Touch the side of a rooftop tank at three in the afternoon in May and you already understand the problem. The water inside is warm, it has been sitting since the last supply turn, and whatever chlorine it arrived with has gone. Heat, bacteria and a water tank in Karachi are a predictable combination.
This article explains the science in plain terms, corrects a few exaggerated claims that circulate online, and sets out what a household can do about it.
In brief
- Bacteria of concern multiply fastest between about 20 and 45°C. Karachi tank water is in that range for most of the year.
- Heat sets the speed of growth. Sludge and biofilm supply the food. Removing the food is what you control.
- Warm water loses its chlorine faster, so stored water has little protection.
- Shade, a tight lid and a sensibly sized tank all help.
- Clean before the hot season, not during a crisis in the middle of it.
What temperature does to bacteria in water
Most of the bacteria that matter for health grow fastest at around body temperature. Below about 20°C they multiply slowly. Above it, growth speeds up steadily until the high forties. The best-documented example is Legionella: the UK Health and Safety Executive states that it multiplies where temperatures are between 20 and 45°C and nutrients are available, and for that reason advises that cold water be stored and distributed below 20°C.
Now compare that with Karachi. Average daily highs are around 35°C in May and June, and summer nights stay in the high twenties. For about seven months of the year even the night-time air stays above 20°C, so a rooftop tank never cools below that, and a dark plastic tank in direct sun runs well above the air temperature. By the HSE’s benchmark, Karachi tank water sits in the growth range most of the year.
A word on “doubling every ten minutes”
You may have read that bacteria in a hot tank double every ten or twenty minutes. Those figures come from laboratory cultures fed with rich nutrients. In stored water, growth is limited by how much food is available. That is not reassuring news for a dirty tank, though, because sludge and biofilm are the food. The practical lesson is that temperature sets the speed and dirt supplies the fuel. You cannot do much about the first. You can remove the second.
Karachi’s year, month by month
These are approximate long-term averages for the city. The last column is our reading of what they mean for a rooftop tank.
| Months | Average daily high | Average night low | What it means for stored water |
|---|---|---|---|
| December to February | About 28°C | 13 to 14°C | Slowest growth of the year; a good time for the second routine cleaning |
| March and April | 32 to 35°C | 19 to 23°C | Tank water moves into the growth range and stays there |
| May and June | About 35°C | 27 to 28°C | Peak risk: warm day and night, long storage, heavy demand |
| July to September | 32 to 33°C | 26 to 28°C | Still warm, with added contamination from rain and flooding |
| October and November | 33 to 35°C | 17 to 22°C | A second hot spell by day; nights begin to cool |
The point of the table is the night-time column. Bacteria do not need extreme heat. They need warmth that never lets up, and from April to October Karachi provides it.
Four ways heat makes stored water worse
- Faster growth. Warm water shortens the time bacteria need to multiply, as long as nutrients are present.
- Chlorine disappears sooner. The disinfectant added at the filter plant breaks down faster in warm water. The WHO guidelines look for at least 0.2 mg/L of free chlorine at the point of delivery; water stored warm for days has usually lost it.
- Biofilm thickens. The slippery layer on tank walls grows faster in warmth and protects the organisms inside it.
- Algae bloom. Heat with even a little light produces green growth, which then dies and feeds bacteria.
The organisms that like warm water
| Organism | Why warmth matters | How it reaches people |
|---|---|---|
| Legionella | Multiplies between 20 and 45°C in stagnant water with sludge, scale or biofilm | Breathing in fine droplets from showers and taps |
| Naegleria fowleri | Thrives in warm, untreated fresh water | Water entering the nose during bathing or ablution |
| E. coli and other gut bacteria | Survive and can regrow in warm, nutrient-rich sediment | Drinking, food preparation, hands |
| Pseudomonas | Grows readily in biofilm in warm plumbing | Skin, eyes and ears during bathing |
Naegleria deserves a special mention because it has killed people in Karachi most years since the first recorded cases in 2012. The US CDC notes that it thrives in warm fresh water and has been found in tap water, and that infection occurs through the nose, not by drinking. Dawn’s report on a June 2025 death quoted the standard public health position: the organism cannot survive in water that is clean, cool and adequately chlorinated. After five deaths in Sindh that year, the health department told residents to clean their tanks before summer and keep water chlorinated.
Which tanks get hottest
| Tank | Heat exposure | Notes |
|---|---|---|
| Dark or thin-walled plastic tank on an open roof | Highest | Absorbs sun all day; lids warp and stop sealing |
| Light-coloured, multi-layer plastic or fibre tank | High | Insulating layers help; still warm by afternoon |
| Rooftop RCC or brick tank | Moderate | Thick walls slow heating but hold warmth overnight |
| Underground RCC tank | Lowest | Cooler and more stable, but collects more sludge and is exposed to seepage |
Exposed pipes on the roof and outside walls heat up too, which is why the first water out of a tap on a summer afternoon can be hot to the touch.
