Study On Optimization Of Disinfection Process And Application Of Replacement Project In Large Water Purification Plant In North China

Disinfection is the most important way to ensure the biological safety risk of factory water in the water treatment process of water purification plant,among whi-ch liquid chlorine disinfection is the most widely used.However,disinfection can produce different kinds of disinfection by-products（DBPs）while ensuring the biolo-gical safety of drinking water.DBPs have attracted more and more attention becau-se among these DBPs,it has been proved that there is a risk of carcinogenesis,tera-togenesis or mutagenicity to human health.The type and concentration of DBPs are determined by the composition of org-anic matter in raw water,water treatment process,disinfection method and factors.This paper takes the M reservoir raw water of the conventional treatment process water plant in H city in the north as the research object.From the perspective of en-gineering practice,the type,content and interannual variation law of orgnic matter in raw water are determined.The results show that the organic matter in the source water of H city is mainly fulvic acid and humic acid with molecular weight of5k Da～10k Da and<1k Da,which are the precursors of chlorinetion DBPs（Cl-DBPs）.As the water treatment plant in H city is a conventional water treatment process,the treatment effect of the above organics is not good,so more Cl-DBPs are formed in the process of liquid chlorine disinfection,mainly trichloromethane（TCM）,chloroa-cetaldehyde（CH）,trichloroacetic acid（TCAA）and dichloroacetic acid（DCAA）.The above months with high Cl-DBPs content are mainly concentrated in spring flood season and summer and autumn flood season.The national standardized assessment results show that the conventional proce-ss water purification plants using chlorine disinfection in northern China are not eq-uipped with sufficient water quality detection equipment to carry out the detection of DBPs due to the problem of funds.Therefore,this paper uses multiple regression method to establish a prediction model based on the conventional indicators of raw water,chlorine dosage and DBPs production,and determines that the dosage of dis-infectant is an important influencing factor.The R² of the two prediction models of TCM and CH are 0.956 and 0.997 respectively.The effects of Cl₂ and permanganate index(COD_Mn)on the formation of TCM and CH are positively correlated,which can promote the formation of DBPs.The establishment of the two models will prov-ide a basis for the water plant without detection ability to predict the generation of DBPs in advance.Take the raw water of M reservior as the object,the production of DBPs by di-fferent disinfection methods was studied.The concentration of DBPs in the effluent of chlorine dioxide disinfection（Cl O₂）could meet the requirements of the national standard,but storage of Cl O₂ and its main raw materials have the potential risk of safety production accidents;The type and content of DBPs in the factory water after sodium hypochlorite disinfection are consistent with that of liquid chlorine disinfec-tion,but the p H can be increased by 0.1-0.2,which can reduce the amount of alkali added in the production process;Although the ultraviolet chlorine coupling disinfe-ction combined technology can reduce the generation of DBPs,the daily power co-nsumption of the ultraviolet lamp is increased by 3872.6k W according to the water supply,the corresponding CO₂ emission is increased by 3052kg,the operation cost of the water plant is greatly increased;Under the condition of adding ammonium sulfate first and then sodium hypochlorite（Cl:N is 4:1）,the disinfection effect is the best,chloramine disinfection is finally determined as the final disinfection method.It is recommended to replace the disinfection method of P water purification plant in two phases.In the first phase,liquid chlorine disinfection is changed to sodium hypochlorite disinfection to solve the safety problem of disinfectant production,which has been completed without water supply interruption;Ammo nium sulfate dosing equipment is added in phase II project to finally realize the replacement fro-m liquid chlorine disinfection to chloramine disinfection.Based on the water supply network of H City,the migration and transformation law of DBPs in the actual network and the influence of secondary chlorination on the generation of DBPs in the network water were studied.The four DBPs showed an upward trend with the increase of hydraulic retention time in the first 24h,and the CH concentration reached the maximum value of 21.2μg/L in 24 hours,increas-ed by 37.2%compared with the factory water,and TCM reached the maximum val-ue of 21.1μg/L at 36h,83%more than the factory water.Similar to the change of TCM,TCAA and DCAA also reached a maximum value at 36h,and then began to decrease.In addition,CH was at risk of exceeding the standard in all seasons,acco-unting for the largest proportion of DBPs in factory water and pipe network water,38.81%and 31.02%respectively;The research results of the impact of flow rate and secondary chlorine addition on DBPs in the pipe network show that,at the same fl-ow rate,DBPs showed an upward trend with the increase of second chlorine additi-on,but the upward trend was small and gentle;DBPs tended to decrease when the secondary chlorine dosage was constant but the flow rate increased.