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newSchoolWorks.com › VoTechWorks › Agriculture, Environment & Public Service › Water / Wastewater Operator › Water and Wastewater Treatment Processes › Lesson 4
AGR-WATE71-03 · Lesson 4 · 14.5 clock hours · lab

Coagulation, Flocculation, and Sedimentation

Objective. Students will perform a jar test to determine optimum coagulant dose for a turbid water sample, then explain how flocculation and sedimentation basin design parameters—including detention time and overflow rate—affect settled-water quality.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Coagulation, Flocculation, and Sedimentation

Removing suspended particles from raw water is one of the most important steps in drinking water treatment. Three linked processes accomplish this: coagulation, flocculation, and sedimentation. Together they reduce turbidity, color, and many pathogens before water ever reaches a filter.

Why Particles Stay Suspended

Most suspended particles in raw water—clay, silt, organic matter, and microorganisms—carry a negative surface charge. Because like charges repel, particles push each other apart and remain dispersed for a very long time. Gravity alone cannot settle them fast enough for practical treatment. Coagulation is the chemical step that overcomes this charge barrier.

Coagulation: Breaking the Charge

A coagulant is a chemical added to the water to neutralize the negative surface charges on particles. The most common coagulant in water treatment is alum (aluminum sulfate). When alum dissolves in water it releases aluminum ions and reacts with the water's alkalinity to form aluminum hydroxide. This reaction consumes alkalinity and lowers pH, so operators must monitor pH closely. The optimal coagulation pH for alum is roughly 6.5–7.5, though the exact range depends on raw-water chemistry.

Coagulants are added at a rapid-mix (flash-mix) point—a zone of very intense turbulence that distributes the chemical throughout the flow within seconds. Inadequate mixing results in poor charge neutralization; excessive coagulant doses can re-stabilize particles (a phenomenon called restabilization) and worsen turbidity.

The Jar Test

Because raw-water quality changes daily, operators cannot rely on a fixed coagulant dose. The jar test is a bench-scale procedure that mimics the full treatment train in miniature. Six jars are filled with identical raw-water samples and dosed with increasing amounts of coagulant. All jars receive the same rapid-mix period (high RPM), then a slow-mix flocculation period (low RPM), then a settling period with no mixing. After settling, operators measure the turbidity of each jar's supernatant. The dose that produces the lowest settled turbidity—while staying within dose limits—is the optimum dose.

Record every step: initial turbidity, dose volume, RPM settings, mixing times, and final turbidity. Plot dose on the x-axis and settled turbidity on the y-axis. The optimum dose corresponds to the lowest point on the curve.

Flocculation: Building Bigger Particles

After coagulation neutralizes surface charges, particles begin to collide and stick together. Flocculation provides gentle, sustained mixing to encourage those collisions and grow larger, heavier aggregates called floc. Too much mixing energy breaks floc apart; too little and particles do not collide often enough. Flocculator paddles or turbines rotate at low speed, usually measured in RPM or expressed as the velocity gradient G (units: s⁻¹). Operators adjust G to balance collision frequency with floc strength. Typical flocculation times range from 20 to 40 minutes, though design varies.

Sedimentation: Letting Gravity Work

Settled water flows into a sedimentation basin (also called a clarifier) where velocity drops sharply and floc settles to the bottom as sludge. Two key design parameters describe basin performance:

Surface Overflow Rate (SOR) — also called surface loading rate — is the flow rate divided by the surface area of the basin:

> SOR = Flow (gal/day) ÷ Surface Area (ft²)

Typical SOR values for conventional clarifiers are roughly 500–1,000 gal/day/ft², though your operator reference manual provides ranges for your state's criteria. A lower SOR means more settling time per unit area and better particle removal.

Detention Time (DT) — also called hydraulic retention time (HRT) — is the theoretical time water spends in the basin:

> DT (hours) = Basin Volume (gal) ÷ Flow Rate (gal/hr)

Typical sedimentation detention times are approximately 2 to 8 hours for conventional basins. Longer detention time generally improves settleability but requires a larger basin.

Sludge that accumulates on the basin floor is removed by mechanical scrapers or by periodic draining and must be disposed of properly according to state and local regulations.

