Almost every disinfection failure described as “the chlorine does not work” is one of three things: the dose was calculated against the target residual without allowing for the water’s chlorine demand, the pH put most of the chlorine into an ineffective form, or the contact time was far shorter than the organism needed. This article is the operational companion to our NaDCC technical article: how to measure demand, how to run the dose calculation, how to prepare and dose a stock solution, how to monitor what you actually achieved, and what each application on a plant or a farm actually requires.
1. Demand, residual and contact time: three quantities, not one
| Quantity | Definition | How it is measured | Why it matters |
|---|---|---|---|
| Chlorine demand | the chlorine consumed by the water itself – organics, ammonia, sulphide, reduced iron and manganese, biofilm | dose to a residual and subtract; or measure by difference after a contact period | if demand exceeds the dose, no residual appears and no disinfection occurs |
| Free residual | chlorine still available as HOCl and OCl– after the demand is satisfied | DPD free chlorine test | this is the number that protects the water, not the dose |
| Contact time (CT) | the time the residual is in contact with the water before use | calculated from flow and volume; verified with a tracer test in large systems | a residual that lasts 30 seconds kills far less than the same residual lasting 30 minutes |
The practical rule is to dose to (target free residual + demand) and then verify the residual. Dosing to the target alone only works if demand is negligible, which is rarely true outside clean municipal water.
2. Measuring demand before you dose
The simplest reliable method is a jar test with the actual product:
- Take a measured volume of the water, typically 1 litre.
- Add a known mass of product, mix, and wait the contact time the process will use.
- Measure free residual. If none is detectable, the dose was entirely consumed – increase and repeat.
- Repeat until a residual is measurable, then compute the demand as the difference between what you added and what remained.
A quick estimate for a well-characterised water can be made from the main consumers: ammonia nitrogen (about 7.6 mg of chlorine per mg of ammonia nitrogen to reach breakpoint), sulphide (about 8.4 mg of chlorine per mg of sulphide), ferrous iron and manganese (about 0.6 – 1.3 mg of chlorine per mg of metal). These are stoichiometric estimates, not design values – the jar test is the confirmation.
3. Breakpoint chlorination
In water containing ammonia, chlorine first forms monochloramine and then dichloramine, both weak disinfectants and both the source of the “swimming pool” odour, before further chlorine destroys them and free residual appears. The dose at which free residual starts to climb is the breakpoint.
| Zone | What is present | Effect | What to do |
|---|---|---|---|
| Below breakpoint | monochloramine, dichloramine | weak disinfection, strong odour, poor residual response | either stay deliberately in the chloramine zone for taste/odour control, or push through |
| At breakpoint | chloramines destroyed | residual starts to rise steeply with dose | the target of a “breakpoint” shock treatment |
| Above breakpoint | free chlorine dominates | effective disinfection, measurable free residual | the normal operating zone for drinking water and pool shock |
This is why pool shock dosing works as a large single dose rather than a gentle increase: the objective is to get past the chloramine zone in one step. It is also why dosing small amounts into ammonia-containing water produces odour complaints without disinfection.
4. Application by application
| Application | Typical target | Contact time | Practical notes |
|---|---|---|---|
| Drinking water, distribution | residual maintained through the network, set by local regulation | 30 minutes minimum before first use in most guidance | product must be approved for drinking water in the destination market; by-product limits apply |
| Emergency / point-of-use water | clear water treated to a measurable residual | 30 minutes, covered vessel | turbidity must be low first; cloudy water consumes chlorine and shelters organisms |
| Swimming pool, routine | free residual in the range set by local pool codes | continuous circulation, not a contact tank | check cyanuric acid and pH before adjusting dose |
| Swimming pool, shock | large single dose to breakpoint | with circulation, pool closed until residual returns to normal | brush surfaces and run the filter; shock oxidises, it does not remove debris |
| Livestock drinking lines | low residual, set by the species and the local guidance | continuous through the line | flush lines first; organic film in the line consumes most of the dose |
| Footbath and vehicle dip | higher concentration, short life | seconds of contact | renew frequently – organic load and dirt kill the bath before the dose does |
| Surface and spill disinfection | contact concentration with visible wetting | contact time stated on the label | blood and other protein soil consumes chlorine; clean before disinfecting |
| Cooling tower shock | intermittent high dose to a target residual | hours, then decay | discharge consent applies to chlorinated blowdown; corrosion inhibitors may be attacked |
| Wastewater, final effluent | residual set by discharge consent | contact tank volume / flow | dechlorinate after the contact tank if the consent requires it |
| Food plant CIP final rinse | approved concentration for food-contact surfaces | per the sanitation procedure | rinse or drain as required; verify by test strip at the point of use |
5. Worked dose calculations
All calculations use available chlorine: a 56% grade delivers 0.56 kg of available chlorine per kilogram of product.
| Case | Given | Calculation | Result |
|---|---|---|---|
| Batch chlorination, clean water | 10 m3, demand 1 mg/L, target residual 1 mg/L, so dose for 2 mg/L | 2 g/m3 x 10 = 20 g chlorine; 20 / 0.56 | 35.7 g of product |
| Batch chlorination, turbid water | 10 m3, demand 6 mg/L, target 1 mg/L, so dose for 7 mg/L | 7 x 10 = 70 g chlorine; 70 / 0.56 | 125 g of product |
| Pool shock | 100 m3, shock to 50 mg/L | 50 x 100 = 5,000 g chlorine; 5,000 / 0.56 | 8.9 kg of product |
| Cooling tower shock | 200 m3 system, 10 mg/L | 10 x 200 = 2,000 g chlorine; 2,000 / 0.56 | 3.6 kg of product |
| Stock solution | make 100 L at 1 g/L available chlorine | 100 g chlorine; 100 / 0.56 | 179 g of product per 100 L |
Note how the same ten cubic metres needs 35.7 g or 125 g depending on turbidity. That factor of three is the whole reason demand has to be measured rather than assumed.
