Salt Conservation, Selectivity Reversal and Breakthrough Detection in Ion Exchange for Nitrate Removal

Abstract

A single-bed ion-exchange process for nitrate removal from water supplies has been studied extensively in the laboratory and in the field. In the conventional mode, nitrate removal is achieved by passing a portion of the feedwater through a fixed bed of chloride-form, strong-base anion resin, and blending the nitrate-free effluent with raw water to produce a blended product water with less than 10 mg/L NO₃-N.

The theoretical and actual effects of sulfate concentration, sulfate/nitrate selectivity, resin type and empty bed contact time were studied during the exhaustion cycle. During pilot-scale field tests in Glendale, Arizona it was found that the end of a run could be reliably determined by bed volumes throughput or by the pH increase (0.6-1.0 pH) which accompanied nitrate breakthrough.

Regeneration was accomplished using NaCl but was not particularly efficient. In fact, for complete regeneration (3.5 eq Cl^-/eq resin), 10 eq of Cl^- were typically required for each eq of nitrate removed from the feedwater. It was discovered that substantial savings in NaCl could be achieved by using dilute (0.25-0.50 N NaCl) regenerants which minimize the effect of electroselectivity reversal on sulfate elution. Also, partial regeneration (1.0 eq Cl^-/eq resin) followed by 7.0 mg/L NO₃-N leakage during exhaustion (using a 20 mg/L NO₃-N feedwater) was found to be feasible and less costly than complete regeneration with zero leakage and bypass blending.

A 4000 m³/day (blended product water) treatment plant to reduce nitrate from 21 mg/L to less than 10 mg/1 in a 600 mg/L TDS background containing 100 mg/L So ^2-₄ would cost about $300,000. Operating costs including capital recovery would be approximately 0.07 $/m³ of product water.