Premilinary Study On The Formation Of NDMA By Henced Coagulation Using Composite Coagulant Treating Micro-polluted Raw Water

In the case of surface water polluted by ammonia nitrogen in China,when the process of prechlorination and inorganic salts/PDMDAAC composite coagulant is used to enhanced coagulation,The interaction of PDMDAAC in composite coagulants with chlorine or chloramine may result in the formation of nitrogen-containing disinfection by-product N-nitrosodimethylamine(NDMA).In this paper,the possibility,regularity and influence of a three component system consisting of ammonia nitrogen in tiny-polluted raw water,added pre-oxidizerant chlorine and PAC/PDMDAAC to form disinfection by-product NDMA in different contact ways were studied.Firstly,a method of NDMA quantitatively analytic method under experimental conditions is established to verify its accuracy.Secondly,the effect of PAC a component of composite coagulant on the formation of NDMA from the reaction of PDMDAAC and chloramines under different reaction conditions was studied.Finally,the possibility,regularity and influencing factors of NDMA formation were investigated and verified at the stage of enhanced coagulation of micro-polluted raw water.Establishment of NDMA analysis method under the experimental conditions On the basis of the existing NDMA detection methods named a method of solid-phase extraction-high performance liquid chromatography,and the requirement of the detection limitation of NDMA,the effect of PAC added of each independent process and the whole process of the method,for UV scanning and liquid chromatographic analysis,on the precision and recoveries was systemically detected,and then the condensing multiple of the solid-phase extractive product was largely improved simultaneously,in order to established an effective method for the determination of trace amount NDMA in the system containing PAC.The results showed that there was no obvious interference of PAC in the method of solid-phase extraction-high performance liquid chromatography detection after adding the actual amount of PAC in use,and when the PAC concentration was added 10 mg/L,the NDMA assay precision was 2.59%.The recoveries of standard additions were 87.96%to 102.36%,and the average recoveries were 92.98%.Compared with the precision and recoveries of measuring NDMA in the system without containing PAC,the smaller the concentration of PAC added was,the closer the value of measured NDMA in the system to the value of that without containing PAC was.Therefore,the detection method of trace amount NDMA(10 ng/L)was established under the experimental conditions.The reaction regulation of composite coagulant and monochloramine Based on the existing research and results oin the reaction between PDMDAAC and monochloramine to form NDMA,and under the condition of using simulated tiny-polluted raw water only concering the ammonia nitrogen content,the operation processes of pre-oxidation and then enhanced coagulation were carried on to explore the effect of PAC contained in composite coagulant on the formation of NDMA vis the reaction of the contained PDMDAAC and monochloramine added.The results showed that after the pre-chlorination of the raw water,the two feeding modes,such as adding PAC/PDMDAAC and only adding PDMDAAC produced most close to the content of NDMA,that is,the content of NDMA(26.82 ng/L).However,the feeding mode of the adding PDMDAAC followed directly by PAC?adding PAC followed directly by PDMDAAC.PDMDAAC and PAC at the same time generated NDMA(32 ng/L).Either in the presence or not of PAC,the formation regulations of NDMA in the reaction system with the concentration of reactants,reaction time and temperature were kept unchanged under different experimental conditions.This indicated that the formation of NDMA is related to the chloramine(chlorine and ammonia nitrogen)concentrations,reaction time and temperature,but not affected by the existence and addition mode of PAC in the system.Therefore,in practical applications,it should be tried to control the NDMA generation concentration to reach its minimium.The possibility and regulation of NDMA formation in the treatment of simulated micro-polluted raw water by enhanced coagulation with composite coagulants Based on the results of the above and the established water quality models of the Yangtze River raw water,the possibility and regulation of NDMA formation in the process of enhanced coagulation treatment of the simulated and real tiny-polluted raw water were studied.The experiment results showed that under the condition using tap water contained 25 ng/L of NDMA to prepare the simulated tiny-polluted raw water of a turbidity about 100NTU,CODMn about 5 mg/L and ammonia 3 mg/L,the formed NDMA content was detected as 18.51 ng/L after the enhanced coagulation treatment only using composite coagulant PAC/PDMDAAC;However,after pre-chlorination and then enhanced coagulation treatment,the amount of NDMA formed was 77.60 ng/L.In the case that the simulated micro-polluted raw water was prepared only using deionized water without NDMA,and no NDMA content was detected after enhanced coagulation treatment only using PAC/PDMDAAC;However,after pre-chlorination and then enhanced coagulation,the NDMA content was 48.33 ng/L.Further study on the enhanced coagulation of the Yangtze River's tiny-polluted raw water with the NDMA content of 41.32 ng/L found that after the enhanced coagulation treatment only using PAC/PDMDAAC,the NDMA content was reduced to about 25.58 ng/L;However,the NDMA content reached 47.32 ng/L under the conditions of pre-chlorination and then enhanced coagulation using PAC/PDMDAAC.This indicated that for the tiny-polluted raw water,NDMA was formed only by the use of pre-chlorination combined with enhance coagulation using composite coagulants.Nevertheless,the enhanced coagulation technology using compound coagulant not only did not form NDMA,but also could effectively remove the existed NDMA in the tiny-polluted raw water bodies.In this paper,the possibility and regulation of NDMA formation in the process of enhanced coagulation treatment of tiny-polluted raw water using the prechlorine oxidation combined with composite coagulant PAC/PDMDAAC,and also various influencing factors were systemically investigated.The acquired results could be the important theoretical basis and technical support for optimizing the application conditions of the enhanced coagulation process using composite coagulant PAC/PDMDAAC,minimizing and avoiding the possibility of forming NDMA,and ensuring the safety of water supply,extensively.