The impact of reclaimed water reuse on circulating water and countermeasures

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Reuse of treated wastewater has a history of nearly 30 years in China. As early as 1973, the Dongfanghong Refinery began to reuse some wastewater that had undergone secondary treatment in its circulating water system. Since then, more and more enterprises have started to use such treated wastewater as makeup water for their systems. However, due to the effectiveness of wastewater treatment processes, the quality of this treated wastewater is unstable, which affects its practical application. Therefore, a wastewater reuse treatment unit with a capacity of 200 t/h was installed in the drainage workshop. Affected by factors such as the source of reclaimed water and its treatment capacity, since 2012, reclaimed water has been primarily used as makeup water for the first circulation system. The amount of water reused is kept between 50 and 90 tons per hour, depending on production needs, with a reuse rate of 30% to 60%. 1 The impact of reclaimed water reuse on the circulating water system: When reclaimed water is used as supplementary water in the circulating water system, it is necessary to ensure that all relevant parameters are within acceptable limits in order to minimize the impact on the water quality of that system. However, in practice, due to the many uncontrollable factors involved in wastewater treatment, the water quality of the reclaimed water can fluctuate frequently. In the short term, this has little effect on the circulating water system; but if such fluctuations persist for an extended period, they can cause abnormalities in the control parameters of the circulating water, thereby posing difficulties in its operation and management. 1.1 The impact of ammonia nitrogen: In the plant where I work, treated wastewater that meets the required standards is generally used as the source water for reclaimed water treatment. However, when the wastewater treatment plant is affected by external water inflows, the effectiveness of ammonia nitrogen removal is compromised; this ultimately results in elevated levels of ammonia nitrogen in the reclaimed water. When ammonia nitrogen levels are too high, it will have an impact on various aspects such as the quality of the circulating water and chemical dosing control. (1) Causes the pH of circulating water to be low. Ammonia nitrogen undergoes nitrification under the action of bacteria, producing H+, which causes abnormal fluctuations in the pH of circulating water. The pH is greatly affected by the high level of ammonia nitrogen in the reclaimed water, resulting in a generally low pH value. If this is not adjusted in time, it will **increase the risk of corrosion in the system**. (2) It increases the difficulty of controlling heterotrophic bacteria in circulating water. Nitrogen and phosphorus are essential nutrients for the growth of bacteria and algae. When the ammonia nitrogen level is high, it provides sufficient nutrients for bacteria; in an environment at their optimal temperature, these bacteria multiply rapidly, which can increase the amount of biological sludge in the system’s pipes. This biological sludge adheres to the surfaces of the equipment, not only affecting heat exchange efficiency but also contributing to corrosion beneath the scale. (3) Increase the consumption of circulating water bactericides. The disinfectants currently used in circulating water systems are mainly oxidizing active chlorine disinfectants. Ammonia nitrogen can undergo redox reactions with active chlorine to produce chloramines, thereby reducing the concentration of free chlorine in the water that actually has a disinfecting effect. On the other hand, to ensure the disinfection effect and maintain a certain concentration of residual chlorine in the circulating water, it is necessary to increase the amount of disinfectant used. This not only raises production costs but also, due to the high chloride levels in the water, accelerates the rate of equipment corrosion. 1.2 Impact of alkaline hardness: The sum of calcium hardness and methyl orange alkalinity is referred to as the circulating water tolerance, which serves as an important indicator for assessing the risk of scaling in circulating water. In accordance with the requirements set out in the \"Regulations on the Operation and Management of Industrial Circulating Water,\" it is recommended that the optimal control range should be below 1,100 mg/L. In actual production processes, based on years of experience with the first cycle of operation, the most suitable control range is below 900 mg/L. If this control limit is exceeded, it will increase the risk of scaling in the system, as well as cause abnormalities in parameters such as the pH of the circulating water. After the reuse of reclaimed water, changes in the alkalinity and hardness of the circulating water occurred, leading to some problems. (1) Increases