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Impact of Fluoride on Impact of Fluoride on Microorganisms in Microorganisms in
Wastewater TreatmentWastewater Treatment
Chandra Khatri, Valeria Ochoa and Reyes Sierra-Alvarez
Department of Chemical and Environmental Engineering, The
University of Arizona
April 19th, 2008
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BackgroundBackgroundFluorideFluoride
• 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 (IC50 = 5-1600 mg/L)
• Fluoride is only lethal to aquatic organisms at higher concentrations (IC50
= 100-800 mg/L)
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BackgroundBackgroundWastewater Treatment PlantsWastewater 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.
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Wastewater Treatment PlantsWastewater Treatment PlantsMain microbial populationsMain microbial populations
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Reactions involvedReactions involved
Methanogenic reactions:
ComplexOrganics
Organic Acids(eg. acetate, propionate, butyrate)
CH4 + CO2
Hydrolysis & Fermentation
MethaneFormation
CH3COOH CH4 + CO2
CO2 + H2 CH4
Acetate Methane Carbon dioxide
Carbon dioxide
Hydrogen Methane
autotropicmethanogenesis
acetoclasticmethanogenesis
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Experimental ObjectiveExperimental Objective
The main objective of this study is to monitor effect of fluoride on anaerobic microorganisms in the sludge digester process
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Experimental MethodExperimental MethodSetup
• Medium with nutrients/minerals
• Substrates: propionate and butyrate
• Inoculum (Aviko sludge)
• Headspace N2/CO2 (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)
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ResultsResultsMethane production & VFA Methane production & VFA
degradationdegradationToxicity of fluoride - propionate as substrate
0
500
1000
1500
2000
2500
0 50 100 150 200 250 300 350 400 450
Time (h)
mg
CO
D/L
Control 5 ppm F 15 ppm F 35 ppm F
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Results (continued…)Results (continued…)
Propionate
IC50 = 37 mg/L
Propionate
0
20
40
60
80
100
0 100 200 300 400 500 600
Fluoride concentration (mg/L)
Acti
vit
y (
%)
Butyrate
0
20
40
60
80
100
0 100 200 300 400 500 600
Fluoride concentration (mg/L)
Acti
vit
y (
%)
Butyrate
IC50 = 26 mg/L
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Conclusion/DiscussionConclusion/Discussion
• Fluoride does have a significant inhibitory effect on anaerobic bacteria involved in wastewater treatment processes.
• IC50 for VFA utilizing anaerobic bacteria was low.
• Fluoride conc. should be reduced to sub-toxic levels to prevent microbial inhibition (eg. precipitation with Ca2+, dilution with fluoride-free effluent streams).
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AcknowledgmentsAcknowledgments
• 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