6. Preparing and dosing a stock solution
| Step | Practice | Reason |
|---|---|---|
| Water | use cool, low-hardness water; add the solid to the water, never water to the solid | avoids local overheating and gas release; hardness can precipitate carbonate at high pH |
| Concentration | keep stock solutions at a few grams per litre of available chlorine, not saturated | stronger solutions decay faster and are hazardous to handle |
| Vessel and lines | plastic (PE, PP, PVC) or suitable stainless; avoid carbon steel and copper alloys | hypochlorous acid attacks most metals, especially at low pH |
| Cover and cool | covered, out of sunlight, used within a few days | chlorine is lost to light, heat and evaporation; solution strength is not stable |
| Agitation | stir until fully dissolved, then let any residue settle before drawing from the tank | undissolved granules block dosing lines and cause local over-dosing |
| Dosing equipment | metering pump with chlorine-compatible seals (PTFE, Viton), plus a check valve | standard elastomers harden and fail |
| Tablet feeders | floating or in-line erosion feeders sized to the flow | tablet dissolution rate depends on flow, temperature and tablet size |
7. Monitoring: what to measure and what the numbers mean
| Measurement | Method | What it tells you |
|---|---|---|
| Free chlorine | DPD colorimetric test, portable or in-line analyser | the fraction that actually disinfects; the number to control |
| Total chlorine | DPD with iodide, or the same kit in total mode | free plus combined; total minus free estimates chloramines |
| Combined chlorine | calculated as total minus free | a rising value means ammonia or amine ingress, or an under-dosed pool |
| pH | meter or test strip | controls the HOCl/OCl– split; above pH 7.5 efficiency drops sharply |
| Cyanuric acid | turbidimetric test | only relevant for stabilised donors (NaDCC, TCCA); high values bind free chlorine |
| Microbial count | heterotrophic plate count, ATP, or organism-specific tests | the outcome measure; residual alone does not prove disinfection |
Two habits make monitoring useful. Measure at the point of use rather than at the dosing point – residual at the outlet of a contact tank says nothing about residual at the far end of a distribution line. And record the demand alongside the dose, so that a rising demand trend (biofilm forming, ammonia ingress) is visible before it becomes a compliance failure.
8. Common errors and their signatures
| Error | Signature | Corrective action |
|---|---|---|
| Dosing to target residual without allowing for demand | no measurable residual after dosing; repeat doses have no effect | measure demand by jar test; dose to demand plus target |
| Disinfecting turbid water | residual disappears, organisms survive | clarify or filter first; chlorine is a disinfectant, not a coagulant |
| Dosing into water at pH above 8 | residual measurable but disinfection poor | correct pH, or accept a much higher dose and cost |
| Small doses into ammonia-containing water | odour, no free residual | go to breakpoint in one step, or remove the ammonia source |
| Using a stabilised donor in a system that is never drained | dose requirement creeps up over months | test cyanuric acid; part-drain, or switch to a non-stabilised donor |
| Storing a stock solution for weeks | test strip shows a lower figure than expected | prepare fresh; assay the stock rather than trusting the label |
| Discharging chlorinated water | consent breach, fish kill | dechlorinate with sodium metabisulphite or allow full decay; measure at the outfall |
9. FAQ
How much NaDCC do I add to one cubic metre of water?
For a target of 2 mg/L available chlorine in clean water, 2 g of chlorine, which is 2 / 0.56 = 3.6 g of a 56% product. If the water is turbid, measure its demand first – it can easily be three times that figure.
Why does my test kit show total chlorine but no free chlorine?
Because the chlorine has combined with ammonia or amines to form chloramines. The free residual is zero, disinfection is poor, and the treatment needs either breakpoint dosing or removal of the ammonia source.
Can I dose the solid directly into a tank?
Not recommended. Pre-dissolve in a small volume of water and add the solution, both for uniformity and to avoid undissolved granules sitting on a liner or at the bottom of a tank.
How often should I change a footbath?
When the measured concentration falls below the target, which in a busy footbath with organic load can be several times a day. Dosing on a schedule without measuring is the usual reason footbaths stop working.
Is a higher residual always better?
No. Above the required residual you add corrosion, by-product formation, odour and cost without improving disinfection, and in drinking water you may breach a limit. The objective is the lowest residual that achieves the microbial target with margin.
What do I do with leftover stock solution?
Use it or dechlorinate it before disposal – sodium metabisulphite or sodium thiosulphate will neutralise it. Never discharge a strong chlorine solution to a drain or surface water without neutralisation.
Conclusion
Dose to demand plus target, verify the residual at the point of use, control pH, and watch the cyanuric acid balance if you are using a stabilised donor. Those four habits prevent most of the failures listed above. For the chemistry and specification see the NaDCC technical article and the NaDCC product page; for choosing between donors see chlorine-releasing disinfectant selection; for dechlorination see sodium metabisulphite for dechlorination.