Safety in the Lab

Personal Protective Equipment (PPE): Wear safety glasses and chemical-resistant gloves whenever handling alum solution or any coagulant chemical. A lab coat or chemical-resistant apron protects skin and clothing from spills.

Turbidimeter: Handle the glass sample cells carefully to avoid breakage. Broken glass is a laceration hazard—dispose of shards in a designated sharps or glass-waste container, never a regular trash can.

Alum: Alum solution is acidic. In the event of skin or eye contact, flush immediately with large amounts of water for at least 15 minutes and notify your instructor. Know the location of the eyewash station before you begin.

Housekeeping: Wipe up spills immediately. Wet floors are a slip hazard. Label all containers, even dilute solutions. Never pour chemicals back into stock bottles.

Bench-Scale Flocculator: Keep hands and loose clothing away from rotating paddle shafts. Secure long hair before operating motorized equipment.

Connecting Lab Results to Full-Scale Operations

The jar test is not just a classroom exercise—operators at full-scale plants run jar tests routinely whenever raw-water turbidity spikes after storms or when source water quality shifts. A properly run jar test can save thousands of dollars in chemical costs while maintaining water quality. Your ability to run a jar test, read a turbidimeter accurately, and calculate SOR and detention time are core competencies tested on both Class I and Class II water operator licensing exams.

Key terms10

CoagulationThe process of adding a chemical (coagulant) to water to neutralize the negative surface charges on suspended particles, allowing them to begin clumping together.
Alum (Aluminum Sulfate)The most widely used coagulant in water treatment; it reacts with alkalinity in the water to form aluminum hydroxide, which destabilizes suspended particles.
FlocculationThe gentle, slow-mixing process following coagulation that encourages destabilized particles to collide and grow into larger, settleable aggregates called floc.
Jar TestA bench-scale laboratory procedure using six identical water samples dosed with varying coagulant amounts to determine the optimum coagulant dose for a given raw-water sample.
Surface Overflow Rate (SOR)A measure of sedimentation basin hydraulic loading calculated by dividing the influent flow rate by the basin surface area, typically expressed in gallons per day per square foot.
Detention Time (DT)The theoretical time a water particle spends inside a treatment basin, calculated by dividing basin volume by flow rate; also called hydraulic retention time (HRT).
TurbidityA measure of water cloudiness caused by suspended particles, measured in Nephelometric Turbidity Units (NTU) using a turbidimeter.
RestabilizationA condition where excess coagulant dose re-charges particle surfaces and causes turbidity to increase again, making it worse than using too little coagulant.
Velocity Gradient (G)A measure of mixing intensity in a flocculator, expressed in units of inverse seconds (s⁻¹); it balances particle collision frequency against the risk of breaking apart fragile floc.
Optimum DoseThe coagulant dose identified during a jar test that produces the lowest settled turbidity in the supernatant while remaining cost-effective and within regulatory limits.

