Bulk Chlorination at Communal Water Points (HTH Dosing)

Dose a communal tank or tapstand with high-test hypochlorite (HTH), the calcium hypochlorite powder, to hit a free residual chlorine of at least 0.5 mg/L after a 30-minute contact time at pH below 8, the Sphere water-quality minimum. HTH is the practical bulk-disinfection choice for the large volumes a water point serves, where individual tablets do not scale.

What is HTH and why use it for bulk chlorination?

HTH is high-test hypochlorite, a calcium hypochlorite powder or granule that is typically 65 to 70 percent available chlorine. It is the workhorse of bulk water treatment because a small mass of powder disinfects a large volume of water, which is exactly the problem at a communal water point serving hundreds of people from a single tank.

At household scale, a chlorine donor like NaDCC in tablet form is precise and simple to brief. At a 10 m³ or 70 m³ tank, counting tablets stops making sense and you switch to a measured stock solution made from HTH. The active agent is the same free chlorine in the water; the delivery format changes with the volume. The trade-off is that HTH demands more care: it is a strong oxidiser, it loses strength in storage if exposed to heat and moisture, and dosing requires a quick calculation rather than a fixed tablet count. Used correctly, it is the most cost-effective way to chlorinate a tank, a tapstand line, or a tankering operation.

What free residual chlorine should you target at a water point?

Target 0.5 mg/L of free residual chlorine after 30 minutes of contact at pH under 8, then confirm at least 0.2 mg/L at the point where people actually collect the water, per the Sphere WASH standards. The 30-minute window is not optional. Chlorine needs that contact time to inactivate the bacteria and viruses in the water, so a tank dosed and drawn from immediately has not been treated, it has merely been mixed.

There is a field reason to dose toward the upper end. A study in South Sudan refugee camps found that 40 to 58 percent of households drawing from chlorinated tapstands had no detectable residual chlorine in their stored water, and that standard doses could not reliably hold 0.2 mg/L 24 hours after distribution; the researchers recommended raising the initial target toward 1.0 mg/L, per the Bulletin of the World Health Organization. The practical reading for a communal water point is this: chlorinate to the standard at the tank, but expect decay between the tapstand and the cup, and dose with that loss in mind rather than to the bare minimum.

How do you calculate the HTH dose?

Work from three numbers: the tank volume in litres, the percent available chlorine of your HTH batch, and the target dose in mg/L. The target dose is not the same as the residual you want to measure later; it is the residual plus the chlorine demand of the water, which is the chlorine consumed by organic matter, ammonia and pathogens before any free residual is left.

A workable field method:

1. Measure the tank volume. Use the rated capacity of the Oxfam steel tank or the bladder, not an estimate.

2. Make a 1 percent stock solution from the HTH powder, mixed in a clearly labelled, dedicated container. A stock solution doses far more accurately than tipping powder straight into a tank.

3. Run a jar test or a pot test to find the demand. Dose a known sample to a few candidate concentrations, wait 30 minutes, then test the free residual chlorine. The dose that leaves 0.5 mg/L is your demand-plus-target figure.

4. Scale that dose to the full tank volume, add the stock solution, mix, and start the 30-minute clock.

5. Test the free residual chlorine before anyone draws water. If it reads under 0.2 mg/L, redose; if it is far over 0.5 mg/L, the water is over-chlorinated and the taste will drive people back to unsafe sources.

The jar test matters because chlorine demand is the variable nobody can read off a chart. The same dose that holds a residual in a clean borehole tank vanishes in a tank filled from a turbid river. CDC household guidance reflects the turbidity effect directly: roughly 2 mg/L of chlorine for water below 10 NTU and about 4 mg/L for water above 10 NTU, per the CDC guidance on making water safe in an emergency. Above that turbidity threshold, raising the chlorine dose alone treats the symptom; clarify the water first.

How does turbidity change the dose?

Turbid water defeats chlorination twice. Suspended particles carry their own chlorine demand, soaking up the dose, and they physically shield pathogens from contact with the disinfectant. That is why high-turbidity water needs a higher dose for the same residual, and why a tank filled with visibly cloudy water cannot be fixed by chlorine alone.

