The short answer
Sediment settles at the base and biofilm forms on the walls; neither is removed by draining, which is why a rinse and refill is not a clean.
A tank is a still body of water in a warm place. Anything that arrives with the supply, or through a broken vent screen, settles and stays.
How it is done
- Sediment at the base, removed by hand rather than by draining
- Biofilm on the walls, which needs scrubbing then disinfection
- The lid seal and vent screens, where most recontamination arrives
- The overflow, which is a route in as well as out
Why it matters
Most recontamination is not from the water; it is from above. A scrubbed tank with a broken vent screen is dirty again within months.
questions
Asked about this
Cloudiness, sediment in an aerator, a taste change, or simply not knowing when it was last done.
No. A filter protects the outlet; it does nothing for what is sitting in the tank.
references
Background reading
- Water tank — Wikipedia
- Biofilm — Wikipedia