Environmental Water Treatment Knowledge: Meaning and Function of Residual Chlorine
Residual chlorine refers to the effective chlorine remaining in water after chlorination disinfection and a certain contact time. The aliases of residual chlorine (calculated as free chlorine) include leftover chlorine and total chlorine. According to the "Standards for Drinking Water Quality" (GB 5749-2022), the limit for free chlorine residual in finished water is ≤2 mg/L (contact time ≥30 minutes) and not less than 0.3 mg/L; the free chlorine residual in pipe-network end water should not be less than 0.05 mg/L. [3][26]
Residual chlorine can be divided into combined residual chlorine (chlorine compounds of chlorine and ammonia in water, also called combined chlorine) and free residual chlorine (OCl⁻, HOCl, Cl₂, etc. in water, also called free chlorine). [3] Free residual chlorine has better disinfection effect; combined residual chlorine has weaker disinfection effect but is more stable.
The significance of maintaining residual chlorine lies in: preventing the regrowth of coliform bacteria; inhibiting the formation of biofilms; stabilizing the water quality of the pipe network; preventing the occurrence of occasional pathogenic bacteria; and serving as a signal for microbial contamination. [3]
According to its existence form and disinfection activity, residual chlorine can be divided into three categories. Free residual chlorine, also called free chlorine, refers to Cl₂, HOCl, OCl⁻, etc. in aqueous solution; it is a residual chlorine component with better disinfection effect and stronger oxidizing property, commonly used to evaluate disinfection effect in sanitary disinfection and swimming pool water quality control. Combined residual chlorine is a chloramine substance generated by the reaction of free residual chlorine with ammonium, amines, or ammonia in water; its bactericidal effect is weaker than hypochlorous acid and may produce odor, making it unwelcome in swimming pools, but due to its stronger stability, it plays a positive role in achieving long-term sterilization in tap water supply systems. Total residual chlorine is the sum of free and combined residual chlorine.
Residual chlorine is mainly prepared by adding chlorine-containing disinfectants to water. [3] Chlorination disinfection is a globally used drinking water disinfection technology; common disinfectants include liquid chlorine, sodium hypochlorite, bleaching powder, and organic chlorine preparations (such as chloramine). [5] Disinfectants (such as sodium hypochlorite) hydrolyze to form hypochlorous acid (HOCl), whose strong oxidizing property denatures proteins on bacteria and viruses, thereby achieving disinfection. [3]
When using the breakpoint chlorination method to improve the quality of heavily polluted raw water, it is often necessary to distinguish between free residual chlorine and combined residual chlorine. This method precisely controls the chlorine dosage so that the ammonia nitrogen in the water is completely oxidized, thereby obtaining a disinfection effect dominated by free residual chlorine; the relationship curve between chlorine dosage and residual chlorine is fold-line shaped. [7]
In terms of detection instruments, applications range from portable rapid detectors to multi-parameter online water quality analyzers. [23-24] Portable residual chlorine detectors are based on the photoelectric colorimetric principle and can quickly determine the residual chlorine/total chlorine content in water on site. Modern online water quality residual chlorine analyzers integrate DPD spectrophotometry or electrochemistry, with functions such as automatic temperature compensation, high/low alarms, and data management, and can realize multi-parameter integrated detection and intelligent early warning of residual chlorine, pH, turbidity, temperature, etc. [23-24]
In industrial pure water production (such as reverse osmosis systems), to prevent residual chlorine from oxidizing and damaging the reverse osmosis membrane, it is often necessary to dechlorinate the influent. Common methods include the use of activated carbon adsorption or the addition of chemical reducing agents such as sodium metabisulfite (SMBS). [17] According to theoretical calculations, 1.34 mg of SMBS can remove 1.0 mg of residual chlorine in the reverse osmosis membrane system, but in engineering practice, 3.0 mg of SMBS must be added to remove 1.0 mg of residual chlorine. SMBS needs to be stored under dry and cool conditions; its aqueous solution undergoes redox reaction with oxygen in the air, and its effective service life must be determined according to the prepared solution concentration. [20]
Residual chlorine is the core indicator of tap water disinfection. Its main function is to provide continuous disinfection capability during pipe network transportation after plant disinfection, preventing secondary pollution and ensuring the microbial safety of drinking water. China’s "Standards for Drinking Water Quality" (GB 5749-2022) stipulates that the free chlorine content in finished water should be maintained between 0.3 mg/L and 2 mg/L, and the free chlorine content in pipe-network end water should not be less than 0.05 mg/L. [3][5-6][8-9][18][25-26] If a slight chlorine smell can be smelled in the water, it usually indicates that the water contains residual chlorine and the water quality is safe and sanitary. [1][5][9][13]
Free residual chlorine is the core indicator for evaluating the disinfection effect of swimming pool water quality. China’s "Hygienic Indicators and Limits for Public Places" (GB 37488-2019) and "Swimming Pool Water Quality Standard" (CJ/T 244-2016) stipulate that the free residual chlorine concentration in artificial swimming pool water should be maintained at 0.3–1.0 mg/L. [10-12][14][16][27-28] Too low a concentration cannot effectively kill pathogenic bacteria and may lead to disease transmission; too high a concentration will irritate the skin, mucous membranes, and respiratory tract of swimmers. [10][12][16][33] In addition, the foot-bath pool, which blocks the spread of contact communicable diseases such as athlete’s foot, has a higher free residual chlorine limit requirement of 5–10 mg/L. [11][33]
To prevent environmental risks from over-disinfection, medical sewage needs to be dechlorinated before being discharged into surface water bodies. According to the "Discharge Standard of Water Pollutants for Medical Institutions," when medical sewage is discharged into surface water bodies, the total residual chlorine concentration must not exceed 0.5 mg/L. [15]
