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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