Company Training No. 3 – Circulating Water Treatment

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The company’s circulating cooling water system utilizes 4 cooling towers with a capacity of 1000 T/h each; the total water volume in the system is approximately 550 T, while the actual circulation rate is 4000 T/h. The current concentration ratio is between 1.5 and 1.6, with plans to maintain it between 3 and 4. The pipes used in the system are made of copper, carbon steel, and stainless steel. No heat exchangers or test plates have been installed in the system, and the operating temperature of the cooling water is between 28°C and 32°C. Generally, when the system operates at a concentration ratio of 1.5 to 1.6 times, the water quality becomes highly corrosive. In an open-type cooling system, microbial growth can occur on a large scale; therefore, controlling bacterial growth is a top priority in such systems. Experience in water treatment shows that proper treatment and management of circulating water play a crucial role in conserving water resources, reducing operating costs, decreasing the frequency of maintenance tasks, and extending the lifespan of equipment, thereby ensuring the safe and efficient operation of industrial processes. Water treatment primarily involves stabilizing the quality of circulating water, so as to keep the scaling and corrosion caused by cooling water on metals and equipment within acceptable limits. This requires the use of advanced monitoring systems to track changes in water quality at all times, as well as the ability to predict such changes in advance. It also demands the use of high-quality chemical treatments for water treatment, along with extensive experience in managing water treatment processes. I. Control of circulating water quality 1. Scale control When cooling water passes through the heat exchange surface, the following reactions occur: Ca2+ + 2HCO3- → CaCO3 + CO2 + H2O; Mg2+ + 2HCO3- → Mg(OH)2 + 2CO2. At the same time, as the cooling water passes through the cooling tower, it undergoes aeration, during which the CO2 dissolved in the water escapes, causing the pH value of the water to rise. Under alkaline conditions, bicarbonates undergo the following reaction: Ca(HCO3)2 + 2OH- → CaCO3 + 2H2O + CO32-. When calcium chloride is present in the water, a displacement reaction occurs as follows: CaCl2 + CO32- → CaCO3 + 2Cl-. If an appropriate amount of phosphate is present in the water, phosphates react with calcium ions to form calcium phosphate, according to the reaction: 2PO43- + 3Ca2+ → Ca3(PO4)2. The calcium carbonate and calcium phosphate formed in these reactions are both slightly soluble salts, with solubilities much lower than those of calcium chloride and bicarbonates. The process by which scale such as calcium carbonate precipitates from water is a process of crystallization and precipitation of slightly soluble salts from a solution. From the perspective of crystallization kinetics, the crystallization process begins with the formation of crystal nuclei, which give rise to a small number of microcrystals. These tiny crystals then continuously collide with each other due to thermal motion (Brownian motion), as well as with the metal walls of the container. These collisions provide opportunities for the crystals to grow, allowing small crystals to transform into larger ones. In other words, for a layer of calcium carbonate scale to form, the small calcium carbonate crystals must arrange themselves in a specific order within the solution. Calcium carbonate is a salt with an ionic lattice; it can combine with other particles only when a small crystal of calcium carbonate with its positively charged Ca2+ ions collides with another small crystal of calcium carbonate whose ions are negatively charged as CO32-. This collision allows them to form larger crystals. If such collisions continue in a certain direction, a scale layer is formed over the heat transfer surface. From the perspective of the crystallization process of CaCO3, by adding certain chemicals to disrupt its crystalline growth, it is possible to control the formation of scale. 2. Corrosion control: There are various forms of metal corrosion: A. Galvanic corrosion, which occurs when different metals are combined together. Galvanic corrosion is also known as bimetallic corrosion or contact corrosion. When two different metals are immersed in a conductive aqueous solution, there is usually a potential difference between the two metals. If these metals come into contact with each other or are connected by a wire, the internal potential difference will drive electrons to flow between them, thereby forming a corrosion cell. B. Oxygen corrosion and oxygen concentration gradient corrosion caused by dissolved oxygen: Since the electrode potential of metals is lower than that of oxygen, the corrosion of metals by dissolved oxygen in water is a type of electrochemical corrosion, in which the metal acts as the anode where corrosion occurs, while oxygen acts as the cathode where reduction takes place. The reaction equations are as follows: Anodic process: M → M2+ + 2e-; Cathodic process: 1/2O2 + H2O + 2e- → 