Common Water Tank Cleaning Mistakes: 8 Practices That Ruin the Result or Put Lives at Risk

Most problems after a tank cleaning do not come from too little effort but from a mistake in how the job was done: water whose chlorine smell will not fade, a lining that peels after a few months, or insects that are back within weeks. Some of these mistakes do more than spoil the result; they put the person doing the work in real danger. Here we collect the common water tank cleaning mistakes we see repeated, with the correct practice backed by reliable references.

The aim is not to turn you into a technician. It is to help you know what to ask of any crew, and what to avoid if you decide to clean a small plastic tank yourself.

What are the most dangerous common tank cleaning mistakes?

Mistake one: mixing chlorine with other cleaners

Some people think adding another cleaner to chlorine makes the cleaning stronger. In reality this is the most dangerous thing you can do in an enclosed space. The Washington State Department of Health explains that mixing bleach with ammonia produces toxic gases called chloramines, and that mixing it with an acid releases chlorine gas. Symptoms of exposure include coughing, shortness of breath and chest pain, and in severe cases fluid can build up in the lungs.

Many bathroom cleaners and descalers contain acids, and some glass cleaners contain ammonia. Inside a poorly ventilated underground tank, the gas concentrates right around the worker. The rule is one product at each stage and a full rinse between the cleaner and the disinfectant.

Mistake two: scented bleach or "colour-safe" bleach

Not every bottle of chlorine on the shelf is suitable for drinking water. In its guidance on disinfecting drinking water, the U.S. Environmental Protection Agency (EPA) recommends plain, unscented household bleach that the label says is suitable for disinfection, and says not to use scented, colour-safe or bleaches with added cleaners. It states that suitable products contain sodium hypochlorite at 6% or 8.25%.

Fragrances and added cleaners are not easily rinsed away and can leave a strange taste and foam. Read the label before buying, and avoid any bottle with words such as "scented" or "with cleaner".

Mistake three: refilling without rinsing after disinfection

Disinfection uses a much higher strength than is suitable for drinking. The World Health Organization's Technical Note 3 says that once the disinfection time has passed the tank is drained completely, then filled with drinking water, left for about 30 minutes and drained again. The WHO also sets a health-based guideline value for chlorine in drinking water of 5 mg/L, noting that most people can taste it at around that level.

Refilling on top of the disinfection solution means water with a sharp smell that may push a household to buy bottled water for days or cause irritation in the shower. For the correct timing of each stage, see how long tank cleaning takes.

Mistake four: cleaning the inside and neglecting the lid and vents

The EPA white paper on water storage says that roof hatches, vent and overflow pipes and wall joints are the main routes of contamination, and that missing screens on vents and overflows and unlocked hatches are among the defects inspectors find most often. Cleaning the walls without fixing these points is like washing the floor while the door stays open to dust.

Make checking the lid, gasket and screens a fixed item on every visit, and use our post-cleaning tank inspection checklist before you open the tank for refilling.

Mistake five: damaging the lining while cleaning

Wire brushes, abrasive powders and high pressure through a narrow nozzle all remove dirt quickly, but they remove part of the lining with it. Hempel's technical guideline lists water pressures above 70 MPa among the classes used to strip coatings. A scratched lining becomes rougher, so biofilm attaches to it faster, and a peeling one lets water reach the concrete.

The full details of setting pressure are in our guide to high-pressure tank washing. If the lining is already damaged, the answer is to redo the tank insulation, not to disinfect again and again.

Mistake six: going down into an underground tank alone

This is the costliest mistake because it concerns lives. The U.S. Occupational Safety and Health Administration (OSHA) standard 1910.146 defines a hazardous atmosphere as one where oxygen falls below 19.5%, and requires at least one attendant outside the confined space for the whole time anyone is inside. The WHO likewise stresses blowing in fresh air before entry and keeping someone by the opening at all times.

• Do not go down into an underground tank without ventilating it first, even if the air seems normal.
• Do not go down without a person outside the opening who can see or hear you continuously.
• Do not go down after someone who has collapsed without respiratory protection; call Civil Defense immediately.
• Do not run a generator or fuel engine near the tank opening.

Other mistakes that keep recurring in homes

• Disinfecting over silt: the disinfectant is used up in the sediment and never reaches the walls; start with removing sludge and silt from the tank.
• Shared tools: using the same bucket or brush as the septic tank in the drinking-water tank.
• Careless disposal: pouring cleaning water and silt into the street instead of the sewer.
• No records: no photos, times or chlorine readings, so nobody can tell what was done.

In Azl's tank cleaning and disinfection service, the procedures are built around avoiding exactly these mistakes; costs vary with tank size, condition and access and are confirmed after inspection. For more general household safety guidance see our health and safety page.