Industrial circulating cooling water fully automatic filter side flow treatment process and equipment

This article reviews the fully automatic filter side flow treatment processes (filtration, membrane separation, chemical precipitation softening, ion exchange, etc.) used to achieve "zero discharge" in industrial circulating cooling water systems, with a focus on the chemical coagulation fiber filtration controlled dosing process (suitable for small and medium-sized circulating cooling water systems) and the chemical coagulation fiber filtration weak acid resin softening process (suitable for large circulating cooling water systems).

The method of increasing the amount of water treatment agent and adding appropriate high-performance dispersants and scale inhibitors can improve the scale inhibition effect, but this is only a temporary and negative treatment method suitable for systems with lower concentration ratios. For cooling water systems operating at high concentration ratios, appropriate processes should be selected for side flow treatment to remove the constantly increasing harmful components in the system, This is equivalent to reusing the sewage after regeneration treatment as supplementary water in the circulating cooling water system, which is truly "zero discharge".

1. Fully automatic filter side flow treatment process
1.1 Filtering method
Filtration is a common side stream treatment method (commonly referred to as side filtration). Its treatment capacity is usually 2%~5% of the circulating water. It can remove most of the Suspended solids, slime and microorganisms in the water, but it cannot reduce the hardness and salt content of the water. During backwashing, impurities will be discharged into the system with the backwashing water. Due to the much higher concentration of impurities in the backwash water compared to the discharged sewage, the system discharges more impurities and consumes less water. This means that the discharge volume can be significantly reduced through side filtration.

Large circulating cooling water system generally adopts gravity valveless filter tank with quartz sand or Anthracite as filter material. Its filtering speed can only be controlled below 10m/h, while the concentration of suspended solids in cooling water can only be controlled below 10mg/L. The increase of filtering and floor area leads to large foundation investment.

Compared with quartz sand, fiber filter media has the characteristics of high porosity, reasonable pore distribution, and large specific surface area. When using fiber filter media, the filtration rate can reach up to 20-85m/h. Due to the flexibility and Compressibility of the fiber, it is gradually compressed with the increase of water flow resistance, so that the upper layer of the filter material has small stress and large pores, while the lower layer has large stress and small pores, which fully reflects the characteristics of the fiber filter material with large sewage holding capacity and long filtering period. Fiber filter media filters usually require the use of air water backwashing, which uses the agitation of the gas to separate the suspended solids from the filter media and then discharge them with the backwash water. Fiber filters have good retention effects on suspended solids, iron, manganese, and microbial sludge, with high filtration accuracy and typically effluent turbidity less than 1 NTU.

In recent years, the role of new ion exchange fiber filter media filters in circulating cooling water side flow treatment has gradually attracted people's attention. In addition to their filtration function, they can also exchange ions with calcium and magnesium ions in water, which has the function of softening water quality.

1.2 Membrane separation method
Anti-Infiltration Act and Electrodialysis are two common membrane separation methods, which can effectively remove hardness, microorganisms and other harmful ingredients in cooling water. They have a high desalination rate, and the water recovery rate can reach 75%~90%.

As the operating cost is constantly increasing due to the easy pollution of the osmosis membrane, the lime softening method is usually used to remove most of the hardness and suspended solids, and then the Anti-Infiltration Act method is used to further reduce the hardness, so as to meet the water quality requirements of the circulating water make-up water.

The disadvantage of membrane separation method is that it has strict requirements for influent water quality, and pressure fluctuations during operation can easily cause membrane damage. Corrosion products and microorganisms in the water can easily block and pollute the pre filtration device and reverse osmosis membrane. Frequent cleaning increases operating costs (the treatment cost may be as high as 5-15 yuan/m3), and the one-time investment cost is high. Therefore, this method is not suitable for large circulating cooling water systems.

1.3 Chemical precipitation softening method
The lime soda softening method is usually used to reduce the carbonate hardness and non carbonate hardness in water. Adding coagulants to the chemical precipitation method can form large flocculent particles such as CaCO3 and Mg (OH) 2 in a colloidal state and adsorb suspended solids in water to settle, achieving the goal of simultaneously reducing turbidity and hardness.

