccessfully applied for the treatment of both lime and alum sludges. Prior to pressure filtration, alumsludges may require the addition of lime to lower the resistance of the sludge to filtration. This isgenerally done by adjusting the pH to approximately 11. Pre-conditioning may increase sludge volumeby as much as 20 to 30 percent. Lime sludges can attain final solids concentrations of 40 to 70percent, while alum sludges may reach 35 to 50 percent total solids. Filter presses require little land,but have high capital costs and are labor intensive (DPRA, 1993a). Capital and O&M costs aregenerally higher than those of comparable non-mechanical dewatering alternatives. As a result,pressure filtration is most applicable for larger water systems.Centrifuges have also been used in the water industry for years. They are capable of producingalum sludges with final solids concentrations of 15 to 30 percent and lime sludges with 65 to 70percent total solids, based on an influent solids concentration of 1 to 10 percent. Centrifugation is acontinuous process that requires very little time (8 to 12 minutes) to achieve optimal sludge solidsconcentration. Centrifuges have low land requirements and high capital costs. They are more laborintensive than non-mechanical alternatives, but less labor-intensive than filter presses. Again, due tothe associated capital and O&M requirements, centrifuges are more suitable for larger water systems.4.2.3 Evaporation Ponds and Drying BedsEvaporation ponds and drying beds are non-mechanical dewatering technologies whereinfavorable climatic conditions are used to dewater waste brines generated by treatment processes suchas reverse osmosis (RO) and ion exchange (IX) (DPRA, 1993a). Ponds and drying beds are notgenerally suitable for dewatering alum and lime sludges.4-3