2OH-. C. Pitting corrosion caused by halide ions: Corrosion caused by halide ions, especially chloride ions, occurs in the form of pitting or crevice corrosion. In such cases, the metal corrodes and dissolves within the pores or gaps, producing Fe2+. This leads to an excess of positive charges in the solution surrounding the corrosion site, which attracts chloride ions from water to that area in order to maintain electrical neutrality. As a result, a high concentration of the metal oxide MCl2 is formed around the corrosion site; subsequently, MCl2 hydrolyzes to form insoluble metal hydroxides and highly corrosive hydrochloric acid: MCl2 + 2H2O → M(OH)2↓ + 2H+ + Cl-. D. Bacterial corrosion is primarily caused by organisms such as iron-feeding bacteria, copper-feeding bacteria, sulfate-reducing bacteria, and nitrifying bacteria. Bacterial corrosion is a special type of corrosion that results in metal degradation due to the direct or indirect involvement of bacteria in the corrosion process (such as by altering electrode potentials and concentration cells). Adding a corrosion inhibitor (also known as a corrosion retardant) to a circulating water system can suppress the corrosion of metals. The corrosion inhibition mechanism of corrosion inhibitors can be viewed from two perspectives: the suppression of electrochemical corrosion and the formation of a protective film on the metal. From the perspective of electrochemical corrosion, corrosion inhibitors suppress the anodic or cathodic processes, inducing polarization on the metal surface and thereby reducing the corrosion current, thus achieving a corrosion-inhibiting effect. From the perspective of film formation theory, corrosion inhibitors form an insoluble protective film on the metal surface, preventing processes such as the diffusion of oxygen in circulating water and the dissolution of the metal. A proper formulation of chemicals is very important; water treatment chemicals used in cooling water are affected by water quality, material leaks, temperature, and the concentration ratio. 3. Microbial control: In the treatment of circulating cooling water, in addition to corrosion control and scaling control, the third issue is microbial control in the water. A. The slime-producing bacteria among bacteria are the most common harmful bacteria in cooling water. In water, they produce a gelatinous, sticky deposit with strong adhesion; these deposits cover the metal surface, reducing the cooling efficiency and preventing scale inhibitors and corrosion inhibitors from performing their functions of preventing scaling and corrosion on the metal surface, thus leading to corrosion beneath the deposits (under-scale corrosion). Iron-depositing bacteria in bacteria grow in iron-containing water, cover the surface of steel, forming oxygen concentration difference corrosion cells; they also remove ferrous ions from the anodic area on the steel surface, accelerating the corrosion of the steel. Others include sulfur-producing bacteria, acid-producing bacteria, and sulfobacteria, all of which are causes of corrosion. B. Fungi, such as molds and yeasts, can attach to wood, pool walls, and heat exchangers, preventing the chemicals from coming into contact with the metal surfaces and thus rendering them ineffective; they can also cause corrosion beneath the deposits. C. Algae will multiply in large numbers on the sun-facing side, forming clusters, remaining suspended in the water, or covering the metal surfaces. Under-scale corrosion caused by deposits in heat exchangers poses a serious threat to such exchangers; therefore, controlling microorganisms is an important aspect of the water treatment process. Use biocides. This is an effective and commonly used method for controlling the growth of microorganisms in water systems. Biocides, also known as bactericides, algaecides, microbial killers, or sterilants, can prevent microbial corrosion and the formation of microbial sludge in water. II. Commissioning of the circulating water system 1. Chemical cleaning: A circulating water system must be cleaned before it is put into use. During installation, equipment and pipelines inevitably accumulate certain microorganisms, biological sludge, air dust, a small amount of scale, and various corrosion products. If these impurities are not cleaned and rinsed away, they will affect the operation of the equipment and accelerate the deposition of suspended particles. In a circulating water system that is in operation, after being used for an extended period of time, when water quality treatment is not adequate, one or more of the following substances can accumulate on the heat transfer surfaces of the heat exchangers: hard deposits such as carbonates, silicates, sulfates, and phosphates, as well as corrosion products resulting from metal oxidation and sludge formed by the growth of bacteria and algae. Even when the water quality is well managed, the turbidity of the circulating water increases after prolonged operation, as changes in its turbidity are influenced by factors such as the turbidity of the make-up water and airborne dust. Therefore, chemical cleaning can improve the coating quality of new systems and reduce corrosion and scaling. For existing old systems, it can ensure long-term safe operation, reduce operating costs, shorten maintenance time, save energy, and extend the service life of the equipment; therefore, sufficient attention must be paid to it in the water treatment process. Currently, there are many methods for cleaning both domestically and internationally, which can generally be divided into two categories: chemical cleaning and physical cleaning. Chemical cleaning is a method that uses chemical agents such as acids, bases, organic chelating agents, and dispersants to dissolve and remove deposits like scale and sludge through chemical reactions. 