8.2 Chemical Feed Equipment, Storage & Metering

8.2 Chemical Feed Equipment, Storage & Metering

Chemical feed is where treatment decisions become physical reality. The Need-to-Know Criteria list "perform routine maintenance on the chemical feed equipment," "monitor the chemical feed equipment," "adjust the chemical dosage," "mix batches of chemical solutions," "add chemicals to hoppers and feed equipment," and "transport, store, and feed chemicals" — a dense cluster of scored tasks.

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Why Positive Displacement, Not Centrifugal

A centrifugal pump's output falls as discharge pressure rises. That is unacceptable for chemical feed, where you need a known volume delivered regardless of what the main is doing. Positive displacement pumps move a fixed volume per stroke or revolution, so delivery is essentially independent of discharge pressure.

Output is set by two controls: stroke length (volume per stroke) and stroke frequency or speed (strokes per minute). Best practice is to keep stroke length in the upper part of its range and vary speed, because very short strokes lose accuracy.

Required accessories on a metering pump discharge:

• Pressure relief valve — a positive displacement pump against a closed valve will burst piping
• Back pressure valve — prevents siphoning and ensures consistent check valve seating
• Pulsation dampener — smooths the pulsating discharge
• Calibration cylinder — the means of verifying actual output
• Anti-siphon / injection valve at the point of application

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Calibrating a Metering Pump: the Draw-Down Test

This is the definitive verification and a very likely exam topic.

1. Fill the calibration cylinder and align the pump suction to draw from it.
2. Run the pump at its normal settings for a measured time (commonly 5 minutes).
3. Record the volume drawn down from the cylinder.
4. Convert to a feed rate and compare with the intended rate.

Worked example. A hypochlorite pump draws down 480 mL in 5 minutes.

$\text{Rate} = \frac{480\text{ mL}}{5\text{ min}} = 96\text{ mL/min}$

$96\text{ mL/min} \times \frac{1,440\text{ min/day}}{3,785\text{ mL/gal}} = 36.5\text{ gal/day}$

If the solution is 12.5% available chlorine (about 1.043 lb chlorine per gallon of 12.5% solution at typical strength), the pounds fed per day follow directly, and that can be checked against the calculated demand. A persistent gap between calculated and measured feed is a maintenance signal, usually worn check valves, a torn diaphragm, or gas binding.

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Common Feed Failures

[!WARNING] Sodium hypochlorite off-gasses oxygen, and the resulting bubbles collect at the pump head and cause vapor lock or gas binding — the number one hypochlorite feed problem. Remedies: a flooded suction (tank higher than the pump), a degassing or self-bleeding valve head, short suction lines, and keeping the solution cool and out of sunlight. Hypochlorite also decomposes with heat, light, and time, losing strength; a solution that has sat all summer in a hot room may be well below its labeled strength, which shows up as an apparent underfeed.

Calcium carbonate scale forms where hypochlorite (high pH) meets hard water at the injection point, plugging the injection quill. Using softened dilution water and periodic acid cleaning both help.

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Dry Chemical Feeders

Dry feeders discharge into a dissolving chamber or slurry tank with a mixer and adequate detention to fully wet and dissolve the chemical before it reaches the process.

Chronic problems:

• Bridging and arching in hoppers, especially with hygroscopic chemicals such as soda ash and lime. Bin vibrators, live-bottom bins, and hopper agitators address it.
• Dusting during delivery and hopper filling — a health hazard requiring dust collection and respiratory protection.
• Slaking for quicklime (CaO), which must be hydrated in a slaker to calcium hydroxide before use, generating substantial heat. Hydrated lime (Ca(OH)₂) skips this step.

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Solution Preparation and Day Tanks

A day tank holds roughly one day's supply of prepared solution. It provides a defined, measurable volume so that consumption can be verified against calculation, and it limits how much chemical can be delivered in a failure.

Solution strength calculation:

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

Worked example. Dissolving 45 lb of dry polymer into 500 gallons of water:

$500\text{ gal} \times 8.34 = 4,170\text{ lb of water}$ $\% = \frac{45}{45 + 4,170} \times 100 = 1.07\%$

Polymer requires aging after mixing — commonly 30 to 60 minutes — for the long-chain molecules to fully uncoil and become active. Feeding polymer immediately after mixing wastes chemical and underperforms.

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Storage, Containment, and Segregation

• Secondary containment must hold at least the volume of the largest tank, and commonly 110% of it.
• Chemically compatible materials throughout: fluorosilicic acid and hypochlorite attack ordinary metals; hypochlorite degrades some plastics under ultraviolet exposure.
• Separate fill connections, clearly and uniquely labeled, for every chemical.
• Segregate incompatible chemicals in separate containment.

[!WARNING] Mixing sodium hypochlorite with any acid liberates chlorine gas. The classic and repeatedly fatal incident is a delivery driver connecting an acid hose to a hypochlorite fill connection. Prevention is physical, not procedural: unique, non-interchangeable fill fittings, distinct locations, unmistakable labeling, and supervised deliveries. Hypochlorite plus ammonia produces chloramine vapors; hypochlorite plus organics can ignite.

Every chemical needs an accessible safety data sheet, and the storage area needs an emergency eyewash and safety shower with appropriate ventilation.