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What is microbiological fouling?

Microbiological fouling in cooling systems is the result of abundant growth of algae, fungi, and bacteria on surfaces. Once-through and open or closed recirculating water systems may support microbial growth, but fouling problems usually develop more quality and are more extensive in open reticulating systems.

Once-through cooling water streams generally contain relatively low levels of the nutrients essential for microbial growth, so growth is relatively slow. Open recirculating systems scrub microbes from the air and, through evaporation, concentrate nutrients present in makeup water. As a result, microbe growth is more rapid. Process leaks may contribute further to the nutrient load of the cooling water. Reuse of wastewater for cooling adds nutrients and also contributes large amounts of microbes to the cooling system.

In addition to the availability of organic and inorganic nutrients, factors such as temperature, normal pH control range, and continuous aeration of the cooling water contribute to an environment that is ideal for microbial growth. Sunlight necessary for growth of algae may also be present. As a result, large, varied microbial populations may develop.

The outcome of uncontrolled microbial growth on surfaces is "slime" formation. Slimes typically are aggregates of biological and nonbiological materials. The biological component, known as the biofilm, consists of microbial cells and their by-products. The predominant by-product, extracellular polymeric substance (EPS), is a mixture of hydrated polymers. These polymers form a gel-like network around the cells and appear to aid attachment to surfaces. The nonbiological components can be organic or inorganic debris from many sources which have become adsorbed to or embedded in the biofilm polymer.

Slimes can form throughout once-through and recirculating systems and may be seen or felt where accessible. In nonexposed areas, slimes can be manifested by decreased heat transfer efficiency or reduced water flow. Wood-destroying organisms may penetrate the timbers of the cooling tower, digesting the wood and causing collapse of the structure. Microbial activity under deposits or within slimes can accelerate corrosion rates and even perforate heat exchanger surfaces.

What is Microbial Control Growth?

Microbiological growth in cooling systems presents a risk for the integrity and efficiency of the system, as well as the health of workers and the general community in close proximity with the cooling tower. Microbiological growth control aims at preventing the growth of bacteria, algae and other living organisms from developing in the systems to levels that can be detrimental to operation and health.

Microbiological growth control is typically conducted with a regimented monitoring program and treatment additives that are selected based on the type of organic contaminant observed in a specific system. Microbiological monitoring in cooling systems can be conducted in the field using various techniques such as petri slides, ATP luminescence or ADN scan. These techniques offer different levels of accuracy, range of detection and speed for making treatment adjustments.

Chemical treatment for microbiological growth can be done with oxidizing and non-oxidizing biocides, as well as biodispersants. Oxidizing biocides, such as chlorine and bromine, are typically fed continuously and can be cost-effective microbiological control methods, but can also cause metal corrosion and wood deterioration. Non-oxidizing biocides are often fed sporadically, sometimes in combination with an oxidizing program, to target specific microbiological forms. Biodispersants do not kill microbiological contaminants, but help free them from surfaces and hiding spots while removing the biofilm (i.e. slime) they may secrete to protect themselves. The use of biodispersants then enhances the effectiveness of biocides used in combination.

As all biocides are designed to attack living organisms, their handling, storage and feed practices must be carefully supervised and implemented, using all appropriate PPE (personal protective equipment) and following the SDS recommendations.