Hypochlorite Feed Drifts After Six Weeks in a Warm Room

Step 1 - chlorine dose required:

• lb/day Cl2 = mg/L x MGD x 8.34 = 2.2 x 4.5 x 8.34 = 82.6 lb/day as Cl2

Step 2 - convert to product weight:

• At 12.5 percent available chlorine: 82.6 / 0.125 = 661 lb/day of solution

Step 3 - convert to volume:

• 1 gal weighs 8.34 x 1.17 = 9.76 lb
• 661 / 9.76 = 67.7 gal/day
• In gph: 67.7 / 24 = 2.82 gph
• In mL/min: 2.82 gal/h x 3,785 mL/gal / 60 min = 178 mL/min

Step 4 - compare to pump:

• Current output 2.45 gph versus 2.82 gph required. The pump is running about 13 percent low, which is close to the residual drop you are seeing but does not explain all of it.
• 2.82 gph on a 0 to 5.0 gph pump = 2.82 / 5.0 = about 56 percent stroke

So at full strength, the pump was set almost exactly right. That is the important finding: the setting did not drift, the chemical did. Note also that the required output sits at 56 percent of the pump's range, which is fine, but if the strength falls further the pump will run out of range.

New requirement at 9.0 percent:

• Product weight: 82.6 / 0.09 = 918 lb/day
• Volume: 918 / 9.76 = 94.1 gal/day
• In gph: 94.1 / 24 = 3.92 gph
• Stroke needed: 3.92 / 5.0 = 78 percent of range

The pump can physically do it, but 78 percent of range leaves little room for a further strength loss or a higher seasonal dose, and control near the top of the range is poorer. Chasing chemical decay with pump stroke is a treadmill.

Why the strength dropped:

• Sodium hypochlorite decays continuously. The rate roughly doubles for every 10 degrees C (18 degrees F) of storage temperature; 85 degrees F is far too warm for 12.5 percent product.
• Sunlight and metal contamination (iron, copper, nickel fittings, or an unlined drum) catalyze decomposition, as does dilution with water containing trace metals.
• At 9.0 percent after six weeks, the loss is consistent with warm storage plus age. This is normal degradation, not a bad batch.

What to do before touching the stroke:

1. Do not just raise the stroke. Raising it to 78 percent means the next drum at 8 percent puts you at 88 percent, and then you have no control margin.
2. Fix the storage first. Move the drum to a cool, shaded, ventilated area, out of direct sun, and keep the room below 80 degrees F if possible. Order smaller, more frequent deliveries so chemical age stays under 30-60 days.
3. Titrate every delivery and log the measured strength, then compute the stroke setting from the measured number rather than the label. This is the single most valuable habit in the whole program.
4. Recheck the calibration. The drawdown showed 2.45 gph at 55 percent, so the pump itself was honest; still re-verify after any adjustment, because drive-tube wear and check-valve fouling change output over time.
5. Consider a switch to 15 percent or to on-site generation only after the storage and titration controls are in place; a stronger product decays faster and is more hazardous.

Interim action: bring in fresh chemical, verify strength, set the pump from the calculated rate, then confirm with a residual and a confirmatory titration a few hours later.

Safety checks:

• Ventilation and temperature. Hypochlorite releases chlorine and chloramine vapors, especially when it decomposes in warm conditions. The room needs continuous or demand ventilation, and 85 degrees F is both a product-quality problem and a worker-exposure problem.
• Incompatible storage. No acids, no ammonia, no reducing agents, and no organics in the same area. Acid plus hypochlorite equals chlorine gas; ammonia plus hypochlorite equals chloramine gas. Check the whole room, not just the adjacent shelf.
• Secondary containment. All drums and the day tank in containment sized for the largest container plus the ability to contain a spill; no floor drains running to soil or storm sewer.
• Eyewash and safety shower within 10 seconds' travel, unobstructed, and tested per your schedule. 12.5 percent hypochlorite causes severe eye damage.
• PPE for anyone handling product: face shield or goggles, chemical-resistant gloves, apron. Confirm the PPE is actually stocked at the room, not in a locked cabinet elsewhere.
• Labels, SDS, and spill kit present and current; drum bungs closed and drums not stored in sunlight.
• Ladder and drum handling - 350 gallons remaining in a drum moved by hand is a strain and a spill waiting to happen; use a drum pump and a cart.

Records to write:

• Date, time, and measured strength of each drum received (the 9.0 percent titration), with the identity of the analyst.
• Metering pump setting, calibration drawdown results, and any adjustment made, with the calculation behind it.
• Daily free chlorine residual leaving the plant, dose in lb/day, and gallons of hypochlorite used, so the chemical usage reconciles against production and strength.
• The deviation: residual fell to 0.55 mg/L against target. Record the cause, the corrective action, and the timeline, and check whether the drop took you below any state minimum residual requirement and whether notification is required.
• Temperature log for the chemical room if you are tracking the decay problem, and the maintenance or relocation action taken.
• Any spill, near miss, or eyewash activation, on the required forms.