8.2 Chemical Dosage & Feed Equipment

8.2 Chemical Dosage & Feed Equipment

Quick Answer: To feed any chemical you need the same equation: pounds per day = dose (mg/L) × flow (MGD) × 8.34. The 8.34 factor is the weight of one gallon of water (8.34 lb/gal). You will use this formula dozens of times on the Class I exam. Chlorine itself comes as gas, sodium hypochlorite solution, or calcium hypochlorite solid, each with its own feed equipment and safety considerations.

The Chemical Feed Formula

The single most important dosage equation on this exam is:

Feed (lb/day) = Dose (mg/L) × Flow (MGD) × 8.34 lb/gal

• Dose is the target concentration in milligrams per liter (equivalent to parts per million, ppm).
• Flow is the treated flow rate in million gallons per day (MGD).
• 8.34 is the weight of one gallon of water in pounds, which converts mg/L (a concentration) into pounds of chemical per day of flow.

Worked Example

A booster station feeds chlorine at 2.0 mg/L into a flow of 0.75 MGD. What is the required chlorine feed in lb/day?

Feed = 2.0 mg/L × 0.75 MGD × 8.34 = 12.51 lb/day

Reverse the equation when you need to find dose from a known feed rate:

Dose (mg/L) = Feed (lb/day) ÷ [Flow (MGD) × 8.34]

Chlorine Demand and Residual

Recall from Section 8.1 that chlorine dose = chlorine demand + chlorine residual. If you dose at 2.5 mg/L and the demand is 1.7 mg/L, the residual leaving the plant is 0.8 mg/L. To raise the residual, you raise the dose — and the feed equation tells you the new lb/day setting.

Forms of Chlorine

Available chlorine is a convention that compares the oxidizing power of a compound to pure Cl₂. Gas chlorine is 100%; a 12.5% NaOCl solution delivers about 12.5% of its weight as equivalent chlorine; Ca(OCl)₂ delivers about 65%.

Because sodium hypochlorite degrades, you cannot assume the label strength after months of storage. Operators who switch from gas to hypochlorite must increase feed pump output to deliver the same chlorine dose, and they re-test residuals more often.

Feed Equipment

• Metering pumps — Positive-displacement pumps, most commonly diaphragm or peristaltic, deliver a measured chemical slug per stroke. Output is adjusted by stroke length and stroke frequency.
• Day tanks — Hold a diluted or day's supply of chemical; let the operator see consumption visually and prevent overfeed from a bulk tank.
• Solution tanks — Where dry calcium hypochlorite is dissolved, or where sodium hypochlorite is diluted before being pumped.
• Scales — Used under cylinders or day tanks to verify actual mass fed. For gas chlorination, the loss-in-weight method (weighing the cylinder over time) is the most reliable feed check.

Calibration: The Drawdown Test

A metering pump's nameplate output is only a starting point. Real output is verified with a drawdown (calibration) test:

1. Isolate a known volume in a calibrated sight glass or burette on the pump suction.
2. Run the pump for a measured time (e.g., 60 seconds).
3. Read the volume lost from the sight glass.
4. Compute actual output: volume ÷ time, then convert to lb/day or gal/day.

Repeating the test at several stroke settings produces a calibration curve so the operator can set the pump to deliver a target dose. Drift from the curve signals worn tubing, clogged check valves, or gas locking — all of which an entry-level operator should recognize and report.

Maintaining an Adequate Chemical Supply

Running out of disinfectant is a violation and a public-health event, so the Criteria list maintaining an adequate supply of chemicals as an operator task in its own right. The arithmetic is simple and worth doing monthly:

Days of supply = chemical on hand ÷ average daily use

A system feeding 30 lb/day of chlorine with six 150-lb cylinders on hand holds 900 lb, or 900 ÷ 30 = 30 days. Compare that with the supplier's lead time, then set a reorder point with margin — commonly reordering at roughly twice the lead time, so a two-week lead time triggers an order at about 28 days remaining.

Practical rules for a small system:

• Track usage, not just deliveries. Daily feed rate from the log is what predicts the next order; a single hot week can double it.
• Order against peak, not average. Summer maximum-day demand raises pounds per day even at a constant dose.
• Watch shelf life. Sodium hypochlorite degrades with time, heat, and light — a 12.5% solution can lose meaningful strength in a matter of weeks in a hot shed, so buy smaller quantities more often and store it cool and dark. Calcium hypochlorite and chlorine gas are far more stable.
• Rotate stock first-in, first-out, and date every container on receipt.
• Keep an emergency reserve so a missed delivery or a supplier outage never forces the system to feed below its required residual.

Chlorine Gas Safety

Chlorine gas is a respiratory hazard. Even small releases irritate the eyes, throat, and lungs; larger releases are life-threatening. Class I operators may not change cylinders on every shift, but they must know the rules:

• Personal protective equipment (PPE) and self-contained breathing apparatus (SCBA) are required for entry into a chlorine room during a release.
• Ventilation — Chlorine rooms need forced, bottom-exhaust ventilation (chlorine is heavier than air) and a powered exhaust fan actuated from outside the room.
• Leak detection — A chlorine leak is located with the ammonia wand test: hold a squeeze bottle of dilute ammonia solution near joints and valve stems. Where chlorine gas is leaking, the two react to form white monochloramine vapor (NH₂Cl) — a visible white cloud that pinpoints the leak.
• Cylinder handling — Chains or brackets secure cylinders upright; a leaking cylinder can be rotated so the leak is on top, since chlorine gas exits above and the liquid is below.
• Emergency response — The chlorine institute's kit (Kit A for 100/150-lb cylinders, Kit B for ton containers) caps or patches leaks; never apply water directly to a chlorine leak, which accelerates corrosion.