16.1 Chemical Feed Systems: Metering Pumps, Dry Feeders & Calibration

Feed Equipment

Solution (Liquid) Feeders

Essential accessories on any solution feed: a calibration cylinder (drawdown tube), a back-pressure valve to give the pump something to work against, a pressure relief valve to protect the pump and piping, an anti-siphon valve to prevent the feed line siphoning the whole day tank into the main, a degassing head or foot valve where the chemical off-gasses (sodium hypochlorite does), a strainer, and an injection quill placed in a turbulent, well-mixed location.

Dry Feeders

Dry chemicals almost always feed into a dissolving or slurry chamber with a controlled water supply and a mixer, then to the point of application. Dry systems fail predictably from bridging or arching in the hopper (solved by a bin vibrator or agitator), flooding (fine material flowing uncontrolled), hygroscopic caking (solved by a dehumidified room or hopper heaters), and plugged slurry lines (solved by adequate velocity, flushing connections, and, for lime, regular acid cleaning).

[!TIP] Gravimetric feeders exist because bulk density lies. A volumetric feeder set to deliver 40 lb/hour of soda ash delivers whatever weight happens to be in that volume. If a new shipment is finer, damper, or more compacted, the actual weight changes and the dose changes with it, silently. A gravimetric feeder measures weight directly and corrects itself.

---

Feed Pacing and Control

[!WARNING] Every chemical feeder must be interlocked to plant flow. If the plant stops and the feeder does not, the chemical accumulates in a static line and is then delivered as a slug when flow resumes. Nearly every documented drinking water chemical overfeed - fluoride, caustic, chlorine - traces to a feeder that kept running into a stopped or greatly reduced flow. Interlocks should be hard-wired, tested on a schedule, and never bypassed for convenience.

---

Calibration by Drawdown

A feeder's dial setting is a claim; a drawdown test is a measurement.

Procedure: isolate the day tank and align the pump suction to a calibration cylinder of known graduation. Run the pump at the setting to be verified for a timed interval. Record the volume drawn down. Return the suction to the day tank.

$\text{Feed rate (gal/day)} = \frac{\text{Volume drawn (mL)}}{\text{Time (min)}} \times \frac{1{,}440}{3{,}785}$

Worked example. A hypochlorite pump draws 420 mL in 5 minutes. Rate = 420 / 5 = 84 mL/min = 84 x 1,440 = 120,960 mL/day = 31.96 gal/day

Now check the dose. The plant is treating 2.2 MGD with 12.5 percent sodium hypochlorite weighing 10.0 lb/gal:

Pounds of chlorine delivered = 31.96 x 10.0 x 0.125 = 39.95 lb/day Dose = 39.95 / (2.2 x 8.34) = 39.95 / 18.35 = 2.18 mg/L

If the target dose was 2.5 mg/L, the pump setting must be increased by a factor of 2.5 / 2.18 = 1.15, and the drawdown repeated to confirm.

Dry feeder calibration uses the same logic with a catch and weigh: divert the discharge into a container for a timed interval, weigh it, and compute pounds per hour or per day.

Calibrate on installation, after any repair or setting change, after a chemical strength or supplier change, and on a routine schedule - monthly is common for critical feeds.

---

Solution Strength Calculations

Two forms recur constantly.

Percent solution:

$\% = \frac{\text{lb of chemical}}{\text{lb of solution}} \times 100$

Example: 60 lb of soda ash in 480 lb of water gives 60 / 540 x 100 = 11.1 percent.

Dilution (mixing) equation:

$C_1 V_1 = C_2 V_2$

Example: how much 12.5 percent hypochlorite is needed to make 60 gallons of 2.0 percent solution? V₁ = (2.0 x 60) / 12.5 = 9.6 gallons of hypochlorite, diluted to 60 gallons total.

Available chlorine and purity must be applied wherever the chemical is not pure:

$\text{lb/day of product} = \frac{\text{Flow (MGD)} \times \text{Dose (mg/L)} \times 8.34}{\text{Purity or available fraction}}$

Example: 3.0 MGD dosed at 2.4 mg/L with 65 percent calcium hypochlorite: Chlorine needed = 3.0 x 2.4 x 8.34 = 60.0 lb/day Product = 60.0 / 0.65 = 92.3 lb/day

---

Common Feed Problems

[!IMPORTANT] Sodium hypochlorite degrades. Strength falls with temperature, light exposure, storage time, and the presence of metal ions, and the degradation also produces chlorate. Store it cool, dark, and no longer than about 30 to 60 days, rotate stock, and verify strength on receipt and periodically thereafter rather than assuming the label. A 12.5 percent product that has aged to 9 percent delivers 28 percent less chlorine at the same pump setting.

---

Chemical Safety and Certification

• NSF/ANSI/CAN Standard 60 certification is required for every direct additive - coagulant, disinfectant, fluoride, caustic, polymer, corrosion inhibitor. NSF/ANSI/CAN 61 covers indirect additives, the materials that contact water: pipe, linings, coatings, gaskets, valves.
• Never mix incompatible chemicals. Sodium hypochlorite plus an acid releases chlorine gas; hypochlorite plus ammonia releases chloramine vapor. Separate storage, separate containment, separate and clearly labeled fill connections, and unique fill fitting types so a delivery driver physically cannot connect to the wrong tank.
• Secondary containment sized for the largest tank plus freeboard, with no floor drain that would carry a spill offsite.
• Eyewash and safety shower within 10 seconds' travel of any corrosive chemical area, tested weekly.
• Compatible materials throughout - PVC, CPVC, PVDF, PTFE, and specific elastomers depending on the chemical. Sodium hypochlorite attacks many metals; caustic attacks aluminum; acids attack concrete.
• Safety data sheets accessible, current, and known to the crew; labeling at every tank, pump, and line.