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The coagulant dosing system includes units for the storage and transportation of chemicals, their preparation, lifting, storage, metering and dosing, as well as mixing. (1) Preparation of the chemical agent 1. Methods for preparing the chemical agent: The dissolution and dilution of coagulants should be carried out using mechanical, hydraulic, or compressed air methods for mixing and dilution, depending on the amount to be added and the properties of the coagulant. (1) Mechanical modulation is suitable for various agents and scales, and is widely used. The mixing impeller can be driven by a motor, and a speed control device can be considered as needed ； Mixing equipment must have anti-corrosion measures, especially when using iron salt agents ； (2) A dissolution tank with hydraulic modulation is used, with the pressure of the water applied being 0.2 MPa; (3) Compressed air cavitation is suitable for preparing various chemicals in sewage treatment plants that handle large volumes of water ； The specific requirements are as follows: ① Air supply rate: 8–10 L/(㎡•s) for the dissolution tank, and 3–5 L/(㎡•s) for the solution tank; ② Flow velocity at the air pipe orifices: 20–30 m/s, with an orifice diameter of 3–4 mm; ③ The compressed air modulation method is not suitable for continuous stirring of lime slurry over extended periods of time. 2. Solution tank and dissolution tank: The function of the solution tank is to prepare a concentrated solution into one with a specific concentration ； The dissolution tank dissolves the bulk or granular reagent into a solution. Calculate respectively according to Equation (2) and Equation (3). W1 = aQ/417cn (2) W2 = (0.2~0.3)W1 (3) Where W1 is the volume of the solution tank, in m3; W2 is the volume of the dissolution tank, in m3. a represents the maximum amount of coagulant that can be added, expressed as an anhydrous substance. The maximum amount of lime used is expressed in terms of CaO, in mg/L. Q is the volume of water to be treated, in m3/h. c is the concentration of the solution; it is generally set at 5%~20% (based on the mass of solid coagulant), or at 5%~7.5% (after accounting for the crystallization water). For lime slurry, the concentration ranges from 2%~5% (based on pure CaO) ； n is the number of times of daily modulation, which should be determined based on factors such as the amount of coagulant added and the mixing conditions; generally, it should not exceed 3 times. (II) Modulation equipment: (1) The slope of the bottom of the dissolution tank and solution tank shall be at least 0.02; there shall be slag discharge pipes at the bottom of the tanks, and the tank walls must be raised to prevent the stirred solution from overflowing ； (2) The inner walls of the dissolution tank and solution tank must be treated to prevent corrosion. Generally, the inner wall is lined with epoxy fiberglass, diabase, acid-resistant mortar, tiles, or PVC sheets; when the corrosivity of the chemicals used is not very high, acid-resistant cement mortar can also be employed. When using ferric trichloride, it is not advisable to use materials such as polyvinyl chloride that soften and deform when exposed to heat ； (3) When the dosage of the chemical is low, a leaching hopper can also be installed at the top of the solution tank to replace the chemical dissolving tank; in use, the chemical is placed in the leaching hopper, and the resulting chemical solution flows into the solution tank after being dissolved by water flow ； (4) The solution tank can be installed on a raised platform to enable the addition of chemicals by gravity. Workbenches should be available around the tank, and an overflow device is advisable at the highest working water level inside the tank ； (5) For solution tanks with a smaller dosage of chemicals, they can be combined with the dissolution tank into one tank ； (6) The polyacrylamide solution tank must be equipped with a stirring device, and the stirring speed is generally 10–15 r/min. (III) Lift and dosing of the liquid medication 1. Design requirements for metering equipment: To achieve automatic control, metering pumps, rotameters, or electromagnetic flowmeters can be used. The design requirements are as follows: (1) Pumps and pipelines should be made of corrosion-resistant metal materials. (2) Metering pumps should be used for the dosing equipment, and automatic control devices should be installed to adjust the dosage automatically. The following points should be noted when using metering pumps: ① Metering pumps are generally diaphragm pumps or plunger pumps. At present, most water treatment plants use plunger metering pumps or diaphragm metering pumps for dosing. Their advantage is reliable operation, and the dosage of coagulant can be adjusted by changing the pumping stroke of the metering pump or via frequency control, allowing for both manual and automatic control. Plunger pumps are suitable for applications requiring extremely high pressure, and can be selected based on specific conditions in the design ； ②In the design, a spare metering pump should be provided; preferably, metering pumps of the same model and specifications should be used, along with an adequate supply of wear parts and spare components ； ③When adding special chemicals (systems for adding alkalis or acids), attention should be paid to the corrosion resistance requirements of the metering pumps and other components in the system ； ④Some metering pumps have a stroke frequency feedback signal, which can be used to calculate the actual filling flow rate. To this end, the metering pump should be linked to water quality control parameters in order to automatically adjust the dosage of chemicals added ； ⑤The dosing of coagulants or coagulant aids should be carried out under automatic control ； ⑥The solution feed pipe must be equipped with the necessary solution filter to prevent clogging of the metering pump. 2. Methods of dosing chemicals: The methods for dosing chemical solutions include gravity dosing and pressure dosing. In either case, chemical transfer equipment must be installed from the dissolution tank to the solution tank, and then to the point where the chemicals are applied. Commonly used chemical transfer devices are centrifugal pumps and water jet pumps. 1) Gravity dosing involves using gravity to introduce the chemical into the suction pipe of the water pump or at the suction flange in the suction well, where it is mixed by the impeller of the water pump. This method is suitable for small and medium-sized water treatment plants where the pump house is located close to the chemical dosing room in the plant. If the water pump room is located far from the water treatment plant, a high-level solution tank can be built to use gravity to feed the chemicals into the water pump pipes. The figure below shows a schematic of gravity dosing in a high-level solution tank: http://www.ep360.cn/public/editor/kindeditor/attached/image/20161219/20161219151751_25088.jpg 1. Dissolution tank ； 2. Solution tank ； 3. Lift pump ； 4. Water-sealed box ； 5. Float valve ； 6. Flow meter ； 7. Control valve ； 8. Pressured water 2) Pressure dosing: Chemicals are added to the raw water pipeline using pumps or water jetters; this method is suitable for adding chemicals to pressurized water pipelines, or in cases where it is necessary to deliver them to water treatment facilities that are located at higher elevations or greater distances away. ①Pump dosing involves pumping the chemical solution from the solution tank into the pressure pipeline; there are two methods for this: using a metering pump directly, or using an acid-resistant pump in combination with a rotameter ； The figure below shows a schematic of the metering pump in use: http://www.ep360.cn/public/editor/kindeditor/attached/image/20161219/20161219151809_63140.jpg 1. Solution tank ； 2. Metering pump ； 3. Pressure water pipe ② Water jet injector: The water jet injector method involves using high-pressure water (pressure > 0.25 MPa); the negative pressure generated as this water passes through the nozzle and throat draws the liquid medication into the pressure water pipe. Metering equipment should be installed for water jet injection; since the water supply pipes in typical water treatment plants have high pressure, this makes use of such systems convenient. The figure below shows a schematic of the water jet injector in use: http://www.ep360.cn/public/editor/kindeditor/attached/image/20161219/20161219151816_83835.jpg 1. Solution tank ； 2. Medication box ； 3、 ； funnel ； 4. Water jet pump ； 5. Pressure water pipe ； 6. High-pressure water pipe sourced from Yijingshui Network.