Technologies and Costs for Removal of Arsenic From Drinking Water

solution strength. Arsenic elutes readily from IX columns, regardless of resin type, mainly becauseit is a divalent ion and as such is subject to selectivity reversal in high ionic strength (> 1M) solution(Clifford and Lin, 1995). Clifford and Lin also found that dilute regenerants tend to be more efficientthan concentrated regenerants in terms of the ratio of regenerant equivalents to resin equivalents. Forexample, they found that two resins (Dowex-11 and Ionac ASB-2) could be regenerated equivalentlyusing either 2 BV of 1.0 N NaCl or 5 BV of 0.25 N NaCl in “co-current flow” operation. Also, arinsing cycle is required after regeneration; typically only a few BV are required for rinsing as well.2.4.9 Regenerant Reuse and TreatmentSpent regenerant is produced during IX bed regeneration. Typically this spent regenerant willhave high concentrations of arsenic and other sorbed contaminants. Spent regenerant must be treatedand/or disposed of appropriately. Spent regenerant may be reused many times. Clifford and others(1998) estimate that regenerants may be used 25 times or more before treatment and disposal arerequired. Regenerants do not need treatment prior to reuse, except to replenish the chlorideconcentration to maintain a 1 M solution. Once the contaminant concentration becomes too high in theregenerant, the spent solution must be treated and/or disposed.Spent brine can be treated by precipitation. Clifford and Lin (1995) have shown that arseniclevels can be substantially reduced using iron and aluminum coagulants as well as lime. Much greaterthan the stoichiometric amounts (up to 20 times as much), however, are needed in actual practice toreduce arsenic to low levels. In addition, pH adjustment may be necessary to ensure optimumcoagulation conditions. Reductions from 90 mg As(V)/L to less than 1.5 mg As(V)/L have been seenusing iron and aluminum metal salts (Clifford and Lin, 1995). Both coagulant types seem to workwell, however, iron precipitates tend to settle better due to their weight. Dried sludge from brinereduced to 1.5 mg As(V)/L using precipitation passed an EP toxicity test with only 1.5 mg/L As(V)in the leachate. In this situation, dried sludge could have been disposed of in a landfill. The problemwith this option is that the brine stream prior to chemical precipitation could be considered hazardouswaste. As discussed in Chapter 4, brine streams with arsenic concentrations greater than 5 mg/L canbe considered hazardous waste. Since arsenic will be even more concentrated in a brine reusescenario, it is unlikely to be used unless the brine could be discharged to the POTW. The TDS contentof the brine may restrict that option.2-25