Cooling Tower Problem#3 - Biological Fouling

Hi friend, in this blog we are going to discuss about one of the most common cooling tower problem is biological fouling. I will update blog about cooling tower in detail and its problems & solution in upcoming blog.

What is biological  fouling?

    Elimination of “biological fouling” and prevention of the incubation of pathogens forms the third leg of the cooling water management triangle.

    There are many species of microorganisms (algae, protozoa, and bacteria) that can thrive in cooling systems under certain circumstances; their growth is helped by favorable water temperature and pH, the oxygen picked up by the spray water, sunlight, and organics that provide food. Generally microbial organisms form colonies at points of low water velocity that leads to uncontrolled microbiological accumulations. The deposits are transferred throughout the piping system, which interfere with heat transfer surfaces and restrict flow through piping, strainers, spray nozzles, and control valves. The deposits are also concern for threatening infectious agents like the bacteria Legionella pneumophillus.

Possible types of micro-organisms that exist in cooling water Micro-organisms	Impact on cooling tower system
Algae	• Provide a nutrient source for bacterial growth • Deposit on surface contributes to localized corrosion process • Loosened deposits can block and foul pipe work and other heat exchange surfaces
Fungi	• Proliferate to high number and foul heat exchanger surfaces
Bacteria	• Some types of pathogenic bacteria such as Legionella may cause health hazards • Sulphate reducing bacteria can reduce sulphate to corrosive hydrogen sulphide • Cathodic depolarization by removal of hydrogen from the cathodic portion of corrosion cell • Acid producing bacteria produce organic acids, which cause localized corrosion of deposit laden distribution piping and also provide the potential for severe pitting corrosion of heat exchanger surface

 

    In order to effectively control the growth of micro-organisms in cooling water, chemical and physical water treatment methods shall be adopted.

Treatment Methods

    Chemical biocides are the most common products to control the growth of micro-organisms. Three general classes of chemicals are used in microbial control 

            1) Oxidizing biocides, 

            2) Non-oxidizing biocides and 

            3) Bio-dispersants.

1) Oxidizing Biocide

    Oxidizing biocides are powerful chemical oxidants, which kill virtually all micro-organisms, including bacteria, algae, fungi and yeasts. 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. Bromine chloride or chlorine dioxide should be considered for use in circulating water treatment for systems with high ammonia concentrations. Ozone is now days widely used to curb microbial growth.

Application of oxidizing biocide

    The most effective use of oxidizing biocides is to maintain a constant level of residual in the system. Oxidizing biocides are usually maintained at a continuous level in the system. Dosage may be adjusted in response to regular testing but fully automatic control of biocide level in using reliable and durable measuring systems is desirable since overdosing can lead to increased corrosion and adversely affect the water treatment performance. Shock dosing is also applicable, which can enhance the effectiveness by faster killing action.

    Since oxidizing biocide may sometimes be corrosive, corrosion inhibitors shall be added and selected to ensure compatibility.

2) Non-Oxidizing Biocide

    Non-oxidizing biocides are organic compounds, which kill micro-organism by targeting specific element of the cell structure or its metabolic or reproductive process. Non-oxidizing biocides are not consumed as fast as the oxidizing types and remain in the system for a significant period of time until they pass out with the blowdown. They often have the added advantage of breaking down into harmless, nontoxic chemicals after accomplishing their bacteria-killing purpose. They are effective where chlorine may not be adequate.

    However, the non-oxidizing biocides are more costly and normally justified only in small systems, as a supplement to an oxidizing biocide in a large system, or when a particular problem exists in a large system and an alternative to the use of chlorine is required.

 

    • Non-oxidizing biocides usually are organic compounds such as ammonium salts, Isothiazolinones, organo-metallics and organo-sulfur compounds.

    • Quaternary ammonium salts or diamines are sometimes found to be toxic, but the low concentration application allows them to maintain in an acceptable limit for discharge.

    • Isothiazolinones are biodegradable, which cause little adverse impacts to the environment.

    • Glutaraldehyde is an effective and rapid-acting biocide and its reactivity prevents it from persisting to harm the environment.

    • Non-oxidizing biocides such as organo-metallics, chlorophenols, organo-sulfur compounds and cationic biocides have not been adopted for use in large circulating water systems because of economics, degree of effectiveness, adverse side effects, or a combination of these reasons.

Generally accepted advantages to using non-oxidizing biocides are:

        1. They are well suited to applications that are highly concentrated or fouled.

        2. Contaminants such as ammonia, organic material, or nitrites cause a chlorine demand but do not affect a properly applied non-oxidizing biocide program.

        3. They are active at low levels and are environmentally degradable, providing minimal discharge problems.

        4. They will not attack organic scale inhibitors and are less corrosive than chlorine at normal dosages.

Disadvantages to using non-oxidizing biocides are:

        1. The cost is considerably higher.

        2. They are pH sensitive.

        3. Some are specific to certain microbial organisms.

        4. Some are not effective against algae.

        5. In order to ensure the effectiveness of non-oxidizing biocides, monitoring of chemical concentration in cooling tower systems is required.

     Non-oxidizing biocides shall be presented in sufficient concentration with adequate time for effective micro-biological control. System capacity, evaporation rate, bleed-off and make up rates shall be considered in calculating dosage concentration and frequency. Also, biocide hydrolysis (chemical degradation) rate affects the residual concentration of biocides containing in the cooling tower system.

    Typically, except in air conditioning cooling systems, non-oxidizing biocides are used only as a supplement to chlorine. If, during operation, it is determined that a non-oxidizing biocide is required because of severe fouling problems, the chemical supplier should be consulted on the method of feed. Normally, the supplier will lease the storage and feed equipment for a minimal charge.

3) Bio-dispersants

    Bio-dispersants do 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 biocides. Bio-dispersants are an effective preventive measure because they make it difficult for micro-organisms to attach to equipment and / or pipe work surfaces to form deposits. Bio-dispersants can greatly enhance performance of biocides particularly oxidizing biocides. Possible bio-dispersants include Acrylates, Ligonsulphonates, Methacrylates and Polycarboxylic acids, etc.