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Treatment of Biofilm in Cooling

Water Systems with Chlorine DioxideDr. Simone Schulte

Kylvattenkonferensen

08 September 2011

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CONTENT

• Biofilm

- Biofilm ecology

- Biofilm in cooling water environment

• Chlorine Dioxide

- Properties

- Application in cooling water systems

• Case History

• Summary

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Biofilm in the Natural Environment

Biofilm Traffic Sign

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Biofilm at Home

(U. Szewzyk, TU Berlin)

(M. Exner, Uni Bonn)(H.-C. Flemming, Uni Duisburg-Essen)

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Biofilm in Drinking Water Outlets

Shut-off valve Rust tubercle in 10 year old drinking water pipe

99 year old cast iron pipe28 year old PVC Pipe

(H.-C. Flemming, Uni Duisburg-Essen)

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Biofilm in Industrial Water Systems

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Negative Effect of Biofilm in Cooling Water

Systems

• Mechanical blocking of piping, heat exchangers

and cooling tower packages with living or dead

biomass

• Loss of heat transfer due to build-up of insulation

layer

• Loss of cooling tower performance

• Microbiologically induced/enforced corrosion

• Mechanical destruction of cooling towers

• Loss of quality in directly cooled products

• Hygiene risk of releasing pathogenic organisms

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Biofilm in Cooling Water Systems

• Inner surface of pipes

• Surface of heat exchangers

• Packages

• Drift eliminator

• Lime scale, corrosion products

• Sludge, sediments

from: Fembacher

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Biofilm Can Be Found Wherever Water Is

1. Sea ice, permafrost: cold-loving microorganisms (Psychrophiles,

< -12°C)

2. Deep sea vent: thermophiles, hyperthermophiles (> +125°C),

barophiles

3. Sulfuric spring: acid-loving microbes (acidophiles, pH < 1)

4. Salt lake: salt-loving microbes (halophiles, almost saturated salt

solutions)

5. Soda lake: alkali-loving microbes (alkalophiles, pH 13)

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• A biofilm is an aggregate of microorganisms

(bacteria) in which cells adhere to each other on a

surface. These adherent cells are frequently

embedded within a self-produced matrix of

extracellular polymeric substances (EPS)

• Biofilm content:

- 95% water - biofilm total

- 95% EPS - of dry content

- Microorganisms and also higher life forms

- Particles (inorganic, organic)

• Scientific word for slime

What is Biofilm?

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What is EPS?

• Organic macromolecules

• Often polysaccharides and

proteins

• Allow attachment of biofilm

on system surfaces

• Gives biofilm its mechanical

stability

• Often stabilized by cations

– e.g. Ca2+ , Fe 2+ / 3+

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COO

CH2

OH

CH2

OH

EPS TA M CM Cy

-

COO-CH2

CH2

OH

COO-

OOC-

+ + + + ++

+

- - - - - --

- - - -

+ +

OH

OH

CH2

OH

CH2

Ca2+

+ ++

-

Electrostatic

Forces

Ionic

Forces

Ionic

Forces

H-Bridging

v.d.Waals Forces

Stabilisation of Biofilms

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Biofilm Model

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Time:

1. Conditioning Film

�Polyasccarides

�Glycoproteins

�Humic acids

SecondsSeconds

4. Detachment &

Dispersion

DaysDays……

3. Growth

HoursHours

2. Microbial

Adhesion

MinutesMinutes

Biofilm Development

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Advantages of Life in a Biofilm

• Enrichment of nutrients

• Micro niches for other species – biodiversity

• Higher resistance against biocides and

other stress

• Protection against desiccation

• Protection against higher life forms (protozoa)

• Big gene pool � Facilitated horizontal gene

transfer

• Symbiosis of species

� Multicellular Behaviour

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Oxidizing Biocides Mechanism

1 - Computer generated image of a bacteria cell

2 - Close-up of oxidizer molecule coming into contact with bacterial wall

3 - Oxidizer is penetrating and creating hole in bacterial wall

4 - Close-up effect of oxidizer on cell wall

5 - Bacterial cell after a few oxidizer molecules come into contact

6 - Destruction of cell after oxidizers attack (cell lysing)

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Biofilm in a copper pipe of a central disinfection

distribution system, continuously exposed to

disinfectant (Courtesy of G.-J. Tuschewitzki)

Advantages of Life in a Biofilm - Resistance

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Limited diffusion in biofilms and reaction with EPS-matrix

(neutralization of the disinfectant)

Survival of Pseudomonas and

micro bacteria embedded in EPS

in PVC-Pipes after 7 days

exposition with10-15 mg/l free

chlorine(Vess et al., 1993)

Diffusion of chlorine in Biofilms

of P. aeruginosa and K. pneumoniae(deBeer et al., 1994)

Resistance Mechanisms of Biofilm

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What is Chlorine Dioxide?

