Guide

How to dose sodium hypochlorite in water and sewage treatment

Short answer

To dose sodium hypochlorite, work out the chlorine you need and divide it by the hypo's tested strength. The formula is kg/day of available chlorine = dose in mg/L × flow in MLD, and each kg/day takes 10 L/day of 10% w/v hypo (100 g/L). The dose is the chlorine demand plus the residual you want, so run a demand test on your own water. CPHEEO gives a normal dose of no more than 2 mg/L for drinking water that is not significantly polluted, and a usual 10 mg/L for sewage after aerobic secondary treatment, each with 30 minutes' contact. Hypo loses strength in storage. Test each load and recalculate.

Updated 28 September 2026 · Rise Chemical India, Kirti Nagar Industrial Area, Delhi

What does 10–12% available chlorine mean on a hypo label?

Sodium hypochlorite (hypo) is sold by its available chlorine, which is its oxidising power expressed as chlorine. The Indian standard for hypo used in water treatment is IS 11673 (Part 2):2019. It states strength as percent mass by volume: Grade 1 is 4.0 to 6.0% and Grade 2 is 12.5 to 15.0%. Its foreword says hypo made by the chemical process usually has 10 to 15%.

Source: Bureau of Indian Standards, 2019

Percent mass by volume (w/v) is also called trade percent. It is g/L ÷ 10, so 10% w/v hypo holds 100 g/L of available chlorine. Available chlorine by weight (w/w) is trade percent ÷ specific gravity, and NaOCl by weight is that figure × 1.05 (Chlorine Institute). OxyChem's tables (60 °F, 15.6 °C) give 100 g/L hypo a specific gravity of 1.140, and put it at 8.77% available chlorine and 9.21% NaOCl by weight. In the same tables, 10.0% available chlorine by weight is 116.1 g/L. So hypo sold as 10% available chlorine by weight carries about 16% more chlorine per litre than 10% w/v hypo. A label that reads 10–12% means 100–120 g/L if the percent is w/v, or about 116–144 g/L if it is by weight.

Source: The Chlorine Institute (copy hosted by Pennsylvania DEP), 2006; Occidental Chemical Corporation (OxyChem); copy hosted by Force Flow, 2000

OxyChem stresses stating the units. Ask for the strength in g/L, or ask whether the percent is w/v or w/w. IS 11673 (Part 2) requires each package to show its available chlorine and its date of packing. CPHEEO (2024) says 12% hypo typically degrades to 11% during shipping, and advises calculating doses from 10%.

Source: Occidental Chemical Corporation (OxyChem); copy hosted by Force Flow, 2000; Bureau of Indian Standards, 2019; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024

What is the difference between chlorine dose, demand and residual?

CPHEEO defines chlorine demand as the chlorine added minus the residual left after a set contact period. Demand varies with dose, contact time, pH and temperature. So dose = demand + residual. Pennsylvania DEP gives an example: a demand of 2.1 mg/L plus a 0.6 mg/L residual needs a dose of 2.7 mg/L.

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; Pennsylvania Department of Environmental Protection, Bureau of Safe Drinking Water, 2022

The chlorine left in the water is total chlorine. It is free chlorine, which disinfects, plus combined chlorine bound to nitrogen (WHO). CPHEEO says combined chlorine is less effective. Free chlorine is hypochlorous acid plus hypochlorite ion, and the two are roughly equal at pH 7.5 and 25 °C (WHO). Hypochlorous acid kills far better. That is why chlorination works better at lower pH (US EPA).

Source: World Health Organization / WEDC, 2013; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; World Health Organization, 2003; US Environmental Protection Agency, Office of Water, 1999

How do you calculate how much hypo to dose?

CPHEEO's rule of thumb: a 1 MLD plant needs 1 kg of chlorine per mg/L (1 mg/L × 1 million litres = 1 kg). So chlorine (kg/day) = dose (mg/L) × flow (MLD). Divide that by the chlorine in each litre of hypo (kg/L) and you get litres per day. Take a 10 MLD water plant at 2 mg/L:

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024

Chlorine needed: 2 × 10 = 20 kg/day.
At 10% w/v (100 g/L, or 0.100 kg/L): 20 ÷ 0.100 = 200 L/day. That is 200 ÷ 24 = 8.3 L/h.
If a fresh load tests at 12.5% (125 g/L): 20 ÷ 0.125 = 160 L/day. If stored hypo has fallen to 8% (80 g/L): 20 ÷ 0.080 = 250 L/day.
By weight, using OxyChem's specific gravity of 1.140 for 100 g/L hypo: 200 × 1.140 = 228 kg/day.

