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Abstract: It discusses the current technologies, application effects, and applicable conditions for the treatment of highly concentrated circulating cooling water in large thermal power plants. In particular, for the first time in China, lime-acid treatment technology with side filtration and chemical dosing was adopted in practice; furthermore, building on the experience gained from weak acid resin treatment, reverse osmosis treatment technology was developed, ultimately enabling zero wastewater discharge from the circulating cooling water system. Keywords: High-concentration ratio treatment technology and practices for circulating cooling water in large power plants; Water-saving circulating water systems; Zero wastewater discharge. Date of receipt: 1998–07–17. 1 Introduction The high-concentration ratio treatment technology for circulating cooling water in large thermal power plants refers to the technology that enables the concentration ratio of circulating cooling water to reach 3 or higher. In accordance with the requirements set out in the 1998 guidelines issued by **Power Company regarding water conservation in thermal power plants, the concentration ratio of circulating cooling water is determined based on different water qualities, condenser tube materials, as well as through testing and technical-economic analysis. Various circulating cooling water treatment solutions should generally achieve the following results: 1) The use of scale and corrosion prevention agents as well as acid treatment, with a concentration ratio of around 3 ； (2) When using lime treatment, the concentration ratio should be around 4 ； (3) Provisions stating that treatment methods such as weak acid resins should be used, with a concentration ratio of over 4. These regulatory provisions clearly show that the treatment technology for circulating cooling water in large thermal power plants is evolving in the direction of water conservation, energy savings, and meeting environmental protection requirements. 2 Key Points of Applying High-Concentration Circulating Cooling Water Treatment in Thermal Power Plants at Home and Abroad 2.1 Key Points of Applications in Foreign Thermal Power Plants 2.1.1 In the eight thermal power plants in Germany, lime softening – combined with scale inhibitors – is used for the treatment of circulating cooling water; the concentration ratio is 3–6, with 4 being the most common value, and the results are satisfactory. 2.1.2 In U.S. thermal power plants, circulating cooling water treatment primarily relies on softening or chemical dosing via side filtration. Its concentration ratio is usually between 3 and 3.5; only in thermal power plants in the western United States is a combined treatment method of lime softening followed by bypass filtration for desalination used, allowing the concentration ratio to exceed 10. 2.1.3 The circulating cooling water at the 4 X 660MW Bessborough Power Plant in New South Wales, Australia, is treated using a bypass process. The plant’s water source is water from Lake Liddell, which has a high salt content; it undergoes lime treatment followed by bypass treatment, enabling the plant to achieve zero wastewater discharge. The general treatment details are as follows: (1) Limestone softening treatment equipment – equipped with two clarifiers with a diameter of 42 m, offering a total processing capacity of 5000 m3/h. (2) Circulating water bypass treatment equipment – 3% of the circulating water volume is treated as bypass flow, using an alkalinity reduction device and a reverse osmosis unit. (a) Filters in the alkali reduction unit: 6 units per unit (660MW), with each unit having a capacity of 267 m3/h. (b) Ion exchanger tanks in the alkali reduction unit: 4 units per unit, with a capacity of 1552 m3/h; these tanks can remove calcium and magnesium ions. (c) For reverse osmosis, two sets are installed per unit; each set has a treatment capacity of 187.5 m3/h for circulating water with a salt content as high as 2500 mg/L. This device can remove 90% of the ions. 82.5% of the treated water, amounting to about 300 m3/h, is fed into the circulating cooling water system. 2.2 Key Points in the Practice of Domestic Thermal Power Plants 2.2.1 Circulating cooling water treatment at Shanxi Shentou No. 2 Power Plant: Lime softening, acid addition, and chemical dosing via side filtration were first employed in the 1990s, yielding good results with a concentration ratio of 4.32–4.64. 2.2.2 Circulating cooling water treatment at Datong No. 2 Power Plant: Weak acid resin treatment was first used at the end of the 1980s, achieving a concentration ratio of 4–4.5. 2.2.3 Circulating cooling water treatment at Zouxian Power Plant in Shanxi: A combination of sulfuric acid and scale inhibitors was used initially, enabling a concentration ratio of around 3.7 ； The concentration ratio at the Dalaat Power Plant in Inner Mongolia also reaches 3.41. 