Our procedure
How we clean a water tank
Eight steps, what each one is actually for, and how you can check it was carried out. We have written this so you can hold any contractor to it — not only us.
What “clean” actually means
Most people picture tank cleaning as removing the dirt they can see: silt on the floor, a film on the surface, whatever fell in through a gap in the lid. That part is the easy half, and it is the only half most cleaners do.
The half that matters is biofilm. It is a thin, slippery layer of bacteria that bonds itself to the inside surface of the tank and builds a protective matrix around itself. A brush slides over it. Chlorine poured into the water struggles to penetrate it, kills whatever is floating, and leaves the layer on the wall intact — where it immediately starts reseeding the water again. This is the whole reason a tank can be cleaned in the morning and taste wrong again the following week, and why people conclude that tank cleaning does not work.
Removing biofilm takes mechanical force applied at the wall. That means pressurised water, not scrubbing. Everything else in the procedure below exists either to make that possible, to get the loosened material out of the building, or to make sure nobody is hurt doing it.
The second thing that separates a real clean from a cosmetic one is where the sludge goes. The shortcut is to open the outlet and let the loosened sediment run out down the supply line. The tank empties, the job looks finished, and the problem has quietly moved into your pipes, your pump, your geyser and your filters. It has to be vacuumed out and carried off the property.
The procedure
Eight steps, and how to check each one
Every tank goes through all eight. The check line under each step is what you can verify yourself, without taking anyone’s word for it.
Survey and written quote
We open the tank, measure it, look at sludge depth, and check how the hatch is placed and where the vacuum hose will have to run. Then you get a figure in writing before any equipment comes off the truck.
The survey is free and takes about twenty minutes.
How to check: Ask for the quote in writing. If a company will not put a number on paper before starting, the number is going to move.
Controlled drainage
The tank is drained to a safe point and the standing water is directed away from the property. Nothing is pushed back down the outlet into your supply line, and the pump is protected from running dry.
20–40 minutes depending on volume.
How to check: Watch where the water goes. If it disappears down your own outlet pipe, the sludge in it has just been sent into your plumbing.
Ventilation and gas test
On underground and sealed tanks, forced ventilation runs before anyone goes near the hatch and the atmosphere inside is tested. A harness and tripod go on, and an attendant stays at the opening for the entire time a person is inside.
15–30 minutes, and it is never skipped.
How to check: If a worker climbs into an underground tank with no blower running and nobody at the hatch, stop the job. This is the step that kills people in Karachi every year.
Jet washing
Walls, floor and ceiling are cut with water at 3,000+ PSI. This is what breaks biofilm off the surface — the slippery bacterial layer bonded to the wall that a brush slides over and a bucket of bleach does not touch. Pressure is dropped for fiberglass, Syntex and Bestank so the surface is not scored.
45 minutes to 2 hours by tank size.
How to check: You should hear a machine, not scrubbing. Afterwards the wall should feel clean, not slick.
Vacuum extraction
All the loosened sludge, sediment, rust and debris is vacuumed into a container and carried off the property. The floor is checked corner by corner afterwards, because sediment collects where the vacuum head is hardest to reach.
30–90 minutes.
How to check: Ask to see what came out. A tank that has gone years between cleans produces a visible quantity of sludge — if nothing was collected, nothing was removed.
Disinfection
Silver-stabilised hydrogen peroxide is applied to every internal surface at a concentration matched to the tank volume, and left for its full contact time. Contact time is the part that gets rushed: a disinfectant that is sprayed and immediately rinsed has not done anything.
Application 15 minutes, then the product sits.
How to check: Ask what the contact time is and whether it was observed. Ask to see the product label.
Rinse and inspection
The tank is rinsed until the water runs clear, so there is no residue and no taste. While it is empty and lit we check the walls and joints for cracks, and we check the lid, hinge and inspection cover — a lid that no longer seats is how a freshly cleaned tank gets dirty again within weeks.
20–40 minutes.
How to check: Get the lid checked. Almost nobody does this and it is the most common reason a clean does not last.
