Application Of Technological Process Reform For Disinfection In Drinking Waster Treatment

Disinfection is an essential process in water treatment.Traditional disinfectant,typically the liquid chlorine,is still applied in a number of water treatment plants in China.However,liquid chlorine,which is a hazardous chemicals,poses severe threat to the environment as well as operators.Besides,disinfection based on liquid chlorine readily form disinfection by-products（DBP）including trihalomethanes（THMs）,haloacetic acid（HAAs）,which cause cancer.Comparatively,sodium hypochlorite,characterized by higher process safety,relatively simple equipment demand,and consistent disinfection effect,is one of the ideal alternative disinfectants.Sodium hypochlorite is adopted as alternative disinfectant to liquid chlorine in this study.By installing and operating sodium hypochlorite-based disinfection system for months in a water treatment plant in Wuhan city,we come to conclusions as follows:（1）The sodium hypochlorite-based disinfection system is primarily comprised of electrolytic sodium hypochlorite generator,solution-storing system,addition system,pipe system,instrument system,automatic control system,etc.Addition of disinfectant is the combination of pre-addition sand pro-addition.Dose of disinfectant is adjusted according to water quality of source water.（2）Source water treated was in the range of 33.16×10⁴ and 47.93×10⁴ t/d,with the average quantity of 41.65×10⁴ t/d.the addition of sodium hypochlorite was in the range of 3.68 and 5.25 mg/L,with the average dose of 4.35mg/L.The annual disinfectant consumed was about 200.10t.（3）p H value in the effluent of sodium hypochlorite-based disinfection system was in the range of 7.50 and 8.09,with the average value of 7.76.Color in the effluent was all below 5 times.Turbidity in the effluent was in the range of 0.03 and 0.27NTU,with the average value of 0.11NTU.COD_Mnin the effluent was in the range of 0.64and 2.24mg/L,with the average concentration of 1.31mg/L.Dissolved iron in the effluent was all below 0.05 mg/L.Total hardness in the effluent was in the range of108 and 154mg/L,with the average value of 137mg/L.Chloride in the effluent was in the range of 12 and 35 mg/L,with the average concentration of 20.1 mg/L.Average concentration of ammonia in the effluent was 0.033mg/L.With regard to microorganism-related water index,total plate count（TPC）,total colibacillus and thermo-tolerant coliform bacteria were all completely killed,with number of these microorganism in the effluent were all zero.On the other hand,all these water quality in the effluent meet the Standards for Drinking Water Quality（GB5749-2006）.（4）Free residual chlorine in the effluent was in the range of 0.7 and 1.16mg/L with the average value of 0.93mg/L.In practice,free residual chlorine in pre-addition phase is maintained at 0.1 to 0.2mg/L,with maintaining at around 1mg/L in pro-addition phase.The concentration of disinfection by-products such as trichloromethane and trichloroacetaldehyde was positively correlated with the residual chlorine in the discharged water,indicating that the disinfection by-products generated by sodium hypochlorite disinfection may come from the reaction between residual chlorine and the precursor of disinfection by-products.（5）Environmental risk of liquid chlorine-based disinfection system basically came from the chlorine storage pool,while that of sodium hypochlorite-based disinfection system mainly comes from solution storage system,such as solution storage boxes.Due to its high toxicity,when environmental emergencies,such as leaking of the disinfectant occur,consequences caused by liquid chlorine is severely worse than by sodium hypochlorite.（6）The cost of Sodium hypochlorite addition is slightly higher than liquid chlorine.On the other hand,sodium hypochlorite-based disinfection system has fundamentally decreased various safety apparatuses needed for liquid chlorine-based disinfection and corresponding expense of maintenance.Sodium hypochlorite-based disinfection system also does not need stringent requirements such as pressure and flow,which is more cost and energy effective.Finally,sodium hypochlorite-based disinfection system completely eliminates environmental treat caused by liquid chlorine-related major risk sources.