Basic Cooling Water Treatment principles

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Basic Cooling Water Treatment principles

• John Cowpar
• Area Manager
• GE Water and Process Technologies

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USING WATER
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POTENTIAL PROBLEMS

• CORROSION
• DEPOSITION - Fouling
• Biofouling
• Scaling

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Scale Formation

• Results in loss of heat transfer efficiency
• Increased running costs
• Danger of under deposit corrosion
• Increased maintenance costs
• Danger of bacteria
• Health implications

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Corrosion

• Destruction of plant
• increased maintenance costs
• Fouling
• loss of efficiency due to increased pumping costs
• loss of heat transfer efficiency
• Increased Biological Nutrients
• fouling and health implications

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Fouling

• Loss of heat transfer efficiency
• increase in running costs
• Under deposit corrosion
• increase in maintenance requirements
• Increased biological nutrients
• health implications
• Blockages in system
• increased operating costs and downtime

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Objectives of Water Treatment

• MINIMISE SCALE
• MINIMISE CORROSION
• MINIMISE FOULING
• MINIMISE BIOFOULING
• MAXIMUM SAFETY
• MAXIMUM EFFICIENCY
• NON-POLLUTING

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WHAT CAUSES OUR PROBLEMS?
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DISSOLVED SOLIDS

• e.g. CALCIUM
• MAGNESIUM
• SODIUM
• CHLORIDE
• BICARBONATE
• SULPHATE
• SILICA
• IRON

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DISSOLVED GASES

• e.g. OXYGEN
• CARBON DIOXIDE
• NITROGEN
• SULPHUR DIOXIDE

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SUSPENDED MATTER

• DUST/DIRT
• CONTAMINANTS e.g. OIL
• BIOLOGICAL e.g. ALGAE, FUNGI, BACTERIA

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TYPICAL WATER ANALYSIS CHART
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Water Analysis

Result pH

7.7 Colour
3.00 HAZEN Turbidity
9.00 F.T.U. Solids - Suspended
5 mg/l Chloride as Cl 44
mg/l Alkalinity as CaC03
144 mg/l Ammoniacal Nitrogen as N
0.140 ug/l Iron (Total) as Fe
311 ug/l Manganese (Total) as
Mn 65 ug/l Nitrate as
N
4.0 mg/l Total Hardness as CaC03
207 mg/l Sulphate
as S04
62.3
mg/l Silica - Reactive as Si02
6.9 mg/l Sulphide as S 0.015
mg.l Carbon Dioxide - Free
2.50 mg.l Solids - Total Diss. at
180C 347 mg/l D.O.
Concentration (Field Det.)
10.7 mg/l Coliforms lt10 /100ml E.
Coli lt10 /100ml Faecal
Streptococci
lt1 /100ml Sulphite Red. Clostridia
300 /20ml
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Hardness

• Hardness is due to calcium and magnesium salts
  dissolved in water
• All hardness salts are less soluble in hot water
  than in cold water (they show inverse solubility)
• Different hardness salts have different levels of
  solubility
• Hardness is normally reported as calcium carbonate

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EVAPORATION
WINDAGE
MAKE UP
BLEED
M E W B
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Useful Equations
ER/100 x Temp Drop(degF)/10
WR x 0.2/100 ( Forced Draught)
WR x 0.6/100 (Natural Draught)
BE/(C-1) -W
ME B W
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SCALE FORMATION

• SCALE CAN BE CONTROLLED BY
• PRE-TREATMENT
• CHEMICALS
• CONCENTRATION FACTOR

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CORROSION

• Iron ore is found in nature and requires a large
  input of energy to convert it into steel.
• Steel corrodes in order to get back to its
  natural (lower energy) state
• Corrosion is an electrochemical process

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CORROSION CAN BE CONTROLLED BY

• REMOVAL OF OXYGEN ?
• ADDITION OF CHEMICALS
• CONTROL OF pH

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Biofouling
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What is Biofouling caused by?

• FUNGI
• ALGAE
• BACTERIA

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FOULING/BIOFOULING

• Can be controlled by
  
• Filtration
• Control of Concentration Factor (bleed)
• Dispersants
• Biocides

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Open Cooling

• When evaporation occurs, the heat of evaporation
  is used to drive off the vapour
• The loss of this energy results in a cooling
  effect in the water
• Pure water is evaporated (gases may also be lost)
• Dissolved solids remain in the water

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Cooling Water
WATER DROPLET COOLS BY
EVAPORATION RADIATION
CONVECTION
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Control of Concentration

• The number of times the solids build in the
  system water is termed the concentration factor
  (CF).
• CF is controlled by bleed
• to increase CF - decrease bleed
• to decrease CF - increase bleed

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Bleed Control

• Effect of too much or too little bleed
• Too much bleed -
• low concentration factor
• waste of water
• waste of treatment
• Too little bleed-
• high concentration factor
• danger of scale and fouling
• increased nutrient in system
• danger of biofouling