3 Current technologies for treating circulating cooling water with high concentration ratios 3.1 Lime softening combined with acid addition, filtration, and chemical dosing 3.1.1 Applicable conditions (1) This method is suitable for areas suffering from severe water shortages, where high-purity lime powder is available, and it is appropriate for raw water with primarily temporary hardness. (2) This method offers a relatively comprehensive treatment approach, with a recirculating water concentration ratio that can reach 4–5 times. (3) Suitable for thermal power plants with a single-unit capacity of 300 MW or 600 MW that use groundwater with very low magnesium content as their water source. Germany uses this method quite frequently; it places great emphasis on the fact that lime softening treatment should be supplemented by scale-inhibiting agents for stabilization, as well as continuous cleaning of the condensers with rubber balls, so as to maintain an operation concentration ratio of 4–6. 3.1.2 Advantages and Disadvantages The advantage of this treatment method is its high processing capacity and relatively low operating costs. The problems are high investment costs, high requirements for the purity of the lime powder supplied by the lime production facility, and a poor working environment. 3.2 100% Weak Acid Resin Exchange Treatment Technology 3.2.1 Applicable Conditions This treatment method is suitable for areas suffering from severe water shortages, where significant water savings are required. It is applicable to water with a concentration ratio maintained at 4–5, carbonate hardness greater than 2 mmol/L, high alkalinity and a moderate hard-alkali ratio, as well as groundwater with high sulfate content and suspended solids levels of less than 5 mg/L. It is used as make-up water for the circulating water systems in thermal power plants, as well in power plants that have access to sulfuric acid in the required form and under suitable transportation conditions. According to statistical analysis of relevant data, in the water quality used as make-up water for thermal power plants, whether it is groundwater or tap water, the carbonate hardness of almost all such waters is greater than or equal to 2 mmol/L. Waters with a hardness-to-alkalinity ratio of 0.8–1.3 account for 70% of the total samples analyzed; this shows that this treatment method is broadly applicable. 3.2.2 Advantages and Disadvantages: This treatment method is highly favored due to its mature technology, reliable operation, simplicity of use, ease of automation, and low environmental impact. The problem is that the investment is somewhat high, and the equipment space is not fully utilized. 3.2.3 Improvement: The improved equipment has been upgraded from a single-flow exchanger to a dual-flow exchanger. In this way, it is equivalent to installing two (single-stream) weak acid resin layers in one (double-stream) exchanger and operating them in parallel, which can nearly double the processing capacity. The advantages of the double-flow exchanger are as follows: (1) it reduces the amount of resin used by about 12.5%; (2) it improves the efficiency of regenerants, allowing for a reduction of sulfuric acid usage by around 15%; (3) the rate of water used for self-cleaning drops from 7% to 5%; (4) the quality of the effluent water is better; (5) it offers economic benefits, with a 20% reduction in investment costs, a 14% decrease in annual operating expenses, and a reduction in the cost per ton of water from 0.325 yuan/m3 to 0.274 yuan/m3. 3.2.4 Improved applications involve thermal power plants that use dual-flow exchangers with weak-acid resins to treat circulating cooling water; an example of this is the Second Yangquan Power Plant in Shanxi, which came online in 1997, with a concentration ratio of 4. 3.3 Sulfuric acid – scale inhibitor stabilization treatment technology 3.3.1 Applicable conditions This treatment method is suitable for situations where it is possible to maintain a circulation cooling water concentration ratio of 3, and it is intended for thermal power plants located in areas facing water shortages where significant water savings can be achieved. 3.3.2 Advantages and disadvantages: This treatment method requires low investment, occupies little space, and has simple technical requirements. The disadvantage is that the content of hydrous salts and neutral salts in the water increases after acid is added; in particular, the SO4 2- content in the circulating cooling water rises as the amount of acid added increases, which is detrimental to circulating water systems operating at high concentration ratios. Secondly, phosphates are nutrients for bacteria and algae, promoting their growth and reproduction. When treated with organophosphorus stabilizers, wastewater is also generated, posing an environmental pollution problem. 