2. Chemical pre-coating: After chemical cleaning, the pipes and metal surfaces of the equipment in the cooling water system become highly reactive, making them extremely susceptible to corrosion caused by factors such as oxygen in the air and water. If passivation measures are not taken promptly, this can lead to more severe damage to the metal than if no cleaning had been done at all. The purpose of pre-coating treatment is to rapidly form a protective film on the metal surface, thereby enhancing the corrosion-inhibiting effect of the corrosion inhibitor. III. Normal operation and maintenance of the circulating water system: Once the system has been cleaned and pre-coated, it enters regular operation. Based on the characteristics of the circulating water system, appropriate water treatment chemicals are selected, and the following measures are taken to ensure its proper operation: 1. Implement strict process management to improve the rate of compliance with standards. While treating circulating water seems simple – merely by adding various corrosion inhibitors, scale inhibitors, and biocides – actually controlling corrosion, scaling, and microbial growth is not an easy task, as water quality can vary greatly and there are many factors that influence it. The key point is that once a problem arises, whether it’s corrosion or scaling, it is irreversible. There’s a problem with the circulating water; it’s persistent and irreversible, with corrosion and scaling occurring, which only leads to a vicious cycle in which the situation gets worse and worse. It’s impossible to restore the corroded metal to its original state ； The scale that forms must be completely removed; it’s also very difficult to dissolve it. Therefore, in management, it is necessary to try as much as possible to avoid such irreversible factors and take precautions in advance. We take the following measures to ensure that the circulating water treatment is in excellent condition: a) Adhere to an analysis and monitoring system. It is stipulated that water quality parameters shall be analyzed on-site, with the company’s laboratory taking samples once a week; this also includes retesting the water samples collected on the same day to ensure the accuracy and reliability of the analyses. b) Pay attention to the implementation and monitoring of manually adjustable parameters (drug concentration, residual chlorine, alkalinity), and implement process management for key operations. We manage quality control by treating the addition of water treatment chemicals as a key process control point, to ensure that the chemical concentration levels meet the required standards. c) The qualification rate of process parameters, the analysis error rate, and process capability are all included in the assessment scope. 2. Optimize the water treatment formula to ensure proper water quality. Water treatment involves two aspects: one is the formulation of chemicals, where corrosion inhibition, scale prevention, and sterilization all need to be taken into account ； Second is the adjustment of water quality parameters. In addition to chemicals, there are also parameters related to corrosion and scaling, such as turbidity, pH, alkalinity, calcium ions, chloride ions, etc. It is necessary to adjust the water quality standards to meet the requirements of the chemical formulation; therefore, attention must be paid to both aspects. In fact, the latter is quite important, as drugs vary in their sensitivity to turbidity, alkalinity, Ca2+, Fe3+, and solubility. 3. Pay attention to collecting and analyzing various types of information in order to continuously improve work performance. The impact of water quality on cooling equipment is gradual, and it often goes unnoticed; however, problems do arise once they occur. There are hardly any efficient ways to correct it. Therefore, it is essential to pay attention to the collection of various types of information in order to prevent problems before they arise and to address them in advance. Systemic corrosion and scaling will always be reflected in the water quality parameters, although this reflection is subtle and easy to overlook. If the total iron concentration in the water rises or remains high, it indicates that corrosion has occurred in the system ； An increase in turbidity, heterotrophic bacteria, sludge, and orthophosphorus in the water, or an imbalance in the concentration ratio of Ca2+ to other ions, all indicate that sedimentation may be occurring and that there could be microbial issues. During major and minor repairs of cooling cycle equipment, the heat exchange units are opened for inspection, which represents a rare opportunity to assess the effectiveness of water treatment. When the heat exchange equipment is turned on, it is necessary to carefully check for signs of corrosion, scaling, or biological fouling; this is what truly determines the effectiveness of water treatment. Sometimes, when possible, one or two dedicated devices are used for testing in order to obtain reliable information. Analyzing the collected scale samples can also help determine the source of the problem. 