Industrial Chlorination and Disinfection — Dose, Contact Time and Residual
Direct answer
Disinfection is not a dose, it is a CT product: concentration of free residual multiplied by contact time. A plant achieving 0.5 mg/L free chlorine for 40 minutes delivers the same CT as 1.0 mg/L for 20 minutes, and both fail if short circuiting in the contact tank means the real detention is a third of the nominal. Choose the chlorine source by logistics and water chemistry, then design the CT. Sodium hypochlorite (10–12 % available chlorine) is the default for continuous dosing because it pumps, but it degrades 2–5 % of strength per month at 20 °C and much faster above 30 °C or in sunlight, so a summer delivery held eight weeks is not the product you costed. Calcium hypochlorite (65–70 %) ships dry, stores for years and cuts freight dramatically, but adds calcium hardness and is a genuine fire hazard in contact with organics. Chlorinated isocyanurates (TCCA, SDIC) carry stabiliser and belong in outdoor pool and remote-site duty, not in potable systems. Chlorine dioxide is the tool for biofilm, high-pH water and THM-sensitive supplies, and it must be generated on site.
Comparison
| Sodium hypochlorite | Calcium hypochlorite | TCCA / SDIC | Chlorine dioxide | |
|---|---|---|---|---|
| Available chlorine | 10–12 % | 65–70 % | 90 % / 56 % | Generated on site |
| Storage life | 1–3 months usable | 2+ years dry | 2+ years dry | None — make and use |
| pH effect | Raises pH | Raises pH | Lowers pH | Near neutral |
| Adds | Sodium, chlorate on ageing | Calcium hardness | Cyanuric acid | Chlorite residual |
| Best for | Continuous municipal / industrial dosing | Remote sites, freight-limited supply | Outdoor pools, sanitising tablets | Biofilm, high pH, low THM duty |
Getting the residual you actually need
Chlorine demand is consumed first by iron, manganese, sulphide, ammonia and organics; only what survives is free residual. Breakpoint chlorination requires roughly 7.6 parts chlorine per part ammonia-nitrogen, and dosing below breakpoint leaves chloramines that smell strongly and disinfect weakly — the classic "over-chlorinated" complaint is almost always under-chlorination. Efficacy also collapses with pH: at pH 7.0 about 75 % of free chlorine is hypochlorous acid, at pH 8.0 only about 22 %. If your water sits at pH 8.2, either correct pH ahead of the contact tank or accept roughly four times the dose for the same kill.
By-products and limits
Free chlorine reacting with natural organic matter forms trihalomethanes and haloacetic acids. Reduce them by removing organics upstream through coagulation rather than by cutting the residual below a safe level. Ageing hypochlorite generates chlorate; buying fresher product in smaller, more frequent deliveries is cheaper than a chlorate exceedance. Where potable water is involved, confirm NSF/ANSI 60 certification on the certificate of analysis for every lot.
Handling
Never store or dose hypochlorite near acid — the reaction releases chlorine gas immediately. Keep hypochlorite in vented HDPE out of sunlight; keep calcium hypochlorite dry, cool and away from oils, glycols and organic dust. Use dedicated transfer equipment for each product. Degassing at the pump head is the most common cause of erratic dosing on hypochlorite skids: fit a flooded suction and a degassing pump head.
What to send with your enquiry
Flow rate, chlorine demand or raw water analysis, target free residual and CT, pH, storage temperature and turnover, delivery form and certification required (NSF/ANSI 60, food contact, pool grade). Tell us your consumption per month so the strength and pack size match your turnover instead of your storage capacity.