Technologies and Costs for Removal of Arsenic From Drinking Water

following dewatering, the dry sludge from this plant could be removed and applied to local farm fieldsas a beneficial amendment.4.4.7 Ion ExchangeIon exchange (IX) involves the use of a synthetic resin in the chloride form for arsenicremoval. With time, the efficiency of the resin is reduced as exchange sites are depleted. The IX resincan be regenerated using a NaCl solution. The regenerant is added at a rate of approximately 2equivalents chloride per equivalent of resin, i.e., 10.2 pounds of salt per cubic foot of resin.Regeneration requires approximately 2.8 bed volumes (BV) of brine and 1.2 BV displacement rinse.Therefore, 4 to 5 BV of waste are produced per regeneration cycle. (AWWARF, 1998). The rinsevolume in another study was 5 BV (Clifford et al, 1997). Using this as an upper bound, the wastevolume could be as high as 7.8 BV.The other important factor is the arsenic concentration in the waste brine. An EPA study (EPAOctober 2000) which found that arsenic concentrations during brine regeneration ranged from 1.83to 38.5 mg/L (average: 16.5 mg/L). The wastewater generated during the other three steps of theregeneration process (i.e., backwash, slow rinse, and fast rinse) evaluated by this study werecharacterized by lower arsenic concentrations (0.0594, 1.332, and 0.108 mg/L, respectively).Therefore, if these waste streams were combined prior to disposal, the overall arsenic concentrationof the brine would be below that observed during the brine regeneration step.Selection of Handling and Disposal OptionsThe November 1999 Technology and Cost Document listed three mechanisms to dispose ofthe brine stream used for regeneration. The options were: sanitary sewer, evaporation pond, andchemical precipitation. Many comments on the proposed rule were considered with waste streamsbeing considered hazardous waste. Waste streams containing less than 0.5% solids are evaluatedagainst the toxicity characteristic directly to determine if the waste is hazardous. Arsenic in theregeneration brine will likely exceed 5 mg/L for most systems with arsenic above 10 Fg/L and sulfatebelow 50 mg/L. Since the brine stream would likely be considered hazardous, the evaporation pondand the chemical precipitation options were eliminated as options for disposal of anion exchangewastes. Discharge to a sanitary sewer was retained because domestic sewage and any mixture ofdomestic sewage and other wastes that pass through a sewer system to a publicly-owned treatmentworks (POTW) for treatment is excluded from being considered solid waste (40 CFR 261.4).4-18