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; Occidental Chemical Corporation (OxyChem); copy hosted by Force Flow, 2000

Now take a 5 MLD STP at CPHEEO's usual sewage dose of 10 mg/L. It needs 10 × 5 = 50 kg/day, or 50 ÷ 0.100 = 500 L/day of 10% w/v hypo. A contact tank that gives 30 minutes at average flow holds 5,000 m³/day ÷ 1,440 min/day × 30 min = about 104 m³.

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; CPHEEO, Ministry of Urban Development, Government of India, 2013

In a tank or sump, 1 mg/L in 1,000 L is 1 g of chlorine, or 10 mL of 10% w/v hypo. So per 1,000 L, 0.5 mg/L needs 5 mL, 2 mg/L needs 20 mL and 5 mg/L needs 50 mL. NSW Health's ready reckoner agrees: 8 mL of 12.5% hypo per 1,000 L for 1 mg/L (1 g ÷ 125 g/L = 8 mL). It calls the figure an estimate. The residual you measure will be lower, by the amount of the chlorine demand.

Source: NSW Health (New South Wales Government, Australia), 2014; Pennsylvania Department of Environmental Protection, Bureau of Safe Drinking Water, 2022

What dose, contact time and residual do water and sewage plants need?

CPHEEO says the actual dose must come from chlorine demand tests. These are the published targets:

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024

Chlorine targets for drinking water and treated sewage

	Drinking water (WTP)	Treated sewage (STP)
Dose	Normally not above 2 mg/L if the supply is not significantly polluted (CPHEEO); 0.2–2 mg/L as hypo at US plants (US EPA)	Usually 10 mg/L after aerobic secondary treatment (CPHEEO, 2013); 5–20 mg/L (US EPA)
Contact time	30 minutes in a contact tank (CPHEEO)	30 minutes at average flow, open baffled tank (CPHEEO, 2013)
Residual	Free chlorine at least 0.2 mg/L at the consumer end, 0.5 mg/L for protection against viruses (IS 10500); 0.2 mg/L post-monsoon, 0.5 mg/L in monsoon or epidemics, at the end of the distribution system (CPHEEO); at least 0.5 mg/L after 30 minutes at pH below 8.0 (WHO)	No minimum for discharge; for reuse, CPHEEO (2013) sets a minimum residual chlorine at the point of use of 1 mg/L for toilet flushing, fire protection, vehicle washing and horticulture or golf courses, and 0.5 mg/L for non-contact impoundments (nil for crop irrigation). Residuals after contact are generally 1 to 1.5 mg/L at most (CPHEEO, 2013).
Upper limit	IS 10500 lists 1 mg/L of free residual chlorine in its 'permissible limit in the absence of alternate source' column (its requirement is the 0.2 mg/L minimum); 5 mg/L free chlorine (WHO guideline value); some people taste it from 0.3 mg/L	1.0 mg/L total residual chlorine for discharge to inland surface water or marine coastal areas (Schedule VI general effluent standards; the 2017 STP standards set none)

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; US Environmental Protection Agency, Office of Water, 1999; CPHEEO, Ministry of Urban Development, Government of India, 2013; US Environmental Protection Agency, Office of Water (copy hosted by Netafim USA), 1999; Bureau of Indian Standards, 2012; World Health Organization, 2022; Government of India (hosted by CPCB), 1993; Ministry of Environment, Forest and Climate Change, Government of India (hosted by CPCB), 13 Oct 2017

For sewage, CPHEEO (2013) requires aerobic polishing of UASB effluent before chlorination. It also says contact tanks must stay uncovered, because chlorine can corrode a concrete roof out of sight. The residuals it reports after contact go up to 1.5 mg/L. That can exceed the 1.0 mg/L total residual chlorine limit in the general effluent standards, so trim the dose or dechlorinate. CPHEEO names sodium bisulfite and sodium thiosulfate as dechlorinating chemicals. In lab jar tests on secondary effluent from one Amritsar STP, 1.5 ppm (mg/L) of NaOCl was the optimised dose for the study's target of 1,000 MPN/100 mL total coliforms, or 3-log removal, at 15, 30 and 60 minutes (Kesar and Bhatti, 2022).

