Research On The Construction Of Coagulation System For Source Water From The South-to-North Water Diversion Project Based On Water Quality Safety

As an important unit of a water purification plant,coagulation process effects the quality of the effluent water as well as the cost of water treatment.The water source of the South-to-North Water Diversion has become the main source for drinking water in Beijing.In order to achieve the balance between coagulant saving and meeting water quality standards,it is necessary to improve the intelligent level of coagulation dosing,and optimize the coagulation dosing based on the features of water sources as well as characteristics of units of water purification plant.The changes of residual aluminum in the whole process was studied with pilot test in Water Plant J.Based on the results,several dosing methods in different circumstances were established.A coagulation dosing model was created based on the online monitoring data of the pilot system.An intelligent control system for coagulation dosing was established.In addition,the optimized dosing based on the quality of effluent and the coagulation enhanced by pre-ozone reaction were studied during the situation of water with low turbidity and low temperature.The main research contents and conclusions are as follows:（1）Analyzing the changes of residual aluminum in the whole process of water purification process,it is found that suspended aluminum particle in the effluent of the sedimentation tank is easily removed by sand filter.However the filter process has a poor removal effect on aluminum particles.Therefore,the content of dissolved aluminum in the effluent of the sedimentation tank is a key factor to control the residual aluminum in the effluent.（2）The dual-dose mode can reduce the concentration of dissolved aluminum in the effluent of the sedimentation tank and improve the quality of the effluent.During the experiment,the residual aluminum in the effluent was reduced to below 107.30μg/L.It is found that the dissolved aluminum in the effluent of the sedimentation tank under low temperature is 19.6%lower than under high temperature,that is,the risk of residual aluminum exceeding the standard under low temperature is lower.The single/dual dose can be selected reasonably during the low temperature period,but dual-dose must be used during the high temperature period,otherwise it will cause the aluminum to exceed the standard.（3）The random forest dosing model and XGBoost dosing model were established respectively using the online monitoring data of the pilot system.It is found that XGBoost had better performance of prediction than rand forest did.Based on the XGBoost algorithm,the predicted value of the ferric chloride dosing model was selected as the input of the polyaluminum chloride dosing model,and a dual-dose model of polyaluminum chloride and ferric chloride was established.An intelligent control system for coagulation dosing was further established.Under the system’s control,the coagulant consumption was reduced by about 21.4%,and the turbidity of the activated carbon filter effluent was stabilized below 0.033 NTU.（4）The effect of coagulation dosing on the effluent water quality during low temperature and low turbidity period was studied.15 mg/L of polyaluminum chloride for single-dose was recommended.12 mg/L for polyaluminum chloride and 7.0 mg/L for ferric chloride were recommended for dual-dose dosage.With recommended dual-dose dosage,the processing capacity of each process unit was fully utilized,the effluent quality requirements were met,and the consumption of coagulant was saved.（5）The pre-ozonation enhanced coagulation technology in the low temperature and low turbidity period was studied.Pre-ozonation can reduce the turbidity of the effluent and head loss of the sand filter（operating for 24 h）.Moreover it also had a better enhanced removal effect on UV₂₅₄.0.2 mg/L-0.9 mg/L was recommended for ozone dosage.