The "Water Cycle" of cooling towers: How to Reduce Wastewater Discharge and Achieve Efficient Utilization of Water Resources? ​

Cooling towers play a crucial role in heat dissipation in industrial production and the operation of modern facilities. However, the problems of water resource consumption and wastewater discharge during its operation have gradually become prominent. In the current situation where global water resources are increasingly tight and environmental protection requirements are becoming more and more strict, exploring the optimization path of the water circulation system of cooling towers to achieve efficient utilization of water resources and reduction of wastewater has become an inevitable trend in the development of the industry.

Overview of the Cooling Tower Water Circulation System

The water circulation system of cooling towers is mainly divided into two types: open and closed. In an open water circulation system, water and air come into direct contact for heat exchange. The working process is as follows: Hot water flows out from the equipment that needs to be cooled and enters 1the top of the cooling tower. It is evenly distributed to the water distribution device through the water distribution system. In the water distribution device, the water is dispersed into fine water droplets or water films and fully contacts with the air flowing upward. Heat is transferred to the air through evaporation and convection. After the water temperature drops, it is collected in the water collection pool at the bottom of the cooling tower and then transported back to the equipment for recycling by the water pump. However, this system has significant drawbacks. As water is directly exposed to the air, the amount of evaporation loss is large. At the same time, it is prone to contamination by dust, impurities and microorganisms, leading to deterioration of water quality. To maintain water quality, frequent sewage discharge is required, which in turn causes a large waste of water resources.

The closed water circulation system effectively avoids these problems. Take the common closed cooling tower as an example. The process fluid to be cooled (such as hot water, water-ethylene glycol solution, etc.) circulates in a closed coil and does not come into direct contact with the outside air and spray water. The spray water system draws water from the water collection tank and evenly sprays it onto the outer surface of the coil through nozzles to form a water film. Under the action of the fan, external air flows through the coil area. At this point, the hot process fluid inside the coil first transfers heat to the spray water through the tube wall (sensible heat transfer). Part of the heated spray water absorbs heat and evaporates into water vapor, taking away a large amount of latent heat, thereby achieving the cooling of the fluid inside the coil. The unevaporated spray water falls back into the water collection trough for recycling. A small amount of concentrated water is regularly discharged through the drain valve, while the make-up water valve automatically replenishes fresh water to maintain the stability of water quality and water level. Closed systems significantly reduce water evaporation and pollution, and greatly improve the efficiency of water resource utilization.

Strategies for reducing wastewater discharge and improving water resource utilization

1.Optimize water treatment technology

Water quality stabilization treatment is a crucial step. By adding appropriate water quality stabilizers, such as scale inhibitors, corrosion inhibitors and biocides and algaecides, the problems of scaling, corrosion and microbial growth in circulating water can be effectively controlled. Scale inhibitors can prevent calcium, magnesium and other ions in water from forming insoluble salts, which adhere to the surface of pipes and equipment, reduce heat exchange efficiency, and thereby decrease the discharge of wastewater caused by cleaning scale. Corrosion inhibitors can form a protective film on the metal surface, inhibit metal corrosion, extend the service life of equipment, and reduce the additional water consumption caused by equipment corrosion maintenance or replacement. The bactericide and algaecide can kill bacteria, algae and other microorganisms in the circulating water, prevent microbial slime from clogging pipes and affecting water quality, and avoid frequent sewage discharge due to water quality deterioration.

In addition, by adopting advanced filtration technologies such as ultrafiltration and reverse osmosis membrane filtration technologies, tiny particles, colloids, organic matter and some ions in the circulating water can be effectively removed, further improving water quality, reducing the impact of impurities on the system, and thereby reducing the amount of sewage discharged. Some enterprises have introduced ultrafiltration devices into the cooling tower circulating water system, which can remove impurities with a particle size larger than 0.01 microns from the water, significantly reducing the turbidity of the circulating water. This greatly reduces the discharge of wastewater caused by water quality issues and simultaneously improves the reuse rate of the circulating water.

2. Implement precise pollution discharge control

The traditional regular sewage discharge method often lacks specificity and will discharge a large amount of water resources even when the water quality is still good. With the help of water quality monitoring sensors and automatic control systems, precise sewage discharge can be achieved. Real-time monitoring of key indicators such as conductivity, pH value, hardness and turbidity of circulating water is carried out. When the water quality parameters exceed the set reasonable range, the automatic drainage device is activated to discharge an appropriate amount of concentrated water and replenish fresh water, ensuring that the water quality is always maintained within the appropriate operating range. A large chemical enterprise has installed an intelligent water quality monitoring and sewage discharge control system in the water circulation system of the cooling tower. It precisely controls the sewage discharge volume based on the changes in conductivity. Compared with the previous regular sewage discharge, the wastewater discharge volume has been reduced by more than 30%, and a large amount of make-up water has been saved at the same time.

3. Carry out the recycling and reuse of wastewater

After undergoing advanced treatment, the wastewater discharged from the cooling tower can be reused for non-potable purposes. For instance, the treated wastewater can be used for irrigation of the factory area's greenery, road washing, toilet flushing, etc. By building dedicated wastewater treatment stations and adopting processes such as neutralization, sedimentation, filtration and disinfection, harmful substances in the wastewater can be removed to meet the corresponding reuse standards. In some water-scarce areas, some power plants use the treated wastewater from cooling towers for irrigating green plants within the factory area. This not only realizes the recycling of water resources, reduces the use of fresh water resources, but also lowers the cost of wastewater treatment, achieving good economic and environmental benefits.

4. Analysis of Successful cases

Upm (China) Co., Ltd. has achieved remarkable results in the treatment of papermaking wastewater and the reuse of cooling tower make-up water. The company adopts a treatment process of disc filter filtration and sterilization and disinfection for papermaking wastewater, ensuring that the treated water quality meets the water quality requirements for cooling tower make-up water. This measure can save 600 to 1,000 cubic meters of water for cooling tower make-up water every day, 250,000 to 300,000 cubic meters of water annually, and reduce the water consumption per unit product by 2.5%, significantly lowering the consumption of clear water and the discharge of wastewater pollutants in power plants.

The circulating cooling tower system invested and constructed by Nanjing Tianjia Environmental Technology Co., Ltd. has also achieved remarkable results. This system employs the "air cooling + circulating pump" integrated control technology. Water is transported to the cooling tower by the circulating pump, and the water flow is forcibly cooled by a high-efficiency fan. The cooled water is then recycled to the test unit through a closed-loop pipeline, forming a full-process recycling mode. Compared with the traditional direct exhaust cooling method, it saves 28,700 cubic meters of water annually, providing a referenceable example for water-saving technological transformation in the industrial field.

The optimization of the water circulation system in cooling towers is of great significance for reducing wastewater discharge and achieving efficient utilization of water resources. By adopting advanced water treatment technologies, precise pollutant discharge control measures, actively promoting wastewater recycling and reuse, and drawing on the practical experience of successful enterprises, various industries can significantly reduce water consumption and minimize negative environmental impacts while ensuring production and operation, moving towards a green and sustainable direction. This is not only a necessary measure to address the challenge of water shortage, but also a key path for enterprises to fulfill their social responsibilities and enhance their own competitiveness, which is worthy of in-depth exploration and wide promotion by the entire industry.