Source: CPHEEO, Ministry of Urban Development, Government of India, 2013; Government of India (hosted by CPCB), 1993; Heliyon (Elsevier), 2022

MoEFCC's notification of 13 October 2017 (G.S.R. 1265(E)) sets faecal coliform in STP effluent at under 1,000 MPN/100 mL anywhere in India, except where the treated effluent is used for industrial purposes. The National Green Tribunal's order of 30 April 2019 applied faecal coliform limits of 100 MPN/100 mL (desirable) and 230 MPN/100 mL (permissible) to all STPs, and said MoEFCC may notify them. The standards it accepted let State Pollution Control Boards go stricter. Check your plant's consent to operate.

Source: Ministry of Environment, Forest and Climate Change, Government of India (hosted by CPCB), 13 Oct 2017; National Green Tribunal, Principal Bench, New Delhi, 30 Apr 2019

How fast does sodium hypochlorite lose strength?

Stronger hypo breaks down faster. IS 11673 (Part 2) gives half-lives at 25 °C of 700 days at 6%, 180 days at 12% (120 g/L) and 100 days at 15% (150 g/L). Heat speeds it up. The Chlorine Institute reports that for solutions of 5 to 16% NaOCl by weight, the rate rises 3 to 4 times per 10 °C. So hypo kept at 15 °C decomposes 14 times slower than hypo at 35 °C. IS 11673 says storage should not exceed 32 °C, and that above 38 °C sodium chlorate formation becomes appreciable.

Source: Bureau of Indian Standards, 2019; The Chlorine Institute (copy hosted by Pennsylvania DEP), 2006

In light, the half-life of 10–15% hypo becomes about 3 or 4 times shorter (Pennsylvania DEP). Nickel, copper and cobalt also speed its breakdown (Chlorine Institute). IS 11673 puts optimum stability near pH 11, with heavy metals at 5 ppm or less. The Chlorine Institute says pH 11.9 to 13.

Source: Pennsylvania Department of Environmental Protection, Bureau of Safe Drinking Water, 2022; The Chlorine Institute (copy hosted by Pennsylvania DEP), 2006; Bureau of Indian Standards, 2019

The sources do not agree on how long hypo keeps:

IS 11673 (Part 2): it must hold its minimum strength for 30 days from packing. After that, the strength is as agreed with the supplier.
US EPA: about 1 month if stored properly (2023). Even in ideal conditions, 12.5% hypo falls to 10% in 30 days (1999).
WHO: use it as soon as possible, for example within a month. Pennsylvania DEP: use it within 30 days and do not store it longer than 45.
CPHEEO (2024): six months, the longest figure here.

Source: Bureau of Indian Standards, 2019; US Environmental Protection Agency, 2023; US Environmental Protection Agency, Office of Water, 1999; World Health Organization, 2022; Pennsylvania Department of Environmental Protection, Bureau of Safe Drinking Water, 2022; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024

Why does old hypo add chlorate to treated water?

Hypo mainly decomposes by turning three hypochlorite ions into one chlorate and two chloride ions (Chlorine Institute). Chlorate builds up in hypo stored for long periods, especially when it is warm. WHO warns that dosing aged hypo can push chlorate in treated water above its guideline value. WHO's provisional guideline value for chlorate is 0.7 mg/L. Levels above 1 mg/L have been reported where hypochlorite was used, but WHO calls them unusual unless the hypo was stored under adverse conditions. For the product itself, IS 11673 (Part 2) allows at most 3.0 g/L of sodium chlorate for Grade 2.

Source: The Chlorine Institute (copy hosted by Pennsylvania DEP), 2006; World Health Organization, 2022; Bureau of Indian Standards, 2019

WHO lists these controls. Buy fresh hypo of appropriate quality. Store it cool and out of direct sunlight, and use it soon. Never add new hypo to a container that still holds old hypo, because that speeds chlorate formation. Powell, an equipment maker, advises two storage tanks used in turn.

Source: World Health Organization, 2022; Powell Fabrication & Manufacturing, 2020

Which tank, pipe and valve materials can you use for hypo?

