Methods are as follows: 1, the use of field simulation device to determine and control the dosage of polyaluminium chloride is a relatively simple method. Common simulation devices are inclined tube settler, filter or a combination of both. When the raw water turbidity is low, the commonly used analog filter (diameter is generally about 100ram); when the raw water turbidity is high, the use of inclined tube precipitator or precipitator and filter in series. The method of using the filter is that a small amount of water is taken out from the mixed water of the water plant and continuously enters the filter to continuously measure the turbidity of the water produced by the filter to determine whether the dosage is appropriate and then feedback to the manufacturer for regulation of dosage. Because it is continuous detection and detection time is short (usually about ten minutes to complete), it can be used for water plant coagulant PAC dosing automatic control system. However, this law still exists feedback lag phenomenon, only lag time is shorter. In addition, analog devices and production equipment, after all, there are some differences. But compared with the laboratory test, closer to the production of the actual situation. Beaker test method: Beaker test method has the characteristics of convenience, flexibility, simplicity and less equipment investment, so it is widely used in some waterworks in our country (especially some small waterworks). The test results of this method are only representative of the water quality at the moment of sampling, and the problem of discontinuity and hysteresis of the quantity of polyaluminum chloride that is determined is solved. Therefore, it is suitable as an aid to evaluate the dosage of polyaluminum chloride, but not suitable for the online real-time control of coagulation dosage. 3. Mathematical model method: The mathematical model method is based on a number of raw water quality parameters (such as turbidity, pH, water temperature, alkalinity, etc.) and water parameters as variables, to establish the correlation between the dose and function, that is, the mathematical model. In water treatment, the best combination of feedforward and feed back control model. Feedforward mathematical model should choose the main parameters that affect the coagulation effect as variables, such as raw water turbidity, pH, water temperature, alkalinity, dissolved oxygen and water. Feedforward control to determine a given amount, and then the sedimentation tank effluent turbidity as a feedforward signal to adjust the feedforward feed.