Microbiology

Closed Systems Water Treatment

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MICROBIOLOGICAL PROBLEMS IN CLOSED COOLING WATER SYSTEMS

Basic Microbiological Control for Recirculating Cooling Systems

The basic objectives of a successful cooling water treatment program are increased process efficiency, increased production output and maximized equipment service life. For a cooling water treatment program to achieve maximum effectiveness, it must address the problem areas of corrosion, deposition and microbiological contamination. Open recirculating cooling water systems are continuously infected with microorganisms. These microorganisms multiply rapidly in warm cooling waters and can lead to fouled heat exchangers, system metal loss due to microbiologically induced corrosion, degradation of cooling tower wood, and clogged screens and filters. Reduced flow rates, a reduction in heat exchanger capacity and the accumulation of microbiological growths on tower decks and tower fill are all signals of a severe microbiological problem.

An effective microbiocide program to control the growth of microorganisms involves three steps:

The identification of the types and concentrations of microorganisms present in the cooling system.
The selection of proper biocides based upon system design, discharge restrictions, and types of microorganisms.
Proper application, dosage and control of the selected biocides.

Microorganisms found in open recirculating cooling water systems will generally fall into one of three categories: Fungi, Algae, or Bacteria.

In closed recirculating systems normally algae are not a problem due to the lack of sunlight, necessary for their life. Fungi are sometimes present, creating the problem of giving nutrients to bacteria.

Bacteria are the main problem, and as the system is closed, anaerobic types are common, like slime forming bacteria, biofilms, sulfate reducing bacteria and iron bacteria.

The anaerobic conditions are giving also shelters for the growth of specific bacteria possibly dangerous to the man.

These organisms can enter a cooling system in a variety of ways. Some will enter the system through the make-up water supply. Others will be scrubbed from the atmosphere during normal operation. Still others may enter the system as a result of the accumulation of organic matter such as insects or grass clippings.

Once inside a system, the propagation of these microorganisms depends upon a variety of factors.

The system must contain enough nutrients to sustain microbiological growth. Most of the protection chemicals used today, being environmentally friendly are supplying sufficient nutrients for biological life to expand. By this procedure we are loosing the corrosion control of thesystem and we are facing fouling problems.
The system must have the correct ratios of oxygen, nitrogen, phosphorous and carbon dioxide to support microbiological life.
The location is an important factor. Sunlight can significantly increase to a rate at which microorganisms propagate.
The system temperature must be within certain ranges for different types of microorganisms. Microorganisms that produce slime tend to flourish between 40deg and 150deg F.

The objective of a effective microbiocide program is to expose the microbial population to a sufficient dose of biocide for a long enough time to achieve the desired effect.

Biocides may be fed continuously through the use of automatic feed equipment. More often they are fed on an intermittent basis because of economic considerations or effluent restrictions. An effective biocide program will include the use of two biocides of differing types which are fed in an alternating manner.

	Corrosion due to Microorganisms
	Microbial population growth in closed systems

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