water treatment

Basic Cooling Water Treatment principles:

Basic Cooling Water Treatment principles John Cowpar Area Manager GE Water and Process Technologies

USING WATER:

USING WATER

POTENTIAL PROBLEMS:

POTENTIAL PROBLEMS CORROSION DEPOSITION - Fouling Biofouling Scaling

Scale Formation :

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

Corrosion:

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

Fouling:

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

Objectives of Water Treatment:

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?

DISSOLVED SOLIDS:

DISSOLVED SOLIDS e.g. CALCIUM MAGNESIUM SODIUM CHLORIDE BICARBONATE SULPHATE SILICA IRON

DISSOLVED GASES:

DISSOLVED GASES e.g. OXYGEN CARBON DIOXIDE NITROGEN SULPHUR DIOXIDE

SUSPENDED MATTER:

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 <10 /100ml E. Coli <10 /100ml Faecal Streptococci <1 /100ml Sulphite Red. Clostridia 300 /20ml

Hardness:

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 BLEED MAKE UP M = E + W + B

Useful Equations:

Useful Equations E=R/100 x Temp Drop(degF)/10 W=R x 0.2/100 ( Forced Draught) W=R x 0.6/100 (Natural Draught) B=E/(C-1) -W M=E + B + W

SCALE FORMATION:

SCALE FORMATION SCALE CAN BE CONTROLLED BY: PRE-TREATMENT CHEMICALS CONCENTRATION FACTOR

CORROSION:

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

CORROSION CAN BE CONTROLLED BY::

CORROSION CAN BE CONTROLLED BY: REMOVAL OF OXYGEN ? ADDITION OF CHEMICALS CONTROL OF pH

Biofouling :

Biofouling

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What is Biofouling caused by? FUNGI ALGAE BACTERIA

FOULING/BIOFOULING :

FOULING/BIOFOULING Can be controlled by Filtration Control of Concentration Factor (bleed) Dispersants Biocides

Open Cooling:

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

Cooling Water:

Cooling Water WATER DROPLET COOLS BY: EVAPORATION RADIATION CONVECTION

Control of Concentration:

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

Bleed Control:

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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1 2 3 4 5 6 x x x x x Concentration Factor Water Use 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

Non-biological Fouling :

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

Non-biological Foulants:

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

MICROBIOLOGY:

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 TO MICROBIAL 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 SRB’s Low residual toxicity Counts approaching potable water standards possible

Non Oxidising Biocides:

Non Oxidising Biocides Screen water Select alternating biocide to prevent resistant strains from developing Effective against SRB’s 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.

Control of Concentration:

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