Process Control Factors for Continuous Microbial Perchlorate Reduction in the Presence of Zero-Valent Iron
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Water is a staple of civilization, and in the event of drinking water contamination the use of the contaminant’s source should be discontinued. Alternatively, a technology that can remove the contaminant from the water must be developed. Perchlorate (ClO4-) is a byproduct of munitions, and pyrotechnics, and has been detected in water sources throughout the United States. It is unlikely that the use of perchlorate will be discontinued as it is linked to the integrity of national security. Due to the toxicity to human health, the United States Environmental Protection Agency (US EPA) announced in February 2011 that perchlorate will be federally regulated. It is expected that the maximum contaminant level (MCL) could be 1 ppb (µg/L). Therefore, it is necessary to develop a safe and inexpensive technology that is capable of completely removing the contaminant. Technologies for the perchlorate removal include: ion exchange, activated carbon adsorption, chemical reduction, and microbial reduction. Several studies demonstrated that zero-valent iron (ZVI) can be used as an electron donor for the microbial perchlorate reduction process. The core of our research approach is on the use of ZVI and mixed microbial culture. Process control parameters influencing microbial perchlorate reduction by a flow-through ZVI column reactor were investigated in order to optimize perchlorate removal in water. Mixed perchlorate reducers were obtained from a wastewater treatment plant (aerobic activated sludge and anaerobic digester) and inoculated into the reactor without further acclimation. Examined parameters include; hydraulic retention time (HRT), pH, nutrient requirement, and both chemical and microbial kinetics. The minimum HRT required for our system that can completely reduce 10 mg/L of perchlorate was 8 hours. Perchlorate removal was reduced by 60% without pH control. As pH was determined to be an important parameter for microbial perchlorate reduction, a viable alternative of pH buffer is also discussed. Unlike other systems that used laboratory cultured microorganisms, our system needed no additional nutrients for the complete reduction of 10 mg/L of perchlorate in water. This is likely due to the plethora of nutrients available within activated sludge based seed cultures. The perchlorate reduction reaction follows the first order kinetics, with an average rate constant (K̅) of 0.761 hr-1. The microbial growth in the column follows the Monod growth kinetics. The average maximum growth rate (µ̅max) and the average half saturation constant (K̅s) were determined to be 0.55 hr-1 and 15.4 mg/L, respectively. Also, a numerical model using Monod kinetics, transport, and attachment and detachment was used to verify the experimental result pertaining to the microbial growth kinetics in the ZVI supported perchlorate reducing column system.