---BIOLOGICAL GROWTHS

BIOLOGICAL GROWTHS

The uncontrolled multiplication of bacteria, algae, fungi and other microorganisms can lead to deposit formations, which contribute to fouling, corrosion and scale. A biological growth has been recognized as an important contributor to impaired heat transfer efficiency in cooling water systems.

1.     How do microorganisms enter a cooling water system?

The make-up water supply, wind and insects can all carry microorganisms into a cooling water system.

2.     What factors contribute to microbial growth?

The main factors are:

§  Degree of infected microbial contamination already build up

§  Nutrients: For instance, hydrocarbons or other carbon sources can serve as food for slime-forming organisms.

§  Atmosphere: Organism growth depends upon the availability of oxygen or carbon dioxide.

§  Location: The factors such as amount of light and moisture significantly affect growth rates.

§  Temperatures: Organisms that compound into masses (slime) tend to flourish between 40 and 150 deg F.

3.     How does microbial slime impact scale formation?

Slime can cause treatment chemicals for scale to be ineffective and hence promotes scale formation.

4.     How microbial slime does cause fouling?

Slime masses themselves are foulants. They provide excellent sites for the deposition of other foulants. Although many organisms tend to die at high temperatures the remaining debris fouls metal surfaces. Generally microbial organisms form colonies at points of low water velocity.  Heat exchangers & cooling towers are therefore subject to microbial contamination.

5.     What factors must be considered in planning an effective microbial control program?

The most important factors are:

§  Types and quantities of microbial organisms

§  Microbial trouble signs such as wood rot, slime deposits and corrosion

§  Operating characteristics of the system, such as temperature flow rate and water composition

§  Types of equipment employed such as cooling towers, spray ponds, open box condensers etc.

6.     What level of microbial count should be maintained in the cooling tower?

Ideally the cooling tower system should not be allowed to have bacterial/microbial growth beyond 50000 counts/ml.

7.     How are microbial treatments selected?

Microbial treatments are selected by first analyzing representative water and slime samples to determine the types of organism present. Three general classes of chemicals are used in microbial control

ü  Oxidizing biocides literally burn up any microbe they come in contact with. Common oxidizers are chlorine, chlorine dioxide, and bromine, ozone, and organo-chlorine slow release compounds. Chlorine is one of the most widely used, cost effective biocides and is available in liquid, gaseous or solid form. Its effectiveness is increased when used with non-oxidizing biocides and biological dispersants. Ozone is now a day widely used to curb microbial growth.

ü  Non-oxidizing biocides kill the micro-organisms. They are effective where chlorine may not be adequate.

ü  Bio-dispersants: These chemicals does not kill organisms, they loosen microbial deposits, which can then be flushed away. They also expose new layers of microbial slime or algae to the attack of oxidizing biocides. These are an effective preventive measure because they make it difficult for the microorganisms to attach to the metal surfaces to form deposit.

A combination of all three generally makes an excellent program.

In fact, it has been unequivocally demonstrated that because of the unique surface characteristics of bio-films, their hydrodynamic and insulating properties far exceed those of an equivalent thickness of scale or corrosion deposits.

Of particular concern are the slime and spore formers which are difficult to control because of the protection afforded by the polysaccharide sheaths that they secrete and the organisms that metabolize either cellulose or lignin, resulting in structural weakness and eventual collapse of wooden tanks or towers.