The Chlorine Institute says mild steel, stainless steel and Hastelloy corrode rapidly in hypo, and the corrosion products speed its decomposition. Where a metal is needed, it recommends titanium. Compatible materials include PVC, CPVC, polypropylene, PVDF, PTFE, EPR, chlorobutyl rubber, Viton A (70 durometer minimum) and FRP with a compatible resin. CPHEEO names ceramics, glass, plastic and special rubber.

Source: The Chlorine Institute (copy hosted by Pennsylvania DEP), 2006; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024

Tanks: rubber-lined steel, FRP with a UV stabiliser or gel coat (shaded if possible), linear or cross-linked HDPE, or titanium (Chlorine Institute).
Pipes: Schedule 80 PVC or CPVC where they are safe from impact, and lined steel where they are not (Chlorine Institute).
Spill containment: 110% of the largest tank (Pennsylvania DEP), or its entire contents plus freeboard (US EPA).

Source: The Chlorine Institute (copy hosted by Pennsylvania DEP), 2006; Pennsylvania Department of Environmental Protection, Bureau of Safe Drinking Water, 2022; US Environmental Protection Agency, Office of Water, 1999

When metals catalyse hypo's decomposition, it gives off oxygen. Powell reports that this oxygen can gas-lock idle dosing pumps and burst closed PVC ball valves, and advises buying filtered hypo low in nickel and copper. The same oxygen can build pressure in sealed containers such as carboys. The Chlorine Institute says vented closures can be used on industrial-strength hypo to avoid it. White powder left where hypo has dried around a pump seal can hold sodium chlorate. Sodium chlorate can explode or ignite if struck, for example by a hammer while the pump is taken apart. Before you open the pump, wash it and the area with plenty of water (Chlorine Institute).

Source: Powell Fabrication & Manufacturing, 2020; The Chlorine Institute (copy hosted by Pennsylvania DEP), 2006

How do you handle hypo safely, and how is it classed for transport?

Never let hypo meet acid, which releases chlorine gas, or ammonia, which releases chloramine gas (US ATSDR; Chlorine Institute). Store it in closed containers in a cool, dark place, away from combustibles, reducing agents, acids and food (ICSC 1119). Give each incompatible chemical its own spill containment (US EPA). Label each tanker fill point with the chemical it takes, so hypo is never unloaded into a tank that holds another chemical (Chlorine Institute).

Source: Agency for Toxic Substances and Disease Registry (US CDC), 21 Oct 2014; The Chlorine Institute (copy hosted by Pennsylvania DEP), 2006; International Labour Organization and World Health Organization, 2017; US Environmental Protection Agency, Office of Water, 1999

Hypo at 10–15% is about pH 13. It causes severe skin burns and eye damage. Wear goggles and a face shield, impervious gloves, boots and an apron or suit, and use ventilation or breathing protection (Chlorine Institute; ICSC 1119). If hypo splashes on skin, rinse with plenty of water for at least 15 minutes, take off contaminated clothes and rinse again. If it gets in the eyes, rinse with plenty of water for several minutes, and remove contact lenses if that is easy. In both cases, get medical help at once (ICSC 1119). Collect spills in non-metallic containers and wash the rest away with plenty of water. Because of the fire hazard, never soak hypo up with sawdust or other combustible material, and rinse splashed clothing with plenty of water before it dries (ICSC 1119). Hypo solution is UN 1791 (US ATSDR). ICSC 1119 is the ILO/WHO card for hypo above 10% active chlorine. It gives the GHS signal word "Danger", hazard class 8 (corrosive), packing group II or III and a density of 1.20 to 1.25 g/mL at 20 °C.

Source: International Labour Organization and World Health Organization, 2017; The Chlorine Institute (copy hosted by Pennsylvania DEP), 2006; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; Agency for Toxic Substances and Disease Registry (US CDC), 21 Oct 2014

Should you use hypo, bleaching powder or chlorine gas?

CPHEEO names chlorine compounds (bleaching powder, calcium or sodium hypochlorite) as the primary disinfectant for supplies up to 1 MLD, and as an emergency disinfectant for large plants. For medium and large supplies, chlorine gas is the common practice, but it needs chlorinators and elaborate safety practices. Bleaching powder has 20 to 30% available chlorine and loses strength quickly. CPHEEO says to store it no more than three months and to test its chlorine content to set the dose. Gas lowers the water's pH and hypo raises it. So without pH correction, gas disinfects more effectively (US EPA).

