Efficiency And Mechanism Of Enhanced Coagulation To Remove Haloacetonitrile Precursors From Water In Yellow River Reservoir

Haloacetonitrile(HANs)is one of the typical representatives of nitrogen-containing disinfection by-products(N-DBPs).It has stronger teratogenicity,reproductive and developmental toxicity and cytotoxicity than trihalomethanes(THMs)and haloacetic acids(HAAs).The removal of HANs precursors can simply and efficiently achieve the control of HANs disinfection by-products,and enhanced coagulation is recognized by the US Environmental Protection Agency(USEPA)as an effective method for controlling DBPs precursors.Therefore,carrying out enhanced coagulation experimental research and exploring its removal efficiency of HANs precursors has important theoretical and application significance for ensuring the safety of drinking water.In order to control the precursors of Hans in the water of the Yellow River Diversion reservoir,a pilot test of enhanced coagulation with four factors and three levels was carried out by using BBD response surface method.In this study,the removal efficiency of raw water flow,mixed GT value,flocculation GT value and dosage of polyaluminum ferric chloride(PAFC)on seven indexes including process parameters,water quality parameters and disinfection by-product parameters was explored,and the process conditions were optimized when the optimal values of each index were obtained respectively and simultaneously.Finally,combined with the change of floc characteristics during the coagulation process and the removal of organic matter before and after coagulation,the enhanced coagulation mechanism and its control effect on haloacetonitrile are analyzed.First,optimize the calculation of the coagulation GT value.The simplified formula shows that the coagulation stirring intensity is inversely proportional to the raw water flow rate and directly proportional to the 3/2 power of the motor stirring frequency.Subsequently,a response surface enhanced coagulation pilot test was carried out,and each index model was analyzed.The seven parameter equations obtained are all significant.The primary and secondary relationship of the degree of influence of various factors on the streaming potential is:coagulant dosage>raw water flow>mixed GT value>flocculation GT value.The primary and secondary relationship of the degree of influence of various factors on the flocculation index is:coagulant dosage>flocculation GT value>mixed GT value>raw water flow.The primary and secondary relationship of the degree of influence of various factors on the COD_Mn removal rate is:coagulant dosage>mixed GT value>flocculation GT value>raw water flow.The primary and secondary relationship of the degree of influence of various factors on the chlorophyll-a removal rate is:coagulant dosage>flocculation GT value>mixed GT value>raw water flow.The primary and secondary relationship of the degree of influence of various factors on the turbidity removal rate is:coagulant dosage>flocculation GT value>mixed GT value>raw water flow.The primary and secondary relationship of the degree of influence of various factors on the HANs formation potential removal rate is:coagulant dosage>flocculation GT value>mixed GT value>raw water flow.The primary and secondary relationship of the degree of influence of various factors on the THMs formation potential removal rate is:coagulant dosage>flocculation GT value>mixed GT value>raw water flow.The optimal operation conditions and the predicted values of various factors are as follows:raw water flow=5.47m³/h,mixed GT value=73809,flocculation GT value=21987,coagulant dosage=8.34mg/L,streaming potential=0.017,flocculation index=1.105,COD_Mn removal rate=27.16%,chlorophyll-a removal rate=72.16%,turbidity removal rate=86.39%,HANs formation potential removal rate=23.94%,THMs formation potential removal rate=47.85%.The content of hydrophilic components in the water of the Yellow River Diversion Reservoir is higher than that of hydrophobic components.Coagulation process has a good removal effect on hydrophobic components,and its removal effect on biopolymer and humic acid like components is higher than other fluorescent substances.The coagulant dosage and p H had significant effects on zeta potential and floc size.PAFC has an ideal coagulation effect in the range of p H=6-8 and dosage=9.5-11mg/L.At this time,its coagulation effect is mainly adsorption bridging and net catching and sweeping.The coagulation process mainly achieves the control of HANs precursors by removing DON,hydrophilic organic matter and small molecular substances in the water.The removal rate of haloacetonitrile precursors by the coagulation process fluctuates in the range of 11%-25%depending on the coagulation conditions.