To ensure water quality safety compliance, in places such as water plants, swimming pools, and water parks, it is necessary to monitor residual chlorine concentration in real time or regularly. Modern water quality monitoring systems can integrate online monitoring devices for parameters such as residual chlorine, pH, and turbidity, realizing real-time data collection and intelligent early warning. [11][23-24]
The detection methods for residual chlorine are mainly divided into spectrophotometry, electrochemistry, and colorimetry. [21] The DPD (N,N-diethyl-1,4-phenylenediamine) spectrophotometric method is a commonly used method in standards such as China’s "Standard Examination Methods for Drinking Water" (GB/T 5750.11-2006) due to its rapid reaction, obvious phenomenon, stable products, and high accuracy and precision. This method reacts residual chlorine with DPD reagent to generate a red compound, and its absorbance is measured for quantitative analysis. [21][23-24] The electrochemical method realizes continuous online monitoring through electrode reactions (such as gold electrode and silver/silver chloride electrode). [23-24] The colorimetric method has the advantages of simplicity and rapidity, but is mostly visual colorimetry with relatively large measurement errors. [21]
Water plants control residual chlorine content by optimizing the disinfection process, such as adopting precise disinfectant dosing equipment, monitoring residual chlorine in real time and adjusting according to water quality conditions. In addition, combined processes such as activated carbon adsorption and ultraviolet disinfection can be used to reduce the generation of disinfection by-products while ensuring disinfection effect. [6]
China currently has 11 valid standards for the determination of residual chlorine in water, involving multiple industries such as environment, health, chemical, electric power, shipbuilding, and urban construction, and the current situation is relatively complex. The naming (such as residual chlorine, total chlorine, free residual chlorine, combined residual chlorine, etc.) and definitions of these standards are not yet completely unified, and there are coordination needs in quality indicators, sample pretreatment, and storage and transportation provisions between methods. [21]
According to China’s "Standards for Drinking Water Quality" (GB 5749-2022), when chlorination disinfection is used, the limit for free chlorine in finished water is ≥0.3 mg/L and ≤2 mg/L; the free chlorine in pipe-network end water should be ≥0.05 mg/L. [5][18][26] For artificial swimming pool water quality, the national standard "Hygienic Indicators and Limits for Public Places" (GB 37488-2019) stipulates that the free residual chlorine in pool water should be maintained at 0.3–1.0 mg/L; [12] the industry standard "Swimming Pool Water Quality Standard" (CJ/T 244-2016) further stipulates a free residual chlorine limit of 0.3–1.0 mg/L and requires combined residual chlorine concentration to be below 0.4 mg/L. [27-28] In medical sewage treatment, the "Discharge Standard of Water Pollutants for Medical Institutions" stipulates that when medical sewage is discharged into surface water bodies, the total residual chlorine concentration must not exceed 0.5 mg/L. [15] Internationally, the World Health Organization (WHO) recommends a maximum allowable concentration of free chlorine in drinking water of 5 mg/L; the US Environmental Protection Agency (EPA) stipulates a maximum allowable concentration of free chlorine in drinking water of 4 mg/L, which can be used as a reference. [29]
In January 2024, due to a metering error in the chlorination disinfectant dosing pump in the water supply pump room of the Wucai Yangguangcheng residential community in Shenyang, the residual chlorine in the finished water exceeded the standard, and the tap water had a noticeable odor. After adjustment by water technical personnel, the residual chlorine indicator returned to normal that night. Subsequent tests showed a residual chlorine content of 0.37 mg/L, meeting the limit requirements of the "Standards for Drinking Water Quality" (GB 5749-2022). [32]
In August 2022, the Comprehensive Law Enforcement Detachment for Health in Fuling District, Chongqing, found in a joint law enforcement inspection that some swimming venues had problems such as substandard foot-bath disinfection and unqualified residual chlorine indicators in swimming pool water, and were ordered to rectify. Law enforcement personnel pointed out that the residual chlorine standard is 0.3–1.0 mg/L; too high or too low is detrimental to health. [14]
In July 2016, a swimming pool in Xi’an allegedly caused 16 children to show symptoms of poisoning after swimming, possibly due to excessive chlorine water addition. According to national standards, the free residual chlorine in pool water should be controlled at 0.3–0.5 mg/L, which can ensure disinfection effect without harming the human body. [33]
In July 2022, due to low residual chlorine in the pool water of a hotel swimming pool in Jiaxing, the disinfection effect could not be achieved, leading to excessive total bacterial count and causing more than 20 children to develop dermatitis. The health supervision department confirmed the water quality was unqualified through testing. [34]
In 2023, Hunan Province reported in a sampling inspection of packaged drinking water that multiple batches of products had unqualified residual chlorine (free chlorine) problems; the reason for exceeding the standard may be that enterprises had lax production process control or used large amounts of chlorine disinfection after the water source was polluted. [30]
In response to online rumors such as "residual chlorine in tap water causes cancer after heating," experts have clarified multiple times. [19] Professor Wenjun Liu of Tsinghua University pointed out that chlorine, as a disinfectant, is widely used globally, and its hazard after dissolving in water is extremely low, its own hazard is negligible, and residual chlorine content that meets national standards is safe. [31] The International Agency for Research on Cancer (IARC) classifies chlorine as a Group 3 carcinogen (suspected human carcinogenicity, insufficient evidence). [18] Dr. Wuxin Yun, a food engineering PhD, also stated that residual chlorine mostly decomposes during heating, and there is currently no epidemiological evidence to support its carcinogenicity. [19]
