Industrial Water Engineering, Inc.

CHEMICAL WATER TREATMENT

toll free 1-800-543-6519

GSA # GS-10F-7884A (1/31/2007)

DOD Cage # 0MV90

 

 

Control of microbiological growth is cooling water systems is important for system efficiency and public health. Cooling waters are open to the atmosphere and sterilization is not possible. Control must be done to numerically keep microorganisms as low as practical and to maintain the system in a clean manner.

Microorganism control can never be achieved by microbiocide feed alone. Cooling towers should be cleaned a minimum of once per year and pressure washed to remove debris, silt, and deposits. Clean systems are easier and less expensive to keep clean. Dirty systems offer a large nutrient supply for microorganisms and chemicals cannot penetrate slime masses. If a system is already fouled, it should be mechanically cleaned prior to treatment.

Dirty towers often contain a variety of pathogenic organisms such as Legionella sp., Pseudomonadas sp., and others. Prior to cleaning, the tower should be disinfected by maintaining 5-ppm free chlorine for a minimum of 4 hours. Chlorine is aggressive to metal alloys at these levels, so care must be taken to drain the system as soon as possible. All workers cleaning the system should be equipped with rubber boots, particle masks, rubber gloves, and goggles. The OSHA Chapter 7 Technical Manual is attached and details information on prevention of Legionella in cooling towers. The ASHRAE paper, Legionellosis: Position Paper is also attached.

Microbiological organisms can be categorized into two types: deposit producing and corrosion producing.  Deposit producing organisms cause slime that entraps mud and debris causing heat exchanger fouling. Sessile organisms adhere to surfaces. Plaktonic organisms are found in the water. Beware which type of organism your biological test is really testing.

BACTERIA: Bacteria can form spores that allow bacteria to reproduce.  Spores are resistant to biocides, sunlight, oxidizing agents, and drying out.  Bacteria can be made up of both spore formers and non-spore formers.  They appear as small, gelatinous mucous and bind suspended material together.

ALGAE: Algae in cooling water is usually blue, blue-green, or diatoms.  Algae form stringy masses that help filter dirt and silt.  Often fungi and bacteria grow in the algae mat.  They require sunlight and often appear on exposed distribution decks and supports.

FUNGI: These are molds and yeasts that are relatively large organisms.  Fungi attack wood and trap mud and silt.

Corrosion causing organisms produce corrosive waste that will attack the metals in a cooling water system.  Most corrosion causing bacteria require a mud deposit to become active.

SULFURIC ACID PRODUCERS: These organisms produce sulfuric or sulfurous acid by the conversion of hydrogen sulfide.   These organisms thrive under tower mud deposits and can rapidly corrode galvanized cooling tower basins.  Thiobacillus is a common producer.

SULFIDE PRODUCERS:  Sulfate Reducing Bacteria or SRBs form hydrogen sulfide from sulfate in the water.  These bacteria cause corrosion on mild and stainless steel and will precipitate zinc and chromates. Common species are Thiobacillus, Clostridium, and Desulfovibrio.

NITRIFYING BACTERIA: These bacteria convert ammonia and ammonia compounds to nitric acid.  The nitric acid will then corrode metal.  Common species are Nitrobacter and Nitrosomanonas.

IRON BACTERIA:  Iron is naturally found in many water supplies as ferrous or soluble iron. Iron bacteria reduce the iron to insoluble or ferric iron.  These bacteria are commonly found in shallow wells. Common species are Gallionella and Crenothrix.

No single biocide program is acceptable for all systems. Many factors need to be taken into consideration when selecting a biocide program.

1. System volume needs to be determined accurately, either using a salt volume test for calculations based on piping and tower volumes.

2. Materials found in system. Is the tower made of wood, PVC, galvanized? Is the piping mild steel and what are the heat exchangers made of. Chlorine is very aggressive to stainless steel and copper alloys if not fed properly.

