Cooling systems provide ideal growth
environments for bacteria, algae, fungi and mollusks which can adhere to
pipeline, heat exchanger and cooling tower surfaces. The result can be
reduced heat transfer efficiency, increased corrosion and potential system
failure. Biological growth, scaling and corrosion are the main maintenance
concerns with cooling towers. Typical treatment involves the application of
chemicals such as chlorine, sulfuric acid, phosphorus and zinc compounds.
Care must be taken in the storage, use or discharge of these chemicals. Care
must also be taken to ensure that the proper mixes and proportion of
chemicals are used and to determine the corresponding blow down rates.
Excessive application of these treatment compounds raises the probability of
corrosion and other undesirable effects. As traditional chemical water
treatments are being restricted because of environmental concerns, Ozone is
gaining acceptance as a viable biocide alternatives.
Ozone produces oxidation by-products and these several secondary compounds
must be accounted for in the set up of cooling tower system Ozonation. Both
iron and manganese will be oxidized by the ozone to form insoluble
particulate matter, which will collect in basins on screens or in any scale
that is formed. Excessive amounts of either of these two elements in the
make-up water will require pretreatment, such as softening, to facilitate
removal. Organic compounds that may either be in the make-up water or
introduced through the atmosphere will react with ozone to form various
substances. These substances, particularly peroxides, aldehydes, ketones and
alcohols are efficient biocides themselves, which will further disinfect the
water. If bromide is present, ozone can convert it to hypobromous acid and
hypobromite ion. These two are also effective biocides and would be
considered helpful in controlling biological fouling, but because they are
such effective biocides, they are generally detrimental in the blowdown
discharge to municipal waste water treatment systems. Excessive ozone
concentrations in the water will further oxidize the hypobromite ion to
bromated, reducing the biocidal effectiveness of this component.
Effect of Ozone on Biofilms
On heat exchanger surfaces, biofilms degrade
heat transfer efficiency, increase pressure drop (and pumping power
required) through exchanger tubes and plates by substantially reducing flow
rates through these machines and can lead to corrosion problems under the
film. Ozone kills organisms by rupturing their cell walls, a process to
which the microorganisms cannot develop immunity. When oxidized, the
mucilaginous material secreted by these microorganisms is loosened from the
heat exchanger surfaces, and the biofilms, along with inorganic precipitates
which were adhered to the secretions, are flushed away by the motion of the
flowing water. Once biofilms have been removed, ozone concentrations of less
than 0.1 mg/lit have proven to be effective in preventing new growth of
bio-fouling material.
Effect of Ozone on Algae
Most algal species are readily oxidized upon
application of ozone; different species will require different exposure
times for removal. The oxidation pocess can proceed to total decomposition
of the algae to carbon-dioxide and water with sufficient concentrations of
ozone and contact time. A combination of controlled sunlight exposure and
low level of residual ozone will minimize algae growth in the cooling tower
fill and basin. Destruction of many algal species using ozonation will
liberate nucleic acids, proteins, polysaccarides, and other bio-polymers.
The production of polysaccarides by exposure of biofilms and algae to ozone
liberatessurface active materials with ability to complex iron, manganese
and calcim effectively removing them from solution in the tower water and
thereby reducing scaling potencial. These surface active substances, through
micro-flocculation, may contribute to the observation that ozone treated
cooling tower water becomes crystal clear.Ozone treatment is a unique and
valuable process. It can accomplish all the aforementioned treatment
objectives without leaving a taste or chemical residue behind. Ozone is an
exceptionally powerful disinfectant and oxidant. It does its job and
disappears. With appropriate pretreatment and careful monitoring and
control, it can leave water free of disinfection by- products as well.
Effect of Ozone on Bacteria
High Levels of bacteria can lead to an increase
in microbially influenced corrosion (MIC). Certain sulfate reducing and iron
metabolizing bacteria influenced can destroy a system's steel or iron piping
in less than one year. Ozone kills bacteria by rapturing their cell walls,
aprocess to which microorganisms cannot develop immunity. Residual ozone
concentrations of 0.4 mg/lit or higher result in 100% kill within 2 to 3
minutes for pseudomonas fluorescens (a biofim producer) in an established
biofilm, while residual concentrations as low as 0.1 mg/lit will remove 70
to 80% of the biofilm in a 3 to 4 hour exposure. Studies have also proven
that maintained ozone concentrations of less than 0.0 mg/lit will reduce the
population of Legionella pneumophila, the bacteria responsible for
Legionnaire's Disease, in cooling tower system water by 80%.