The order of operations at a communal point is settle or filter, then chlorinate. Let the tank stand so solids settle, or pre-treat heavily turbid intake, before you dose. Chlorinating muddy water to a target residual wastes HTH and still leaves shielded organisms behind. For household-level treatment of genuinely turbid flood water, a flocculant-disinfectant is the better tool, but at a communal tank the discipline is to reduce turbidity at the inlet so the bulk HTH dose can do its job.

How do you keep the residual holding to the cup?

A water point that tests clean at the tapstand still fails if the residual is gone by the time water reaches the household. The South Sudan storage finding is the warning: most stored water in that study had lost its residual entirely, per the Bulletin of the World Health Organization. The CDC household benchmark is a minimum of 0.2 mg/L free chlorine in stored water 24 hours after treatment, per the CDC guidance on making water safe in an emergency, and holding that line is where most communal systems break.

Three things protect the residual to the cup:

• Dose for decay, not just for the standard. Aim toward the upper Sphere range at the tank so there is residual left after the walk home and a night in storage.

• Protect collection and storage. Sphere calls for covered, narrow-necked containers and at least two clean collection containers per household; recontamination during transport and dipping is a leading cause of residual loss.

• Test continuously, not once. Run free residual chlorine checks at the tank and spot-check stored household water, so you are dosing to the reality of the water and not to a number on a form.

Siting helps too. Sphere sets a water point within 500 m of dwellings with no more than 30 minutes of queueing, per the Sphere standards; shorter carries and shorter queues mean less time for the residual to decay and less incentive to draw from an unsafe nearer source.

Where does bulk chlorination fit in a refugee-settlement supply chain?

In Uganda's refugee settlements, where boreholes and tankered supply feed communal tanks across large dispersed sites, HTH is the disinfection backbone. The supply problem is keeping consistent-strength HTH, accurate testing kits and the right storage tanks in country and at the water point, so dosing is done to a known concentration rather than guessed. SLS supplies the full chain for this, the WASH product range covering chlorine products, Oxfam storage tanks and field testing kits, with in-country stock for partners working across Uganda's refugee settlements. The difference between a water point that holds 0.5 mg/L and one that reads zero is rarely the chemistry; it is whether the right products and a working test kit reached the tank.

Frequently asked questions

What free residual chlorine should a communal water point hold?

At least 0.5 mg/L after 30 minutes of contact at pH below 8, and at least 0.2 mg/L at the point of collection, the Sphere water-quality minimum. During outbreaks and where storage decay is high, dose toward the upper end of the range.

How much HTH do I add to a tank?

There is no fixed figure; it depends on tank volume, the percent available chlorine of your HTH, and the water's chlorine demand. Make a 1 percent stock solution, run a jar test to find the dose that leaves 0.5 mg/L after 30 minutes, then scale it to the full tank.

Why does chlorinated water lose its residual before reaching the household?

Chlorine is consumed by organic matter and recontamination during transport and storage. In South Sudan camps, 40 to 58 percent of households drawing from chlorinated tapstands had no detectable residual in stored water, per the Bulletin of the World Health Organization, which is why dosing for decay and protecting storage matter as much as the dose itself.

Can I bulk-chlorinate turbid water?

Not effectively. Suspended solids carry chlorine demand and shield pathogens, so settle or filter the water first, then chlorinate. CDC dosing reflects this, around 2 mg/L for water below 10 NTU and about 4 mg/L above 10 NTU.

How do I confirm the dose worked?

Test the free residual chlorine with a DPD or pool-tester kit before anyone draws water, after the full 30-minute contact time. A reading of at least 0.2 mg/L at the tapstand and a target of 0.5 mg/L in the tank means the dose held; under that, redose.

Specialized Logistics Solutions is an in-country distributor based in Juba, authorised for Aquatabs (Medentech/Kersia), P&G Purifier of Water, Oxfam tanks and Butyl Products, Multiquip and Aussie Pumps, and a UNGM-registered vendor (No. 380716). SLS deployed more than 52 million Aquatabs tablets, a treatment capacity exceeding one billion litres, across South Sudan in the 2024-2025 cholera response, and stocks the chlorine products, storage tanks and testing kits that bulk chlorination at a water point depends on. Request a quotation from SLS to supply your water points from one authorised, in-country source.