Chlorine is our workhorse disinfectant — but "3 ppm on the meter" means nothing until you know which chlorine you measured. This covers when disinfection is called for, the difference between free and combined chlorine, which one to test for and when, and how to validate that the injection rate is actually landing where you set it.
Narrated overview of this module. The article below has the full procedure and the numbers.
Chlorine isn't a default additive — it's dosed for a reason. The common ones on our systems:
Public/community water systems must carry a residual. Chlorine provides both primary disinfection (the kill) and a lasting residual that protects water through storage and distribution to the tap.
New wells, repaired wells, or any positive bacteria hit get shock chlorination, then a maintained residual if the source stays suspect. Also knocks back iron/sulfur bacteria that foul equipment.
Chlorine controls biological growth in tanks and piping. Thin-film composite RO membranes are destroyed by free chlorine — so where chlorine is used ahead of an RO, it's fully dechlorinated (SBS or carbon) before the membranes. Verifying zero free chlorine at the membrane is its own critical check.
After construction, media changeouts, or tank service, we sanitize to a high dose, hold contact time, then flush and confirm a safe residual before returning to service.
Add chlorine to water carrying ammonia and organics, and the first chlorine you add doesn't disinfect — it reacts. It forms chloramines (combined chlorine), then, as you add more, destroys them. Only after that demand is satisfied — the breakpoint — does added chlorine start showing up as free residual. Dose below breakpoint and your free chlorine reads near zero no matter how much you're feeding.
Practical read: if your free chlorine won't climb, you may be stuck left of the breakpoint. The fix is usually more chlorine to push past demand — not less — but confirm with the free/total gap before you chase the dial.
Both are DPD tests and take seconds (free = DPD-1; total = DPD-1 + DPD-3, or the total tablet). The question is which number answers what you're asking:
Disinfection is dose and time together. The regulatory yardstick is CT — concentration of residual multiplied by the contact time the water sees before the first user. A modest residual with enough contact time can outperform a high residual with none.
Required CT rises as water gets colder and as pH climbs (HOCl, the potent form, gives way to weaker OCl⁻ above ~pH 7.5). This is why free chlorine paired with a contact tank or a long main beats a residual measured with no time behind it — and why we design contact volume in, not just dose.
Same discipline as any chemical feed: set the pump to a calculated rate, then prove the result two ways — the pump's mechanical output (drawdown) and the water's actual response (residual test). Both have to agree.
The demand-plus-residual you want, converted to a pump rate off your hypochlorite strength:
Example — 50 gpm, target 2.0 ppm, 12.5% sodium hypochlorite: (50 × 3.785 × 2.0) ÷ (10,000 × 0.125) = 0.30 ml/min of neat bleach. As with antiscalant, dilute the hypochlorite to bring that into the pump's accurate range — and remember bleach loses strength on the shelf, so dose to measured strength, not the label.
Neat hypochlorite feed rate + the drawdown you should measure in 60 seconds.
Illustrative — dose to measured hypochlorite strength; confirm targets against the site's disinfection requirement.
Free chlorine is the protection you still have. Total is what you started with — always know which one the meter just told you.