4 Lime softening – acid addition – chemical dosing via side filtration: Practical application of this approach to treat circulating cooling water at the Second Power Plant in Shentou, Shanxi Province in the 1990s. 4.1 Overview The treatment of circulating cooling water at this plant involved high-pressure water flushing, rubber ball cleaning, condenser treatment, lime and acid treatment, as well as chemical dosing via side filtration. In particular, for the make-up water of the circulating water system, the treatment technique of lime plus acid – backwashing with chemicals is employed. This treatment technology yields good results; the concentration ratio of the circulating water is K=4.32–4.64. As a result, the total water consumption of this million-kilowatt-class power plant is 3508 m3/h, with the supplementary water volume for the circulating water system being 2223 m3/h, accounting for 63% of the total amount – which corresponds to 0.975 m3 per SGW. The water consumption during operation (with 5500 operating hours per year) is 5.588 kg/kWh. 4.2 High-pressure water flushing of the condensers and gel ball cleaning of the condensers: 4.2.1 Each of the 2 X 500MW units is equipped with 2 condensers, each having a cooling area of 12040 m2; thus, the total cooling area is 24080 m2. There are 2 X 15720 copper tubes, with specifications of medium 25 X 1 X 10000. The volume of circulating cooling water required is 50400 m3/h. 4.2.2 High-pressure water flushing of the condensers: As part of preventive maintenance, the copper tubes are flushed using high-pressure water at 42 MPa, yielding significant results. 4.2.3 Cleaning the condenser with rubber balls: The plant’s condenser is equipped with a rubber ball cleaning system, which is used for regular cleaning during operation; it was found that on about 1/5 of the copper tubes, visible signs of wear caused by the rubber balls could be seen. 4.3 Lime and acid pre-treatment of make-up water in the circulating water system 4.3.1 Pre-treatment process flow The pre-treatment capacity is 3300 m3/h; the amount of make-up water required in summer is 2223 m3/h, while it is 2800 m3/h during hot periods. The lime and acid pre-treatment equipment used in this plant was imported from the German company MAN, and its pre-treatment process flow is as follows... 4.3.2 Equipment specifications for pre-treatment (1) Hydraulic circular clarifier This type of clarifier has advantages such as simpler operation and reduced need for maintenance compared to mechanical accelerated clarifiers. (2) As can be seen from Table 3, the gravity filter tanks manufactured by the German company MAN require a large amount of water for flushing as well as high water consumption. However, the company has taken into account the need for equipment to recycle wastewater generated during backwashing, by installing a 700 m3 wastewater tank and two wastewater recycling pumps with a capacity of 100 m3/h each. Calculations show that approximately 2,000 m3 of backwash wastewater can be recovered per day. (3) The lime system equipment is equipped according to the unit size and arranged in a modular fashion. Each unit is equipped with equipment such as lime silos, arch-breaking devices, screw feeders, measuring systems, dust removal devices, and exhaust systems, which effectively addresses the issue of dust contamination in dry powder measurement. The prominent equipment from the German company MAN in lime system installations is the BAV2402 type vibrating feed hopper, which effectively solves the problems of flow obstruction and arching in lime silos. This device features easy adjustment, reliable operation, low noise, and also has the capability of breaking arches in the feed material. (4) The chemical dosing system equipment is configured according to the unit and arranged in a modular fashion. Depending on the changes in the raw water flow rate, lime, ferrous sulfate, and chlorine are added in proportion. In addition, the German company MAN has installed an additional dielectric charging system to improve the agglomeration effect. 4.3.3 Quality of water used for replenishing circulating water: After pretreatment with lime and acid, the quality of the water obtained is as follows: calcium ions at 0.21 – 2 mmol/L, magnesium ions at 0.585 mmol/L; sulfate ions at 0.8 mmol/L, bicarbonate ions at 0.55 mmol/L. Chloride ions at 0.352 mmol/L, with undissolved substances at 1–2 mg/L and organic matter at 2 mg/L. The pH value is 8.3 at 25°C; phenolphthalein alkalinity is 0, while methyl orange alkalinity is 0.55 mmol/L. Silica content is 8 mmol/L. 4.4 Filtration and chemical treatment of the bypass stream in circulating cooling water systems 4.4.1 Purpose of treatment: According to available data, when circulating cooling water becomes contaminated during its circulation, failing to meet the specified quality standards, or when the concentration ratio needs to be increased resulting in an increase in the turbidity of the water, it is necessary to employ bypass filtration for treatment. If alkalinity reduction and desalination are also required, bypass treatment must be considered. 