Material Insights · Understanding the Material World

Chlorine and Water and sanitation

Chlorine is the standard disinfectant for piped water because it leaves a residual that keeps protecting water in the pipes, so water utilities treat it as a non-negotiable input; because chlorine is bought from nearby plants, a single outage can leave utilities short within weeks, as in the US Pacific Northwest in 2021.

Chlorine (chemical) and Water and sanitation (industry): 4 relationships.

How they are linked

1. Disinfectant with a residual

Physical flow · Strong · Chlorine → water utilities and sewage plants

Chlorine kills pathogens and, unlike most alternatives, stays in the water as a residual through the distribution network; three enterprise types in water and sanitation depend on it, and water utilities rate it 5 of 5 on criticality.

  • 5 of 5 criticality for water utilities (residual disinfection)
  • 0.2 mg/L minimum free residual chlorine at the consumer end in chlorinated supplies (IS 10500:2012)
  • 1.0 to 2.5 mg/L typical chlorine dose in CPHEEO design guidance

Indian design guidance calls for at least 30 minutes of contact before distribution and a residual of about 0.2 to 0.5 mg/L at the tap. Sewage treatment operators use chlorine to disinfect effluent (4 of 5 on criticality) and treatment-chemical suppliers sell it on (3 of 5). Large plants above about 5 MLD use chlorine gas as the cheapest form, while small and rural schemes use bleaching powder or sodium hypochlorite.

Enterprise types that depend on it:

  • Water utilities (municipal boards and private operators): residual disinfection (criticality 5 of 5)
  • Sewerage and sewage-treatment plant operators: effluent disinfection (criticality 4 of 5)
  • Water-treatment chemical suppliers: disinfection (criticality 3 of 5)

Sources: Material Insights; InfraLens; Bureau of Indian Standards

2. Dosing set by water standards

Policy · Medium · Drinking water standards → chlorine dose and by-products

Regulators set both a minimum residual to keep water safe and limits on chlorine and its by-products, so standards decide how much chlorine utilities use and in what form.

  • 5 mg/L WHO guideline maximum for free chlorine
  • 100 µg/L UK legal limit for total trihalomethanes

Free chlorine reacts with organic matter to form disinfection by-products such as trihalomethanes, which pushes utilities to treat raw water better before dosing or to switch to chloramine for the residual. Chlorine gas also needs safety infrastructure at treatment plants: a containment room, an emergency scrubber, gas detectors and trained staff.

Sources: InfraLens; CIWEM; Drinking Water Inspectorate (UK)

3. Partial alternatives

Substitution · Weak · UV, ozone and chloramine → part of chlorine's role

UV and ozone can do primary disinfection, but free chlorine or monochloramine is still added after them to hold a residual in the pipes, so they reduce chlorine use rather than replace it.

Chloramine, itself made with chlorine, decays more slowly and gives a longer-lasting residual with fewer trihalomethanes, but it is a much weaker disinfectant needing very long contact times. On-site electrolysis of brine to make hypochlorite avoids storing and handling chlorine gas but still relies on the same chemistry.

Sources: InfraLens; CIWEM; Drinking Water Inspectorate (UK)

4. Local supply outages

Shared risk · Asymmetric · Chlor-alkali plant outage → utility shortages

Chlorine is hazardous to move and store, so utilities tend to buy from a few nearby producers, and a single plant failure can tighten supply for a whole region.

  • up to 20 days time officials said supplies could take to return to normal, June 2021

An electrical failure at a Westlake Chemical plant in mid-June 2021 disrupted chlorine supply to water and wastewater systems in Washington, Oregon, Idaho and parts of California, and state water bodies and the US EPA surveyed utility stocks. Most of the more than 75 systems that responded had enough supply through early July, but the episode showed how thin regional buffers are.

Sources: Water Online

For investors

Key learnings

  1. Water utilities are a steady chlorine buyer whose demand barely responds to price, since residual disinfection has no full substitute.
  2. Chlorine supply to utilities is regional, so a single chlor-alkali plant outage, as in June 2021, can create local shortages regardless of global balance.
  3. Tighter by-product rules and on-site hypochlorite generation shift utility spending from bulk chlorine gas towards treatment kit and hypochlorite.

What to track

  • Chlor-alkali plant outages and force majeure notices: Risk of regional shortages for water customers (As announced · company notices, Argus, ICIS)
  • Municipal water treatment capacity added (MLD): Growth in steady chlorine and hypochlorite demand (Yearly · Jal Jeevan Mission, AMRUT dashboards)

For policymakers

Key learnings

  1. Indian standards require at least 0.2 mg/L free residual chlorine at the consumer end in chlorinated supplies, so chlorine supply is part of public health security.
  2. Utilities should hold chlorine or hypochlorite stocks and have more than one supplier, because regional outages can last weeks.
  3. Chlorine gas at large plants needs containment, scrubbers, gas detection and trained staff, so bleaching powder, hypochlorite or on-site generation suit small and rural schemes.

What to track

  • Share of water samples meeting residual chlorine norms: Whether disinfection is reaching consumers (Monthly · Jal Jeevan Mission water quality portal)
  • Domestic chlor-alkali capacity and utilisation: Headroom to supply utilities if a plant fails (Yearly · Alkali Manufacturers Association of India)

India

IS 10500:2012 sets a minimum of 0.2 mg/L free residual chlorine at the consumer end in chlorinated supplies; large Indian plants use chlorine gas, while small and rural schemes use bleaching powder or hypochlorite.

Sources

  1. Water and sanitation: materials and inputs, Material Insights (2026)
  2. CPHEEO Manual on Water Supply and Treatment, chapter 9 (disinfection): chapter summary, InfraLens
  3. Chlorination and chloramination of drinking water: policy position statement, CIWEM (2017)
  4. US Department of Homeland Security reporting chlorine shortage in California, Water Online (2021)
  5. IS 10500:2012 Drinking water: specification (second revision), Bureau of Indian Standards (2012)
  6. Chlorine disinfection (private water supplies treatment guide), Drinking Water Inspectorate (UK)