• Chemical formula - ClO2

• Orange-brown gas at room temperature

• Dissolves in water to give a yellow-green solution

• Very old disinfectant: 1811 - Discovered by Sir Humphrey Davy

• It has one free electron - radical character - strong oxidiser

• Selective reactivity

• Due to its free electron no chlorination but oxidation

ClO2 + e- ↔ ClO2-

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Activity in fuction of pH

0.0%

20.0%

40.0%

60.0%

80.0%

100.0%

120.0%

6 6.5 7 7.5 8 8.5 9 9.5 10

pH

PERCENT

% HOCl

% HOBr

% CLO2

ClO2 is Unaffected by pH

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– Ammonia

– Ammonium salts

– Alkanes

– Alkenes

– Alkynes

– Alcohols

– Primary amines

– Glycols

– Ethers

– Unsubstituted aromatics

– Most inorganic compounds

– Starch

– Organic acids

– Diols

– Saturated aliphatics

• In cooling systems, ClO2 is the recommended

biocide:

●ClO2 will not react with:

Selective Reactivity of ClO2

– where these substances are present

– where biofilm is present

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Potential Applications in Cooling Water SystemsMajor benefits in contaminated cooling towers

• Recommended for environmentally

sensitive areas

• Forms less disinfections by-products

(DBPs) such as chloramines, THMs,

AOXs, Bromates

• Proven to be the biocidal treatment for

contaminated cooling systems, in short

retention time systems such as once

through systems and contaminated river-

and sea water systems

• Effective micro- and macro fouling

inhibition

• Quicker kill rates than Cl2, Br2

• Non reactive with a lot of (in)organics

• Quicker kill rates on larvae and adult

mussels

• Highly effective in both neutral and

alkaline cooling systems

• Disinfection rate unaffected by pH range

• No or less additional biofilm removal

treatment needed

• Biofilm removal

• Improved corrosion rates• Lower Redox potential

• Lower dose rates (than Cl2, Br2)

BenefitsFeatures

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Case History

• Company: Infracor Marl, Germany

• Cooling system: RKW 383

• Feed water: River water after sand filtration

• System volume: 2.500 m³

• Recirculation rate: Max. 5.000 m³/h

• Biocide changeover from chlorine/bromide to chlorine

dioxide treatment

- Goal 1: Less TOX-concentration in the effluent

- Goal 2: Biofilm removal

- Goal 3: Less consumption of biocides

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Chlorine Dioxide Consumption and

TOX-Concentration

100%

189%

149%

27%

169%

221216

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80

130%

50%

100%

150%

200%

15.03.10 18.03.10 19.04.10 13.05.10 15.06.10 20.07.10 17.08.10 16.09.10 12.10.10

TOX

[% compared to

before ClO2]

0

50

100

150

200

250

ClO2-

consumption

[kg/week]

AOX%

ClO2kg/ Week

Detection limit of

TOX determination applied

Changeover

to ClO2

Kossmann and Duve, Infracor 2011

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Effects of Treatment with Chlorine Dioxide

• In the long run biocide changeover “chlorine /

bromide” to chlorine dioxide is characterized by:

– Low viable counts (e.g. Legionella <100 cfu/

100ml)

– Low TOX values (<50 µg/l)

– Low ClO2 consumption

• First months are characterized by biofilm removal

resulting in:

– Considerable increase in ClO2 consumption

– Considerable increase in TOX values

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Operational Parameter for ClO2 Dosage:

Redox Potential

• Dosage of ClO2 in relation to the redox potential

• ‘On’ and ‘off’ switch points due to specific water parameters

• Redox potential is a general parameter for the biocidal potential

of the water – the higher the redox potential is the higher is the

biocidal activity

Runtime ca. 2 h

Temperature [°C]

Redox potential in

reject [mV]

Runtime [sec]Kossmann and Duve, Infracor 2011

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Cleaning Effect of Chlorine Dioxide

Drift eliminator before

treatment

Drift eliminator after two

weeks of treatment

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Why is ClO2 Not Always Used?

• ClO2 expands 1000 x when

formed

• Pressure build-up

• ClO2 in gas is dangerous to

health and live

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How Has Safety Been Improved?

• Move the reaction from the work floor to under water

• Reduce reactor volume

• Eliminate pressure vessel

• Eliminate transport of ClO2 solutions

• Reduced reagent handling

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Summary

• A cooling tower is a perfect place for microbiological growth

• You will never be able to eliminate bugs

• Microbiological growth will cause system failures

• Almost all microorganisms on earth live in biofilm

• Almost all microorganisms in water are released from biofilm

• Chlorine dioxide is a powerful tool for microbiological control

• Chlorine dioxide is beneficial in contaminated systems

• Chlorine dioxide is able to remove biofilm from system surfaces

• With Ashland’s new chlorine dioxide generation system, biocidal

water treatment becomes safer and easy to handle

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