Due to the high hardness of carbonates and low hardness of non carbonates in circulating cooling water, and the fact that bypass treatment does not require deep softening, it is feasible to use lime softening method for both wastewater and raw water softening treatment in large-scale circulating cooling water systems.

The disadvantages of the chemical precipitation method are that the residual hardness and pH value of the effluent are high (requiring acid adjustment), the amount of sludge is too large, and the addition of acid increases the salt content in the cooling water (including chloride ions and sulfates), exacerbating the corrosion tendency of the cooling water. The presence of phosphate corrosion and scale inhibitors in cooling water as a side flow treatment process may interfere with the softening treatment process; And lime milk also has adverse effects on the performance of scale inhibitors (such as removing calcium and magnesium while also removing scale inhibitors, intensifying the decomposition of scale inhibitors, etc.), which exacerbates the consumption of scale inhibitors themselves due to adsorption and precipitation, and affects the treatment efficiency of the cooling system.

1.4 Ion exchange method
The ion exchange method used for the preparation of pure water requires a large amount of acid and alkali in daily operation, and generates a large amount of wastewater. The treatment cost is high. If Na type resin is used for softening treatment, the cost of purchasing industrial salt is high. During regeneration, a large amount of Cl - ions may be introduced, increasing the corrosion tendency of cooling water.

The application of weak acid cation exchange resins in water softening treatment is becoming increasingly widespread. The carboxylic acid group of weak acid resin has a high affinity for Ca2+and Mg2+, which can effectively remove carbonate from water. Theoretically, the hardness can be removed to the level equivalent to HCO3 alkalinity. Its working exchange capacity is more than twice that of strong acid resin, and the consumption of regenerant is about 1.05-1.10 times the theoretical value, making it very economical. The regenerant can be hydrochloric acid or sulfuric acid (the cost of the former is three times that of the latter, but when using sulfuric acid, the concentration and flow rate of the regenerant must be strictly controlled to suppress the generation of CaSO4. The use of a dual flow ion exchanger can save investment, water consumption, and land occupation, and achieve high regeneration efficiency.

1.5 Other methods
By heating and evaporating the sewage water through steam compression condensation, harmful components in the cooling water can be concentrated, and 95% of the sewage water can be recovered in the form of condensate and returned to the system as circulating water and boiler make-up water. However, this method requires too much energy and can only be used in areas with special water shortages. Controlling the release of water treatment agents with phosphate can achieve constant rate dosing. Adopting automatic dosing devices and improving water treatment technology can help improve the quality of circulating cooling water, increase concentration times, and save water consumption. In addition, petrochemical enterprises often use oil separators to remove oil that enters the cooling water system due to leaks, but there have been no reports on the side flow removal process for orthophosphate and silicon dioxide in cooling water.

Combination of 2 side flow treatment processes and equipment
2.1 Fiber filtration controlled release dosing process
2.1.1 Principle
The circulating cooling water is mixed evenly in the pipeline after being added with coagulants through a jet filter, and then enters a high-efficiency fiber filter. Micro flocculation reaction occurs in the reaction zone above the filter bed, and the suspended solids in the water form flocculent particles that are intercepted and removed together with microbial sludge by the fiber filter material. The turbidity of the filtered water is less than 1 NTU. By using this method, the orthophosphate in the side filter water can be adsorbed and removed by the fiber filter material and its intercepted flocculent particles. When the filtered side flow cooling water passes through the dosing tank, the glassy "rust scale cleaning" water treatment agent dissolves into the water at a certain rate, releasing polyphosphates with corrosion and scale inhibition effects, which can achieve constant rate dosing.

This process has the characteristics of simple process, compact layout, and convenient and flexible operation. It can simultaneously remove harmful components such as suspended solids, orthophosphate, and microbial sludge in industrial circulating cooling water, and supplement the active components of polyphosphate to improve water quality and improve corrosion and scale inhibition effects.