4.4.2 Treatment methods: Circulating water bypass filtration and chemical dosing involve diverting a certain percentage of the water from the circulating water system as bypass water for filtration and chemical treatment. 4.4.3 The amount of water to be filtered through the bypass system shall be determined in accordance with the provisions regarding bypass water treatment in the **Standard GB50050-95, ‘Design Code for Industrial Circulating Cooling Water Treatment’. For open-type circulating cooling water systems, this amount should be calculated using the relevant formula, or it can be set at 1%–5% of the total circulating water volume. The plant uses 0.75% of the circulating water volume as the amount of water for side filtration. The circulating water volume for the 2 x 500MW thermal power units is 116,960 m3/h; therefore, the amount of water used for side filtration is approximately 900 m3/h, which is far less than the value specified in the standards. This is because the side filtration and chemical dosing system in question is Czech equipment that was introduced along with the entire thermal power generation equipment, representing a new technology for circulating water treatment that has been used for the first time in China. As such, it is not appropriate to change the amount of water used for side filtration. 4.4.4 Side-stream treatment process: diversion at the outlet of the circulation pump – 3 out of 6 units, each being a 3000-type double-layer double-flow mechanical filter (with a flow rate of 150 m3/h per unit) – chemical dosing (DDF-2A scale inhibitor and chlorine tincture) – the pool in front of the circulation pump, as shown in Figure 1. 4.4.5 Double-layer double-flow mechanical filter (1) Structural characteristics The double-layer double-flow mechanical filter is different from ordinary mechanical filters. The layout of the transmission ducts between the upper space of the filter bed and the bottom of the nozzle, as well as between the lower space and the bottom of the nozzle, is all located inside the filter. See Figure 2 for details. (2) Parameters of the double-layer, dual-flow filter: rated output of 150–180 m3/h; operating flow rate of 180 m/h. Filter media heights: 400 mm for anthracite and 200 mm for quartz sand. Particle size of the filter media: 1.7–3.0 mm for anthracite, and 8–12 mm as well as 2–4 mm for quartz sand. (3) Application of the double-layer, dual-flow mechanical filter as a bypass filter in the circulating water system of 2 X 500MW thermal power units. Each 500 MW unit is equipped with 3 double-layer, dual-flow filters; each filter bed has a flow rate of 150 m3/h, resulting in a total flow rate of 450 m3/h per unit. During one month of operation, the amount of water treated is 32.4 X 10^4 m3. These filters are used to remove mechanical impurities and suspended solids from the circulating water, thereby reducing its turbidity – the target being a reduction of 5 mg/L in turbidity. The turbidity levels at the inlet and outlet of the filters during 1993–1994 are shown in Table 4. As can be seen from this table, the double-layer, dual-flow filters are highly effective in removing contaminants and reducing turbidity, with each filter bed achieving a turbidity reduction of around 70%. This improves the quality of the water and extends the service life of the condensers. 4.4.6 The treated water from the circulating water bypass dosing process, after passing through a double-layer dual-flow filter, is mixed with DDF-2A scale inhibitor and chlorine, and then fed into the pre-tank of the circulating water pump. The purpose of adding chemicals is to reduce the alkalinity and hardness of the circulating water. The results are as follows: the average hardness before treatment was 5.8 mmol/L, while it was 5.64 mmol/L after treatment. The average phenolphthalein alkalinity before treatment was 0.16 mmol/L, and it dropped to 0.13 mmol/L after treatment. The average methyl orange alkalinity before treatment was 1.53 mmol/L, and it became 1.49 mmol/L after treatment. This indicates that the treatment was effective. 4.5 Performance of circulating cooling water treatment: The circulating water for the 2 X 500MW thermal power units at Jinotou No. 2 Power Plant shows good performance after treatment. 