Household water tanks: cleaning and disinfecting step by step, following the WHO guide

Water may arrive clean, but an uncovered or neglected tank contaminates it again. The World Health Organization guide sets out a four-step approach: selecting the tank, cleaning, disinfecting, and safely disposing of the wastewater1.

1) A hygienic tank to begin with

• The tank must be covered, with an access point fitted with a lockable lid1.
• Easy to clean, with no sharp corners that trap dirt1.
• Permanent tanks are usually fitted with a washout valve drawing from the base; use it for emptying rather than the normal outlet1.

In practice: make sure the ground tank lid seals well and its neck stands above ground level to keep out washing water and rain, and keep it away from the septic tank.

2) Calculate your tank volume

Both formulas are from the WHO guide1. Example: a 3 × 2 × 2 m ground tank = 12,000 litres.

3) Cleaning

1. Empty the tank completely through the washout valve.
2. Scrub the internal surfaces with hot water and detergent (household laundry powder will do) using a stiff brush or a high-pressure jet; a brush on a long pole may allow cleaning without entering the tank1.
3. Rinse with water (preferably hot) and keep flushing until no trace of detergent remains1.
4. Clean the hoses, pump and connected pipes by flushing hot water and detergent, then clean water1.

4) Chlorine disinfection

The most common method is chlorination, and the most common product is high-strength calcium hypochlorite (HSCH)1:

1. Fill a quarter of the tank with clean water.
2. Add 80 g of granular HSCH for every 1,000 litres of total tank capacity.
3. Fill the tank completely, close the lid and leave it for 24 hours. (If the tank is needed urgently: double the dose and the time drops to 8 hours.)
4. Circulate the disinfecting water through the pump and hoses for about an hour.
5. Empty the tank completely, fill it with drinking water, leave for about 30 minutes, then empty it again. The tank is now ready.

All steps and figures from the WHO guide1. Example: a 12,000-litre tank needs 12 × 80 = 960 g.

5) Safety when entering a tank

Working inside tanks is difficult and dangerous, so the guide recommends1:

• Blowing fresh air into the tank for a period before anyone enters.
• Wearing protective clothing: gloves, boots, a hat and glasses.
• Someone staying next to the hatch at all times in case of an accident.

How often to clean depends on water quality, the tank's condition and local health authority instructions; any change in the water's colour, smell or taste calls for an immediate tank check.