How often should a water tank be cleaned in Kerala?

A sealed tank of clean water still accumulates three things: sediment that settles out of the supply, biofilm — the slippery layer that forms on any permanently wet surface — and whatever gets in through a gap. Six months is the interval at which that is a routine clean rather than a problem.

It is worth being straight about where the number comes from, because a lot of sites state it as law. There is no Indian standard that fixes a cleaning frequency for a domestic overhead tank. IS 10500 is a drinking-water QUALITY standard and says nothing about cleaning dates. WHO guidance points to three months, Indian practice has settled on six, and six is what we work to.

Two things push this district toward the shorter end, and both are measurable rather than atmospheric. Stored water sits warm most of the year, which speeds up everything biological. And the groundwater here genuinely is difficult: the Kerala Ground Water Department has recorded iron in hard-rock terrain wells ranging from trace levels to several milligrams per litre, against a BIS acceptable limit of 0.3 mg/L, which is where the reddish-brown staining and the metallic taste people blame on the tank actually come from.

On the coastal strip it is harder again. A published study of groundwater at Alappad, in this district, found nearly half of the samples taken during the monsoon unsuitable for drinking on a water-quality index, with hardness far above the acceptable limit and a signature consistent with seawater intrusion. That is not an argument for cleaning the tank more often on its own — a clean does not fix the supply — but it is the reason a tank here silts and stains faster than one inland.

• Every six months — a normal household overhead or sump tank.
• Every three to four months — if the tank is in direct sun, the supply is bore water, or the house has had a stomach-bug run through it.
• Immediately — after a flood, after any work that opened the tank or the line, or if you can see growth or smell the water.

None of these are subtle once you know to look, and all of them are reasons not to wait for the six-month mark.

• The water smells earthy, musty or of chlorine that was not there before.
• A metallic taste, or reddish-brown staining in the sink, on tiles or in the washing.
• Visible particles, or a film on the surface when you open the lid.
• Slime on the tank wall at the waterline — that is biofilm, and it is the thing a rinse does not remove.
• Anyone in the house with a repeated stomach upset that nothing else explains.
• A lid that does not seal, or a missing insect mesh on the overflow — those are how contamination gets in, and finding one means the tank needs opening now.

Worth knowing, because "tank cleaning" is sold at very different levels of thoroughness and the cheap version is usually a drain and a rinse.

1. Drain the tank down and remove the residual sludge from the base rather than flushing it into the house lines.
2. Scrub every internal surface — walls, base and the underside of the lid — to take the biofilm off. This is the step that separates a clean from a rinse.
3. Disinfect, then leave the contact time the product actually requires instead of rinsing it straight out.
4. Rinse out and flush, so nothing is left in the water you then drink.
5. Check the fittings: lid seal, overflow mesh, inlet and any cracks. A tank cleaned annually with an unsealed lid is being recontaminated between cleans.
6. Refill and run the taps through.

For a typical two-tank house — an overhead tank and a sump — that is roughly three to four hours, and you are without stored water for most of it. Fill what you need for the afternoon before the team starts.

Tank cleaning is priced by capacity, access and how long it has been left. As a general guide across the market, a small domestic overhead tank sits in the low hundreds of rupees, a typical thousand-litre household tank runs to roughly ₹700–₹1,500, and a large sump or a commercial installation runs into a few thousand. Those are market ranges rather than our quote — the point of publishing them is so you can tell whether a number you have been given is reasonable.

We quote a fixed price before starting rather than a rate that moves once the lid is off, and for anything unusual — a very large tank, an awkward access, a commercial building — we look at it first and quote after. The visit for that is free and carries no obligation.