4.5.1 Inspection and analysis of the condenser (1) During the inspection on July 21, 1994, the copper tubes at the water inlet and outlet of the condenser were in their natural color. The inside of the copper pipe is covered with soft sludge ranging in thickness from 0.05 to 0.11 mm; the pipe opening feels smooth to the touch, with no debris or blockages present. (2) After the circulating cooling water was treated (including bypass filtration) and all systems were properly commissioned in October 1993, the compliance rate of the water quality of the circulating water increased from 50% to around 98%. The amount of sediment accumulated inside the copper tubes of the condenser was significantly reduced, which enabled the concentration ratio of the circulating water to rise from 2.5 times to 4 times as much, resulting in substantial water savings as well as safer and more economical operation. 5 Practical Application of Weak Acid Resin Treatment for Circulating Cooling Water at Datong No. 2 Power Plant in the Late 1980s 5.1 Overview The plant’s 4 X 200MW wet-cooled units were put into operation in June 1984, December 1984, October 1985, and December 1986, respectively. The weak acid treatment system for circulating cooling water utilizes a total of 4X4, that is, 16 Φ3000 single-flow weak acid ion exchangers. Each exchanger requires approximately 10 tons of D113 weak acid resin, which are installed together and put into use simultaneously. Through years of operational experience, the plant has been able to operate with a circulating water concentration ratio of 4–5 times, achieving good scale inhibition effects; moreover, the circulating water system itself exhibits significant water savings (a water saving rate of 29%). 5.2 Water Quality and Removal Rates (1) The designed water quality is that of groundwater, with the following main components: pH value of 7.88, dissolved solids of 401 mg/L, hardness of 3.96 mmol/L, alkalinity of 4.20 mmol/L, calcium ions at 1.82 mmol/L, magnesium ions at 2.14 mmol/L, and chloride ions at 37 mg/L. (2) The water quality for weak-acid softening is as follows: pH value of 5.81, dissolved solids of 234 mg/L, hardness of 0.4 mmol/L, alkalinity of 0.55 mmol/L, calcium ions at 0.15 mmol/L, magnesium ions at 0.25 mmol/L, and chloride ions at 37 mmol/L. (3) Removal rates: 41.65% for dissolved solids, 89.9% for hardness, 86.9% for alkalinity, 91.76% for calcium ions, and 88.32% for magnesium ions. 5.3 Equipment and Systems 5.3.1 The circulating water system for the plant’s 4 × 200 MW wet-cooling units operates on a one-unit-per-tower basis. The treatment process for the circulating cooling water using weak-acid hydrogen ion exchangers relies on a parallel motherline system; there are 16 Φ3024 mm weak-acid hydrogen ion exchangers in total, along with 2 Φ3024 mm resin loading/unloading tanks. The weak acid hydrogen ion exchangers are filled with D113 weak acid resin, with a loading height of 1400 mm. Each unit has a designed output of 165 t/h, giving a total designed output of 2000 t/h. 5.3.2 The process for treating weak acid in the make-up water of the circulating water system is shown in Figure 3. As can be seen from Figure 3, the treated weak acid softened water is pumped out through three channels and eventually discharged into the circulating water system as make-up water. Weak acid resins are regenerated using sulfur, and the regeneration process is a single-step countercurrent regeneration. 5.4 Operating characteristics of weak acid resins The main factors affecting the operating characteristics of D113 weak acid resins include: operating flow rate, water temperature, resin layer height, resin particle size, mechanical strength, and the ratio of raw water hardness to alkalinity. When the temperature of the raw water in this plant is 12–15°C and the ratio of hardness to alkalinity of the raw water lies within the range of 0.9–1.1, the operating flow rate is 25–30 m/h, with no significant effect on the working exchange capacity. 5.4.1 Operating parameters: The operating cycle is approximately 50 hours; the water production volume during this cycle is 8600 tons. The working exchange capacity of the resin is 3100–3200 mmol/m3, the acid consumption is 50–55 g/mol, and the acid regeneration efficiency is 98%. 5.4.2 Self-water usage rate: The self-water usage rate includes the total amount of water used in processes such as backwashing, regeneration, displacement, and rinsing; it is also related to the level of regeneration. When the regeneration level is 156.8 g (sulfuric acid)/L (resin), the self-water usage rate is 5.64%; when it is 183.5 g/L, N = 8.33%. 