Chlorination and De-Chlorination Process in Water Treatment

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Chlorination and De-Chlorination are two essential stages in the treatment and preparation of water, particularly in pharmaceutical, industrial, and municipal water systems. These processes ensure that water is both microbiologically safe and chemically suitable for subsequent purification stages such as Reverse Osmosis (RO) and Deionization (DI). Understanding the principles, chemistry, and control parameters of these steps is vital for maintaining consistent water quality under Good Manufacturing Practices (GMP).

What is Chlorination?

Chlorination is the process of adding chlorine or chlorine compounds to water for the purpose of disinfection. Chlorine is a powerful oxidizing agent that destroys pathogenic microorganisms such as bacteria, viruses, and protozoa, making the water safe for use.

Purpose of Chlorination

  • To disinfect and eliminate disease-causing microorganisms.
  • To oxidize organic matter, iron, and manganese compounds present in raw water.
  • To maintain a protective residual that prevents microbial regrowth during water storage and transport.

Common Forms of Chlorine Used

  • Chlorine Gas (Cl₂): Used in large-scale treatment systems for continuous dosing.
  • Sodium Hypochlorite (NaOCl): Commonly used liquid form for smaller or pharmaceutical systems.
  • Calcium Hypochlorite (Ca(OCl)₂): Used in tablet or powder form for emergency or batch dosing.

Chemical Reactions in Chlorination

When chlorine is added to water, it hydrolyzes and forms hypochlorous acid (HOCl) and hypochlorite ions (OCl⁻), which are the active disinfecting species.

Cl₂ + H₂O → HOCl + H⁺ + Cl⁻  
HOCl ⇌ H⁺ + OCl⁻

The disinfecting power of chlorine depends on the pH of the water. At lower pH values (6.5–7.0), hypochlorous acid (HOCl) predominates and provides maximum germicidal activity.

Mechanism of Microbial Kill

  • Chlorine oxidizes cellular enzymes and disrupts metabolism.
  • It damages cell membranes and denatures nucleic acids (DNA/RNA).
  • Destroys spore-forming bacteria and biofilm-producing organisms with sufficient contact time (CT value).

Types of Chlorination

  • Pre-Chlorination: Chlorine is added before filtration to control algae and microorganisms.
  • Post-Chlorination: Chlorine is added after filtration or treatment for final disinfection.
  • Breakpoint Chlorination: Sufficient chlorine is added to oxidize all organic and ammonia compounds, leaving only free chlorine residual.

Monitoring of Chlorine Levels

It is essential to control chlorine dosage and residual levels to achieve efficient disinfection without overdosing. Common parameters monitored include:

  • Free Chlorine: Active and available chlorine for disinfection.
  • Combined Chlorine: Chlorine combined with ammonia and nitrogen compounds (less effective).
  • Total Residual Chlorine: Sum of free and combined chlorine.

Recommended Control Limits (Pharmaceutical Applications)

Water Stage Free Chlorine (mg/L) Residual Chlorine (mg/L)
Raw Water (Pre-Treatment) 1.0 – 2.0 0.2 – 0.5
After Carbon Filtration < 0.5 < 0.1
Before Reverse Osmosis Nil Nil

What is De-Chlorination?

De-Chlorination is the process of removing or neutralizing residual chlorine from treated water before it enters sensitive purification systems such as RO or ion exchange units. This step is critical because even small traces of chlorine can damage RO membranes, resins, and interfere with microbiological processes.

Purpose of De-Chlorination

  • To protect RO and DI membranes from oxidation.
  • To ensure chlorine-free water for pharmaceutical manufacturing and testing.
  • To maintain compliance with USP, EP, and WHO water standards.

Chemical Methods for De-Chlorination

1. Using Sodium Metabisulfite (Na₂S₂O₅)

One of the most common dechlorinating agents used in pharmaceutical water systems.

Na₂S₂O₅ + Cl₂ + H₂O → 2NaHSO₄ + 2HCl

Dosage: 1.34 mg of sodium metabisulfite neutralizes 1 mg of chlorine approximately.

2. Using Sodium Thiosulfate (Na₂S₂O₃·5H₂O)

Another effective reducing agent used for laboratory and sampling dechlorination.

Na₂S₂O₃ + 4Cl₂ + 5H₂O → 2NaHSO₄ + 8HCl

3. Activated Carbon Filtration

Granular Activated Carbon (GAC) filters effectively remove chlorine by catalytic reduction. This is widely used in continuous systems as a pre-treatment step to RO.

De-Chlorination in Pharmaceutical Systems

In a pharmaceutical water treatment plant, the typical sequence is as follows:

Raw Water → Sand Filter → Activated Carbon Filter → Softener → RO → EDI → Storage & Distribution

The Activated Carbon Filter (ACF) plays a dual role by removing both chlorine and organic compounds, ensuring water entering the RO system is chlorine-free.

Verification and Testing

De-chlorination effectiveness must be routinely verified using analytical test kits or DPD colorimetric methods. The acceptable level of chlorine before RO should be:

  • Free Chlorine: Non-detectable (0.00 mg/L)
  • Residual Chlorine: Non-detectable (0.00 mg/L)

Impact of Improper Control

  • High chlorine damages RO membranes and reduces lifespan.
  • Inadequate chlorination leads to microbial contamination.
  • Over-dechlorination may affect oxidation-reduction balance in storage systems.

Conclusion

Both Chlorination and De-Chlorination are critical steps in ensuring safe, high-quality water for pharmaceutical and industrial use. Chlorination provides primary microbial control, while De-Chlorination protects downstream purification equipment from oxidative damage. Proper monitoring, dosage control, and analytical verification are essential for compliance with regulatory and quality standards such as USP, EP, WHO, and GMP guidelines.

References

  • USP <1231> – Water for Pharmaceutical Purposes
  • IS 10500: Indian Standards for Drinking Water
  • APHA – Standard Methods for the Examination of Water and Wastewater
  • WHO – Guidelines for Drinking-Water Quality

💬 About the Author

Siva Sankar is a Pharmaceutical Microbiology Consultant and Auditor with extensive experience in sterility testing, validation, and GMP compliance. He provides consultancy, training, and documentation services for pharmaceutical microbiology and cleanroom practices.

📧 Contact: siva17092@gmail.com
Mobile: 09505626106

📱 Disclaimer: This article is for educational purposes and does not replace your laboratory’s SOPs or regulatory guidance. Always follow validated methods and manufacturer instructions.

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