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x
While increasing concentration factor reduces
water use, it also increases nutrients in the
system water, encouraging growth of bacteria and
slimes. Therefore, we normally run most cooling
systems between 2 and 5
Water Use
x
x
x
x
x
1 2 3 4
5 6
Concentration Factor
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Non-biological Fouling

• Treated by addition of dispersants
• dispersants (antifoulants) coat the particles and
  so keep them apart
• The dispersed particles are then removed from the
  system water
• either with the bleed or via a side stream filter

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Non-biological Foulants

• Silt
• Rust
• Process contamination
• all removed by dispersant/bleed
• Oil
• Grease
• a different chemical is required but the
  principle is the same

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MICROBIOLOGY
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Microbiology in Industrial Cooling Systems

• Problematic Microorganisms
• The Biofouling Process
• Water Treatment Biocides
• Biocide Programming
• Monitoring and Control

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FUNGI

• Although yeast and some aquatic fungi are
  normally unicellular, most fungi are filamentous
  organisms
• Fungi form solid structures which can reach a
  considerable size
• Some wood destroying fungi exist, associated with
  deterioration of tower timber
• Fungi require presence of organic energy source
• Exist at between 5 to 38 C and pH 2 to 9 with an
  optimum of 5 to 6

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ALGAE

• Classified as plants as they grow
  by photosynthesis
• Range in size from unicellular microscopic
  organisms to plants that can be up tp 50m in
  length

Single cells
Multi cellular
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ALGAE

• Algae cannot survive in the absence of air, water
  or sunlight
• Basic difference is that algae utilise CO2 and
  water using sunlight as the energy source to
  assimilate food

• Large quantities of polysaccharides (slime) can
  be produced during algal metabolism
• Plug screens, restrict flow and accelerate
  corrosion
• Provide excellent food source
• Exist between 5 to 65 C and pH 4 to 9

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BACTERIA

• Universally distributed in nature
• Great variety of micro organisms
• Multiply by cell division
• Slime formation
• Pseudomonas (utilise hydrocarbon contaminants)
• Sulphur bacteria - anaerobic sulphate reducing
  bacteria
• Nitrogen cycle bacteria

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FACTORS CONTRIBUTING TOMICROBIAL GROWTH

• Rate of incoming contamination
• Amount of nutrient present
• pH
• Temperature
• Sunlight
• Availability of oxygen/carbon dioxide
• Water velocities

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THE BIOFOULING PROCESS

• Bacteria prefer to colonise surfaces
• enables production of biofilm which acts to
  protect and entrap food sources
• Planktonic bacteria
• free swimming in bulk water
• Sessile bacteria
• attached to surfaces

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EFFECTS OF BIOFOULING

• Fouling of tower, distribution pipework, heat
  exchangers
• Reduction in heat transfer efficiency
• Lost production
• Under deposit corrosion
• Inactivation/interference with inhibitors

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WATER TREATMENT BIOCIDES

• Oxidising Biocides
• Have the ability to oxidise organic matter eg.
  protein groups
• Non-Oxidising Biocides
• Prevent normal cell metabolism in any of the
  following ways
• Alter permeability of cell wall
• Destroy protein groups
• Precipitate protein
• Block metabolic enzyme reactions

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OXIDISING BIOCIDES

• Sodium Hypochlorite
• Hypobromous Acid
• Chlorine dioxide
• Ozone
• Hydrogen Peroxide

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Oxidising Biocides

• Rapid kill
• Cost effective
• Tolerant of contamination
• e.g. Bromine, Chlorine Dioxide
• Minimal environmental impact
• e.g. Bromine, Ozone, Peroxide, Chlorine
  Dioxide
• Ineffective against SRBs
• Low residual toxicity
• Counts approaching potable water standards
  possible

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Non Oxidising Biocides

• Screen water
• Select alternating biocide to prevent resistant
  strains from developing
• Effective against SRBs
• Can protect system long after dosing.
• Contain biodispersant
• Higher dosage for kill possible
• Environmentally some have rapid breakdown e.g.
  DBNPA

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BIODISPERSANTS

• Improves penetration of biocide within bacterial
  slime
• Disperse released bacteria and biofilm into bulk
  water for removal by blowdown
• Reduces ability for bacteria to attach to system
  surface
• Improves performance of both non oxidising and
  particularly oxidising biocides

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Physical Methods

• Ultra Violet and Ultra Filtration
• Only Effective At Point Of Use
• Cannot Kill Sessile Organisms
• Offer No Protection To Isolated Parts Of System
  (Static Areas)
• Environmentally Acceptable.

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Control of Concentration

• The number of times the solids build in the
  system water is termed the concentration factor
  (CF).
• CF is controlled by bleed
• to increase CF - decrease bleed
• to decrease CF - increase bleed

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