6.3 Chemical Preparation, Feed, and Uses

Start with the treatment purpose

The WPI Class I outline names acids, bases, oxidants, coagulants, and disinfectants and also expects operators to mix solution batches, add dry products to feed equipment, and operate chemical-feed pumps. Product controls vary by supplier, plant, and jurisdiction. The operator matches the chemical to an authorized purpose and follows the current Safety Data Sheet (SDS) and plant standard operating procedure (SOP).

Categories can overlap. Chlorine is both an oxidant and a disinfectant. Ferric chloride is a coagulant but can also lower pH. Carbon dioxide behaves as an acidifying agent without being handled like a liquid mineral acid. The exam-worthy question is the intended process effect and evidence of control—not the assumption that every member of a category is interchangeable.

The pre-feed identity gate

Before connecting, transferring, or preparing any product, stop and verify:

1. Purchase name, chemical name, lot, delivered concentration, and certificate or receiving record match the order and tank label.
2. The receiving tank has enough confirmed capacity and contains a compatible, correctly identified material.
3. Hose, coupling, piping, valve lineup, day tank, and feeder are dedicated or explicitly approved for that chemical.
4. The current SDS and plant SOP specify hazards, personal protective equipment, ventilation, transfer controls, dilution sequence if any, spill response, and required authorization.
5. Interlocks, containment, eyewash/shower access, communications, and emergency equipment are available as the SOP requires.

Never infer compatibility from color, trade name, or prior practice. Hypochlorite contacting acid can release toxic chlorine gas; contact with ammonia can create hazardous chloramine vapors. CDC/NIOSH incident guidance reinforces nonmixing and SDS-based controls. This section intentionally does not provide a physical mixing sequence: an operator must use the exact approved procedure for the actual product and system.

Batch math is not a mixing instruction

For a problem involving one approved stock solution and a specified final solution, the WPI table gives C1 × V1 = C2 × V2, with compatible concentration and volume units. If an approved 12.5% stock is used to produce 200 L at 5.0%, the calculated stock component is:

V1 = (5.0% × 200 L) ÷ 12.5% = 80 L.

The arithmetic identifies a component volume only. It does not specify the order of addition, acceptable water quality, temperature control, headspace, agitation, tank material, or PPE. Those controls come solely from the SDS, manufacturer information, and approved SOP. Confirm that the final volume—not an assumed added-water volume—is the equation's basis, and never apply this simple relationship to reacting products or an unapproved blend.

Record product lot and strength, tank readings, calculated components, required checks, time, destination, and unusual observations. Label the completed tank immediately under the site procedure. A wrong concentration can make an otherwise correct pump setting deliver the wrong process dose.

Feed equipment: prove delivery

Liquid metering pumps may be manually set, flow-paced, or adjusted by feedback from a valid process analyzer. Dry feeders meter mass or volume from a hopper into a wetting or solution system. For either type, a display setting is not proof of output. Compare a timed calibration or approved drawdown, tank or scale usage, plant flow, and process response. EPA tribal drinking-water maintenance guidance similarly directs operators to inspect feeders and compare measured chemical use with water produced and intended dose.

For dry product, inspect storage condition, dust controls, hopper level, evidence of moisture, bridging or rat-holing, feeder motion, and downstream wetting according to the SOP. Never reach into, strike, or clear energized equipment; isolate it under the site's authorized energy-control procedure. For liquids, check for leaks, gas binding, loss of prime, plugged valves, and command-output disagreement.

Scenario: concentration changed, setting did not

A replacement delivery has a different verified active strength, but the operator leaves the volumetric feeder at its old rate. Plant flow is unchanged, yet the delivered active mass changes because solution volume and active concentration are separate variables. Quarantine any questionable connection; verify identity and records; then recalculate, approve, calibrate, and confirm under the SOP. Do not alter the certificate value or analyzer record to make the old setting appear correct.

Final control loop

• Plan: define the authorized process goal and verified input data.
• Prepare: follow product-specific SDS and SOP controls; do not improvise.
• Feed: use an approved setting within calibrated equipment capacity and working interlocks.
• Verify: compare actual delivery, inventory, flow, and the correct downstream measurement after travel or reaction time.
• Record and respond: document the result, investigate deviations, and make only authorized changes.

This loop applies whether the chemical adjusts pH, aids coagulation, oxidizes iron, or provides a disinfectant residual. Safe chemical operation is not simply calculating a dose; it is maintaining identity, containment, accurate delivery, and evidence that the intended water-quality change actually occurred.