Total residual chlorine (total residual chlorine)
Also known as: TRC · residual chlorine
Total residual chlorine is the free plus combined chlorine remaining in water after disinfection. The inland surface water and marine coastal effluent limit is 1.0 mg/L, as excess chlorine is toxic to aquatic life.
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What is Total residual chlorine?
Total residual chlorine (TRC) is the amount of chlorine — both free chlorine and combined chlorine (chloramines) — that remains in water after it has been disinfected with chlorine. Chlorine is widely used to kill pathogens in water and effluent, but the chlorine left over after disinfection is itself a pollutant. The effluent limit is 1.0 mg/L for inland surface water and marine coastal discharge.
The concern is that residual chlorine is toxic to aquatic life — it is an oxidant that damages fish gills and kills aquatic organisms at low concentrations, which is precisely why it works as a disinfectant but also why it must not be discharged in excess. A second concern is that chlorine reacts with organic matter in water to form chlorinated by-products (trihalomethanes, chlorophenols, and — where it contacts the right precursors — chlorinated dioxins), some of which are toxic or carcinogenic.
For recyclers, total residual chlorine is relevant wherever chlorine is used for disinfection of treated effluent or process water, or where chlorine-based cleaning and treatment chemicals are used. It also connects to the broader chlorine/chloride theme: in any thermal process, chloride is the dioxin precursor, and chlorine chemistry must be managed carefully. A treated effluent that has been over-chlorinated for disinfection can fail the residual chlorine limit and the bio-assay test even though disinfection succeeded.
The practical control is dechlorination before discharge. Where chlorine disinfection is used, the residual is removed by adding a reducing agent (sodium thiosulphate, sulphur dioxide, or sodium bisulphite) or by activated-carbon contact, bringing TRC below 1.0 mg/L before the effluent reaches the receiving water. The cleaner alternative, increasingly preferred, is to use non-chlorine disinfection (UV or ozone) that leaves no toxic residual. The key operational point is that disinfection efficacy and residual-chlorine compliance are separate requirements — both must be met.
Common questions about Total residual chlorine
Plain-English answers to what people most often ask.
What is the total residual chlorine limit in effluent in India?
How is residual chlorine removed before discharge?
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