Perchlorate (ClO4-) contaminated water is becoming a wide-spread problem as more sites are being identified worldwide. Biological perchlorate reduction is a promising alternative to conventional physical/chemical treatment processes and has the advantage of reducing perchlorate to the benign products, chloride and oxygen. A number of bacteria are capable of reducing perchlorate using a variety of electron donors including organic carbon compounds, hydrogen, iron, and reduced sulfur compounds. Previous studies in our laboratory successfully used a novel, sulfur oxidizing bacterial consortium (SUPeRB) to reduce perchlorate in both batch culture and in packed bed reactors (PBR). There were two main objectives of this research. The first objective was to construct and operate an ex-situ pilot scale PBR using SUPeRB cultures, with elemental sulfur pellets and crushed oyster shells as a packing material. The second objective was to investigate the role of the oyster shell as a buffer, organic carbon source, adsorbent, and/or attachment site to gain a better understanding of the SUPeRB process.
The first study examined the scale up of a PBR for treatment of water from a perchlorate and RDX contaminated aquifer in Massachusetts with low-level background nitrate levels. The pilot-scale PBR (~250-L) was constructed with elemental sulfur and crushed oyster shell packing media and was inoculated with SUPeRB cultures enriched from a wastewater seed. Sodium sulfite provided a good method of dissolved oxygen removal in batch cultures, but was found to promote the growth of sulfate reducing bacteria, which inhibited perchlorate reduction in the pilot system. After terminating sulfite addition, the PBR successfully removed 96% of the influent perchlorate in the groundwater at an empty bed contact time (EBCT) of 12 hours (effluent perchlorate of 4.2 µg L-1 ). Simultaneous perchlorate and nitrate degradation was observed in the lower half of the reactor before reactions shifted to sulfur disproportionation. Analyses of water quality profiles were supported by molecular analysis showing distinct groupings of perchlorate and nitrate degrading organisms in the bottom of the PBR, while sulfur disproportionation was the primary biological process occurring in the top of the reactor.
The use of crushed oyster shells as an alkalinity source in the SUPeRB process was found to enhance perchlorate degradation. The second study examined the role of oyster shells as a buffer, organic carbon source, attachment site, and adsorbent in the SUPeRB process. Perchlorate degradation was monitored in microcosms comparing the base case (sulfur and oyster shells) to systematic variations. The necessity for direct microbial attachment was examined by isolating sulfur pellets, oyster shells, or bacteria from the culture using membranes. The oyster shell maintained a favorable pH for perchlorate reduction (k=23.7 day-1 g protein-1 ), but this could not completely explain the enhanced perchlorate reduction rates. SUPeRB cultures were found to be capable of mixotrophic metabolism, which increased rates of perchlorate reduction fivefold. Heating oyster shells impaired perchlorate degradation due to the diminished availability of organic carbon for cellular synthesis. Oyster shells reduced bacterial toxicity, possibly by hydrogen sulfide adsorption. The necessity for direct microbial attachment to the solid oyster shell matrix was unclear, though proximity to oyster shells was more important than to sulfur.