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; US Environmental Protection Agency, Office of Water, 1999

What sodium hypochlorite does Rise Chemical India supply?

Rise Chemical India makes sodium hypochlorite at its own factory in Bharatpur, Rajasthan, at 10–12% available chlorine (ask for the g/L figure when you enquire), and delivers it by tanker or in 35 kg and 50 kg carboys. The minimum order in India is 10 tonnes. For transport it is UN1791, Class 8. The company is ISO 9001:2015 certified. Its office at Plot No. 1/6, M-11 Kirti Nagar Industrial Area, Delhi 110015 handles every enquiry, quote and order (phone and WhatsApp +91 95829 99838). It sells in India and exports, and quotes each order on quantity and destination: send an enquiry.

Questions buyers ask

How much 10% sodium hypochlorite should I add per 1,000 litres of water?

Add 10 mL of 10% w/v hypo (100 g/L of available chlorine) to 1,000 litres for each 1 mg/L of chlorine you want. So 5 mL gives 0.5 mg/L, 20 mL gives 2 mg/L and 50 mL gives 5 mg/L. NSW Health's ready reckoner uses the same arithmetic for 12.5% hypo: 8 mL per 1,000 litres for 1 mg/L. The residual you measure after 30 minutes will be lower, because the water's chlorine demand uses up part of the dose.

How do I calculate the sodium hypochlorite dosing rate for my plant?

Multiply the dose in mg/L by the flow in MLD to get kg/day of available chlorine. CPHEEO's rule is 1 kg per MLD for each mg/L. Then divide by the hypo's available chlorine in kg/L, which is 0.100 for 10% w/v hypo. A 10 MLD plant dosed at 2 mg/L needs 20 kg/day of chlorine, or 200 L/day (8.3 L/h) of 10% hypo. CPHEEO advises calculating from 10% even when you buy 12% hypo, because hypo loses strength in transit.

What chlorine dose is used for treated sewage at an STP?

CPHEEO's 2013 sewerage manual gives 10 mg/L for effluent from aerobic secondary treatment, with 30 minutes' contact at average flow in an open, baffled tank. The US EPA gives 5 to 20 mg/L. Adjust the dose to meet the faecal coliform limit in your plant's consent to operate. The published limits are under 1,000 MPN/100 mL in the 2017 MoEFCC notification, and 100 MPN/100 mL desirable and 230 MPN/100 mL permissible in the NGT's 2019 order. Check the consent for a chlorine limit too: the general effluent standards cap total residual chlorine at 1.0 mg/L for discharge to inland surface water.

What residual chlorine is required in drinking water in India?

IS 10500:2012 requires at least 0.2 mg/L of free residual chlorine at the consumer end, and at least 0.5 mg/L where protection against viral infection is needed. CPHEEO asks for 0.2 mg/L at the end of the distribution system post-monsoon, and 0.5 mg/L during the monsoon or an epidemic. WHO recommends at least 0.5 mg/L of free chlorine after 30 minutes' contact at pH below 8.0.

How long can sodium hypochlorite be stored?

Plan to use it within about a month. IS 11673 (Part 2) requires hypo to hold its minimum strength for 30 days from packing. The US EPA and WHO both point to about a month, and Pennsylvania DEP says not to store it longer than 45 days. CPHEEO's manual gives a shelf life of six months, the longest of these figures. Strength keeps falling the whole time, though, so test stored hypo before you set the dose. Heat shortens storage life. The Chlorine Institute reports that for 5 to 16% NaOCl solutions, decomposition rises 3 to 4 times per 10 °C, and IS 11673 says storage should not exceed 32 °C.

Can sodium hypochlorite be mixed or stored with acids or ammonia?

No. Hypo releases chlorine gas when it meets acid, and chloramine gas when it meets ammonia (US ATSDR). ICSC 1119 says to store it separated from acids, reducing agents, combustibles and food, in closed containers in a cool, dark place. The US EPA asks for separate spill containment for each incompatible chemical.

Products in this guide

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Sources

Sources were checked on 28 September 2026.