Worked examples4

1. A jar test is run with six alum doses: 5, 10, 15, 20, 25, and 30 mg/L. The settled turbidities (NTU) after the test are 12, 7, 3, 1.8, 2.5, and 4.0 respectively. The raw-water turbidity was 45 NTU. What is the optimum dose, and what is the percent turbidity removal at that dose?
  1. List the dose-turbidity pairs and identify the lowest settled turbidity: 20 mg/L gave 1.8 NTU — the lowest value in the set.
  2. The optimum dose is therefore 20 mg/L.
  3. Calculate percent turbidity removal: % Removal = [(Initial − Final) ÷ Initial] × 100
  4. % Removal = [(45 − 1.8) ÷ 45] × 100 = [43.2 ÷ 45] × 100 = 96%
Answer: Optimum dose = 20 mg/L; turbidity removal = 96%.
2. A rectangular sedimentation basin is 80 ft long, 20 ft wide, and 12 ft deep. The plant flow is 1.2 million gallons per day (MGD). Calculate the surface overflow rate (SOR) in gal/day/ft².
  1. Identify the surface area of the basin (length × width): 80 ft × 20 ft = 1,600 ft².
  2. Convert flow to gal/day: 1.2 MGD = 1,200,000 gal/day.
  3. Apply the SOR formula: SOR = Flow ÷ Surface Area = 1,200,000 gal/day ÷ 1,600 ft².
  4. SOR = 750 gal/day/ft².
Answer: Surface overflow rate = 750 gal/day/ft².
3. Using the same basin from the previous example (80 ft × 20 ft × 12 ft, flow = 1.2 MGD), calculate the theoretical detention time in hours. (Use the conversion: 1 ft³ = 7.48 gallons.)
  1. Calculate basin volume in cubic feet: 80 × 20 × 12 = 19,200 ft³.
  2. Convert volume to gallons: 19,200 ft³ × 7.48 gal/ft³ = 143,616 gallons.
  3. Convert flow to gal/hr: 1,200,000 gal/day ÷ 24 hr/day = 50,000 gal/hr.
  4. Apply detention time formula: DT = Volume ÷ Flow Rate = 143,616 gal ÷ 50,000 gal/hr = 2.87 hours.
Answer: Theoretical detention time ≈ 2.87 hours.
4. A raw-water sample has a turbidity of 60 NTU. After the jar test, the best jar shows a settled turbidity of 8 NTU. Does this meet the minimum 80% turbidity removal target for the jar-test check-off? Show your calculation.
  1. Use the percent removal formula: % Removal = [(Initial − Final) ÷ Initial] × 100.
  2. Substitute values: % Removal = [(60 − 8) ÷ 60] × 100.
  3. Calculate numerator: 60 − 8 = 52.
  4. Divide: 52 ÷ 60 = 0.8667; multiply by 100 = 86.7%.
  5. Compare to the 80% target: 86.7% > 80%, so the target IS met.
Answer: Turbidity removal = 86.7%; this meets the ≥ 80% requirement.

The math in this lesson1

Learners pick the optimum jar-test dose and compute turbidity removal (45 − 1.8) ÷ 45 = 96%.
7.RP.3
Brush up in the math halls at newSchoolWorks.com:
In class: Conduct a standard six-jar jar-test procedure on a prepared turbid sample using alum as the coagulant; record turbidity readings before and after each dose, plot results, and identify the optimum dose. · Calculate surface overflow rate and detention time for a given rectangular sedimentation basin using influent flow and basin dimensions; compare calculated values to typical design ranges found in the operator reference manual. · Adjust the simulated rapid-mix speed and flocculator paddle speed on a bench-scale flocculation demonstration unit (or video simulation), observe floc formation under each condition, and document findings in a lab notebook.
Materials: Jar-test apparatus (six-position stirrer), alum solution, turbid water sample, turbidimeter, graduated cylinders, operator reference manual (AWWA or state equivalent), calculator, lab notebook.
Check: Jar-test report identifies the correct optimum dose (within the instructor-established acceptable range), turbidity removal ≥ 80% at that dose, and both sedimentation calculations are correct; lab notebook entry is complete and legible.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Water Treatment Processes
1. What is the primary purpose of rapid mixing during coagulation?
Water Treatment Processes
2. During a jar test, a jar dosed at 30 mg/L shows higher settled turbidity than the jar dosed at 20 mg/L. This result most likely indicates which of the following?
Water Treatment – Hydraulics and Basin Design
3. A sedimentation basin receives a flow of 800,000 gal/day and has a surface area of 1,000 ft². What is the surface overflow rate?
Water Treatment Chemistry
4. Which pH range is generally considered optimal for alum coagulation in drinking water treatment?
Water Treatment Processes
5. Which of the following best describes the role of flocculation in the treatment process?
Water Treatment – Hydraulics and Basin Design
6. A sedimentation basin has a volume of 200,000 gallons and receives a flow of 100,000 gal/hr. What is the theoretical detention time?
Safety and Health
7. What minimum personal protective equipment (PPE) should an operator wear when handling alum solution during a jar test?
Water Treatment Processes
8. An operator plots jar-test results and finds that settled turbidity decreases from 5 mg/L to 10 mg/L to 20 mg/L to 30 mg/L as the alum dose increases from 10 to 40 mg/L. What does this trend indicate?
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