1 Impact of Fluoride on Microorganisms in Wastewater Treatment Chandra Khatri, Valeria Ochoa and Reyes Sierra-Alvarez Department of Chemical and Environmental Engineering, The University of Arizona April 19 th, 2008

2 Background Fluoride Fluoride is a common contaminant in effluents from semiconductor manufacturing and other industrial processes. Maximum contaminant level in water is 4 mg/L and drinking water standard is 2 mg/L Fluoride has an inhibitory effect on cell growth and fermentation of oral microorganisms (IC 50 = mg/L) Fluoride is only lethal to aquatic organisms at higher concentrations (IC 50 = mg/L)

3 Background Wastewater Treatment Plants Biological processes are commonly applied in the treatment of wastewaters. Anaerobic sludge digestion is used to  stabilize excess sewage  improve sludge dewatering  produce methane There is almost no data on the toxicity of fluoride to anaerobic microorganisms in wastewater treatment systems.

4 Wastewater Treatment Plants Main microbial populations

55 Reactions involved Methanogenic reactions: Complex Organics Organic Acids (eg. acetate, propionate, butyrate) CH 4 + CO 2 Hydrolysis & Fermentation Methane Formation CH 3 COOH CH 4 + CO 2 CO 2 + H 2 CH 4 AcetateMethaneCarbon dioxide Carbon dioxide HydrogenMethane autotropic methanogenesis acetoclastic methanogenesis

6 Experimental Objective The main objective of this study is to monitor effect of fluoride on anaerobic microorganisms in the sludge digester process

7 Experimental Method Setup Medium with nutrients/minerals Substrates: propionate and butyrate Inoculum (Aviko sludge) Headspace N 2 /CO 2 (80/20) or Air Controls Medium lacking fluoride Treatment Medium with increasing fluoride conc. (triplicates) Assays/Tests Gas chromatography was used to measure the production of methane and VFA degradation Controls (― F)Treatments (+ F)

8 Results Methane production & VFA degradation

9 Results (continued…) Propionate IC 50 = 37 mg/L Butyrate IC 50 = 26 mg/L

10 Conclusion/Discussion Fluoride does have a significant inhibitory effect on anaerobic bacteria involved in wastewater treatment processes. IC 50 for VFA utilizing anaerobic bacteria was low. Fluoride conc. should be reduced to sub-toxic levels to prevent microbial inhibition (eg. precipitation with Ca 2+, dilution with fluoride-free effluent streams).

11 Acknowledgments Valeria Ochoa Dr. Reyes Sierra and Dr. Jim Field SRC/Sematech Engineering Research Center for Environmentally Benign Semiconductor Manufacturing (ERC) Many of the other graduate students and faculty in the lab