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

USING WATER

POTENTIAL PROBLEMS CORROSION DEPOSITION - Fouling Biofouling Scaling

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 Destruction of plant Fouling Increased Biological Nutrients 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 Loss of heat transfer efficiency Under deposit corrosion 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 MINIMISE SCALE MINIMISE CORROSION MINIMISE FOULING MINIMISE BIOFOULING MAXIMUM SAFETY MAXIMUM EFFICIENCY NON-POLLUTING

WHAT CAUSES OUR PROBLEMS?

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

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

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

TYPICAL WATER ANALYSIS CHART

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 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

EVAPORATION WINDAGE MAKE UP M = E + W + B BLEED

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 CAN BE CONTROLLED BY: PRE-TREATMENT CHEMICALS CONCENTRATION FACTOR

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: REMOVAL OF OXYGEN ? ADDITION OF CHEMICALS CONTROL OF pH

Biofouling

What is Biofouling caused by? FUNGI ALGAE BACTERIA

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

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 WATER DROPLET COOLS BY: EVAPORATION RADIATION CONVECTION

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 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

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

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

MICROBIOLOGY

Microbiology in Industrial Cooling Systems Problematic Microorganisms The Biofouling Process Water Treatment Biocides Biocide Programming Monitoring and Control

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

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

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

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

FACTORS CONTRIBUTING TO MICROBIAL GROWTH Rate of incoming contamination Amount of nutrient present pH Temperature Sunlight Availability of oxygen/carbon dioxide Water velocities

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

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

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

OXIDISING BIOCIDES Sodium Hypochlorite Hypobromous Acid Chlorine dioxide Ozone Hydrogen Peroxide

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 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

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

Ultra Violet and Ultra Filtration 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 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