Why Karachi’s supply pattern makes it worse
Heat would matter less if water moved through the tank quickly. It does not. A World Bank document describes supply in Karachi as ranging from two hours every two days to four hours a day, so households store several days’ worth. Long outages stretch that further, a link we explore in load shedding and water contamination. And the water often arrives already carrying bacteria and sediment, as our KWSB water supply guide explains. Warm, stagnant, already seeded and sitting on a layer of sludge: that is the situation in an uncleaned tank by June.
Heat, bacteria and your water tank in Karachi: reducing the heat
- Shade the tank. A simple roofed frame or shade net over a rooftop tank lowers its temperature noticeably.
- Choose light colours and insulated walls when replacing a tank. Avoid thin, translucent ones that let light through.
- Keep the lid tight to block light, dust and insects.
- Size the tank to the household. A tank far bigger than you need means older, warmer water. Aim for turnover every few days.
- Insulate or shade exposed pipes where practical.
- Run a tap for a minute after returning from a trip, to flush water that has been standing in hot pipes.
- Avoid getting water up the nose from tank-fed taps in summer, particularly for children.
What you can do about the fuel
Removing sludge and biofilm takes away what bacteria feed on, and it is the step that makes the biggest difference. Clean before the hot season, and again after the monsoon if the tank was exposed. For most houses that means every four to six months; details are in how often to clean a water tank in Karachi, and the rainy season is covered in our monsoon risk guide.
A proper cleaning has to strip the biofilm physically. That is the purpose of jet washing at 3,000+ PSI in our cleaning procedure, followed by vacuum extraction of the sludge and disinfection with silver-stabilised hydrogen peroxide, which we describe in this article on how the disinfectant works. Dropping chlorine tablets into a dirty tank is not a substitute, because dirt consumes chlorine before it can disinfect.
The symptoms to watch in summer
Repeated stomach upsets, a musty smell, green-tinged water and fast-clogging filters are the usual warnings. The full list is in ten signs your water may be making you sick, and the infections involved are described in waterborne disease in Karachi.
Booking a pre-summer clean
April and May are the sensible months. Our branches cover Gulshan-e-Iqbal, Gulistan-e-Jauhar and East Karachi and DHA, Clifton and South Karachi, and the price calculator gives a fixed quote for a standard home.
Frequently asked questions
Does hot water in the tank kill bacteria?
No. Tank water in summer is warm, not hot enough to disinfect. Temperatures of 30 to 45°C encourage growth. Hot water systems are kept at 60°C or above for that reason.
Is an underground tank safer than a rooftop one in summer?
It stays cooler, which helps. It also gathers more sludge and is more exposed to seepage, and in most houses its water is pumped to the rooftop tank anyway. Both need cleaning.
Should I add chlorine to my tank in summer?
Chlorine tablets used as directed are a reasonable extra precaution, and the health department recommends chlorination. They work properly only in a tank that is already clean.
Do I need to clean more often in summer?
Plan the main cleaning just before the hot months. Shared, tanker-fed and unshaded rooftop tanks benefit from a three-month cycle through summer.
Does painting the tank white help?
A light, reflective surface absorbs less heat than a dark one, so it helps a little. Shade helps more.
Is warm tank water dangerous to bathe in?
Warm water itself is not the problem. The concern is what grows in warm, dirty, unchlorinated water, and that it should not be forced up the nose. Keep the tank clean and take particular care with children.
Sources
Figures in this article were checked in October 2026 against the following:
- UK Health and Safety Executive: Managing legionella in hot and cold water systems
- WHO: Guidelines for Drinking-water Quality
- US Centers for Disease Control and Prevention: About Naegleria fowleri infection
- Dawn, 5 June 2025: Young man dies of Naegleria fowleri in Karachi
- Geo News, 13 September 2025: Naegleria claims another life in Karachi, toll hits five in Sindh
- World Bank, 2019: Karachi Water and Sewerage Services Improvement Project, project information document
About Khan Tank Cleaning
Written by the team at Khan Tank Cleaning, which has worked on Karachi’s water tanks since 2005 under founder Mateen Khan. Some of the organisations we clean for are listed on our clients page. To talk to a branch, use the contact page or call 0333 0293174 (DHA Phase 2) or 0340 2717530 (Gulshan-e-Iqbal).
Mateen Khan is the Founder & Owner of Khan Tank Cleaning, Karachi’s first dedicated professional water tank cleaning company (est. 2005). With 20+ years of industry experience, he leads an ISO 9001 & ISO 45001 certified team that follows a strict 8-step process using industrial equipment and food-grade disinfectants. Mateen is committed to making clean, safe water accessible to every household and business in Karachi.