5.4.2 Water quality of the effluent: At a regeneration level of 183.5%, the hardness removal rate reaches 91.25%, while the alkalinity decreases to 90.7%. The hardness drops from 4.0 mmol/L to 0.35 mmol/L, and the alkalinity falls from 4.3 mmol/L to 0.40 mmol/L. 5.5 Water quality control parameters: The control of circulating water quality should focus on two aspects – first, it is necessary to control the quality of the make-up water, as this is a prerequisite and foundation for preventing corrosion and scaling in the circulating water system ； On the other hand, it is necessary to monitor and treat the quality of the circulating water, keeping track of and controlling its quality in real time, so that the carbonate hardness of the circulating water remains below its limit at all times, thereby preventing scaling in the copper tubes of the condenser. 5.5.1 Control parameters for the quality of make-up water The control parameters for the quality of make-up water (i.e., weakly acidic softened water) are as follows: alkalinity at 0.8 mmol/L, hardness at 0.5 mmol/L, and calcium ion concentration at 0.2 mmol/L. Years of experience at this plant have shown that when the ratio of hardness to alkalinity of the raw water lies within the range of 0.88–1.02, it is possible to keep the hardness of the water exiting the weakly acidic hydrogen ion exchanger at 1.0 mmol/L; exceeding this value indicates that the exchanger has reached its limit of effectiveness. The regeneration level is generally around 156.8 g/L. The alkalinity of the weakly acidic softened water ranges from 0.35 to 1.35 mmol/L, its hardness is 0.3–0.6 mmol/L, the calcium ion concentration is 0.1–0.3 mmol/L, the pH value is 5.7–6.5, the dissolved solids content is 206–260 mg/L, and the chloride ion concentration is 35–49 mg/L. 5.5.2 Quality control parameters for circulating water: Through laboratory tests on scaling behavior, it was determined that, under the aforementioned parameters of the make-up water, the maximum allowable alkalinity for circulating water is 4.3 mmol/L, the maximum allowable hardness is 2.58 mmol/L, and the maximum allowable concentration ratio is 5.56 times. Based on this, the quality control parameters for circulating water are set as follows: concentration ratio of 4–4.5 times, total alkalinity of 3.2–3.6 mmol/L, and hardness… 0–2.6 mmol/L; calcium ions. Low molecular weight: (b) Affinity order – aldehydes > alcohols > amines. 6.3.2 Overview of specific applications: (1) Use of reverse osmosis in the treatment of circulating cooling water in thermal power plants abroad. South Africa is a country with severely limited water resources. Cooling in thermal power plants mainly relies on water-saving air cooling technologies, dry ash removal technologies, and bypass purification technologies for circulating cooling water. In addition, reverse osmosis technology is also used to treat circulating cooling water. The 28 thermal power plants owned by South Africa’s Electricity Supply Commission (ESKOM) generate nearly 96% of the electricity in South Africa’s power industry. ESKOM treats the circulating cooling water in these power plants using both reverse osmosis and electrodialysis methods; the treated fresh water is then reused in the cooling water systems. (2) Application of reverse osmosis in the treatment of wastewater from circulating water systems in domestic thermal power plants. The Xibaipo Power Plant in Hebei Province had to adopt a zero-waste-discharge scheme for its circulating water system in order to meet local environmental protection requirements, and this system was put into operation in 1998. The basic principle of this water treatment process is to treat the wastewater from the wet cooling towers of Units 1 and 2 using weak acid resins, which is then used as make-up water for the wet cooling towers of Units 3 and 4 ； As for the wastewater from the wet cooling towers of thermal power units No. 3 and No. 4, it is treated using reverse osmosis membrane technology to achieve concentration; the resulting concentrated water is used as water for ash washing in the thermal power plant, while the fresh water obtained is used as a source of water for chemical makeup in the plant. In addition, the infiltration water from the wet ash storage area is pumped back to be used for ash flushing in the power plant and as a water source, thereby essentially meeting the requirements of a zero-waste discharge system for circulating water. **In summary, in the water treatment systems of domestic thermal power plants, reverse osmosis technology is mostly used for the treatment of boiler make-up water. For example, the 600MW power plant in Haibowan, Inner Mongolia, uses water with a high salt content of 800–1000 mg/L; as a result, reverse osmosis treatment technology is employed, with a treatment capacity of 3 × 45 t/h and a desalination rate of over 95%. The Xi Baipo Power Plant in Hebei, which came online in 1998 and uses reverse osmosis to treat the wastewater from its circulating water system, represents a good start; it also adds new elements to the comprehensive water-saving technologies used in thermal power generation.