Jar Testing and Dose Setting: Finding the Right Chlorine Dose for Your Source

Tony Miller

4 days ago

7 min read

A chlorine jar test finds the lowest dose that holds free residual chlorine (FRC) at 0.2 to 0.5 mg/L after 30 minutes' contact, for one specific water source. Because organic load, turbidity and pH shift that dose, no single figure carries safely across a South Sudan operation's water points.

Why does a fixed chlorine dose fail across sources?

Because the water is not the same everywhere, and chlorine reacts with everything in it before it starts protecting anyone. Organic matter, iron, manganese and ammonia all consume chlorine on contact, a reaction water engineers call chlorine demand. A borehole with low organic load and a river source with high organic load can need very different doses to reach the same FRC target, even though both are treated by the same tablet count or bucket recipe.

The consequence of skipping this step is documented, not theoretical. A study in the Bulletin of the World Health Organization found that in South Sudan refugee camps, 40 to 58% of households collecting water from chlorinated tapstands had no detectable residual chlorine in their stored water at home. The researchers concluded that a standard fixed dose could not guarantee 0.2 mg/L FRC 24 hours after distribution under the conditions in those camps, and recommended raising the initial target toward 1.0 mg/L instead of relying on the textbook figure. That is a fixed-dose failure at outbreak scale, not a testing-error story, and exactly why a dose set once, at one source, on one day, should never be assumed to hold at the next.

What is chlorine demand, and why does dose have to be set per source?

Chlorine demand is the portion of a dose consumed reacting with organic matter, sediment and dissolved minerals before any chlorine is left over to disinfect. What is left after demand is satisfied is free residual chlorine, the fraction that actually protects the water until it is drunk. Dose is therefore demand plus target residual, and demand is a property of the source, not a constant.

Turbidity is the clearest driver of demand in the field. Manufacturer instructions for Aquatabs 67 mg NaDCC tablets direct users to filter cloudy water through clean cloth before treatment and then dose it at roughly double the tablet count used for clear water in the same container, because suspended solids shield pathogens and consume chlorine that would otherwise become residual. The Sphere/IFRC water quality criteria set turbidity at 5 NTU or below as part of the standard a treated source should meet; above that band, dosing harder stops being reliable and pre-treatment becomes the answer instead. A jar test is how you find out, for your actual source on the day, which side of that line you are on.

How do you run a jar test or dose trial at a water point?

The method is the same whether you are setting a dose for a bucket of HTH solution at a communal tapstand, a batch of Aquatabs for a tank, or a P&G Purifier of Water sachet trial on a badly turbid source. Six steps:

Collect a real sample. Draw water from the actual source you are about to treat, at the turbidity it presents that day, not from a cleaner reference container.
Split it into equal volumes. Four to six identical containers of the same measured volume, side by side.
Dose each container differently. Apply an increasing, known amount of chlorine stock solution (or tablet count) to each container, moving in equal steps from a low dose to a high one.
Mix and hold for the full contact time. Thirty minutes, matching the Sphere/IFRC contact-time standard. Do not shortcut this step to get a faster answer; a dose that looks adequate at ten minutes can still fall away before thirty.
Test the residual in every container. A DPD1 tablet with a comparator, or a photometer, gives an FRC reading for each dose tested.
Compare dose against residual. Plot the results and find where the curve flattens once demand is satisfied, the point past which extra dose buys residual chlorine roughly one-for-one rather than being consumed.

How do you read the 30-minute result and pick the working dose?

Choose the lowest tested dose whose FRC lands inside the target band once contact time is complete, not the dose that looks best on paper before the reaction finishes. The Sphere/IFRC standard sets that band at 0.5 mg/L or above at pH below 8 after 30 minutes, and no less than 0.2 mg/L at the point of delivery, a different measurement to the point of dosing. During an active cholera or acute watery diarrhoea response, weigh the case for dosing toward the higher end of that band or beyond it, given the same Bulletin of the WHO finding that 0.2 mg/L at dosing routinely fails to survive to the household.

