Microbiological Control in Cooling water Systems

Abstract

One of the primary problems affecting efficient operation of cooling water system is accumulation and excessive growth of microorganisms. In the operating plant, biological fouling may severely affect process equipment, leading to a progressive reduction in performance and efficiency. Cooling water systems usually provide optimum conditions for the microbial growth. Ranges of temperature and pH, continuous aeration as well as an abundance of organic and inorganic nutrients and sunlight make cooling towers an ideal place for proliferation of many biological species. As a result, diverse microbial populations may develop. A number of associated problems can occur due to biological factors in cooling systems, including:-Reduction of heat transfer in heat exchangers due to slime and biofilm formation;-Restriction of water circulation in condenser tubes;-Plugging tubes in heat exchangers causing shutdowns and loss of production in power utilities;-Clogging of water lines in cooling water system due to algal mats;-Pitting corrosion of metals caused by MIC;-Biodeterioration of wooden components of cooling towers;-Spread of Legionnaires Disease The main source of microbiological contamination is usually the make-up water from the water source (lakes, rivers, wells, ocean, etc). Another source of contamination is an atmosphere. Many microorganisms are removed from air when it is washed with water in the cooling tower. Biological problems in cooling water systems are usually site-specific depending both on biological aspects of the cooling water and environmental and technological conditions in a particular system. Each cooling water system presents a unique combination of equipment, water chemistry, water microbiology, contaminants, temperature, blowdown, make-up, etc. Monitoring of cooling water parameters is an essential part of the process of selection of cooling water treatment measures. Control of biological problems in cooling water needs a multifaceted approach including selection of cooling tower design, proper construction materials, cooling system maintenance, application of biocides/microbicides, condenser tubes cleaning, etc. The primary method of biofouling control is usually proper biomonitoring and application of appropriate cooling water biocides/ antimicrobials. The choice of biocidal program is determined by many factors, but three stand out above all others: efficiency, cost and environmental acceptability. These and other related aspects of biocide implementation in cooling water system will be reviewed in the current chapter. 5.2.2 Cooling water systems Cooling water systems are critical components of most major industries, including energy generating, general manufacturing, oil production, chemicals, steel, metalworking, etc. The main function of a cooling system is to remove heat from a process or equipment. Most often the cooling medium is water. Appliances that have to be cooled include condensers and heat exchangers, oil, air, gas and liquid coolers, motors and compressors, furnaces, rolling mills, chemical reactors, etc. The cold water enters these appliances and is heated by contact with hot surfaces. At the outlet the heated water can be handled in one of three ways: 1) the hot water is discharged into a river or other receiving body (open once-through cooling system); 2) hot water is cooled by contact with secondary fluid and is returned to the appliance without coming into contact with the atmosphere (closed system); 3) hot water is cooled in a cooling tower through partial evaporative condensation and then returned to the appliances (open recirculating water system). The systems of the third type are used most often. During evaporation at the cooling tower the system looses some pure water which initiates an increase in concentration of the remaining dissolved solids. In order to control this concentration some water must be removed from the system (blowdown) and replenished with fresh water from the water source (make-up). The ratio of the make-up to the blowdown is called ''cycles of concentration'', which is a very important parameter in cooling water treatment practices relating to the level of closure of the system. What is happening in cooling water? In correlation with cycles of concentration, dissolved solids concentrate, which lead to corrosion and deposition problems. Deposition is based on two mechanisms-scaling and fouling. 121