3. What are the objectives of the program?

4. How much money is the owner willing to spend?

5. How are you going to monitor the program?

6. How much support from site-personnel is available to implement the program?

7. How often can biocide be applied to the system?

8. How often will the tower be cleaned?

9. What is the present condition of the system? How deep is the mud in the basin? Visible growth and locations? Plate counts or ATP test results?

10. Primary biological concerns? Algal masses? Pseudomonas, especially sessile microbes and biofilms? Sulfate Reducing Bacteria? Legionella?

11. Contamination from process leaks? If ammonia or oil enters the water, oxidizers will be used up and the system will have high demands.

12. Is a biodispersant required due to system design?

13. Flow rates and water distribution? If biocide cannot contact a surface, it cannot kill the organism.

14. Water chemistry. Temperature, pH, dissolved solids, foreign matter?

 

Non-oxidizing biocides should be added in pairs to assure that all biological species are controlled.   Prior to addition, it is important to determine the volume of water in the cooling water system.   Once the volume is determined, the half-life or time for half the water to be lost from the system must be determined.  Next, the water treatment company selects the amount of biocide to maintain a lethal concentration for a minimum specified time. Contact time is critical to performance.

Never use biocides except in accordance with labeling requirements.  Compounds are deadly if mishandled or misapplied.

Biocide programs may be verified for performance.  General plate counts should be below 100,000 colonies per mL.  The tower should not have thick visible algal growth.  A slight algal sheen is acceptable.  Basin walls should not be slick.   No visible slime deposits should be present.

Wet/dry areas and areas under deposits are often not in contact with lethal doses of biocides.  Microbiological growth in these areas is common and may not be controlled with normal label feed rates.  Efforts to improve water flow and keep sediment out of the system will be helpful.

Non-oxidizing biocides are any chemical that is not an oxidizing agent used to control microorganisms.  Many different types exist and each controls specific organisms in specific applications.  All are registered by the U.S. Environmental Protection Agency and must be used in accordance with labeling.  Overfeed is expensive and illegal. Biocides may be categorized into two classes, oxidizers and non-oxidizers. These are summarized below:

Quaternary Ammonium Compounds: Common compounds are alkylbenzyldimethyl ammonium chloride or benzalkonium chloride. These are fatty acids and are generally used to control bacteria and algae. Many will cause foam and react with anionic inhibitors. Quats kill by forming an electrostatic bond with the cell wall and this effects permeability of protein formation. Quats are excellent algaecides and some are good bactericides. Optimum pH is 6.5 to 8.5. Quats are of limit benefits for Sulfate Reducing Bacteria (SRB) or Pseudomonas species.

Polyquats: Common compounds are poly[oxyethylene(dimethyliminio)ethylene, dichloride. These are also cationic polymers for cooling towers and air-washers. These compounds are relatively safe and often used in swimming pools for algae control. Polyquats have little foam and may react with anionic polymers. May be applied from pH 7.5 to 9.0.

Metals: Copper and copper sulfate are used to control algae. Water pH is critical. Use of copper salts in cooling water will result in pitting as the copper plates on steel surfaces. Copper compounds are generally limited to lakes, ponds, and decorative fountains.

Fatty Amines: These are cocoamine quats and are very effective on bacteria.  They cause foam and react with anionic inhibitors.

Methylene Bis Thiocyanates and similar: Very effective on bacteria and algae that is active.  Alkaline pHs reduce half-life and are not usually used at pHs above 7.5.

Organosulfur-These are carbamate types and are effective on bacteria. These will cause precipitation of metals and other corrosion inhibitors. Optimum pH is 7.0 to 8.5.

Isothiazoline Compounds: Very effective on bacteria and some algae. Effective over a broad pH 6.5 to 9.0. While effective biocides, these require extreme caution when handling.

Glutaraldehyde Compounds: Very effective on bacteria and has good penetration into biomass. Effective over a wide pH from 6.5 to 9.0. These are also fact acting biocides and will be effective in 3 to 4 hours under most conditions.

