3.3 Chlorination & Dechlorination Feed Equipment
Key Takeaways
- Chemical-feed safety, actual feed verification, contact, and residual measurement form one control system.
- Chlorine dose depends on mass feed, flow, and active product strength.
- Dechlorination is verified by residual, not assumed from pump operation.
- Gas and liquid systems have different hazards but both require compatible equipment and emergency controls.
3.3 Chlorination & Dechlorination Feed Equipment
2025 WPI alignment: This section teaches chlorination equipment for chlorine, hypochlorite, or chloramine and dechlorination equipment for bisulfite, thiosulfate, or sulfur dioxide in the official Equipment Evaluation, Maintenance, and/or Operation content area.
Why this job task matters
Chemical disinfection equipment must meter a known active dose, provide safe containment and contact, verify residual, and remove excess chlorine when the permit requires dechlorination.
Core operating concepts
| Concept | What the operator must understand |
|---|---|
| Chlorine gas system | Vacuum regulators, ejectors, scales, valves, detectors, ventilation, and scrubber or response provisions manage a highly toxic compressed chemical. |
| Sodium hypochlorite | Liquid feed avoids gas cylinders but degrades with heat, light, age, and contamination and can release gas or crystallize. |
| Chloramine | Combined residual forms when chlorine reacts with ammonia; the intended chemistry and permit govern its use. |
| Dechlorination | Sulfur dioxide, sodium bisulfite, or sodium thiosulfate reduces chlorine residual before discharge when required. |
| Feed pacing | Dose may be paced to flow and trimmed by residual, but analyzers, sample transport, and lag must be understood. |
| Containment and compatibility | Tubing, gaskets, pumps, ventilation, secondary containment, eyewash/shower, and PPE must suit the chemical. |
Operating and maintenance workflow
- Verify storage quantity, scale or level, leaks, ventilation, detectors, containment, feed-pump or gas-feed status, and emergency equipment.
- Confirm flow signal, target dose, active product strength, calibration-column result, and chemical-pump stroke or speed.
- Check contact-basin level, mixer and baffle condition, sample-line flow, residual analyzer, and an independent grab residual.
- Trend feed rate, calculated dose, demand, residual, microbial result, and dechlorination response together.
- Respond to leaks or alarms under the chemical emergency plan; do not enter an unsafe atmosphere to investigate.
- Document chemical delivery, batch strength, calibration, maintenance, analyzer checks, and abnormal residual or discharge events.
Diagnostic evidence
| Signal | Likely meaning | Defensible first response |
|---|---|---|
| Dose calculation is normal but residual falls | Demand increased, product weakened, feed failed, or measurement is wrong | Verify product, actual feed, flow, sample, and demand before increasing dose. |
| Analyzer exceeds grab result | Sample line, reagents, calibration, or analyzer condition may be wrong | Use a fresh approved grab method and service the analyzer path. |
| Residual remains high after dechlorination | Reducing-agent feed, mixing, pacing, or analyzer lag may be inadequate | Verify actual chemical delivery and contact before changing the ratio. |
| Hypochlorite pump loses output | Air/gas binding, crystallized checks, empty tank, or suction leak may exist | Isolate as needed and inspect the metering system under the SOP. |
Calculation, control, or records connection
Use the WPI feed-rate relationship: lb/day = dose (mg/L) × flow (MGD) × 8.34 / purity as a decimal when purity correction applies. Keep “as delivered product” separate from “active chemical.” A 12.5 percent solution is 0.125 in a purity denominator, not 12.5. A calibration-column draw verifies actual volume delivered; a stroke setting alone does not prove feed.
Worked operator scenario
Effluent residual rises even though the chlorine pump setting is unchanged. Flow has dropped by half, and dechlorination pacing stayed fixed. The operator verifies both analyzers with grab tests, recognizes that a constant mass feed at lower flow doubles the nominal dose, and corrects flow pacing under the SOP. Simply increasing bisulfite could hide the chlorine-control fault and waste chemical.
Common exam traps
- A pump’s percent stroke is not the same as chemical dose in mg/L.
- Never treat a chlorine detector alarm as a routine instrument nuisance until the atmosphere is proven safe.
- Dechlorination needs mixing and measurement; feeding a reducing agent does not by itself prove zero residual.
- Use active strength as a decimal and preserve units in purity-corrected calculations.
Field-to-exam checklist
- Chemical-feed safety, actual feed verification, contact, and residual measurement form one control system.
- Chlorine dose depends on mass feed, flow, and active product strength.
- Dechlorination is verified by residual, not assumed from pump operation.
- Gas and liquid systems have different hazards but both require compatible equipment and emergency controls.
Inventory reconciliation is an independent feed check: unexplained disagreement among scale weight, pump calibration, flow-paced demand, and residual warrants investigation before changing the setpoint.
Gas-feed architecture and hypochlorite decay
Gas chlorine is fed under vacuum, not pressure. The vacuum regulator mounts directly on the cylinder or ton-container valve, the rotameter meters the gas, and the ejector — driven by motive water — creates the vacuum that pulls gas through the whole train. The safety logic follows from that arrangement: if motive water is lost, vacuum collapses and the regulator stops gas flow rather than releasing it. A vacuum-break or check assembly prevents water from backing into the gas line.
Ton containers carry two valves on the head. In the normal orientation the upper valve draws gas and the lower valve draws liquid; withdrawing from the wrong valve floods the regulator with liquid chlorine, which then vaporizes downstream and damages the equipment. Container orientation is therefore part of the delivery checklist, not a detail.
Sodium hypochlorite degrades in storage. Strength is usually quoted as trade percent (available chlorine per volume) rather than weight percent, and the two are not the same number. Decomposition accelerates with heat, ultraviolet light, elapsed time, and trace metal contamination such as iron or copper. Two operational consequences follow: the actual dose drops even though the pump setting has not changed, and the decomposition releases oxygen gas that collects at the pump head and vapor-locks a diaphragm metering pump. Store cool and dark, rotate stock, degas the pump head, and re-titrate the strength of a delivered batch rather than trusting the bill of lading.
Worked product feed. 3.0 MGD dosed at 6.0 mg/L needs 6.0 x 3.0 x 8.34 = 150 lb/day of available chlorine. At 12.5 percent trade strength, the as-delivered product requirement is 150 / 0.125 = 1,200 lb/day of solution — the purity denominator is 0.125, never 12.5.
A constant chlorine mass feed continues while plant flow drops by half. What happens to the nominal dose in mg/L?
What best verifies that a dechlorination system is accomplishing its purpose?