Research On Optimal Control Of The Risk Of Re-flocculation Using Coagulation-Ultrafiltration Process In Drinking Water

Re-flocculation refers to the phenomenon that flocculation occurs after the membrane passes through the ultrafiltration membrane due to incomplete coagulant reaction in the coagulation-ultrafiltration process.The occurrence of re-flocculation will affect subsequent processes and cause corrosion of the pipe network,and at the same time cause water quality degradation to endanger people’s health.Therefore,the residue of coagulant is controlled to reduce the risk of re-flocculation.For the coagulation-ultrafiltration process in drinking water treatment Particularly important.Aiming at the re-flocculation problem,this paper takes the coagulation-ultrafiltration process as the research object,and aims to reduce the risk of re-flocculation and improve the efficiency of the coagulation-ultrafiltration process by changing the operating parameters of the coagulation stage.In addition,single factor experiments and Box-Behnken design experiments were performed for coagulant dosage,pH value,slow stirring rate and time,and a response surface model was established and optimized for "re-flocculation".The main research contents of this paper The conclusions are as follows:(1)The optimal coagulant was screened by comparing the coagulation effect and the risk of "re-flocculation" of the three coagulants.The results show that in coagulation-ultrafiltration,all three coagulants have a significant "re-flocculation" risk.PSAF has a lower dosage than PAFC(9:1)and PAFC(5:5).Volume and lower membrane fouling resistance,the difference in TOC removal efficiency is small,taking into account factors such as reduced dose addition and continuous production,PSAF is selected as the optimal coagulant,and the optimal dosage is 0.20 mmol/L.(2)Single factor study of the coagulation operation process for PSAF coagulation-ultrafiltration experiments,and analysis of the influence mechanism of each factor.The results show that:(1)the membrane resistance during the ultrafiltration process mainly comes from the filter cake pollution caused by flocs,and the irreversible pollution is relatively small;(2)compared with residual iron,residual aluminum is more likely to exceed the re-flocculation limit,and the risk of re-flocculation is greater;(3)In the single factor experiment,when the pH value,slow stirring rate and time are 7.5,60 rpm,and 15 min,respectively,the coagulation efficiency is higher and the risk of re-flocculation is lower.Box-Behnken design test,and response surface analysis based on the experimental results.Finally,design-Expert 8.0.6 software was used to perform multi-objective optimization based on the response surface model.The optimal operating conditions were finally obtained: the dosage of coagulant was 0.1905 mmol/L,pH 7.71,the best coagulation operation process is after 200 rpm rapid stirring for 1 min,and 72 rpm slow stirring for 15.3 min.Finally,through the verification experiments under the best operating parameters,it is found that the model estimates are close to the actual values,indicating that the optimization method is practically usable.