4.2 Chemical Feed Equipment: Feeders, Day Tanks, Carrier Water & Feed-Rate Calibration

The equipment that actually delivers the dose

The pounds formula tells you how much chemical to feed. The feed system determines whether that amount actually reaches the water at a steady, verified rate. WPI's outline lists "chemical feed equipment" and "chemical feed pumps" as inspect-maintain-operate items, and separately requires operators to "mix batches of chemical solutions," "add chemicals to hoppers and feed equipment," and "calibrate inline instrumentation."

Dry chemical feeders

Used for lime, soda ash, powdered activated carbon, dry alum, potassium permanganate, and dry polymer.

• Volumetric feeders meter a volume of material per unit time using a screw or auger, a rotating disc, a belt with an adjustable gate, or a vibrating pan. They are simple and inexpensive. Their weakness is that they meter volume while the recipe is written in pounds: if the material's bulk density changes because of moisture, compaction, or a different lot, the mass delivered changes even though the setting has not. Accuracy is roughly plus or minus 2 percent at best.
• Gravimetric feeders meter by weight using a belt on a load cell or a loss-in-weight hopper. They correct automatically for bulk density and achieve roughly plus or minus 1 percent. They cost more and have more to maintain.

Dry feeders need help staying accurate. Bridging and rat-holing in the hopper starve the feeder; bin activators, vibrators, and air pads break the arch. Hygroscopic materials such as soda ash and some polymers cake with humidity, so hoppers get dehumidified air or heaters. Downstream, the dry chemical drops into a dissolving or slurry chamber where carrier water wets it, and lime slurry lines must be sized and sloped to keep solids moving, because lime scale will close a line completely.

Solution feeders and metering pumps

Used for liquid alum, ferric, sodium hypochlorite, caustic soda, fluoride, phosphate, ammonia, and polymer solutions.

Two problems dominate. Vapor lock occurs with sodium hypochlorite, which off-gasses oxygen, increasingly so as it ages and as temperature rises. Gas collects under the pump head and the pump loses prime while continuing to stroke. Remedies are a flooded suction, a degassing or auto-vent valve head, a peristaltic pump, or fresher product. Backpressure problems occur when the injection point pressure is too low or fluctuates, so the pump siphons or over-delivers. A backpressure valve on the discharge and an anti-siphon valve at the injection point correct it.

Storage, day tanks, and containment

• Bulk storage is sized for delivery economics and reliable resupply, commonly 15 to 30 days.
• A day tank holds roughly one day of usage close to the feeder. It limits the quantity a failure can release, gives a clean volume for volumetric drawdown calibration, and makes daily usage directly measurable.
• Every tank needs level indication the operator can read without climbing, high-level alarm and overfill protection during deliveries, and a vent appropriate to the chemical.
• Secondary containment must hold the largest tank's volume plus freeboard. Incompatible chemicals must never share containment: sodium hypochlorite and any acid in the same dike will generate chlorine gas if both fail.
• Fill connections must be unique and labeled so a delivery driver cannot pump acid into a caustic tank. Cross-connected fill lines have caused fatalities.
• Materials of construction matter: 50 percent caustic soda crystallizes below about 55 degrees Fahrenheit and needs heat tracing; hypochlorite degrades with heat, light, and time and should be stored cool and dark; fluorosilicic acid and ferric chloride are aggressive to most metals.

Carrier water and the injection point

The chemical still has to enter the process water correctly.

• Carrier water dilutes and transports the chemical from the feeder to the injection point. It must be non-scaling where lime or fluoride is fed, so softened or otherwise conditioned carrier water is often required to keep lines open.
• Injection quills extend into the center of the pipe so the chemical enters the turbulent core rather than running down the wall, where it corrodes the pipe and mixes poorly.
• The point matters as much as the dose. Coagulant belongs at the rapid mix where energy is highest; permanganate belongs far enough upstream that oxidation finishes before the filters; powdered activated carbon belongs well away from any oxidant; corrosion inhibitor belongs after filtration; fluoride belongs where it will not be lost to sludge.
• Chemical incompatibility in the pipe is real: feeding fluoride and lime through the same carrier line produces immediate precipitation and plugging.

Calibrating feed rate

Never trust a dial setting. Two field methods are standard, and both appear on the exam.

1. Volumetric drawdown, for liquid feed. Isolate the day tank and let the pump draw from a graduated calibration cylinder.

• Record the starting level, run for a timed interval, and record the ending level.
• Volume drawn divided by time gives the actual feed rate in mL/min or gal/hr.
• Convert to pounds per day: lbs/day = (gal/day) x (specific gravity x 8.34) x (decimal strength)

Worked example. A hypochlorite pump draws 620 mL in 5.0 minutes from the calibration cylinder. The product is 12.5 percent available chlorine with a specific gravity of 1.20.

• 620 mL divided by 5.0 min = 124 mL/min
• 124 mL/min x 1,440 min/day = 178,560 mL/day, divided by 3,785 mL/gal = 47.2 gal/day
• 47.2 gal/day x (1.20 x 8.34 lb/gal) x 0.125 = 47.2 x 10.008 x 0.125 = 59.0 lbs/day of available chlorine

If the plant treats 4.0 MGD, the delivered dose is 59.0 divided by (4.0 x 8.34), which is 1.77 mg/L. Compare that against the target dose; the difference is your calibration error.

2. Catch and weigh, for dry feed. Divert the feeder discharge into a tared container for a measured time, weigh the catch, and scale up:

lbs/day = (grams caught / 453.6) / (minutes caught) x 1,440

Worked example. A lime feeder discharges 1,260 grams in 4.0 minutes.

• 1,260 divided by 453.6 = 2.778 lbs in 4.0 minutes
• 2.778 divided by 4.0 = 0.6944 lbs/min, times 1,440 = 1,000 lbs/day

Calibration is repeated at several settings to build a calibration curve, and it is repeated after any pump rebuild, valve replacement, chemical lot change, or significant change in injection point pressure. Records of each calibration belong in the equipment file. WPI lists completing equipment maintenance and repair records including work orders as a job task, and a feed calibration record is the document that proves your reported dose was real.

Routine maintenance and safety

• Rebuild metering pump check valves and diaphragms on a schedule; worn check balls are the single most common cause of drifting feed rate.
• Replace peristaltic tubing before it fails, since failure dumps chemical into the pump housing.
• Flush and clean lime and fluoride lines on a schedule to prevent scale closure.
• Verify calibration cylinders, rotameters, and mass flow meters against a known volume.
• Wear the PPE the safety data sheet specifies, including a face shield, chemical goggles, and gloves and apron of the correct material, for every connection break, and know where the nearest eyewash and safety shower are before you start.