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Microbial hazards are very serious in circulating cooling water systems; some believe that in terms of chemical treatment, microbial control can be described as something where success and failure go hand in hand. Some foreign articles also suggest that “the biggest challenge comes from microbial reproduction.” Compared to scale and electrochemical corrosion, its hazards are even greater. The harms that microorganisms cause to systems are nothing more than dirt and corrosion. When it manifests itself, it is often combined with the hazards of scale and electrochemical corrosion. Microbial sludge is primarily composed of microbial cells along with the sticky substances that bind them together (such as polysaccharides and proteins). In open-loop circulating cooling water systems, failures caused by sludge-forming bacteria are the most common, followed by those caused by algae, molds (filamentous fungi), and ball-shaped bacteria (filamentous fungi). 1 Growth environments of various microorganisms: The growth temperature for the vast majority of water microorganisms is between 20 and 40℃ ； pH 5–9; some microorganisms can survive even at a temperature of 70℃ ； Reproduction is also suitable at pH values between 2 and 3. Therefore, microbial activity occurs and grows in every circulating cooling water system. 2 Main hazards of microbial growth and sludge: ① Formation of dirt deposits, which reduces the cooling efficiency of the cooling water, increases energy consumption, and affects normal production. ②A large amount of sludge will block the channels for cooling water in the heat exchanger (cooler), preventing the cooling water from functioning properly ； A small amount of sludge reduces the cross-sectional area of the cooling water channels, thereby decreasing the flow rate of the cooling water and its cooling efficiency, and increasing the pump pressure. ③ Sludge accumulates on the surface of the cooling tower fill or between the fill elements, blocking the flow of cooling water and reducing the cooling efficiency of the tower. ④ Sludge covers the metal surfaces inside the heat exchanger, preventing corrosion inhibitors and scale inhibitors from reaching those surfaces to exert their protective effects. It also stops biocides from killing the microorganisms present in the sludge or beneath it, thereby reducing the effectiveness of these chemicals and exacerbating corrosion beneath the scale. ⑤ Sludge covers the metal surface, forming differential corrosion cells that cause corrosion of these metal devices. Scaling, corrosion, and microbial issues are the three major hazards that impede the proper operation of circulating cooling water systems. As can be seen from the above analysis, these three problems affect each other; therefore, the chemical treatment strategy for circulating cooling water systems must be a comprehensive one. An effective treatment plan for such systems needs to provide good scale inhibition and corrosion prevention effects, as well as efficient biocidal and sterilization capabilities.