Keeping Stored Water Safe: Chlorination, Residual and Cleaning for Emergency Tanks

Tony Miller

Oct 10

7 min read

Stored water is safe only if it still carries free residual chlorine when someone collects it. Sphere asks for at least 0.2 mg/L at the point of delivery, and WHO asks for a covered tank with a lockable access point, cleaned and disinfected before first fill. Treating the water once is not enough.

Why does treated water become unsafe in storage?

Because chlorine is consumed, and a tank is a place where contamination gets in. Chlorine reacts with organic matter and with anything that enters through an open hatch, and the residual is gone long before the water is. The WHO/WEDC technical note on measuring chlorine levels says disinfection is only one defence: effort must also go into preventing contamination during collection and storage, and an unmonitored supply can lead people to believe water is safe when it is hazardous.

The decay is measurable and fast. In a study of South Sudan refugee camps, 40% to 58% of households collecting from chlorinated tapstands had no detectable residual chlorine in their stored water (Bulletin of the World Health Organization, 2015). The same paper found that a tapstand delivering 1.0 mg/L kept the residual above 0.2 mg/L for an average of 13.8 hours, with camps ranging from 10.4 to 19.7 hours. Covering household storage containers improved residual levels.

Our view: a tank is a buffer, not a safeguard. The tank, the dose and the residual test are one system, and a site that has bought the tank but not the test kit has bought half of it.

What residual should a stored-water tank hold?

At least 0.2 mg/L free chlorine where people collect, and a higher figure at the dosing point so that the far end still passes. The Sphere/IFRC WASH standards set free residual chlorine at 0.5 mg/L or more after 30 minutes of contact at pH below 8, and no less than 0.2 mg/L at the point of delivery. The WHO/WEDC note frames the same range for a small communal supply: 0.2 to 0.5 mg/L at the consumer's collection point, higher near the dosing point and lower at the far extremities.

Two rules follow from those sources:

Chlorine needs at least 30 minutes of contact, and the WHO/WEDC checklist says to apply it after any other treatment and before storage and use.
Never add solid chlorine directly to a supply, because it will not mix and dissolve. Make it up as a paste first.

The South Sudan study went further and recommended raising the initial target to 1.0 mg/L, because standard doses could not hold 0.2 mg/L for a day. We read that as an instruction to test at the household, not only at the tapstand. Our guide to why chlorine disappears before it reaches the cup covers the mechanism, and the existing free residual chlorine field guide for the South Sudan cholera response covers the response routine.

How do you clean and disinfect a tank before first use?

Follow the four-step WHO method: select, clean, disinfect, dispose. The WHO technical note on cleaning and disinfecting water storage tanks and tankers (updated July 2013) sets it out as follows.

Select. Tanks should be easy to clean, covered, and fitted with an access point with a lockable lid. Reused tanks should only have held food-grade liquids, never fuel or sewage. A new emergency tank from stock skips the selection step but not the next two.
Clean. Empty the tank, then scrub all internal surfaces with detergent and hot water, taking special care in corners and joints. Flush until no traces of detergent remain. Clean the hoses, pumps and pipes the same way.
Disinfect. Fill the tank a quarter full with clean water. The note's figure is 80 grams of granular high-strength calcium hypochlorite per 1,000 litres of total capacity. Fill the tank completely, close the lid and leave it for 24 hours. If the tank is needed urgently, double the quantity and the contact time drops to 8 hours.
Dispose. Empty the tank, dispose of the strongly chlorinated water safely, fill with drinking water, leave it for about 30 minutes and empty it once more. The tank is then ready.

Scaling that figure is simple arithmetic, ours rather than the note's: 80 g per 1,000 litres means 880 g for an 11 m³ tank and 3.6 kg for a 45 m³ tank. Wash water should go to a sewer, a septic soakaway or a tanker, not to rivers or ponds, because the chlorine level and the organic load can kill fish and plant life.

Entering a tank is a hazard in its own right. The WHO note says to blow fresh air in before anyone enters, to have the cleaner wear protective clothing, and to keep someone outside the hatch at all times. Do not treat that as optional on a busy site.

