5.3 Chemical Feed Systems & Calibration
Key Takeaways
- Dry feeders are classified as volumetric (by volume) or gravimetric (by weight, which is more accurate).
- Liquid feeders include positive displacement diaphragm pumps and peristaltic pumps that prevent gas locking.
- Drawdown calibration tests physically verify pump flow rates using a graduated cylinder.
- Chlorine gas safety relies on vacuum-type chlorinators and checking for leaks with ammonia.
5.3 Chemical Feed Systems & Calibration
Why the Topic Matters for the Exam
Chemical feed systems introduce precise amounts of coagulants, disinfectants, pH adjusters, and fluoride. Accurate dosing is essential; under-dosing leads to treatment failure and disease outbreaks, while over-dosing wastes chemicals, causes taste issues, and threatens public health. For the operator certification exam, understanding chemical feeder types, calibration calculations, and safe chemical handling procedures is a core requirement.
Dry and Liquid Feeder Types
Chemical feeders are broadly categorized into dry and liquid systems. Dry feeders deliver crystalline or powdered chemicals like lime, soda ash, and alum, and are classified as volumetric or gravimetric.
A volumetric dry feeder delivers chemical by volume, using a rotating screw, disk, or belt running at a controlled speed. Volumetric feeders are simple and inexpensive but do not account for density changes. If chemical compacts or absorbs moisture, the mass fed varies, causing dosing errors.
A gravimetric dry feeder measures and feeds chemicals by weight. It utilizes a weighing belt or scale-mounted hopper to ensure a constant mass of chemical is delivered. They automatically adjust speed to compensate for density changes, moisture content, or particle size, making them highly accurate and preferred for large municipal systems requiring tight dosage control.
Liquid chemical feed systems typically utilize positive displacement pumps to inject liquid chemicals like liquid alum, sodium hypochlorite, and polymers. The most common type is the diaphragm metering pump, which uses a reciprocating diaphragm to displace a precise volume of liquid. However, diaphragm pumps can suffer from gas locking when pumping off-gassing chemicals like sodium hypochlorite. To prevent gas locking, peristaltic pumps (or hose pumps) are often used. Peristaltic pumps use rollers that rotate and compress a flexible tube, squeezing the liquid forward. This design handles off-gassing chemicals easily because it has no check valves to lock and maintains a positive seal, reducing maintenance downtime associated with air binding.
Feeder Calibration and Dosage Calculations
Feeder calibration is the physical process of verifying that a feed pump is delivering the exact amount of chemical calculated. The standard chemical dosage formula is:
Feed Rate (lb/day) = Flow (MGD) × Dosage (mg/L) × 8.34 lb/gal
To verify feed rate, operators perform a drawdown calibration test. They measure the volume drop in a suction-side graduated cylinder over a set time:
Flow Rate (mL/min) = Volume drop (mL) ÷ Time elapsed (min)
To convert this flow rate to gallons per day (gpd), the formula is:
Flow Rate (gpd) = [Flow Rate (mL/min) × 1440 min/day] ÷ 3785.4 mL/gal
If the operator needs to convert a dry chemical feed rate from grams per minute (g/min) to pounds per day (lbs/day):
Feed Rate (lb/day) = Feed Rate (g/min) × 3.17
Chemical Safety and Handling
Safety is paramount in chemical feed rooms. Secondary containment must be provided for all liquid storage tanks to capture leaks. Personal protective equipment (PPE), including face shields, chemical-resistant aprons, and neoprene gloves, is mandatory when handling chemicals. Emergency eyewash and shower stations must be within 10 seconds of chemical areas and tested weekly.
When using chlorine gas, vacuum-type chlorinators are used because they operate under a vacuum; if a leak occurs in the system, air is drawn in rather than chlorine gas escaping. To detect small chlorine gas leaks, operators wave a rag wetted with an ammonia solution near the suspected leak. If chlorine gas is present, it reacts with ammonia to form a visible white cloud of ammonium chloride smoke.
| Feeder Type | Operating Method | Common Chemical | Advantage | Disadvantage |
|---|---|---|---|---|
| Volumetric Dry | Rotates screw/disc by volume | Lime, Alum (dry) | Simple, low cost | Density changes cause inaccuracy |
| Gravimetric Dry | Weighs belt or hopper | Soda ash, Fluoride | Highly accurate | High initial cost, complex |
| Diaphragm Liquid | Reciprocating diaphragm | Alum, Polymer | High pressure discharge | Gas lock from off-gassing |
| Peristaltic Liquid | Squeezes flexible tubing | Sodium hypochlorite | No gas lock, simple | Tubing wears out and ruptures |
Realistic Operational Scenario
In a realistic operational scenario, an operator must verify a liquid alum feed pump's dosage. The plant flow rate is 2.0 MGD, and the target alum dose is 15.0 mg/L. The required alum feed rate is calculated as: Feed Rate = 2.0 MGD * 15.0 mg/L * 8.34 = 250.2 lbs/day. The alum solution has a specific gravity of 1.33 and is 48% active, meaning each gallon contains 5.32 pounds of active alum. The pump is set to deliver 47.0 gallons per day. To verify, the operator performs a 5-minute drawdown test and measures a drop of 620 mL. The flow rate is 124 mL/min. Converting this yields: 124 mL/min * 1440 min/day / 3785.4 mL/gal = 47.1 gpd. This matches the pump setting, confirming the calibration is correct and the target dosage is being delivered.
An operator performs a drawdown test on a chemical feed pump and records a drop of 180 mL in 3 minutes. What is the feed rate in milliliters per minute?
What type of dry chemical feeder is the most accurate because it adjusts for changes in chemical density by weighing the material?