10.4 pH Adjustment & Chemical Feed Control

10.4 pH Adjustment & Chemical Feed Control

2025 WPI alignment: This section teaches chemical dosing for pH adjustment in Treatment Process Evaluation and Adjustment, the 40-question area containing 6 recall, 34 application, and 5 calculation items.

Why this process task matters

pH adjustment uses acids, bases, alkalinity sources, or process changes to keep treatment chemistry and discharge within the required range. Operators distinguish pH from alkalinity and control nonlinear response safely.

Process-control model

Evaluation and adjustment sequence

1. Verify pH with a calibrated instrument and representative grab and review temperature compensation.
2. Check alkalinity and identify the process or waste stream causing the shift.
3. Inspect chemical identity, strength, compatibility, containment, pump calibration, injection, and mixer.
4. Use a titration or approved bench test to set a conservative starting dose.
5. Apply gradual flow-paced feed with high/low alarms and interlocks appropriate to the hazard.
6. Confirm downstream pH, alkalinity, process performance, inventory, and absence of overshoot before further change.

Diagnostic evidence

Calculation and mass-balance connection

There is no safe universal formula that converts a pH change directly to mg/L of acid or base because pH is logarithmic and buffering varies. Use titration data supplied by the problem or a blending/dose relationship. For feed mass, apply WPI’s dose-flow formula and correct for purity as stated. Always use product density and active concentration if converting mass demand to liquid pump rate.

Worked operating scenario

An automated caustic loop oscillates from pH 6.2 to 9.0. The downstream probe is far from the injection point but the sample line adds delay, and the pump is oversized. The operator verifies probe calibration and mixing, places the loop in the approved stable mode, reduces aggressive gain or feed increments, and uses titration data. Alternating more acid and more caustic would amplify the problem.

Common exam traps

• pH and alkalinity measure different properties.
• A one-unit pH change is not a fixed chemical dose.
• Do not feed acid and base against each other to mask poor control.
• Concentrated acid/base handling requires compatibility, containment, PPE, and emergency wash access.

Field-to-exam checklist

• Verify pH and understand buffering before selecting dose.
• Use representative titration/bench data rather than a universal conversion.
• Account for mixing, measurement delay, pump size, and nonlinear response.
• Find and correct upstream causes when possible.

Neutralization safety and sequencing

Concentrated acid and base can react violently and generate heat when mixed. Use separate compatible systems, prevent cross-connections, and dilute only according to the product and facility procedure. Never attempt to correct an overshoot by pouring the opposite chemical into a tank without engineered mixing and dose control. If a pH excursion threatens biology or discharge, manage routing and notification while the feed fault is isolated; do not sacrifice worker safety for rapid neutralization.

Because pH is logarithmic, equal numerical steps do not represent equal chemical demand. Titration or controlled testing is more informative than scaling dose from the pH difference alone.

Reagent selection and buffer arithmetic

Bases differ in speed, solids production, and forgiveness.

Acids differ the same way. Sulfuric acid is inexpensive but adds sulfate, which can feed sulfide generation downstream. Carbon dioxide is self-limiting — in most waters it cannot drive pH much below about 6 — which makes it inherently overshoot-resistant where a modest reduction is all that is needed.

Work the alkalinity, not the pH. Because pH is logarithmic and buffering varies, no universal formula converts a pH change into a chemical dose. Alkalinity arithmetic, by contrast, is straightforward.

Worked alkalinity dose. To raise alkalinity by 50 mg/L as CaCO₃ in a 3.0 MGD stream: 50 x 3.0 x 8.34 = 1,251 lb/day expressed as CaCO₃. Sodium bicarbonate has an equivalent weight of 84 against 50 for CaCO₃, so each pound of product supplies 50/84 = 0.6 lb of CaCO₃-equivalent alkalinity. Required product = 1,251 / 0.6 = about 2,100 lb/day.

Probe placement often beats controller tuning. A pH electrode located before mixing is complete reads a local, unrepresentative value, and no amount of retuning fixes a measurement problem. Installing a static mixer or moving the electrode into a well-mixed zone stabilizes far more loops than a gain change does.