4.1 Coagulation Chemistry & Jar Testing

4.1 Coagulation Chemistry & Jar Testing

Quick Answer: Coagulation adds metal-salt or polymer chemicals to destabilize colloids so particles can collide and form floc. Operators choose charge neutralization or sweep floc mechanisms, control pH and alkalinity, and use jar tests to set dose. Alum, ferric salts, and PACl are the workhorse coagulants on FDEP Class B/C treatment exams.

Coagulation is the first chemical step in conventional and direct filtration trains. On Florida operator exams it appears under coagulation/flocculation/sedimentation process control—not as isolated chemistry trivia. Expect scenarios: storm turbidity spike, low-alkalinity raw water, rising TOC, or a jar test that does not match plant performance.

Why Colloids Will Not Settle Alone

Raw water carries colloidal particles—clay, silt, organic debris, bacteria, and some viruses—that stay suspended for days because:

1. They are very small (roughly 0.001–1 µm for true colloids).
2. Most natural particles carry a negative surface charge, so they repel each other and rarely collide hard enough to stick.

Gravity settling alone cannot meet turbidity or pathogen-removal goals. Coagulation destabilizes those particles so subsequent flocculation and sedimentation (or filtration) can remove them.

Destabilization Mechanisms Operators Must Name

Charge Neutralization

Metal coagulants hydrolyze and form positively charged species that adsorb to particle surfaces, reducing the repulsive charge. Destabilized particles can then approach closely enough for van der Waals attraction to hold them. Charge neutralization typically works at lower doses and is sensitive to overdosing (restabilization if you reverse the charge).

Sweep Floc (Enmeshment)

At higher coagulant doses, metal hydroxides precipitate as a bulky Al(OH)₃ or Fe(OH)₃ floc that sweeps and enmeshes particles as it settles. Sweep floc is more forgiving of dose error than pure charge neutralization but uses more chemical, produces more sludge, and still needs correct pH.

Bridging with Polymers

Organic polymers (cationic, anionic, nonionic) can bridge between particles or strengthen metal floc. Many plants use a primary metal coagulant plus a flocculant aid polymer. Polymers do not replace understanding metal chemistry on the exam—they refine floc strength and settling.

Common Coagulants

Aluminum Sulfate (Alum)

Alum (Al₂(SO₄)₃ · 14H₂O is the commercial hydrate operators usually mean) is the classic coagulant. In water it hydrolyzes, consumes alkalinity, and forms aluminum hydroxide floc when pH is in a workable range.

Exam associations:

• Effective pH window commonly cited near ~5.5–7.5 (plant optimum is verified by jar test; many systems target roughly 6.0–7.0 for alum).
• Consumes about 0.5 mg/L alkalinity as CaCO₃ per 1 mg/L alum (order-of-magnitude rule operators memorize; exact stoichiometry depends on product strength and reactions).
• Residual aluminum can rise if pH is outside the optimum precipitation zone.

Ferric Chloride and Ferric Sulfate

Ferric salts form iron hydroxide floc and often work over a wider pH range than alum (commonly effective from roughly ~4–9, depending on water and dose). They may be preferred for some high-organic or colored waters and for plants that want denser floc. Drawbacks include more corrosive feed solutions, staining potential, and residual iron if under- or over-treated.

Polyaluminum Chloride (PACl) and Other Prehydrolyzed Products

PACl and related polyaluminum products are partially prehydrolyzed. Benefits often claimed (and tested conceptually on exams):

• Lower alkalinity consumption than traditional alum in many waters
• Effective over a somewhat different pH window
• Less pH depression in low-alkalinity sources

They cost more per pound of product in many contracts, so jar tests and total cost of treatment (including sludge and pH adjustment) drive selection—not brand loyalty.

Coagulant Aids and Other Chemicals

• Lime, soda ash, caustic soda — raise alkalinity/pH so coagulation does not crash the water
• Acid — occasionally used when pH is too high for optimum coagulation
• Polymers — primary coagulants (some cationic polymers) or flocculant/filter aids
• Activated silica / bentonite — historical or niche aids that add weight or nucleation sites

Alkalinity Consumption and pH Optimum

Coagulation is inseparable from pH and alkalinity:

• Metal salt hydrolysis produces acid → pH drops unless alkalinity buffers the water.
• If raw alkalinity is low, the pH may fall out of the coagulant’s effective window → poor floc, high settled turbidity, residual metal.
• Operators then add alkali (lime, soda ash, or caustic) before or with coagulant, or choose a coagulant with lower alkalinity demand (e.g., some PACl products).

