To disinfect a water tank after cleaning, add chlorine at about 50 mg/L, which is 80 grams of high-strength calcium hypochlorite granules or 1 litre of plain 5% bleach for every 1,000 litres of tank capacity. Fill the tank, leave it for 24 hours, drain it, then fill once with clean water, wait 30 minutes and drain again. That is the method in the World Health Organization’s technical note on tank cleaning (WHO Technical Note 3).
Disinfection is the step that kills what scrubbing leaves behind. It only works on a tank that is already physically clean. This guide explains the chemistry in plain terms, gives the amounts for common tank sizes, and covers the safety rules that matter. It is written by a chemical engineer who has supervised this work in Karachi since 2007.
Understanding Water Tank Cleaning
Cleaning and disinfection are two different jobs.
- Cleaning is physical. It removes sludge from the floor and the slippery film of bacteria, called biofilm, from the walls.
- Disinfection is chemical. It kills the bacteria, viruses and other organisms that remain on the cleaned surfaces.
The order cannot be reversed or shortened. Chlorine reacts with any organic matter it meets. In a dirty tank it is used up by sludge and biofilm within minutes and never reaches the organisms inside them. This is why adding bleach or tablets to an uncleaned tank achieves very little, and why every proper cleaning ends with disinfection. The physical steps are described on our service procedure page.
Why chlorine
Chlorine is the standard disinfectant for drinking water around the world. It is effective against the bacteria and viruses that cause typhoid, cholera, hepatitis A and diarrhoea, it is cheap, and its strength can be measured. It comes in three common forms:
| Product | Chemical | Available chlorine | Notes |
|---|---|---|---|
| High-strength granules or powder (HTH) | Calcium hypochlorite | About 65 to 70% | Stores well if kept dry and sealed. The product the WHO method is written for. |
| Liquid bleach | Sodium hypochlorite | About 5% in household bleach, 10 to 12% in commercial grades | Easy to measure. Loses strength with age and heat. Use plain, unscented bleach only. |
| Bleaching powder | Chlorinated lime | About 25 to 35% when fresh | Cheap and widely sold. Weakens quickly once opened and leaves a lime sediment. |
We disinfect with calcium hypochlorite or sodium hypochlorite at 50 to 200 parts per million, depending on the method and the tank. Clients who specify a chlorine-free product can have silver hydrogen peroxide instead. The full list is on our cleaning chemicals page, and the alternatives are discussed in eco-friendly products for tank cleaning.
Preparing for Disinfection
1. Clean the tank first
Drain the tank, remove all sludge, scrub or pressure-wash the walls, floor and corners, and remove the dirty water. The WHO note suggests scrubbing with detergent and hot water, then rinsing until no trace of detergent remains. If the tank has scale and you used an acid descaler, rinse it out completely before any chlorine goes in. Acid and chlorine together release toxic gas. See dealing with hard water stains and deposits.
2. Know the capacity
You need the tank’s volume in litres to work out the dose. Plastic tanks have it printed on the side. For a rectangular tank, multiply length × width × depth in feet and then by 28.3. One imperial gallon is 4.55 litres. More on this in how to choose the right size water tank.
3. Gather what you need
- The chlorine product, in date and stored sealed
- A clean plastic bucket and a plastic or wooden stirrer. No metal.
- Kitchen scales or a measuring jug
- Rubber gloves, goggles and a mask
- Clean water for filling and rinsing
- Stored water for the household for the next day
4. Plan the timing
The full WHO method needs the tank out of use for 24 hours, or 8 hours with a double dose. Fill buckets and bottles beforehand. Schedule it when a refill is certain. See the best time to clean your water tank.
Steps to Disinfect Your Water Tank
Method A: the fill-and-soak method (based on the WHO note)
- Quarter-fill the cleaned tank with clean water.
- Add the chlorine. Dissolve granules or powder in a bucket of water first, let any sediment settle, and pour the clear liquid into the tank. Pour liquid bleach straight in.
