Reading an FRC Test: DPD Field Testing Step by Step

Q: What free residual chlorine should a field test show?

Between 0.2 and 0.5 mg/L at the point where people collect water, per the WHO/WEDC technical note on measuring chlorine levels in water supplies (2013). Sphere sets at least 0.5 mg/L after 30 minutes of contact at pH below 8 and at least 0.2 mg/L at delivery, with turbidity at or below 5 NTU.

Tony Miller
2 days ago
4 min read

A DPD test measures free residual chlorine by colouring a 10 ml sample pink, with the strength of that colour read against a comparator disc or a photometer. The field target is 0.2 to 0.5 mg/L at the point where people collect water, and the sample must be read immediately after mixing.

What does a DPD test actually measure?

DPD (diethyl paraphenylene diamine) reacts with free chlorine and colours the sample red in proportion to the concentration present, so what you read is the disinfectant still available to kill pathogens, not the dose you put in. A DPD No.1 tablet gives free chlorine; total chlorine needs a second reagent and a second reading. The WHO/WEDC technical note on measuring chlorine levels in water supplies (2013) calls the DPD comparator test the quickest and simplest field method, and puts the residual at collection between 0.2 and 0.5 mg/L.

Comparator or photometer: which should the team carry?

Carry both, for different jobs. The comparator is a two-chamber block read by eye against printed standards, graduated in steps of 0.2, 0.4, 0.6, 0.8 and 1.0 mg/L with a higher range to 8.0 mg/L, as illustrated in the same WHO technical note. It needs no power and costs little. A photometer runs the same chemistry electronically across a finer range, 0.02 to 2.00 mg/L on Hach's USEPA DPD Method 8021 procedure for free chlorine, low range.

Our view: comparators for volume monitoring at tapstands and in households, one photometer per team as the reference for readings near the 0.2 mg/L threshold, where a colour judged by eye in poor light decides whether a site passes. Both sit in the SLS WASH products range.

How do you run the test, step by step?

Rinse both chambers three times with the water you are about to test.
Fill the second chamber with untreated sample and no tablet. That is your blank for colour comparison.
Drop one DPD No.1 tablet into the first chamber with a few drops of sample and crush it thoroughly.
Fill that chamber to the 10 ml mark, cap it and invert to mix.
Read straight away against the standards, in daylight but out of direct sun.
Record the value with the location, time and water temperature. An unrecorded reading is not monitoring.

Steps 2 to 5 follow the comparator sequence set out in the WHO/WEDC technical note (2013), which also advises turbidity below 5 NTU and pH between 6.8 and 7.2 for chlorine to work properly.

Where and how often should you test?

Test just after chlorine is dosed, at the consumer outlet nearest the chlorination point, and at the furthest points of the network where the residual is lowest. On a new or rehabilitated system, test daily until you are confident, then at least weekly, per the same WHO note. During an outbreak, add household testing at 24 hours. That reading tells you whether the programme is protecting anyone.

The reading is under target. Now what?

Below 0.2 mg/L at collection, treat the water as unprotected and re-dose. Work three causes in order. Contact time: chlorine needs at least 30 minutes at 18°C, longer in colder water. Turbidity: settle or filter first, because one Aquatabs 67mg tablet treats 8 to 10 litres of clear water and cloudy water must go through clean cloth before dosing. Then the dose. If the tapstand reads well but the jerrycan is bare a day later, the dose is too low for the conditions. In South Sudan refugee camps, 40% to 58% of households drawing from chlorinated tapstands had no detectable residual in stored water, and researchers recommended raising the initial target toward 1.0 mg/L, per the Bulletin of the World Health Organization (2015). Defend at least 0.2 mg/L at the point of delivery, the Sphere minimum, and check again in the household where the residual has had time to decay.

The reading is over target. Now what?

A reading above 0.5 mg/L at collection is not a health emergency. The WHO guideline value for chlorine in drinking water is 5 mg/L, and most people can taste it at that level, per the WHO Guidelines for Drinking-water Quality chemical fact sheet. The risk is rejection: over-chlorinated water tastes and smells wrong, and people go back to the unprotected source. Fix it at the mixing point and let over-dosed batches stand or be blended down, rather than cutting doses across a whole response.

Frequently asked questions

What free residual chlorine should a field test show?

Between 0.2 and 0.5 mg/L where people collect water, per the WHO/WEDC technical note (2013). Sphere sets at least 0.5 mg/L after 30 minutes of contact at pH below 8 and 0.2 mg/L at delivery, with turbidity at or below 5 NTU, per the Sphere/IFRC WASH standards (2018).

Is DPD No.1 free chlorine or total chlorine?

DPD No.1 measures free chlorine. Total chlorine needs an additional reagent and a second reading, and free chlorine is the value emergency monitoring is judged against.

How often should chlorine residual be tested?

Daily on a new or rehabilitated system until it is proven, then at least weekly, per the WHO/WEDC technical note (2013). Add household testing at 24 hours during an outbreak.

What if the test shows no chlorine at all?

Treat the water as unprotected and re-dose, then check contact time, turbidity and dose rate. Sphere sets the floor at 0.2 mg/L free chlorine at the point of delivery, per the Sphere Handbook. In South Sudan camps, 40 to 58 per cent of households drawing from chlorinated tapstands had no detectable residual left in stored water, per the Bulletin of the World Health Organization, so a household check some hours after collection is the one that tells you whether the dose held.