Report and invoice
You get a dated service report with the photographs we took inside the tank, a recommended date for the next clean, and a numbered SRB/FBR tax invoice. For a building, the invoice goes to the society and the report is something the committee can circulate to residents.
Handed over on the day.
How to check: Keep the report. It is your record of when the tank was last done, and the only way to answer that question honestly next year.
The step that gets people killed
An underground tank is a confined space, and the hazard inside it is not dirt. Organic matter decomposing in standing water consumes oxygen, and the atmosphere left in a sealed concrete tank can be low enough to cause collapse within a minute or two. There is no warning smell and there is no time to climb back out.
Almost every fatality in this trade follows the same sequence. One worker goes down. He stops responding. A second man goes in to pull him out, without equipment, and the same thing happens to him. It is the rescue attempt that turns one death into two.
Forced ventilation, a gas test before entry, a harness and tripod, and an attendant who stays at the hatch and never goes in exist precisely to stop that sequence. This is what our ISO 45001 certification covers, and it is the single reason most operators in Karachi will only clean accessible rooftop tanks — they are not equipped to enter the underground one, so they do not.
If you take one thing from this page: if you see someone climb into an underground tank with no blower running and nobody stationed at the opening, stop the job. It does not matter whose crew it is.
Chemicals
What goes into your tank
You are entitled to know exactly what is being put into water your family drinks, and to see the label before it happens.
Silver-stabilised hydrogen peroxide
Our primary disinfectant. EPA-registered for potable water treatment. It breaks down into water and oxygen, so there is no residue and no taste, and it is more effective against biofilm than bleach at comparable concentrations. Dosed to the measured volume of the tank, not by eye.
Food-grade chlorine, where needed
Used as a secondary treatment on heavily contaminated tanks, at controlled concentration and followed by a full rinse cycle. Food-grade only — never the industrial product, and never poured from an unlabelled container.
What we will not use
No industrial acids, no raw household bleach, no unlabelled product. If a contractor cannot show you a label and a safety data sheet for what is going into your drinking water, that is the end of the conversation, not a detail to sort out later.
Contact time is the part that gets skipped
A disinfectant sprayed on and rinsed straight off has not done anything. It has to sit on the surface for its full contact time. Ask what that time is and whether it was observed — it is the easiest corner to cut and the hardest for a customer to spot.
Hold us to it
Five questions for whoever cleans your tank
Ask these of any company, including us. A contractor doing the work properly will be glad you asked.
Ask before they start
- Will you give me the price in writing before you begin?
- Are you entering the underground tank, and what equipment do you use to do it safely?
- What disinfectant are you using, and can I see the label?
- Where does the sludge go?
- Will I get a dated report and a tax invoice?
Check after they finish
- Shine a torch in from the hatch — the floor should be bare
- The wall should not feel slick to the touch
- There should be a visible quantity of sludge that came out
- The lid should seat properly and close flush
- You should be holding a report with a next-service date on it
Timing
How long it takes
| Tank | Typical duration | Where the time goes |
|---|---|---|
| Rooftop plastic or fiber | 2 – 4 hours | Mostly washing and disinfection contact time |
| Underground RCC | 4 – 8 hours | Ventilation, safe entry and vacuum extraction, not washing |
| Shared building tank | 4 – 8 hours | Scheduled around the building supply so residents lose least |
| Industrial or multi-tank | Quoted after survey | Planned around your shutdown |
We would rather quote eight hours and take six than quote three and rush the disinfection contact time.
Questions
About the process
What is biofilm, and why does bleach not remove it?
Is silver-stabilised hydrogen peroxide safe for drinking water?
Why is entering an underground tank dangerous?
How do I know whether the job was actually done properly?
How long does the whole thing take?
Do you need my water supply turned off?
How often should this be done?
Ready to book? Price your tank online in about thirty seconds, or call the branch nearest you — DHA Phase 2 for South Karachi, Gulshan-e-Iqbal for East & Central Karachi.
Get an instant priceHave it done properly
Free survey, a written figure that does not change, and a dated report with photographs of the inside of your tank at the end of it.