Water Tank Disinfection in Riyadh

No, and the order here is not a detail. Chlorine-based disinfectant reacts with any organic matter it meets, so on a surface covered in silt or biofilm it is used up on the dirt before it reaches the micro-organisms, leaving the tank as it was with an added taste and smell in the water.

That is why the work starts with draining, pumping silt, scrubbing and rinsing, and disinfection comes onto a clean surface where the solution touches the material directly. Pouring chlorine into a full, dirty tank only masks the odour for a few days; it returns because its source is still on the floor.

The reverse case is acceptable: if the tank was washed carefully a short while ago and the water is clear, and something then calls for disinfection such as a repair or stagnation, a focused disinfection after a light rinse may be enough. The inspection decides that, not a general rule.

The first case is any intervention inside the tank: waterproofing, repairs, replacing a float valve or running new pipework. Workers enter with boots, tools and materials, and new materials can carry dust and debris, so the tank is disinfected before the water goes back into drinking use.

The second is stagnation: a house left empty for weeks in a Riyadh summer, a roof tank with low use, or a closed guest wing. Standing water loses the disinfectant residue it arrived with from the mains, and heat lets biofilm grow easily on the walls, so topping up with new water over old is not enough.

The third is a contaminant getting in: a lid left open, an insect or bird that fell in, or surface water that seeped through a crack after rain. Last is the odour that returns after a sound wash, which suggests biofilm in the pores of the wall or in a spot the scrubbing did not reach.

• After waterproofing, repairs or fitting changes
• After the family has been away and water has stagnated
• After a foreign object or surface water gets in
• When an odour returns after a good wash
• Before first use of a new or reconditioned tank

The process starts with working out the volume: tank length, width and the usual water depth. The dose of disinfectant follows from the volume, since the right dose determines both effectiveness and taste. More is not safer; it means water that is harder to rinse and fittings that corrode faster.

After washing, the solution is prepared at the right dilution and applied to walls, roof, floor, corners, manhole rims and the underside of the lid, then left for the contact time set by the product and the air temperature. During that time the tank is kept closed as far as possible, and ventilation is observed for anyone working near the opening.

The solution is then discharged and the tank is rinsed with clean water again and again until the disinfectant smell is gone from the rinse water, after which it is refilled. During refilling the taps are opened in turn so fresh water passes through the pipes. Disinfecting the internal house pipework is a separate decision taken case by case.

The description of the smell sets the direction. A faint chlorine smell in water from the mains is normal and fades with airing. An earthy or musty smell usually points to algae or biofilm in the roof tank, a case for washing and then disinfecting. A rotten-egg smell in hot water alone is more likely the water heater.

A sulphur smell in both hot and cold water together with stagnation indicates rotting silt on the tank floor, so disinfecting without pumping the silt will only quiet it for a while. A metallic or rusty taste usually belongs to corroded metal pipes, which tank disinfection cannot cure however sealed the tank is.

So before anything else we ask what kind of smell it is and where it appears: all taps or one, hot or cold, and whether it began after a particular event. Those questions decide whether disinfection will help or the search should turn to the heater or the pipes.

• Faint chlorine in mains water: normal, fades with airing
• Earthy or musty: algae or biofilm in the tank
• Sulphur in hot water only: check the heater first
• Sulphur in hot and cold: rotting silt on the floor
• Metallic or rusty: check the pipework

The aim is for the water to return to ordinary household use after a full rinse and a refill with fresh water. We do not treat the tank as ready until the rinse water and the first refill water are free of any disinfectant smell. If you notice a faint smell after refilling, let the taps run until it clears before drinking.

Before refilling, anything sensitive to chlorine is removed or bypassed: carbon filter cartridges and reverse-osmosis membranes, and they stay out until the water runs clear. After refilling, open taps and showers throughout the house and clean the aerators, since particles may come loose from old pipes.

Do not run your own cleaning experiments in the tank afterwards. An acidic descaler with chlorine gives off a toxic gas, and ammonia with bleach produces irritating fumes. In a closed tank one mistake is enough to endanger whoever stands at the opening. Leave work inside the tank to those with proper ventilation and protective equipment.