- Fill the tank to the top so the solution touches every surface, including the underside of the lid.
- Open each tap in the house until you smell chlorine, then close it. This disinfects the pipes as well.
- Leave it for 24 hours. Nobody should drink or use this water. If the tank is needed urgently, double the chlorine and the time comes down to 8 hours.
- Drain the tank completely to a proper drain.
- Refill with clean water, leave for 30 minutes, and drain again.
- Refill for use.
How much chlorine to use
| Tank capacity | Calcium hypochlorite granules (65 to 70%) | Plain household bleach (5%) | Fresh bleaching powder (about 30%) |
|---|---|---|---|
| 500 litres | 40 g | 0.5 litre | about 85 g |
| 1,000 litres | 80 g | 1 litre | about 170 g |
| 1,500 litres | 120 g | 1.5 litres | about 250 g |
| 2,000 litres | 160 g | 2 litres | about 335 g |
| 1,000 gallons (4,550 litres) | 365 g | 4.5 litres | about 760 g |
| 2,000 gallons (9,100 litres) | 730 g | 9 litres | about 1.5 kg |
The granule figures are the WHO’s 80 grams per 1,000 litres, which gives roughly 50 mg/L of chlorine. The bleach and bleaching powder figures are the equivalent amounts for the same strength. Bleaching powder that has been open for weeks may have lost half its chlorine, so buy it fresh and keep it sealed.
Method B: the surface method
Filling a large underground tank with chlorinated water and then throwing it away wastes thousands of litres. For large tanks, professionals apply a stronger solution, up to 200 mg/L, directly to the cleaned walls, floor and roof by spray or brush, leave it for the contact time, and then rinse. This uses a fraction of the water and puts the chlorine exactly where the organisms are. It also means working inside the tank with strong chlorine, so it needs protective gear, ventilation and training. It is not a do-it-yourself method.
Safety Measures during Disinfection
- Never mix chlorine with anything else. Acid, including toilet cleaner and descalers, releases chlorine gas. Ammonia-based cleaners release chloramine gas. Both can be fatal in an enclosed space.
- Add chlorine to water, not water to chlorine. Wetting a heap of dry granules can cause a violent reaction.
- Wear gloves, goggles and a mask. Concentrated chlorine burns skin and eyes.
- Work with the wind behind you and keep your face away from the opening when pouring.
- Do not enter an underground tank. It is a confined space, and chlorine fumes are heavier than air and collect at the bottom. The WHO note calls work inside tanks “difficult and dangerous”. Our safety page explains the precautions a trained crew takes.
- Keep children and pets away from the tank, the bucket and the stored chemical.
- Store chlorine dry, cool, sealed and away from fuel, acids and metal.
- Label the taps or tell everyone in the house not to use the water during the contact time.
Post-Disinfection Steps
Dispose of the chlorinated water carefully
The WHO note warns that the disinfecting water contains a high concentration of chlorine and should not be emptied into rivers or ponds, where it can kill fish and plants. Send it to a sewer drain. Do not run it onto a lawn or into a planted bed. See waste disposal after tank cleaning.
Rinse and refill
After the 30-minute rinse fill has been drained, refill the tank. Run each tap for a minute to clear the chlorinated water from the pipes.
Check the water
A faint chlorine smell in the first tankful is normal and harmless. Pakistan’s drinking water standard is 0.2 to 0.5 mg/L of residual chlorine at the consumer end (S.R.O. 1063(I)/2010). A pool test kit or chlorine test strips will show the level. If the smell is strong, drain part of the tank and top up with fresh water.
Seal the tank
Close the lid properly and check the mesh on the vent and overflow. A disinfected tank with an open lid is contaminated again within days.
When a Tank Needs Disinfecting
- After every cleaning, every 6 months. See how often to clean your water tank.
- Before a new tank is used for the first time.
- After any repair, waterproofing or plumbing work inside the tank.
- After sewage, floodwater or a dead animal has been in the tank.
- After a house has been empty for months.
- Before summer. Naegleria fowleri causes deaths in Karachi most years, and health experts advise residents to keep underground and overhead tanks chlorinated (ARY News).
Maintenance and Regular Inspections
Disinfection is not permanent. Chlorine fades, and each new delivery of water brings new organisms. Between cleanings:
- Keep lids sealed and openings meshed.
- Look inside once a month.
- If your area’s water is known to be poorly chlorinated, ask a doctor or the water utility about safe routine chlorination for household tanks, and measure the result with test strips. Routine dosing is far lower than the cleaning dose in this article.
- Boil or filter drinking water regardless. See filtration and cleaning in Karachi homes.
The monthly routine is in how to maintain your water tank after cleaning.
Water Tank Disinfection in Karachi
Three local conditions make disinfection more important here than in most cities.
- The supply is often contaminated. In 2017, 107 of 118 Karachi water samples tested by the Pakistan Council of Research in Water Resources were unsafe, and 88% contained coliform bacteria above WHO limits (Dialogue Earth).
- Much of the water is not filtered or chlorinated at source. In 2018, officials told a Supreme Court-appointed commission that not even half of the water supplied to Karachi was filtered (ProPakistani).
- The climate is hot. Chlorine disappears faster and bacteria multiply faster in warm tanks.
For how different organisms behave in tanks, see bacteria in Karachi’s water tanks and how to combat them.
Doing it yourself or calling a team
A small overhead plastic tank is within reach of a careful adult using Method A. Underground tanks, concrete tanks, large tanks and any tank that has been badly contaminated should be left to a trained crew. Chlorine disinfection is part of every job we do, not an add-on, and a standard house takes 2 to 4 hours for both tanks. See why hire a professional tank cleaning company.
Frequently Asked Questions
How much bleach do I need to disinfect a 1,000-litre tank?
One litre of plain, unscented 5% household bleach, left for 24 hours in a full tank, then drained and rinsed.
Can I disinfect a water tank with bleaching powder?
Yes, if it is fresh. Use about 170 grams per 1,000 litres, dissolve it in a bucket first, let the lime settle and pour in only the clear liquid.
How long should chlorine stay in the tank?
24 hours at the standard dose, or 8 hours at double the dose.
Is the water safe to drink after disinfection?
Yes, once the tank has been drained, rinsed and refilled. A slight chlorine smell is normal. In Karachi you should still boil or filter drinking water, because the incoming supply is not reliably safe.
Can I just add chlorine without cleaning the tank?
No. Sludge and biofilm use up the chlorine before it reaches the bacteria. Clean first, then disinfect.
What is the best disinfectant for a water tank?
A chlorine-based product, calcium hypochlorite or sodium hypochlorite, used at the correct dose on a clean tank. It is the method set out by the WHO.
Does disinfection kill algae?
Chlorine kills algae on cleaned surfaces, but it will grow back if light still reaches the water. See preventing algae growth.
Professional Tank Disinfection in Karachi
Call or WhatsApp 0313-2796778 for cleaning and disinfection of home, commercial and industrial tanks, or send your details through the contact page. We work across the city, including FB Area, Korangi and Landhi. House rates are on the charges page, and commercial clients receive a cleaning certificate recording the chemical used.
About the author: Ahmar Shoaib is the founder of Karachi Tank Cleaning. He holds a BS in Chemical Engineering from Karachi University and has run the company’s cleaning and disinfection work since 2007. This article follows our editorial policy.
Sources: WHO Technical Note No. 3, Cleaning and disinfecting water storage tanks and tankers (WHO/WEDC, 2013); National Standards for Drinking Water Quality, S.R.O. 1063(I)/2010; PCRWR water testing for the Sindh High Court judicial commission (2017), reported by Dialogue Earth; Judicial Commission on Water and Sanitation proceedings, reported by ProPakistani (January 2018); ARY News (July 2026).