Do not pick the lowest dose in the set just because it clears the minimum. Chlorine that only just meets 0.2 mg/L at the point of treatment is likely to fall below it by the time water reaches a jerrycan, sits through a hot afternoon, and is poured out at home. Build in headroom deliberately rather than discovering the shortfall in a downstream field test.

How do you set and record the working dose once you have it?

A jar test result is only useful once it becomes an instruction someone at the tapstand can follow without repeating the trial. Convert the chosen dose into a fixed recipe, millilitres of stock solution per bucket, or tablets per tank of a known volume, and post it at the point of treatment. Log the source, date, turbidity on the day and resulting dose in a treatment register, so the next operator inherits the answer rather than guessing. Where SLS supplies the testing kits, dosing measures and stock chlorine alongside Aquatabs and P&G Purifier of Water for a site, that register is what lets a dose be defended if a supervisor asks how it was set.

When do you re-test and change the dose?

Whenever the source changes character, not on a fixed calendar alone. Re-run the jar test after a rain event that raises turbidity, when a response switches from a borehole to a river or pond intake, when a new water point opens on an unfamiliar source, and whenever routine FRC checks at the tapstand start missing target even though the dose has not changed. A dose that held all dry season can fail the day the source turns turbid, and the only way to know the new number is to test again. This is separate from monitoring chlorine decay once water is already in household storage; that downstream loss is its own problem, and the fix here is getting the dose right at the point of treatment in the first place.

What equipment does a jar test actually need?

A DPD1 tablet kit with a comparator or a photometer, a turbidity tube or turbidimeter, a stopwatch or phone timer, a graduated dosing syringe or measuring cylinder, four to six clean sample containers of equal volume, and a known-strength stock chlorine solution or a fixed batch of Aquatabs tablets to test with. None of it is specialised equipment, and none of it is expensive relative to the cost of a dose that fails downstream. SLS stocks Aquatabs, P&G Purifier of Water and the testing kits to run this procedure from Juba stock, which matters in South Sudan when a field team needs to re-dose a source the same week turbidity changes, not after a resupply cycle from outside the country.

Frequently asked questions

What is a chlorine jar test?

A jar test doses several identical samples of the same water source at increasing chlorine levels, holds them for the full contact time, then measures free residual chlorine in each to find the lowest dose that reaches the target band. It answers the question a fixed dosing chart cannot: what this specific source needs today.

What free residual chlorine level should the dose be aiming for?

The Sphere/IFRC standard is 0.5 mg/L or above after 30 minutes contact at pH below 8, and no less than 0.2 mg/L at the point of delivery. In an active outbreak, dose toward the top of that band or higher, because research in South Sudan refugee camps found that a dose set to just clear 0.2 mg/L routinely failed to hold that level 24 hours after distribution.

Is there an upper limit on how hard you can dose?

Yes. Chlorine levels up to 4 mg/L are considered safe in drinking water, which gives a jar test real headroom to dose toward, but taste and acceptance fall off well before that ceiling, so the working dose should be the lowest one that reliably clears the target, not the highest one that is still technically safe.

Can you jar-test using Aquatabs instead of bulk chlorine solution?

Yes, and it is the practical method for a tank or drum rather than a communal HTH bucket. One 67 mg Aquatabs tablet treats 8 to 10 litres of clear water with 30 minutes' contact; cloudy water is filtered first and then dosed at roughly double the tablet count for the same volume. Running that ratio across a small set of test containers at different tablet counts is a jar test in miniature.

How is chlorine demand different from the chlorine dose?

Demand is the portion of the dose consumed by organic matter, sediment and dissolved minerals before any chlorine is left to disinfect. Dose is demand plus the residual you want left over, so two sources with identical target residuals can need very different doses if their demand differs.