10.2 Coagulation & Flocculation for Solids/Phosphorus Removal

10.2 Coagulation & Flocculation for Solids/Phosphorus Removal

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

Why this process task matters

Coagulation destabilizes particles or precipitates dissolved phosphorus, and flocculation provides controlled collisions to form separable floc. Dose, pH/alkalinity, mixing, polymer, solids separation, and sludge production must be evaluated together.

Process-control model

Evaluation and adjustment sequence

1. Characterize flow, turbidity/TSS, soluble and total phosphorus, pH, alkalinity, temperature, and downstream separation.
2. Verify actual chemical identity, active strength, feed calibration, injection point, and mixing equipment.
3. Use a representative jar test to compare dose, pH adjustment, polymer, settling/filtering, and residual target.
4. Implement a controlled full-scale change paced to flow and load within permit/design limits.
5. Watch floc, clarifier/filter headloss, effluent turbidity/phosphorus, pH, residual metal where applicable, and sludge production.
6. Confirm improvement over representative time and document the dose-response relationship.

Diagnostic evidence

Calculation and mass-balance connection

Use chemical feed mass = dose × flow × 8.34 / active purity decimal when required. Jar-test dose must be scaled carefully from mg/L to full flow. A stoichiometric metal-to-phosphorus ratio may be given, but real full-scale dose also reflects competing reactions and separation; use the supplied relationship and verify by testing instead of memorizing one universal ratio.

Worked operating scenario

Effluent phosphorus rises and an operator doubles alum, but pH falls and filter runs shorten with no soluble-P improvement. The better response is to verify feed and analytical fractions, check alkalinity and rapid mix, and run a jar test. More coagulant created extra solids and unfavorable pH without fixing the limiting chemistry.

Common exam traps

• Coagulation and flocculation use different mixing intensities and purposes.
• Total phosphorus can follow suspended solids while soluble phosphorus reveals precipitation performance.
• A clear jar at one dose does not authorize unlimited full-scale feed.
• Chemical phosphorus removal increases sludge mass and alkalinity demand.

Field-to-exam checklist

• Optimize dose, pH, alkalinity, injection, mixing, flocculation, and separation as one process.
• Use jar tests and representative fractionated analyses.
• Verify actual chemical mass feed and active strength.
• Account for chemical sludge and downstream filter/solids impact.

Reading a jar test

A jar test is most useful when mixing energy, timing, settling or filtration, temperature, and analytical endpoints resemble the intended full-scale decision. Record not only the clearest jar but also floc formation time, size, strength, settled volume, pH, and soluble versus total phosphorus. The selected dose should have an operating margin without sitting on a sharp failure edge. Full-scale verification is still required because plant hydraulics and recycle streams differ from a beaker.

Scale trials with controlled evidence

A jar test compares chemistry under known rapid-mix, flocculation, settling, dose, pH, and temperature conditions. Use a blank and several doses, observe floc formation and breakage, and measure the parameter the plant is trying to remove. Scale the selected dose to actual flow and active product strength, then introduce it under controlled full-scale conditions. Confirm downstream turbidity or phosphorus, filterability, sludge production, pH, and alkalinity because a beaker does not reproduce plant hydraulics or recycle loads.

Chemical properties, ratios, and mixing energy

Metal coagulants consume alkalinity and depress pH. Aluminum sulfate — commonly delivered as a roughly 48.5 percent liquid — consumes on the order of 0.45 to 0.5 mg of alkalinity as CaCO₃ per mg of alum added, and ferric chloride behaves similarly. In a low-alkalinity effluent, a coagulant increase intended to remove phosphorus can move pH out of the range where the coagulant works, so dose and buffering must be adjusted as one decision rather than two.

The metal-to-phosphorus ratio is not 1:1 in practice. The precipitation stoichiometry for AlPO₄ or FePO₄ is one mole of metal per mole of phosphorus, but competing hydroxide formation means practical molar ratios of roughly 1.5:1 to 2.5:1 are typical for moderate targets, and the ratio rises steeply as the effluent target falls. Removing phosphorus from 1.0 to 0.5 mg/L costs far less chemical per unit removed than going from 0.3 to 0.1 mg/L — an important expectation when a permit tightens.

Mixing energy separates the two unit processes. Rapid mix uses a high velocity gradient — commonly several hundred to over 1,000 reciprocal seconds — applied for seconds, purely to disperse chemical before it reacts locally. Flocculation uses a low gradient, commonly on the order of 20 to 80 reciprocal seconds, applied for 10 to 30 minutes, to grow floc without shearing it. Tapered flocculation steps the gradient down from inlet to outlet so that fragile large floc is not broken just before separation.

Worked product feed. A 4.0 MGD stream dosed at 12 mg/L of ferric chloride as delivered product requires 12 x 4.0 x 8.34 = 400 lb/day of product. At a solution density near 11.9 lb/gal that is roughly 34 gallons per day, a rate small enough that a calibration-column check is the only reliable verification.