Discussion on the recovery technology for concentrated wastewater from circulating cooling water systems

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In a circulating cooling water system, heat is removed through evaporation, which causes the salts in the cooling water to become increasingly concentrated. As the water is reused over and over again, when the calcium and magnesium ions in the cooling water reach saturation, they precipitate on the surface of the heat exchangers and within the pipes, thereby affecting the lifespan of the equipment as well as the overall functionality of the system. This is why the concept of concentration ratio was introduced. If the concentration ratio is set too high, the tendency for scaling and corrosion in the system increases, leading to system instability ； The concentration ratio is set too low, resulting in excessive drainage volume of the system, which makes its use uneconomical. Once the system reaches an appropriate design concentration ratio, it is necessary to discharge a certain amount of system water and introduce fresh water in order to maintain stable water quality in the system. Nevertheless, for circulating cooling water systems, the volume of wastewater generated is excessive. Given the scarcity of water resources in today’s society, it is an urgent issue that we need to find an effective method for recycling the concentrated wastewater from these systems. The concentrated wastewater reuse technology, in other words, involves removing the excess salts from concentrated wastewater ； Among the currently available desalination technologies, common methods for removing salts include reverse osmosis and ion exchange, both of which have many limitations. If reverse osmosis is used, the system requires high standards for water pretreatment; high-pressure pumps consume a lot of energy, the yield of water is low, and the cost of producing water is high ； The ion exchange method involves high costs for regenerating acids and bases; the regeneration waste fluid can easily cause secondary environmental pollution, and the system requires high levels of operational complexity. The quality of water in the circulating cooling water system is poor: it has a high level of suspended solids, high turbidity, high magnesium ion content, high alkalinity and hardness, high levels of various ions, as well as high levels of disinfectants. The complex composition of this water directly affects the application of desalination processes. Compared to traditional desalination methods, capacitive desalination requires less energy, has lower costs, and is easy to regenerate without the need for chemical agents; it is an economical and effective approach. Capacitive electrolysis uses a combination of graphite electrodes and ion membranes, which is known as a membrane electrode ； The membrane electrode combines the advantages of capacitive adsorption with the function of ion-exchange membrane dialysis, which is why it is called capacitive deionization technology. Arsenic, nitrates, fluorides, perchlorates, ammonia nitrogen, sulfates, metal ions, and other ionic compounds present in water can all be treated using capacitive precipitation technology. During the electroadsorption desalination process using MCDI, the anion membrane allows only anions to pass through while blocking cations, whereas the cation membrane permits only cations to pass through while blocking anions. During the desorption of ions by capacitive MCDI, when the electrodes are reversed, the ion membrane prevents ions from adhering to the opposite electrode, resulting in more thorough desorption of the ions. This also enhances the desalination capacity of the electroadsorption device during continuous adsorption and desorption processes. The distance between the membrane carbon electrodes is just one layer of membrane, which is virtually zero. This replaces the traditional design in which spiral or other types of liquid channels were provided between the electrode plates; in that design, the wastewater to be treated flowed over the electrode plates from all sides for adsorption. The greatest advantage of this module is its ease of disassembly, allowing the number of membrane electrodes to be adjusted as needed. Moreover, the close distance between the electrode plates ensures that it retains good adsorption capacity for ions even when dealing with solutions flowing at high velocities. The unique desalination mechanism of capacitive separation is applicable to various water qualities, and it offers significant advantages when used for the recycling of concentrated wastewater in closed-loop cooling water systems. We should pay attention to this technology and expand its application in the field of reducing emissions from circulating cooling water concentrate in the future.