chlorine dioxide for dbp reduction · 2018. 4. 4. · • chlorine dioxide oxidizes the precursors...

Chlorine Dioxide for DBP Reduction

November 14, 2017Carl Scharfe PE

AWWA 2017 Annual Conference

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Outline• Chlorine Dioxide Background Information• Use for DBPs (THMM) Pre-cursor control• Regulatory Considerations• Chlorine Dioxide Generation at Smithfield WTP• Performance (2016-2017)• Pros/Cons• Acknowledge Dale Boyette, ORC at Smithfield

WTP

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Chlorine Dioxide Basics• Chlorine Dioxide = ClO2• Chlorite = ClO2

-

• Chlorate = ClO3-

• Chlorine dioxide typically converted 50 to 70 percent to chlorite and 30 percent to chlorate in drinking water.

• Oxidizes organic and inorganic compounds (vs addition or substitution as with chlorine).

• Germicidal efficacy: O3 > ClO2 > HOCl > OCL- > NH2Cl• Does not make chloramines when exposed to ammonia.• Chlorine dioxide must be generated on-site.

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Chlorine Dioxide Basics • Noncompressible Yellow Orange Gas• Volatile (~ 10oC Boiling Point)• Freezing Point (-59oC)• Explosive Characteristics

o In air - > 4% by volumeo In aqueous solution - > 10 g/L by itselfo > 3 g/L closed unvented containers (> 25oC)

• Irritating Agent – Threshold Limit 0.1 mg/L.• High concentrations – cough, shortness of breath. • Aqueous Solution Stable in the Dark at 4oC.

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Chlorine Dioxide Basics

Reacts with chlorine/hypochlorous acid to produce chlorate. 2ClO2 + HOCl + H2O 2ClO3- + Cl- + 3H+

Exists as a stable free radical

Reacts with organic/inorganic compounds to produce chlorite.2ClO2 + 2X- 2ClO2

- + X2

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DBPs Precursor Removal• Humic/fulvic substances are precursors to Total

Trihalomethanes (TTHMs) and HAAs.• Chlorine dioxide oxidizes the precursors to make

them inactive for THMM formation later when free chlorine is added.

• So strategy - add chlorine dioxide to raw water and inactivate/block precursors and use as preoxidantbefore adding chlorine.

• High oxidizing power (almost 4 times chlorine) and fast reaction.

• Potential to get 50% to 70% reduction in THMM formation. Potential to get 30 to 50% reduction in HAAs.

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Regulatory Considerations• Chlorite (ClO2

- ) MCL = 1.0 mg/L.• Chlorine dioxide (ClO2) MRDL = 0.8 mg/L (residual goal).• So, practically limited to a low dose (~ 1.0 to 1.5 mg/L).• If using chlorine dioxide need to sample finished water

and distribution system.oOnce/day at entrance to distribution.oMonthly at first customer, average residence time

location and max residence time location.• Need to get DEQ approval to install/operate system• Update risk management plan.

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Chlorine Dioxide Generation Sodium Chlorite & Chlorine Gas (focus of this presentation)NaClO2 + 1/2Cl2 → ClO2 + NaCl

Sodium Chlorite & Hydrochloric Acid5NaClO2 + 4HCl → 4ClO2 + 5NaCl + 2H2O

Sodium Chlorite, Bleach & Hydrochloric Acid2NaClO2 + HOCl + HCl → 2ClO2 + H2O + 2NaCl

Sodium Chlorate & Peroxide NaClO3 + ½ H2O2 + ½ H2SO4→ ClO2 + 1/2Na2SO4 + H2O + 1/2O2

Chlorite & Electricity2 NaClO2 + 2 H2O → 2 ClO2 + 2 NaOH + H2

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Sodium Chlorite + Chlorine Gas• 0.5 gal 25 % Sodium Chlorite + 1-

2 lb Chlorine gas = 1 lb Chlorine Dioxide.

• Water feed 2 to 10 gpm at 100 psi.

• 500 to 3000 ppm CLO2concentration to raw water pipe. pH ~ 3.5 to 4.2

• PVC or CPVC solution feed pipes.• Sodium chlorite line use FEP

(fluorinated ethylene propylene) or teflon tubing. Straight PE degrades quickly.

• Gas monitor and room fan recommended.

• Use 25% solution - little freezing concern.

• Flammable if spilled on organic materials (paper, wood, etc) and allowed to dry. Special fire fighting techniques to avoid steam.

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Smithfield CLO2 (Why?)• Johnston County only buy water if <90% of TTHM

standard (i.e. 72 ug/L or below). In 2013, 4 month period TTHMs high and Smithfield could not sell water.

• 2013 Jar testing = Need 100 to 120 mg/L alum dose & 22 mg/L Caustic to reduce TOC to ~ 2 mg/L. Significant sludge increase would occur.

• Goal: pre-oxidize TOC and reduce TTHM and sludge volume.

• Jar testing by staff showed 30% reduction in THMMs at 1.0 mg/L dose CLO2

• Potential to reduce potassium permanganate feed and chlorine gas in parts of plant.

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Smithfield Retrofit• Added Oxychem II generator using sodium

chlorite and chlorine gas and water feed • Converted existing room at WTP to chlorine

dioxide generation room.• Cleaned room & added floor drain. Added fan,

water line, water booster pump, new outside loading ramp & double doors.

• Added chlorine dioxide gas monitor for safety. • Piped chlorine gas from adjacent chlorine gas

room.• Generator provided at no cost if chemical

purchased from same vendor.• Required ATC from DEQ as this is new process.

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WTP Schematic

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System Operation• System started April 12, 2016. • Raw water average monthly flows 2.36 to 3.76 MGD• Influent TOC 5.5 to 7.82 mg/L.• Permanganate feed stopped. • CLO2 raw water doses 0.75 to 1.45 mg/L. • Generator set for target pH of 3.7 to 4.2.• Alum average doses 38 to 75 mg/L.• Polymer fed into last flocculator at 0.2 mg/L• Pre-chlorine dose (top of filters) reduced 1.1 to 0.7 mg/L• Post chlorine dose (filter effluent) reduced 35%

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Performance

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Performance

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Performance

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Performance

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Pros • TTHM reduction in distribution system and no loss of revenue from Johnston County.

• Reduction in chlorine doses on top of filters and after filters.

• No drop in Fe and Mn removal with chlorine dioxide versus permanganate(~ equal removal).

• Potential future CT credit.• Achieve equal TOC reduction at a lower

alum dose than without chlorine dioxide. Reduces sludge production.

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Cons• Did see some increase in HAA formation at

Smithfield. If chlorine gas is overfed in generator then get lower pH in sedimentation basins and could get more HAAs.

• Need to keep on top of the generator operation for gas and sodium chlorite feed balancing.

• Chlorine dioxide residual can result in taste and odor complaints (>0.7 mg/L)

• Chlorite analysis is very specific and you need to be careful with sample collection and lab analysis.

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Summary

• Chlorine Dioxide was viable option at Smithfield WTP (chlorine gas and spare room available).

• 2017 data 20 to 41% THMM reduction.• Alum doses 50 to 75 mg/L versus 100 or 110 mg/L.• Pursuing potential CT credits now.• May be a viable option for use at your WTP for THMM/HAA reduction.• Generation method is site specific (use gas or hypochlorite?)• Chlorine Dioxide doses of 1.0 to 1.5 mg/L maximum.• Need regulatory approval to add to your system.

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Questions??

• Carl Scharfe, PE919 828 0531cscharfe@thewootencompany.com