the risk of scaling. When the quality parameters of reclaimed water are normal, the tolerance level remains around 800 mg/L; however, if the alkalinity of the reclaimed water is high, it leads to an increase in the tolerance level for circulating water. In April 2013, when the alkalinity of the recycled water was above the specified limit, the total alkalinity of the circulating water remained above 500 mg/L throughout the month; the acceptable level was even higher at over 900 mg/L, and the excess levels persisted for an extended period. (2) Causes a high pH. The pH of circulating water is primarily influenced by the concentrations of OH-, HCO3-, and CO32- in the reclaimed water. In the same water system, the level of alkalinity also indirectly reflects the pH value: higher alkalinity corresponds to a relatively higher pH, while lower alkalinity results in a relatively lower pH. Under normal conditions, the pH in water is determined by OH-, HCO3-, and CO32- ions, and a certain equilibrium is achieved: 2HCO3- → H2O + CO2 + CO32-. When the pH is too high, the concentration of CO32- increases; since the solubility product of CaCO3 is low, this inevitably increases the risk of scaling in the system. In April 2013, when the alkalinity of the recycled water was below the required standard, this led to a high pH level in the circulating water; the average pH for that month was around 9.1, which was close to the upper limit set for process control. 1.3 Influence of conductivity: Conductivity reflects the amount of various conductive ions present in water. A high conductivity indicates a high level of salts in the water, and high salt content inevitably increases the potential risk of corrosion in the system. Before reusing the recycled water, the conductivity generally stays around 1,800–2,000 μS/cm per cycle. However, once water that meets the required standards is introduced, its conductivity is usually 2–3 times higher than that of fresh water used for replenishment, which causes the conductivity of the recycled water to rise rapidly to 2,300–2,500 μS/cm. When the conductivity of the circulating water exceeds the control limit of 1,200 μS/cm, it will further lead to a significant increase in the conductivity of the circulating water. Excessively high conductivity forces the circulating water system to require appropriate amounts of wastewater discharge and water replacement, which not only wastes fresh water but also increases the cost of chemicals. The concentration ratio of the recycled water decreases after wastewater discharge, which will further increase water consumption in later stages. 1.4 Effects of residual chlorine: Before being pumped to the circulating water system by lift pumps, the reclaimed water also needs to be disinfected; the disinfectant currently used is hypochlorite. Since the laboratory analysis plan only involves regular testing of bacterial levels, the monitoring of residual chlorine requires team members to take samples and conduct analyses on their own, which leads to irregularities in the monitoring of residual chlorine in reclaimed water. Additionally, differences in individual handling techniques can reduce the accuracy of residual chlorine measurements, and the addition of chlorine compounds can easily cause fluctuations in the residual chlorine levels in reclaimed water. By observing the trends in residual chlorine levels in reclaimed water and recycled water in 2011, it was found that the control of residual chlorine in reclaimed water remained on the high side throughout that year, with an average value of 8.0 mg/L; there were also significant fluctuations. The use of recycled water led to variations in the residual chlorine levels in this water. (1) Causes an increase in chloride levels in the circulating water. For heat exchangers made of stainless steel, the presence of chloride ions is an important factor that induces pitting corrosion in such heat exchangers. If the residual stress in the metal material has not been eliminated or if high temperatures are present, the presence of chloride ions can lead to stress corrosion cracking. In circulating water systems, the concentrations of chloride and sulfate ions are generally controlled. However, since sulfate has little direct impact on such systems – its effect being roughly 1/10 that of chloride – and because the amount of sulfate introduced by reclaimed water is small in daily operations, attention should be focused primarily on the concentration of chloride ions. Before the introduction of reclaimed water, the mass concentration of chloride ions in the circulating water generally remained between 150 and 250 mg/L. However, after the use of reclaimed water, this concentration rose to between 300 and 450 mg/L. When the chloride ion concentration in the reclaimed water exceeded 200 mg/L, it led to an further increase in the chloride ion concentration in the circulating water. In 2011, due to high chloride ion concentrations in the reclaimed water, the chloride ion concentration in the circulating water reached as high as 536 mg/L, which resulted in corrosion rates exceeding the allowable levels in some months. (2) Fluctuations in residual chlorine in reclaimed water increase the difficulty of adding concentrated chlorine to the circulating water. Based on production requirements, the circulating water system adds chloramine as a disinfectant daily, according to the test results. Since hypochlorite is a solid in block form, its solubility in water is affected by various factors such as water temperature and flow rate, making it difficult to control its dosage quantitatively, as is the case with chlorine-based disinfectants. Fluctuations in the residual chlorine level of reclaimed water make it more difficult to determine the daily dosage of hypochlorite, and it is hard to keep this residual chlorine level under control. 1.5 Impact of COD: Since the reclaimed water source is treated wastewater, COD is a very important control parameter that is prone to exceeding specified limits. After reusing reclaimed water with excessive COD levels, the circulating water system is affected to a certain extent in terms of water quality. (1) Increased the amount of fungicide used. Since oxidizing disinfectants are used in the circulating water system, they tend to react with these organic substances. To maintain the desired concentration of the disinfectant, it is necessary to increase the amount of hypochlorite added, which undoubtedly raises the operating costs. (2) It promoted the growth of microorganisms. Organic matter provides microorganisms with abundant nutrients; when the environmental temperature is suitable, these microorganisms multiply in large numbers, increasing the amount of sludge in the system. This can lead to a decrease in the heat exchange efficiency of the heat exchangers, as well as cause biological corrosion. In April 2014, the high COD level in the recycled water caused the adhesion rate of the circulating water tubes to exceed acceptable levels significantly. 2 Strengthen daily management measures for reclaimed water reuse. Reclaimed water reuse increases the complexity of managing circulating water systems, but given the scarcity of water resources, saving water and reducing emissions is a trend for enterprises’ future development. Various problems arising from the use of reclaimed water can be further reduced by strengthening daily on-site management and continuously optimizing process operations, thereby minimizing the impact of reclaimed water on circulating water systems. 2.1 Strengthening the control of water sources for reclaimed water systems: The key to wastewater reuse lies in using reclaimed water that meets the required standards. In the design of the wastewater reuse system at my factory, the impact of salt content on the circulating water system was not taken into account. Moreover, under current conditions, wastewater treatment plants have limited capacity to handle saline wastewater; relying solely on treated wastewater that meets the standards as a source for reclaimed water systems cannot ensure that the salt content remains within acceptable levels. To this end, a dedicated DN 200 pipeline was installed from the emergency tank to the drainage system, used to transport the clean wastewater collected in the emergency tank to the drainage workshop, where it is utilized to dilute the reclaimed water source, thereby reducing the levels of salt, ammonia nitrogen, and COD in the reclaimed water to some extent. The water quality of the wastewater reused after the improvement of water sources has seen significant improvement. However, as various facilities are currently focusing on water conservation and emission reduction, fewer and fewer clean wastewater streams are available to be collected in the emergency tanks on a daily basis. When the wastewater treatment plant is overwhelmed and operations become abnormal, this reduction in the amount of clean wastewater available can lead to instability in the quality of the reused water. Therefore, ensuring the stability of the water supply for reclaimed water treatment systems is key to maintaining the quality of the reclaimed water at acceptable levels. It is recommended to improve the management of wastewater separation and cleaning in these systems; sewage treatment plants should enhance water quality monitoring, and when abnormalities are detected, they should promptly increase the amount of clean wastewater used for dilution to ensure that the output water meets the required standards. 