Microbiological pollution of water is of serious concern to mankind. This not only causes spread of diseases but also microbiologically mediated corrosion and fouling leading to bio-film formation. Bio-films are very common in natural and industrial water and if not controlled, may lead to fatal diseases as well as serious problems in the vital industrial operations.

BIOFILM is an ideal environment for survival a number of organisms including bacteria, protozoa, amoebae, fungi, barnacles and other organisms and create nuisances in man-made environments. The bio-films are formed as results of gummy substances excreted by microorganisms, having a property of adherence. The growth of a bio-film can progress to a point where it provides a foundation for the growth of seaweed, barnacles, and other organisms. Bio-films are reservoirs of pathogenic organisms giving protection against disinfectant action. The metabolic activities of these organisms results into formation of acidic environment which enhances the dissolution of material from the surfaces. Bio-film results into bio-fouling causing serious damages to surfaces, pipelines, water conducing systems in many industrial processes including the food, pharmaceutical, paint, oil processing and manufacturing, and engineering industries using water either in the process (chemical, pharmaceutical and food) or as a utility (cooling, lubrication etc). Power generation stations are frequent targets of fouling invertebrates, including zebra mussels, colonial hydroids, bryozoans, and sponges. An optimal combination of abundant food, warm temperatures, flowing water can result in serious growths in the intake chambers, filters, condenser tube sheets, and other structures. While the populations may die off seasonally, they leave behind dormant reproductive bodies capable of producing a new, larger generation when conditions are favorable.

Biofouling and Cooling Water Systems

The industrial cooling water systems provide an ideal medium (32-38 oC water temperature and 8-9 in pH) for biofouling. The water source contains all the naturally occurring living organisms and nutrients that are responsible for uncontrolled multiplication of the living organisms. Apart from biofouling, the biological growth plays an important role in promoting corrosion through the formation of acid metabolic products resulting in corrosion damage to the heat transfer and other process equipments.

Most of these problems are due to variety of nuisance bacteria entering in the system through various sources. The most commonly encountered bacteria in re-circulatory water systems are species of Bacillus, Proteus, Pseudomonas, Escherichia, Enterobacter, Salmonella, Staphylococcus, Thiobacillus, Flavobacterium, Sphaerotillus, Galionella, Leptothrix, Ferrobacillus, Nocardia, Streptomyces, Nitrobacter, Nitrosomonas, Azotobacter, Legionella, Desulfovibrio, Clostridium etc.

Among fungi are species of sperigillus, Penicillium, Chaectomium, Cladosporium, Basidiomycetes etc, while among algae are species of Anabena, Nostoc, Oscillitoria, Ulothrix, Chlorella and many more.

Bacterial population in re-circulatory water systems can exceed one million per ml and the best levels that should be achieved in conventional treatment program are in thousands. According to the Center for Disease Control, when bacterial populations reach or exceed 500,000 colonies per ml there is an enhanced risk of Legionella growth. High levels of bacteria can also pose increased risks for microbiologically influenced corrosion and the formation of bio-film on wetted surfaces.

Biofouling has been recognized as an important contributor to impaired heat transfer causing decrease in efficiency and increased power consumption. 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 inorganic scales or corrosion deposits. Biofouling can also destroy cooling tower lumber. More seriously, such contaminants can be harmful to humans coming in contact like operational and maintenance personnel. The most notable example has been the outbreaks of Legionnaires' disease, affecting people in hotels, hospitals, office buildings, and other locations, who have come into contact with cooled air from an air conditioning system contaminated from cooling tower water of the air conditioning plant.

Microbial fouling with direct or indirect corrosive effects and interference in re-circulation is controlled by using chemicals collectively known as biocides, which have either cidal of inhibitory action on microorganisms. Various factors influence biocidal activities of these chemicals and among these, development of resistance and immunity is the most important one and is mainly responsible for inactivation of biocides. Bacteria can develop resistance to destroy biocides by: