Bacteria get into Karachi’s water tanks with the supply, through open lids and through cracks, and they multiply in the sludge on the floor and the slimy film on the walls. To get rid of them you have to do two things in order: physically remove the sludge and film, then disinfect the cleaned surfaces with chlorine. Chlorine added to a dirty tank does not work, because the film shields the bacteria inside it.
This article explains which organisms matter, how to find out whether your tank has them, what makes them grow, and how to keep their numbers down. It is written by a chemical engineer who has run tank disinfection work in Karachi since 2007.
Understanding Bacteria in Karachi’s Water Tanks
Not all bacteria are harmful
Water is never sterile. Most bacteria in a tank are harmless environmental types. The concern is with the minority that cause disease and with the conditions that let any bacteria build up to large numbers.
The organisms that matter
| Organism | Where it comes from | What it does |
|---|---|---|
| Total coliforms | Soil, plants, sewage, biofilm | Mostly harmless. Their presence shows that contamination has a way in. |
| E. coli | Human and animal faeces | Proof of faecal contamination. Some strains cause severe diarrhoea. |
| Salmonella Typhi | Human sewage | Typhoid fever. A drug-resistant strain has circulated in Sindh since 2016. |
| Vibrio cholerae | Human sewage | Cholera |
| Shigella, Campylobacter | Sewage, bird and animal droppings | Dysentery and diarrhoea |
| Legionella | Grows in warm, stagnant water and biofilm | Pneumonia, caught by inhaling spray. See the Legionella threat. |
| Pseudomonas | Biofilm in tanks, pipes and filters | Skin, ear and eye infections, mainly in vulnerable people |
Tanks also carry organisms that are not bacteria but are handled in the same way: viruses such as hepatitis A and E, parasites such as Giardia and Cryptosporidium, and the amoeba Naegleria fowleri. Cryptosporidium deserves a mention because it spreads through faeces of infected people and animals (CDC) and is far more resistant to chlorine than bacteria are, which is one reason filtration and boiling still matter for drinking water.
How big the problem is
When the Pakistan Council of Research in Water Resources tested 118 Karachi water samples in 2017, 88.1% had coliform bacteria above WHO limits and 33.4% contained E. coli (Dialogue Earth). Sindh’s household survey later found E. coli in 79.1% of Karachi’s water at source and 85.5% at household level (Geo News). The second pair of numbers is the one that concerns tank owners: contamination increases between the supply point and the glass.
How bacteria get into a tank
- With line water. Sewer and water lines leak side by side, and empty water lines draw sewage in between supply periods.
- With tanker water, from the source, the tanker body or a hose dragged along the ground.
- Through the underground tank’s walls and cover, as seepage from drains or as street water running in. See detecting and preventing leaks.
- Through open rooftop lids, with dust, insects, lizards and bird droppings. See bird contamination in rooftop tanks.
- On hands, buckets and ropes dipped into the water.
Biofilm: why tanks stay contaminated
Bacteria in water do not just float. Within days of a tank being filled, some attach to the walls and floor and produce a sticky coating around themselves. Others join, and the coating thickens into a slippery layer you can feel with a finger. This is biofilm.
Biofilm changes everything about control:
- It protects the bacteria inside from chlorine. The outer layer uses up the disinfectant before it gets through.
- It feeds them, by trapping nutrients from the water.
- It constantly sheds bacteria back into the water, so each fresh fill is seeded within hours.
- It grows faster on rough surfaces such as concrete, scale and rust.
Sludge on the floor does the same job on a larger scale. This is why a tank can be drained and refilled with clean water and still test positive a day later.
Bacterial Contamination Assessment
What you can check yourself
You cannot see bacteria, but you can see the conditions they need.
- Slippery walls when you run a clean finger along them
- Sediment on the floor
- Cloudy, coloured or smelly water
- Open or cracked lids, missing mesh
- Stomach illness that keeps coming back in the household
More signs are listed in signs your family may be drinking contaminated water. Clear, odourless water is not proof of safety. The health consequences are described in the importance of water tank cleaning for health and hygiene.
Laboratory testing
The standard test is for total coliforms and E. coli in a 100 ml sample.
| Result | What it means | What to do |
|---|---|---|
| No coliforms, no E. coli | Meets the national standard on the day of sampling | Keep to your cleaning schedule |
| Coliforms present, no E. coli | Biofilm in the system or a route for contamination | Clean and disinfect the tanks, check lids and seals, retest |
| E. coli present | Faecal contamination. Not safe to drink. | Boil all drinking water. Clean and disinfect immediately. Find the source. |
Pakistan’s National Standards for Drinking Water Quality require that neither be detectable in any 100 ml sample (S.R.O. 1063(I)/2010).
How to sample
- Get a sterile bottle from the laboratory.
- To test the tank’s effect, take one sample from the incoming line or tanker and one from a kitchen tap fed by the overhead tank.
- Remove any aerator, run the tap for two minutes, and fill the bottle without touching the inside of the cap or neck.
- Keep it cool and deliver it the same day.
Factors Contributing to Bacterial Growth
| Factor | Why it matters in Karachi |
|---|---|
| Warmth | Rooftop tanks stay warm for most of the year, and bacteria multiply faster in warm water. |
| Storage time | Intermittent supply means water sits for days. The city gets about 650 million gallons a day against a need of 1,200 million. |
| Nutrients | Sludge, algae, rust and sewage traces feed growth. |
| No disinfectant | Much of the supply is not filtered or chlorinated at source, and chlorine fades quickly in hot storage. |
| Rough surfaces | Concrete, scale and corrosion give biofilm a grip. See Karachi’s hard water problem. |
| Light | Algae grows and feeds bacteria. See preventing algae growth. |
| Access | Open lids and cracked covers keep re-seeding the tank. |
Preventive Measures to Combat Bacterial Contamination
1. Clean the tank properly
The sequence that works is:
- Drain the tank.
- Remove the sludge.
- Pressure-wash or scrub the walls, floor and corners to strip the biofilm.
- Remove the dirty water completely.
- Disinfect with chlorine.
- Rinse and refill.
Steps 2 to 4 remove the shelter. Step 5 kills what is left exposed. Our crews follow this on every job, using a pressure washer, a wet vacuum and a chlorine solution of 50 to 200 parts per million. See the service procedure and the chemicals we use.
2. Disinfect at the right strength
The World Health Organization’s method is 80 grams of high-strength calcium hypochlorite per 1,000 litres of tank capacity, left for 24 hours, then drained and rinsed (WHO Technical Note 3). Amounts for bleach and bleaching powder are in how to disinfect a water tank after cleaning.
3. Repeat on a schedule
Biofilm starts to re-form as soon as the tank refills. Cleaning every 6 months removes it before it matures. Tanker-fed tanks and shared building tanks need it more often. What a household gains is set out in the benefits of regular water tank cleaning. See how often to clean your water tank.
4. Close the routes in
- Tight lids and raised, sealed covers
- Mesh on vents and overflows
- Cracks sealed and waterproofing maintained
- No buckets or hands in the water
5. Slow the growth
- Shade the overhead tank.
- Do not store more water than you use in a few days. See choosing the right size water tank.
- Use opaque tanks so algae cannot grow.
- Remove scale and rust.
6. Treat drinking water as a second barrier
Even a well-kept tank holds Karachi supply water. Boil drinking water for at least one minute, or use a UV, UF or RO unit that is properly maintained. See filtration and cleaning in Karachi homes.
What does not work
- Adding bleach to a dirty tank. The sludge and film consume it.
- Draining and refilling without scrubbing. The biofilm stays on the walls.
- Relying on a kitchen filter. It treats one tap.
- Judging by appearance.
Government and Municipal Efforts
The Karachi Water and Sewerage Corporation, which replaced the old water board in 2023 (Dawn), is responsible for treating and chlorinating the public supply. Courts have pressed it repeatedly. In 2017 a judicial commission reported that the water supplied to Karachi was substandard, and the Supreme Court removed the water board’s managing director (Dawn). In 2018, filtration plant officials told the commission that less than half of the city’s water was being filtered (ProPakistani).
During Naegleria season, health experts urge residents to chlorinate their underground and overhead tanks (ARY News). The responsibility for private tanks rests with their owners. The framework is explained in government regulations on water quality in Karachi.
Future Prospects and Innovations
- Rapid test kits that show the presence of E. coli at home within a day or two are becoming available and affordable.
- Chlorine and temperature sensors can report a tank’s condition to a phone.
- Chlorine-free disinfectants such as silver hydrogen peroxide are used where clients ask for them. See eco-friendly products for tank cleaning.
- Smoother tank linings and coatings give biofilm less to hold on to.
- Mechanical cleaning with pressure washers and vacuums is replacing the bucket and brush. See technology in modern tank cleaning.
None of these replaces the basic routine. They make it easier to check that it is working.
Frequently Asked Questions
How do I kill bacteria in my water tank?
Clean it first, then disinfect. Drain the tank, remove sludge, scrub or pressure-wash the walls, then apply chlorine at about 50 mg/L for 24 hours, drain and rinse.
Can I just add chlorine tablets?
Not as a substitute for cleaning. Chlorine cannot get through sludge and biofilm.
How do I know if my tank water has bacteria?
Only a laboratory test for coliforms and E. coli can tell you. Slimy walls, sediment and recurring stomach illness are warning signs.
Does boiling water kill the bacteria from the tank?
Yes, for the water you boil. It does not help with water used for bathing, washing dishes or brushing teeth.
How fast do bacteria come back after cleaning?
New biofilm starts forming within days, but it takes months to become thick and protective. That is why cleaning every 6 months works.
Are bacteria in tank water dangerous for bathing?
Some are. Dirty water can cause skin, eye and ear infections, and warm unchlorinated water can carry Naegleria, which infects through the nose.
Tank Cleaning and Disinfection in Karachi
Call or WhatsApp 0313-2796778. We clean and disinfect tanks across the city, including Liaquatabad, Garden and Orangi Town. Rates are on our charges page, and you inspect the tank before you pay, as set out in our service guarantee.
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: PCRWR testing (2017), reported by Dialogue Earth; Sindh MICS figures reported by Geo News (May 2025); CDC, About Cryptosporidium; National Standards for Drinking Water Quality, S.R.O. 1063(I)/2010; WHO Technical Note No. 3 (2013); Dawn (March 2017 and July 2023); ProPakistani (January 2018); ARY News (July 2026).