2.2 Strengthening the monitoring of reclaimed water quality: Since the recycling water facility conducts analyses of reclaimed water quality only once a week, the wastewater treatment unit carries out daily analyses of the key quality indicators of reclaimed water. In daily operations, team members should not only pay attention to the amount of reclaimed water being reused but also monitor its ammonia nitrogen and COD levels on a daily basis. If these indicators exceed the specified limits or are too high, it is necessary to contact the wastewater treatment equipment to make adjustments; if the indicators remain above the limits for consecutive days, the use of reclaimed water should be suspended to prevent any impact on the quality of the circulating water. It is absolutely unacceptable to use reclaimed water that does not meet the required standards just to ensure an adequate supply of such water. 2.3 Formulate a reasonable chemical dosing plan; chemical dosing is crucial for stabilizing the quality of circulating water. When using reclaimed water, it is first necessary to pay close attention to the trends in the quality of the circulating water. When the indicators related to the reclaimed water are on the high side, it is important to proactively adjust the amount and frequency of use of corrosion and scale inhibitors as well as disinfectants. Secondly, control plans for adding chemicals in case of common water quality abnormalities should be developed gradually – for example, controls over the amount of alkali or acid to be added when pH is too low or too high, and methods for adding hypochlorite when residual chlorine levels are low. These measures aim to ensure that chemical addition is carried out accurately and steadily, while also taking into account practical experience. Thirdly, it is essential to regularly assess whether the current formulation of water treatment chemicals is suitable for the amount of reclaimed water being used and its quality, based on the operating conditions of the circulating water system. This helps to improve the effectiveness of corrosion and scale inhibitors and reduce the risk of corrosion and scaling in the system. 2.4 Strengthen the daily operation management of circulating water: (1) Manage and make proper use of the side filtration facilities to increase the amount of water that can be filtered by the circulating water system, ensuring that this value is greater than 3.0%. Identify the optimal backwashing cycle for the filter; depending on the water quality, the frequency and duration of backwashing can be increased appropriately. (2) In view of the high salt content in reclaimed water, its tendency to form scale, and the resulting corrosion beneath the scale, the circulating water system is regularly cleaned by sludge removal on a quarterly basis, with enhanced process monitoring to ensure the effectiveness of the cleaning. (3) To address the issue of too rapid an increase in the concentration ratio of circulating water, a method of making continuous small-scale water exchanges can be employed to maintain stability in this concentration ratio. 2.5 Gradual increase in the reuse rate of reclaimed water: The level of reclaimed water reuse should be linked to the management of the circulating water system and its capacity to handle certain water quality parameters; it’s not true that the higher the amount, the better. Maintaining an equilibrium among the various ions in the water is what’s most important. In practical use, it is advisable to adjust the amount of reclaimed water used in stages, gradually increasing the proportion of recycled water. At each stage, attention should be paid to changes in indicators such as bacteria levels in the circulating water, corrosion rates, and the amount of chemicals added, so as to accumulate experience for increasing the usage volume in the future. 3 Conclusion (1) With the scarcity of water resources, the reuse of reclaimed water will be a major trend in future social development, as well as a key method for enterprises to save water and reduce emissions. Nearly two years of production experience have shown that the reuse of qualified, high-quality treated reclaimed water not only helps to maintain the quality of circulating water at acceptable levels, but also significantly reduces the consumption of fresh water; the economic and social benefits associated with energy savings are evident. (2) At the current stage, the reuse rate of reclaimed water remains at 50%–60%; over the past two years, the overall compliance rate of the quality of recycled water has been 99.61%. In the future, various measures will still need to be taken to further improve the compliance rate of reclaimed water quality as well as the stability of its supply. It is also necessary to strengthen operational management and water quality monitoring during the use of reclaimed water, make appropriate adjustments, and strive to achieve a 100% reuse rate for this water. (3) Pay attention to the technological development of advanced treatment for reclaimed water in China, strive to introduce sophisticated treatment processes and equipment, continuously improve the volume of reclaimed water that can be treated as well as the stability of its quality, thereby laying a solid foundation for the reuse of reclaimed water in other closed-loop water systems in the future.