A. Active chlorine: refers to the oxidizing chlorine content produced by the addition of chlorine disinfectant to water
B. Residual chlorine: after the effective chlorine and water contact for a certain period of time, in addition to the water bacteria, microorganisms, organic matter, inorganic materials and other effects of consumption of a part of the chlorine, there is still a part of the chlorine, this part of the chlorine is called residual chlorine
C.Total chlorine:a) dissociative residual chlorine（Cl2、HClO、ClO-)
b) combined residual chlorine（NH2Cl、NHCl2、NCl3）
What we usually mean by residual chlorine is dissociative residual chlorine
2. The Standard
After adding chlorine disinfectant, not only should the microorganism index of the factory water meet the requirements, but also a certain residual chlorine value should be maintained to ensure that the breeding of microorganisms can be avoided in the pipeline transmission process, so as to reach a healthy drinking water.
《Drinking water hygiene standard》GB 5749-2006 sets the following requirements for residual chlorine and total chlorine in drinking water:
Dissociative residual chlorine：The contact time with water should be at least 30 min, and the residual chlorine value in the factory water should be ≥0.3 mg/L，the maximum 4 mg/L, residual chlorine in the end of the pipe network is ≥0.05mg/L
Total chloride:The contact time with water should be at least 120 min, and the residual chlorine value in the factory water should be ≥0.5 mg/L，the maximum 3 mg/L, residual chlorine in the end of the pipe network is ≥0.05mg/L
3. The Sterilization Effect
The main principle of free residual chlorine sterilization is that strong oxidizing HClO and ClO- invade the bacteria, reach the bacteria, and its oxidation, destroy the bacterial enzyme system, so as to achieve the purpose of killing bacteria, free residual chlorine sterilization effect is fast, according to the static experimental results, free residual chlorine disinfection, 5min can kill more than 99% of bacteria;The oxidizing capacity of the combined residual chlorine is weak, the sterilization speed is not as high as that of the free residual chlorine, and 60% of the bacteria can be killed within 5min. However, the same sterilization effect can be achieved by increasing the concentration of the combined residual chlorine or prolonging the sterilization time to ensure a certain CT value.The residence time of clear pool of water plant is in 2 hours above commonly, when use compound sex remaining chlorine disinfection, can assure germicidal effect.
4. Production And Control Of Residual Chlorine And Total Chlorine
The amount of chlorine added to water can be divided into two parts, namely the amount of chlorine needed and residual chlorine
The amount of chlorine required is the amount consumed to kill bacteria, oxidize organic matter, and reduce substances
Residual chlorine is the part of the body that is used to inhibit bacterial growth
When the water is mainly polluted by bacteria, organic matter and reducing substances, and is free of ammonia nitrogen and nitrogen compounds, after adding chlorine disinfectant (excluding chlorine dioxide), the residual chlorine is mainly free residual chlorine
The relationship between chlorine dosage and residual chlorine is shown in figure 1:
When the water ammonia nitrogen or nitrogen organic matter, adding the chlorine disinfectants (does not contain chlorine dioxide), bacteria and reducing material consumption of chlorine (OA) first, and then react with ammonia nitrogen material such as to generate a chloramine (AH, combined residual chlorine), after the chlorine dosing amount continues to increase, combining sexual residual chlorine into nitrogen (HB), when adding the volume continued to increase, will start to appear free residual chlorine (BC), referred to as a fold point chlorine.
The relationship between chlorine amount and residual chlorine is shown in figure 2:
When ammonia nitrogen is present in the water, the ratio between the amount of chlorine added and the content of ammonia nitrogen is 7.6:1 if ammonia nitrogen is greater than 0.3mg/L, the effective chlorine added must be greater than 2.3kg/kt (10% sodium hypochlorite chlorine added is 23kg/kt) to overfold the point, with the increase of ammonia nitrogen, the amount of chlorine added will be greater.Due to the presence of ammonia nitrogen, water will often appear free residual chlorine value is very low, even less than measurement, and the total chlorine value is already high, at this point, the increasing dosing amount, still there will be a total chlorine is higher and higher and free residual chlorine no situation, to meet the requirements of factory water dissociation of residual chlorine dosing quantity is likely to be the usual several times.When ammonia nitrogen or nitrogen compounds exist in the water, it is recommended to control the total chlorine in the factory water. On the one hand, when the disinfection contact time meets the requirements, the total chlorine can meet the sterilization requirements.On the other hand, the dosage can be reduced a lot, reducing the cost while reducing the production of chloroform and other byproducts.
5. Monitoring of residual chlorine and total chlorine
At present, the commonly used detection methods of residual chlorine in the laboratory are 3,3 ',5,5 '-tetramethylbenzidine colorimetric method and DPD method (portable residual chlorine meter), 3,3',5,5 '-tetramethylbenzidine colorimetric method, which can determine residual chlorine and total chlorine after 10min after the determination of residual chlorine, and the portable residual chlorine meter can determine residual chlorine and total chlorine. The difference lies in the use of different reagent powder packages and reaction time.In the on-line determination of residual chlorine method, the residual chlorine meter of DPD method is the same as the portable residual chlorine meter.Whether in the laboratory or in the on-line instrument, the determination of total chlorine can be easily realized on the basis of residual chlorine.
The oxidation of free residual chlorine is stronger than that of combined residual chlorine, and the sterilization speed is faster than that of combined residual chlorine. However, under the condition of ensuring the CT value (sterilization time and concentration), it can completely achieve the sterilization effect and guarantee the safety of drinking water.Therefore, when ammonia nitrogen and nitrogen-containing organic matter exist in water and affect normal chlorine addition, it is suggested to control total chlorine in factory water, which can not only reduce the amount of chlorine addition, ensure the stability of total chlorine in factory water, but also avoid the generation of disinfection by-products caused by the increase of chlorine addition.