Effect of Ozone on Scaling
Another common trouble in a cooling tower
system which requires prevention is mineral build-up, commonly referred to
as scaling. Minerals such as calcium and magnesium, which are normal
dissolved solids in makeup water, are deposited by two different mechanisms,
thermal and biological. As the water in a tower evaporates, dissolved solids
become concentrated in the circulating water. When the concentration of
these reaches the solubility limit of water, they begin to precipitate out
of solution. When biofilms are present on the walls and other components of
the tower, the biofilms acts as a binder, cementing the mineral
micro-crystals to the deposition of organic and inorganic matter increases
scale thickness versus heat transfer efficiency degradation are well
documented. Ozone will oxidize the biological matter and by removing the
glue, the mineral scale is allowed to fall away from the affected surfaces.
However, if the scaling is not due to the presence of biofilms, ozone will
probably be ineffective in removing the scale; Biofilm is rarely the
dominant fraction of scale formation where the temperature of the heat
exchanger is in excess of 135 degrees Fahrenheit. Scale formation minerals
are less soluble at these higher temperatures and will deposit from solution
directly onto hot heat exchange surfaces.
Effect of Ozone on Corrosion
There have been several studies on corrosion
rates in Ozonated systems conducted and reported in trade journals and other
literature. The initial premise for cooling tower system using ozone for
water treatment was that because ozone is a powerful oxidizer, those metals
capable of developing a passive oxide film would be protected from ozone
residual. But, further it was observed from the tests that the corrosion
rates of 1010 carbon steel, copper, brass, 90/10 cupro-nickel, and 304
stainless steel using ozone and chlorine in physically separate pilot
cooling tower systems. In testing periods of 14 and 35 days, they observed
corrosion rates were comparable with control tests without treatment.
Corrosion rates on mild steel were reported to be lower when using ozone
(4.6 mpy) as opposed chlorine alone (28 mpy). Reported test results for an
open cooling tower system in which a 0.05 mg/lit ozone residual was
maintained. Copper alloy samples had; lower corrosion rates than samples in
water without ozone. There was no detectable corrosion on Cr-Ni steel alloys
or on titanium, each of which developed a protective oxide layer.
Ozone is not a corrosion inhibitor; however the higher concentrations ratios
resulting from the reduced blowdown volumes raise the pH of the circulating
water, which helps protect the system form corrosion. It is clear that
maintaining a small ozone residual (0.1 gm/lit or less), or of the oxidizing
ozone residuals, will maintain algae and Biofilm free surfaces, and thereby
avoid or substantially reduce the microbially influenced corrosion (MIC) or
under-film corrosive attack. Any corrosion that occurs in a clean system
will be uniform throughout the system, as opposed to localized, and the
corrosion that does occur will be very unlikely to cause component failures.
The heat transfer efficiency, and overall system efficiency, will be
restored and maintained at original installation levels with clean heat
transfer surfaces after elimination of biological growth in the cooling
tower system.
Advantages Of Using Ozone For
Cooling Tower Water Treatment
- Eliminates the use of chemicals except for pH balancing.
- Ozone is produced on-site and thus requires no storage of dangerous
chemicals; ends discharge liabilities and chemical storage record
keeping.
- Destroys all types of microorganisms instantly and decomposes organic
waste by oxidation.
- Micro-organisms cannot resist and survive in ozone atmosphere after
prolonged exposure to ozone.
- Removes existing calcium carbonate scale by destroying the biomass
glue bonding agent.
- High efficiency as disinfectant. A residual ozone concentration of
0.1 to 0.2 ppm is in most cases very effective in keeping the cooling
tower and the cooling circuit clean.
- Due to good bio-film removal capacities very effective against
Legionella which is responsible for Legionnaire's Disease, in cooling
tower system water by 80%.
- Ozone is absolutely safe and environment friendly as it decomposes
back to Oxygen and does not leave any residue behind unlike other
chemical agents.
- Ozone protects the cooling tower against corrosion.
Use Of Ozone Reduces Energy Costs
- Increasing the heat transfer efficiency of the chiller.
- Reducing the quantity of make-up water to the cooling tower by
permitting more cycles between blowdowns.
- Eliminating the cost and problems in ordering, shipping, handling,
storage and disposal of regular chemicals.
- Reducing power consumption by keeping the chiller heat transfer
efficiency high through cleaner condenser tubes.
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