4. Pay attention to the impact of process conditions on water treatment efficiency. Circulating water is used as a cooling medium in heat exchange equipment, and issues such as corrosion and scaling are closely related to the process conditions of the equipment itself; this effect is particularly significant in equipment designed for low flow rates and high temperatures. This is one of the reasons why equipment operating under different conditions within the same circulating water system exhibits significantly different tendencies toward corrosion or scaling. 5. Concentration ratio management: The ratio of the salt content in the recycled water to that in the make-up water is the concentration ratio. This is an important technical and economic indicator in circulating water treatment. The control method is to strictly prohibit arbitrary drainage and random connection of water pipes, ensuring a closed-loop circulation of the system. Address any leaks promptly. The designed concentration ratio is 3.0; at the initial stage of operation of the equipment’s circulation system, when the heat load is low, it is difficult to achieve this ratio, so it is kept within the design specification (a concentration ratio of 3.0). As the equipment’s cooling system becomes fully operational, the concentration multiplier gradually increases; when it reaches the designed capacity, this multiplier can be as high as 4.0–5.0 times. Since the circulating water concentration ratio is not only a water quality indicator but also an economic indicator for operation. According to the water balance of the circulating water system: M = E + B, where M represents the amount of water to be added ； E is the evaporation amount ； B is the total amount of waste discharged. The evaporation rate E is a function of the water circulation volume, the temperature difference between the water entering and leaving the tower, and the latent heat of evaporation. The wastewater discharge volume is given by B = E/(K-1), where K is the concentration factor. The amount of water to be added is given by M = E + B = E + E/(K–1). Using these formulas, it is possible to determine the relationship between the amount of water added, the amount of wastewater discharged, and the concentration factor. The following table shows this relationship:

Concentration Factor: 1.5, 1.8, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0
Ratio of water added to the circulation volume/%: 5.2, 4.7, 3.5, 2.6, 2.3, 2.2, 2.1, 2.0
Ratio of wastewater discharged to the circulation volume/%: 3.4, 2.3, 1.7, 0.8, 0.5, 0.4, 0.3, 0.2

As can be seen from the table, as the concentration factor increases, both the amount of water added and the amount of wastewater discharged decrease. This means that the amount of chemicals required for water treatment also decreases, thereby reducing the total cost associated with water treatment (water + chemicals). Therefore, under the condition of ensuring excellent water treatment performance, appropriately increasing the concentration ratio is a key focus in water treatment operations. The management concept for the concentration ratio in water treatment is to maximize this ratio as much as possible, while ensuring the quality of the water. As mentioned earlier, when water quality indicators are poor – such as high levels of total iron ions resulting from corrosion, turbidity caused by bacteria and algae, high levels of silt, or other factors that cause certain indicators to exceed acceptable levels – it becomes necessary to discharge wastewater, which reduces the concentration factor. Therefore, the concentration factor can only be increased under the condition of good water quality. Only in this way can a complete and effective water treatment plan be established for the operation and management of circulating water. 6. Water quality management of make-up water: Changes in the quality of make-up water can also affect the quality of circulating water. When parameters such as pH, suspended solids, total iron, and chloride ions in the make-up water exceed the specified limits, it can lead to changes in the quality of circulating water, or even a deterioration in its quality. 7. The key to water treatment technology during normal operation is how to properly manage the relationship among water quality parameters, chemical dosing schemes, and concentration ratios in chemical water treatment. Water quality parameters, chemical treatment plans, and concentration ratios complement each other and constrain one another.