Residual Chlorine in Practice: From Tap Standards and Pool Limits to Dechlorination Before Reverse Osmosis
Three answers to one question
Residual chlorine is the effective chlorine left in water after chlorination disinfection and a defined contact time. Depending on how it is bound, it splits into free residual chlorine, the Cl2, HOCl and OCl- present in solution, and combined residual chlorine, the chloramines formed when free chlorine reacts with ammonium, amines or ammonia. Free chlorine disinfects far better. Combined chlorine is weaker but far more persistent, which is why it earns its keep in long distribution mains and gets complained about in swimming pools. Total residual chlorine is simply the sum of the two, and it is also known as leftover chlorine or total chlorine when calculated as free chlorine.
Holding a residual matters for more than box-ticking. It suppresses coliform regrowth, inhibits biofilm formation, stabilises water quality through the pipe network, guards against occasional pathogenic ingress, and acts as an early signal that something has contaminated the supply.
The limits you have to design around
China's Standards for Drinking Water Quality (GB 5749-2022) sets free chlorine in finished water at not less than 0.3 mg/L and not more than 2 mg/L, with pipe-network end water at not less than 0.05 mg/L. For pools, the Hygienic Indicators and Limits for Public Places (GB 37488-2019) and the Swimming Pool Water Quality Standard (CJ/T 244-2016) both hold free residual chlorine at 0.3–1.0 mg/L, and CJ/T 244-2016 additionally requires combined residual chlorine below 0.4 mg/L. Foot-bath pools, which exist to block contact transmission of things like athlete's foot, run much hotter at 5–10 mg/L. Medical sewage discharged into surface water bodies must stay at or under 0.5 mg/L total residual chlorine.
Internationally the numbers differ: the World Health Organization allows up to 5 mg/L free chlorine in drinking water, while the US Environmental Protection Agency (EPA) caps it at 4 mg/L. If you operate cross-border assets, design to the tighter of the two. China currently has 11 valid standards for determining residual chlorine in water, spanning environment, health, chemical, electric power, shipbuilding and urban construction, and their naming and definitions are not yet fully harmonised, which is worth knowing before you compare results across labs.
How the number is produced
Chlorination remains the dominant drinking water disinfection technology worldwide, delivered as liquid chlorine, sodium hypochlorite, bleaching powder or organic chlorine preparations such as chloramine. The disinfectant hydrolyses to hypochlorous acid (HOCl), and its oxidising power denatures the proteins of bacteria and viruses. Operators move the dose; the residual follows.
Breakpoint chlorination is the tool for heavily polluted raw water. The dose is pushed until ammonia nitrogen is fully oxidised and the residual becomes free-chlorine dominated, tracing the familiar fold-line curve of residual versus dose. Knowing which fraction you are actually measuring is the whole point of the exercise, because a combined-chlorine residual and a free-chlorine residual of the same number behave very differently in the network.
Measurement: DPD, electrodes and colour discs
Three families dominate. The DPD (N,N-diethyl-1,4-phenylenediamine) spectrophotometric method is the reference in China's Standard Examination Methods for Drinking Water (GB/T 5750.11-2006): residual chlorine reacts with DPD reagent to form a red compound whose absorbance is read for quantitative analysis. It is fast, the phenomenon is obvious, the product is stable, and accuracy and precision are good. Electrochemical methods run continuously online through electrode reactions, typically a gold electrode against a silver/silver chloride electrode. Colorimetry is simple and quick but is mostly visual, so the measurement error is relatively large.
Instrumentation now spans portable photoelectric colorimeters for spot checks through to multi-parameter online analysers that combine DPD spectrophotometry or electrochemistry with automatic temperature compensation, high and low alarms and data management, covering residual chlorine, pH, turbidity and temperature in one housing.
Protecting downstream membranes
In industrial pure water production, residual chlorine oxidises and damages polyamide reverse osmosis membranes, so the feed has to be dechlorinated. Activated carbon adsorption works, and so does dosing a chemical reducing agent such as sodium metabisulfite (SMBS). Theory says 1.34 mg of SMBS removes 1.0 mg of residual chlorine in an RO system, but engineering practice needs 3.0 mg of SMBS to remove that same 1.0 mg. SMBS must be stored dry and cool, because its aqueous solution undergoes redox reaction with oxygen in the air, and the effective service life has to be judged from the prepared solution concentration.
When the number goes wrong
Real incidents are usually mundane. In January 2024 a metering error in the chlorination disinfectant dosing pump at the water supply pump room of a residential community in Shenyang pushed finished-water residual chlorine over the limit and gave the tap water a noticeable smell; technicians corrected the dose and the indicator returned to normal that night, with a subsequent test at 0.37 mg/L, inside the GB 5749-2022 band. In August 2022 a joint law enforcement inspection in Fuling District, Chongqing ordered swimming venues to rectify substandard foot-bath disinfection and unqualified pool residual chlorine. In July 2016 a Xi'an pool was linked to poisoning symptoms in 16 children after apparent over-dosing. In July 2022 low residual chlorine at a hotel pool in Jiaxing failed to disinfect, total bacterial count went over and more than 20 children developed dermatitis. In 2023 a Hunan sampling round found unqualified free chlorine in several batches of packaged drinking water, traced to loose process control or heavy chlorination after source contamination.
The cancer rumour, and what the evidence says
Claims that heating residual chlorine in tap water produces carcinogens have been refuted repeatedly. Professor Wenjun Liu of Tsinghua University notes that chlorine is used as a disinfectant worldwide, that its hazard after dissolving in water is extremely low and its own hazard is negligible, and that residual chlorine meeting the national standard is safe. The International Agency for Research on Cancer (IARC) lists chlorine as a Group 3 carcinogen, meaning suspected human carcinogenicity with insufficient evidence. Food engineering PhD Wuxin Yun adds that most residual chlorine decomposes during heating and that no epidemiological evidence supports carcinogenicity.
Controlling it rather than chasing it
Water plants hold residual chlorine in band by optimising the disinfection process: precise disinfectant dosing equipment, real-time residual monitoring, and dose adjustment against actual water quality. Where disinfection by-products are the concern, combined processes such as activated carbon adsorption and ultraviolet disinfection reduce by-product formation while keeping the disinfection effect. The target is a residual that is high enough to protect the network and low enough not to become the next problem.