Process-control idea: When jar tests show good floc only after alkali addition, the plant problem is not “more alum forever”—it is insufficient buffer capacity or wrong pH set point.

Jar Testing — The Operator’s Decision Lab

A jar test (multiple stirrer) simulates rapid mix → flocculation → settling at bench scale so you can choose chemical type and dose before changing the plant.

Standard Procedure Concepts

1. Collect representative raw water at the current plant condition (after a storm pulse if that is what you must treat).
2. Measure raw parameters: turbidity, pH, alkalinity, temperature, color/TOC if organics matter, and any oxidant residual that affects demand.
3. Fill identical jars (often 1 or 2 L) with the same raw water.
4. Dose each jar with a different coagulant concentration (and, in separate series, different pH, polymer, or coagulant type).
5. Rapid mix briefly at high G (seconds) to disperse chemical.
6. Flocculate at lower G for a timed period matching plant detention (often 15–30 minutes in stages).
7. Settle without mixing for a timed period matching clarifier detention (often 15–60 minutes depending on what you are modeling).
8. Sample supernatant turbidity (and sometimes pH, residual Al/Fe, UV254/TOC) from the same depth in each jar.

Interpreting Results

Look for the lowest settled turbidity at the lowest practical dose, then check secondary goals:

• Floc appearance — pin floc that will not settle vs large, dense, settleable floc
• Supernatant clarity — primary metric for sedimentation plants
• pH after coagulation — still in the workable window?
• Residual coagulant metal — high residual suggests wrong pH or incomplete reaction
• TOC/UV254 reduction — for enhanced coagulation / DBP precursor control
• Sludge volume — excessive dose wastes chemical and creates disposal cost

Classic jar-test reading errors on exams:

• Picking the jar with the biggest floc when a lower dose actually produced clearer supernatant
• Ignoring that plant detention time and G differ from the bench, so plant dose may need fine-tuning after transfer
• Running jars on yesterday’s water when today’s storm front has tripled turbidity and TOC

Enhanced Coagulation Context

For surface-water plants with elevated TOC, regulations and good practice may require enhanced coagulation—optimized dose/pH to remove a required percentage of TOC based on raw TOC and alkalinity matrices. Jar tests for enhanced coagulation track TOC or SUVA/UV254 removal, not turbidity alone.

Dose Calculation Concepts

Operators convert between concentration, mass feed, and solution feed rates. Core relationships:

Mass feed rate

lb/day = flow (MGD) × dose (mg/L) × 8.34

Solution feed (when feeding a liquid of known strength)

If a product is 50% active and density is known, convert mg/L as product or as active ingredient carefully—exam traps mix “as product” vs “as dry alum.” Always match the basis used in the jar test.

Worked concept: 5.0 MGD plant, jar-test optimum 25 mg/L alum as product:

lb/day = 5.0 × 25 × 8.34 = 1,042.5 lb/day

If liquid alum is fed, the operator calculates gph from product density and strength so the same 25 mg/L reaches the rapid mix—not a guessed pump dial setting.

Also track:

• Day tank inventory and dilution accuracy
• Calibration of chemical feed pumps (drawdown tests)
• Flow pacing—dose is mg/L, so feed rate must track plant flow

Florida Groundwater vs Surface Water Coagulant Choices

Florida’s source mix changes how often coagulation is the “star” process:

Exam scenario style: After a hurricane, a central Florida surface plant sees raw turbidity climb from 3 NTU to 40 NTU and alkalinity drop slightly from runoff. Correct operator theme: repeat jar tests, raise coagulant dose as indicated, protect pH/alkalinity, watch clarifier and filter loading—not “leave the dose the same because last month it worked.”

Contrast: A deep well plant with stable 0.2 NTU raw turbidity and no color problem does not invent a coagulation crisis; focus stays on disinfection CT, iron/manganese if present, and distribution stability.

Plant Application Points That Connect to Later Sections

• Coagulant is added at rapid mix (high energy, short time) so chemical disperses before flocculation.
• Underdosing leaves stable colloids → high settled and filtered turbidity.
• Overdosing wastes money, creates sludge, can restabilize particles, and may raise residual metal.
• Coagulation sets up §4.2 flocculation and §4.3 sedimentation; no amount of paddle speed fixes a wrong chemical dose.

Master destabilization language, coagulant families, alkalinity/pH coupling, jar-test interpretation, and Florida source-driven choices. Those ideas carry most coagulation chemistry items on Class B and Class C exams.