How do you keep the water chlorinated once it is in the tank?

Dose at the filling point, mix, wait, test, and re-test downstream. The sequence that works:

Fill, then dose for the full volume with the chlorine product your agency has specified.
Allow at least 30 minutes of contact before releasing water.
Test free chlorine at the outlet with a DPD comparator.
Test again at the furthest tapstand and, in an outbreak, in a sample of households.
Log every reading with time and location. An unlogged reading is not monitoring.

The WHO/WEDC note advises checking the residual daily until you are sure the system is stable, and testing just after dosing and at the points where residual is likely to be lowest. A tank topped up from several trucks or boreholes needs a test at every fill, which is our rule rather than the note's.

Where do Aquatabs fit with emergency tanks?

Aquatabs suit household containers and small volumes, and bulk hypochlorite suits the large tanks. The Aquatabs 67 mg instruction sheet says one tablet treats 8 to 10 litres of clear water, to be left for 30 minutes, with cloudy water first strained. Each tablet contains 67 mg of sodium dichloroisocyanurate (NaDCC).

The arithmetic, ours, shows the limit. At one tablet per 8 to 10 litres, an 11 m³ tank of clear water would take roughly 1,100 to 1,375 tablets, and a 45 m³ tank 4,500 to 5,600. That is possible, but it is why large tanks are normally dosed with bulk hypochlorite while tablets are kept for jerrycans, buckets and household storage, where the 24-hour residual is actually lost.

The same sheet carries the storage rule that applies to every container: do not dip hands, cups or other objects into the water, pour into a clean cup if there is no spigot, and where the container is uncovered, use the water on the day it was treated. Aquatabs 67 mg tablets are available from SLS Juba stock, and SLS is an authorised distributor of Aquatabs from Medentech and Kersia.

Which tank features protect stored water?

A cover, a lockable access point and an outlet that does not need a bucket. The WHO tank note names covers and lockable access points as the baseline. The Oxfam Supply Centre tank range offers corrugated steel tanks in 11, 45, 70 and 95 m³ capacities with PVC or EPDM rubber liners, and lists steel conical, steel low-profile and UPVC conical roof options. The roof is the cover the WHO note asks for, so specify it rather than leaving a tank open to dust, birds and wind-blown debris.

SLS supplies Oxfam and Butyl tanks. If you are choosing between tank types or sizing one to a population, the SLS WASH products range is the place to start, and the question to settle before ordering is whether the roof and liner option you need is in the kit.

What does outbreak-scale supply look like?

SLS deployed more than 52 million Aquatabs and 3.28 million P&G Purifier of Water sachets in the 2024-2025 South Sudan response, a distribution figure and not measured consumption. The lesson for tank managers is that supply of tablets is only the front half. The Juba cholera response case study covers the supply chain; the residual still has to be proven by testing at the household.

Frequently asked questions

What chlorine level should stored drinking water have?

At least 0.2 mg/L free chlorine at the point of delivery, with 0.5 mg/L or more after 30 minutes of contact at pH below 8, per the Sphere/IFRC WASH standards.

How do you clean an emergency water tank?

Empty it, scrub the inside with detergent and hot water, flush until no detergent remains, disinfect with chlorine for 24 hours (8 hours at double dose), then drain, refill and drain again after 30 minutes, per the WHO technical note on tank cleaning.

How long does chlorine stay in stored water?

It varies by site. In South Sudan refugee camps a 1.0 mg/L dose kept the residual above 0.2 mg/L for an average of 13.8 hours, with camps ranging from 10.4 to 19.7 hours, per the Bulletin of the World Health Organization (2015).

Can Aquatabs treat a large storage tank?

They can, at one 67 mg tablet per 8 to 10 litres of clear water, per the Aquatabs instruction sheet, but the tablet count rises quickly with tank size. Large tanks are normally dosed with bulk hypochlorite and tablets are kept for household containers.

Does a tank need a lid?

Yes. The WHO tank note says tanks must be covered and fitted with an access point with a lockable lid so that water stays clean in storage, per WHO/WEDC.