DBNPA-2-2-Dibromo-3-nitrilopropionamide. This is a good general purpose product. Above pH 8.0 it hydrolyzes rapidly. This compound is useful for rapid clean-up of bacterial slime.

DTEA-2-(Declythio)ethanamine-DTEA is very effective at high pHs. DTEA is specific for sessile bacteria and removes biofilm and biogrowth. DTEA functions by reversible chelants complexes found in biomass and bioslimes. Can be thought of as biocidal soap.

TBZ-2-(tert-butylamino)-4-chloro-6-(ethylamino)-s-triazine: TBZ is an effective algalcide. It does not foam and is very effective when used with oxidizers.

TCMTB: This is a good slime controller, especially for algal slimes. It is effective from pH 6.0 to 9.0. It is also very effective to control fungi on cooling tower wood.

OXIDIZERS

Oxidizing biocides are any chemical that is an oxidizing agent.  These products destroy the cell walls and destroy nutrients in the water.  The products are surface active and are said to "burn" the organisms.  Care must be taken to use the oxidizing agent in the proper manner.  Overfeed with corrode metal by oxidation and destroy scale and corrosion inhibitors.

Chorine and compounds: Forms include chlorine gas, sodium and calcium salts, and organic chlorine compounds.  Use is effective under pH 7.5. Reacts with ammonia.

Chlorine Dioxide: Produced on-site with sodium chlorate and does not react with ammonia.  Similar to chlorine. Chlorine dioxide is expensive and hazardous to administer.

Bromine and compounds: Most commonly, sodium bromide salt (40%) is activated by addition of a chlorine source such as bleach, chlorine gas, or other oxidants. The resulting bromine will react very similar to chlorine but is effective at a higher pH.

Stabilized Bromines-single-package: Single package bromine compounds are based on bromo-amine chemistry. Unlike chloro-amines, bromo-amines are effective biocides. These products are available in single-package and are safe to handle.

The easiest and most accurate method to determine system volume is by using the salt volume test.

1. Measure amount of chloride in cooling tower, record as mg/L as NaCl. This is the initial reading.

2. Add a known amount of salt to tower. A 4-pound box is adequate for small systems. Allow to circulate for 15 minutes.

3. Measure chloride every 15 minutes until reading has stabilized. Record mg/L as NaCl. This is the final reading.

4. CALCULATE VOLUME:

 

Based on your selection of biocide, determine the amount and frequency to add. If the EPA label required a feed rate of 3 to 6 ounces per 1000 gallons, then you will need to add the biocide at the rate of 72 ounces to 144 ounces for the tower with 24,000 gallons. You also need to add the biocide at the frequency required by the label. If the label says that the biocide should be added every 1 to 6 days, then it must be added at least 1 time in 6 days.

Biocides can be added either manually or with automatic feed equipment.

Prior to using a biocide, always read the EPA label, the Material Data Safety Sheet, and any product literature. Respect all requirements outlined on this information. All biocides must be used in accordance with label requirements. It is against the law not to follow the requirements shown on the label.

Biocides must be added in accordance with label requirements. Records must be kept in accordance with you State Department of Agriculture rules, the EPA rules, and your company requirements. Please consult your State requirements for complete information.

A cooling tower is a device to conserve water.  Hot water enters the distribution deck where the water is distributed to the fill area.  Within the fill, the water breaks into thin-film or droplets for air-cooling.  Air is drawn through the fill with a large fan.  Water is evaporated and the water is cooled 10 to 15^o F.  Air is filled with dust, pollen, cottonwood fiber, debris, and other foreign objects.  The air is washed. Clean air exits the tower and the debris stay in the water.  The foreign matter settles in sumps, heat exchangers, and other low flow area.  Over a year’s time, mud accumulations of several inches are common in the cooling tower sump.  According to ASHRAE, energy cost savings associated with maintaining a clean system is significant.  Additionally, microorganisms such as the deadly Legionella pneumophila are easier to control.