2.1.2 Cost and Benefit Analysis
The operating cost mainly includes the cost of coagulants and the electricity cost of "rust cleaning", with a total cost of less than 0.5 yuan/m3, which is a relatively economical treatment method.

The benefits include saving water and sewage fees by increasing the concentration factor, reducing the amount of supplementary water and sewage, reducing the cost of water treatment agent consumption, improving water quality stability, extending equipment service life, improving heat transfer efficiency, and saving manpower.
This process is suitable for small and medium-sized circulating cooling water systems without professional technical personnel management.

2.2 Fiber filtration - weak acid resin softening process
2.2.1 Principle
After adding sedimentation and coagulants to the circulating cooling water through a jet filter, it is mixed evenly in the pipeline and enters a high-efficiency fiber filter. Partial softening reaction and micro flocculation reaction occur in the reaction zone above the filter bed, and the generated CaCO3 and Mg (OH) 2 particles are intercepted and removed along with the suspended solids in the water by the fiber filter material. The hardness (permanent hardness, i.e. non carbonate hardness) of the filtered water is reduced by about 1mmol/L, turbidity is less than 1NTU, and alkalinity is appropriately increased The pH value rises to around 9.5. The use of weak acid (acrylic acid) cation exchange resin can further reduce the hardness and alkalinity of water to below 1mmol/L, and the pH value of the effluent is 3-6. After meeting the water quality requirements for supplementary water, it can be returned to the cooling water system, and more than 50% of the water treatment agent can be retained in the effluent.

2.2.2 Cost and Benefit Analysis
The operating cost includes the consumption of precipitants and coagulants, as well as the cost of weak acid resin regeneration (using sulfuric acid as the regeneration agent). The cost of the agent is 0.63 yuan/m3, and with the addition of electricity bills, the total cost is less than 1.0 yuan/m3, which is more economical than using other methods such as reverse osmosis.

The benefits include water and sewage fees saved by reducing supplementary water and sewage, costs saved by reducing water treatment agent consumption, and benefits generated by improving water quality stability, extending equipment service life, and improving heat transfer efficiency.

2.2.3 Equipment characteristics
The main equipment includes high-efficiency fiber filters and weak acid cation exchangers, and the process and equipment are characterized by simple, compact, and convenient and flexible operation. The "integrated" equipment that combines the reactor and filter enables the process to be completed in the integrated equipment, making it easy to achieve industrialization. Using the upper space of the filter material layer in the high-efficiency fiber filter as the reaction zone of the "three in one" treatment process can eliminate a large volume reaction clarification tank. Due to the existence of the "micro flocculation effect", the small sediment particles generated by the reaction can be removed through the deep and high-precision filtration of the fiber filter material, which has the advantages of high filtration accuracy, long operating cycle, and large pollutant capacity.

Weak acid cation exchange resin has no effect on removing permanent hardness, and does not produce strong acids when softening water quality. The resin is prone to regeneration after failure, and the consumption of acid is usually only 1.1 times the theoretical value, making it more economical. This process is flexible in operation and can be adjusted according to water quality conditions. Its own water consumption is less than 10%, making it suitable for large-scale industrial circulating cooling water systems.

3 Conclusion
① The use of side flow treatment processes (including filtration, membrane separation, chemical precipitation softening, ion exchange, etc.) to purify and soften the sewage from the circulating cooling water system as supplementary water for reuse can significantly reduce sewage discharge and improve water quality stability, which is of great significance for improving the reuse rate of industrial water and saving water resources.

② For small and medium-sized circulating cooling water systems, a chemical precipitation fiber filtration controlled release dosing process that can remove suspended solids, reduce turbidity, and achieve automatic dosing can be adopted; For large-scale circulating cooling water systems, a chemical precipitation fiber filtration weak acid resin exchange process that can simultaneously remove suspended solids, hardness, etc. can be used to fundamentally improve water quality and achieve true "zero discharge".