What Is Turbidity, and What NTU Can You Actually Chlorinate?
Turbidity measures how cloudy water is, in Nephelometric Turbidity Units (NTU). Sphere sets the ceiling for effective chlorination at 5 NTU: suspended solids above that shield pathogens from chlorine and burn through the dose, so turbid source water needs pre-treatment first, not a heavier dose of tablets.
What is turbidity, and how is NTU measured?
Turbidity is the cloudiness caused by particles suspended in water: silt, clay, organic matter, algae. It is measured in NTU, a reading produced by shining a light through a sample and recording how much scatters off the particles it hits. Clear drinking water sits below 1 NTU; a river in flood can run into the hundreds. Sphere's emergency water-quality standard sets turbidity at or below 5 NTU at the point of delivery, alongside a free residual chlorine (FRC) target of at least 0.2 mg/L at delivery and 0.5 mg/L after 30 minutes' contact, per the Sphere/IFRC WASH standards (2018). Without a photometer, field teams read turbidity with a sight tube: a clear tube marked with a pattern, lowered into or filled with the sample until the pattern disappears from view. The shallower the depth at which it vanishes, the higher the turbidity.
Why does turbidity matter for chlorination?
Two mechanisms work against you. Chlorine is an oxidiser, so it reacts with whatever organic material is suspended in the water before it ever reaches a pathogen, and a turbid sample burns through the dose on solids rather than disinfection. Particles can also physically shelter bacteria and cysts inside their structure, keeping chlorine from making contact at all. The WHO/WEDC technical note on measuring chlorine levels in water supplies (2013) states it directly: turbidity should be below 5 NTU and pH between 6.8 and 7.2 for chlorine to work as a disinfectant. Chlorinate above that and you are not treating the water correctly, you are dosing blind and hoping the residual holds.
What NTU can you actually chlorinate?
Below 5 NTU, dose as normal and expect the standard 30-minute contact time to hold. Roughly 5 to 10 NTU is the boundary Aquatabs' own instructions already build in: one 67mg tablet treats 8 to 10 litres of clear water, and the dose doubles for cloudy water once it has been strained through clean cloth first. CDC's guidance for making water safe in an emergency is consistent: filter cloudy water through a clean cloth or let it settle before treating, and double the disinfectant dose if it still looks murky. Beyond that point, doubling the dose stops being a fix. Chlorine demand climbs faster than any dose you can reasonably add without pushing taste and odour to where people reject the water outright, and shielded pathogens can still get through. That is the point to pre-treat, not the point to chlorinate harder.
What do you do with water too turbid to chlorinate?
Settle it, filter it, or coagulate it before dosing. Storage and settlement over a few hours removes the coarsest particles; cloth filtration takes out more before the tablet goes in. Where a source runs consistently turbid, coagulation-flocculation is the more reliable route: P&G Purifier of Water, developed with the CDC, combines a flocculant with a disinfectant in a single sachet, clearing suspended solids and disinfecting a 10-litre batch in one step, including a large share of the protozoa that chlorine alone does not reliably remove, per CDC-affiliated CSDW. SLS delivered 3.28 million P&G sachets to 27,344 households in the 2024-2025 South Sudan cholera response, much of it into flood-affected, visibly turbid source water across South Sudan. Keep Aquatabs for clear sources and PUR for turbid ones; treating both the same way is the most common field error we see.
Frequently asked questions
What NTU is safe to chlorinate directly?
Sphere sets the ceiling at 5 NTU at the point of delivery, per the Sphere/IFRC WASH standards (2018), and the WHO/WEDC technical note on chlorine (2013) gives the same threshold for effective disinfection. Above it, pre-treat before dosing.
Can I just add more chlorine to turbid water instead of pre-treating?
Only within a narrow margin. Aquatabs' instructions double the dose for cloudy water once it has been strained through cloth, and CDC's emergency guidance does the same for household bleach. Past that, chlorine demand and taste rejection outrun the benefit, and shielded pathogens can still survive.
What's the difference between Aquatabs and P&G Purifier of Water for turbid water?
Aquatabs disinfects; it does not clarify. P&G Purifier of Water coagulates and flocculates suspended solids out of the water in the same step as disinfecting it, which is why it, not a doubled Aquatabs dose, is the right call for a consistently turbid source, per CDC-affiliated CSDW.
How do you measure turbidity without a lab?
A turbidity tube gives a usable field estimate: a clear tube marked with a pattern that disappears from view as the water column deepens with cloudiness, read with no power or reagents needed. A turbidimeter gives a precise NTU reading where one is available.
Related reading
Specialized Logistics Solutions is an in-country distributor based in Juba, authorised for Aquatabs (Medentech/Kersia), P&G Purifier of Water and Oxfam tanks (Butyl Products), and a UNGM-registered vendor (No. 380716). In the 2024-2025 South Sudan cholera response SLS delivered 3.28 million P&G Purifier of Water sachets to 27,344 households across flood-affected and turbid-water areas, alongside more than 52 million Aquatabs tablets for clear sources. SLS operations across South Sudan draw both products, testing kits and turbidity tubes from one Juba-based chain. Request a quotation from SLS to stock the right product for your source water.
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