13.1 Chemical Addition Systems & Feed Equipment (Subclass 8)
Key Takeaways
- Positive displacement diaphragm metering pumps deliver a fixed volume per stroke and are calibrated by adjusting stroke length and stroke frequency, so output is verified by drawdown against a calibration cylinder rather than assumed from the dial setting.
- A day tank holds no more than one day of chemical use, is filled from bulk storage under operator control, and provides the volume reference used for drawdown calibration and for detecting a leak.
- Injection points must be located where mixing is immediate and where the chemical will not damage downstream equipment, and each requires a corporation stop or removable quill so the injector can be cleared of scale.
- Sodium hypochlorite off-gases and forms scale in feed lines, so hypochlorite systems use degassing pump heads, flooded suction and periodic acid cleaning of the injector.
- Acids and hypochlorite must never share containment, since mixing them liberates chlorine gas, and every chemical system requires compatible materials, secondary containment, eyewash and safety shower, and a current safety data sheet.
What Subclass 8 Actually Tests
Chemical Addition is one of the larger drinking water subclass examinations at 40 questions, and it appears at nearly every plant because almost every treatment technology feeds something. The knowledge statements center on the ability to adjust chemical feed rates, confirm chemical strength, calibrate equipment, prepare chemicals, and interpret safety data sheets. Dosage arithmetic is covered with the mathematics chapter; this section covers the hardware and the practice.
Feed Equipment by Type
| Equipment | Principle | Typical service | Control range |
|---|---|---|---|
| Diaphragm metering pump | Positive displacement; fixed volume per stroke | Hypochlorite, caustic, polymer, fluoride, corrosion inhibitor | About 10 to 1 by stroke length, extended by frequency control |
| Peristaltic (hose) pump | Roller squeezes a flexible tube | Polymer, hypochlorite, small doses; tolerates off-gassing well | Wide, limited by tube life |
| Progressive cavity pump | Rotor turning in a stator | Thick polymer, lime slurry | Broad, set by speed |
| Volumetric dry feeder | Screw or belt delivers a fixed volume of powder | Lime, soda ash, powdered activated carbon | Moderate accuracy, roughly plus or minus 2 percent |
| Gravimetric dry feeder | Loss-in-weight control | Lime and soda ash where accuracy matters | High accuracy, roughly plus or minus 1 percent |
| Gas chlorinator | Vacuum regulator and ejector | Chlorine and sulfur dioxide | Covered in the gaseous disinfection section |
Storage, Day Tanks and Transfer
Bulk storage sizing is a compliance decision as much as an economic one: a plant that runs out of coagulant is out of compliance. Standard practice is 30 days of storage at average use, with the caveat that sodium hypochlorite degrades in storage, losing strength faster when warm, when exposed to light, and when diluted with hard water. Hypochlorite inventory is therefore sized for weeks, not months.
A day tank sits between bulk storage and the metering pump:
- Holds no more than one operating day of chemical.
- Is filled deliberately by the operator or under interlocked control, never left connected to bulk storage.
- Carries a calibrated sight glass or level element, which is what makes drawdown calibration and leak detection possible.
- Limits the volume that can be released in a single failure, which is why secondary containment is sized to the largest single container.
Calibrating a Metering Pump by Drawdown
The dial setting on a metering pump is an indication, not a measurement. Feed rate is verified as follows:
- Isolate the pump suction from the day tank and place it on a graduated calibration cylinder filled with the same chemical.
- Run the pump at the operating setting for a measured time, commonly five minutes.
- Record the volume drawn from the cylinder.
- Convert to gallons per day and compare against the required feed rate.
- Adjust stroke length or frequency and repeat until the delivered rate matches, then record the setting and the verified rate in the log.
Calibration is repeated after any change of chemical lot or strength, after maintenance on the pump head or check valves, and on a routine schedule. A drawdown result that is far below the setting normally indicates worn check valves, a gas bubble in the pump head, or a clogged injector.
Injection Points and Their Failures
- Inject where immediate mixing occurs, such as a turbulent zone, a static mixer or the suction side of a pump where permitted for the chemical.
- Use a removable quill or corporation stop so the injector can be pulled and cleaned without shutting down the main.
- Keep chemicals that react with each other far apart. Feeding hypochlorite and an acid at nearly the same point can generate chlorine gas and will destroy the intended treatment.
- Watch for injector scaling: hypochlorite raises local pH and precipitates calcium carbonate at the injection point, gradually restricting flow. The symptom is a rising discharge pressure and a falling delivered dose despite an unchanged pump setting, and the remedy is a scheduled acid soak of the quill.
- Off-gassing in hypochlorite lines causes vapor lock. Flooded suction, a degassing pump head, foot valves and correctly sloped tubing all address it.
Chemical Safety Discipline
| Rule | Reason |
|---|---|
| Never store or contain acids with hypochlorite | Mixing releases chlorine gas |
| Never store calcium hypochlorite with organics, oils or rags | Strong oxidizer; fire and explosion hazard |
| Add acid to water, never water to acid | Violent exothermic reaction and spattering |
| Confirm connections before every bulk delivery | Cross-connecting a delivery to the wrong tank has caused fatal gas releases at water plants |
| Verify material compatibility for tanks, tubing and gaskets | Hypochlorite attacks many metals; caustic attacks aluminum; fluorosilicic acid attacks concrete |
| Maintain eyewash, safety shower, ventilation, spill kit and current safety data sheets | Required by the hazard communication and confined space programs described in the safety section |
Exam tactic: when a scenario shows a stable pump setting but a declining chlorine residual, look for injector scaling, a failed check valve, off-gassing in the suction line, or hypochlorite that has lost strength in storage before concluding the demand changed.
A diaphragm metering pump feeding sodium hypochlorite shows an unchanged dial setting, but the free chlorine residual has fallen over three weeks and pump discharge pressure has risen. What is the most probable cause?
Which practice correctly describes drawdown calibration of a chemical metering pump?
Why must sodium hypochlorite storage and an acid such as hydrochloric or fluorosilicic acid never share a containment area at a treatment plant?