Post on 06-Jun-2020
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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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