Perchlorate reduction using salt-tolerant cultures

Abstract

The wide use of ion-exchange processes to remove perchlorate from drinking water creates an urgency for the regeneration or treatment of perchlorate-laden ion-exchange resins and/or regenerant brines. The use of biological processes with a salt-tolerant culture NP30 has been demonstrated as a promising cost-effective approach. In this study, the kinetics and ecology of NP30 were studied. A pure culture was isolated from the mixed culture, identified and characterized. Perchlorate–laden ion-exchange resins were effectively regenerated by the mixed culture in laboratory batch reactors. A numerical model was developed to describe the regeneration process and for design predictions. A unique “resin phase” regeneration, in which the culture degraded perchlorate on the resin instead of only what desorbed into the bulk medium, was proposed in the model. The model generated an acceptable correlation to experimental data and the degradation from the “resin phase” accounted for the majority of the perchlorate removal. The microbial composition of NP30 and the changes during a pilot plant experiment treating perchlorate- and nitrate-laden ion-exchange brine were analyzed using DGGE (denaturing gradient gel electrophoresis) and FISH (fluorescence in situ hybridization). Halomonas was the dominant (>18%) nitrate-reducing organism and Azoarcus/Denitromonas was the dominant (>22%) perchlorate-reducing organism. A shift towards nitrate-reducing organisms with time in the reactors was observed and attributed to the non-obvious perchlorate reduction seen in operation data. A pure salt-tolerant, perchlorate-reducing strain P4B1 (Marinobacter multirespiro sp. nov. proposed name) was successfully isolated from the mixed culture. P4B1 could grow in the presence of 1.8%-10.2% NaCl. A molar Mg²⁺/Na⁺ ratio of ~0.11 optimized the perchlorate degradation and cell growth when perchlorate was the sole electron acceptor. It could use perchlorate, nitrate and oxygen as electron acceptors. P4B1 preferred perchlorate to nitrate as the electron acceptor. A perchlorate reductase, which is only induced by perchlorate, is active in both perchlorate and nitrate reduction. When nitrate was used as the sole electron acceptor, the strain eventually lost the ability to reduce nitrate. The maximum specific substrate utilization rate (Vm) and the half saturation coefficient (Ks) for P4B1 were determined to be 0.050 ±0.007 mg ClO₄⁻/mg VSS-hr and 22±12 mg ClO₄⁻/L respectively.