Coagulation, Flocculation & Jar Testing

Learn how coagulation and flocculation remove fine particles from drinking water and how jar testing helps operators evaluate coagulant dose, pH, mixing, floc formation, and settling.

Coagulation and flocculation are central processes in many drinking water treatment plants. They prepare fine suspended and colloidal particles for removal by clarification and filtration.

Many particles in raw water are too small to settle efficiently on their own. They can remain suspended because of their small size and surface electrical characteristics. Coagulation changes the conditions that keep these particles stable. Flocculation then brings destabilized particles together so they form larger, settleable floc.

Why Coagulation Is Needed

Raw water can contain:

clay;
silt;
organic matter;
microorganisms;
color-causing material;
other fine particles.

Some of these particles are so small that gravity alone cannot remove them effectively within a practical treatment time.

Colloidal Particles

Colloids are very small particles that remain suspended in water for long periods.

They can resist settling because:

they are extremely small;
their surface charges can cause particles to repel each other;
normal hydraulic motion can keep them suspended.

What Coagulation Does

Coagulation destabilizes particles so they can begin sticking together.

The process commonly involves:

adding a coagulant;
rapidly mixing the chemical into the water;
allowing chemical reactions and particle destabilization to occur.

Common Coagulants

Coagulants used in drinking-water treatment can include compounds based on:

aluminum;
iron;
other approved treatment chemicals.

The best chemical and dose depend on the raw water and treatment process.

Coagulant Dose

Too little coagulant may leave particles insufficiently destabilized.

Too much coagulant can:

waste chemical;
produce poor floc under some conditions;
increase sludge production;
change finished-water chemistry.

The correct dose is therefore not simply the highest dose tested.

Rapid Mixing

Coagulant must be dispersed quickly and uniformly through the water.

Rapid mixing helps:

distribute the chemical;
bring the coagulant into contact with particles;
start destabilization reactions.

Importance of Mixing

If mixing is inadequate:

some water may receive too little chemical;
some water may receive too much;
particle destabilization may be uneven.

Coagulation and pH

Coagulant performance is strongly influenced by pH.

A chemical dose that performs well at one pH may perform poorly if raw-water pH changes.

Alkalinity and Coagulation

Some coagulants consume alkalinity during treatment reactions.

If alkalinity is too low, coagulant addition can cause a large pH decrease.

Buffering Capacity

Alkalinity helps resist rapid pH change.

Operators should understand the relationship among:

coagulant dose;
alkalinity;
pH;
treatment performance.

What Flocculation Does

Flocculation follows coagulation.

During flocculation, water is mixed more slowly so destabilized particles can collide and attach to one another.

The objective is to form particles large enough for effective downstream removal.

Coagulation Versus Flocculation

Remember the distinction:

coagulation destabilizes particles;
flocculation grows larger particle groups.

These are related but separate processes.

Floc Formation

Good floc often develops gradually.

Operators may observe:

small particles beginning to combine;
larger visible floc;
improved settling;
clearer water above settled floc.

Floc Strength

Floc must be strong enough to survive transport through the treatment process.

Excessive mixing can break floc apart.

Mixing Intensity

Flocculation generally uses lower mixing intensity than rapid mixing.

If mixing is too weak:

particle collisions may be insufficient;
floc may remain small.

If mixing is too strong:

formed floc may break;
settling performance may worsen.

Flocculation Time

Floc needs enough time to form before clarification.

Actual flocculation time depends on:

plant design;
flow;
basin volume;
water temperature;
mixing conditions.

Detention-Time Relationship

A simplified detention-time relationship is:

Detention Time = Basin Volume ÷ Flow

Units must be consistent.

Temperature Effects

Cold water can change coagulation and flocculation behavior.

Possible effects include:

slower chemical reactions;
slower particle movement;
different floc characteristics.

Seasonal temperature changes may therefore require treatment adjustments.

Raw-Water Turbidity

Changes in raw-water turbidity can affect coagulant demand and floc formation.

A storm can rapidly increase:

suspended solids;
organic matter;
treatment demand.

Natural Organic Matter

Natural organic matter can affect:

coagulant demand;
color;
chemical reactions;
downstream treatment.

Why Jar Testing Is Useful

A jar test simulates coagulation, flocculation, and settling on a small scale.

It helps operators compare treatment conditions before changing the full-scale plant.

What a Jar Test Can Evaluate

Jar tests can compare:

coagulant dose;
coagulant type;
pH adjustment;
polymer dose where applicable;
mixing conditions;
settling performance.

Basic Jar-Test Procedure

A typical jar test can include:

collect representative raw water;
fill identical jars with equal volumes;
apply different treatment conditions;
rapid mix;
flocculate with slower mixing;
allow settling;
compare results.

Use Representative Raw Water

Jar-test results are useful only if the sample represents the water being treated.

A stale or unrepresentative sample can produce misleading results.

Equal Sample Volumes

Each jar should begin with the same water volume so chemical doses can be compared fairly.

Change One Variable at a Time

When possible, keep most conditions constant and change the variable being evaluated.

For example, when testing coagulant dose, keep:

sample volume;
mixing time;
mixing speed;
settling time

consistent among jars.

Jar-Test Dose Series

An operator may test several coagulant doses such as:

10 mg/L;
15 mg/L;
20 mg/L;
25 mg/L;
30 mg/L.

The exact range should reflect actual water conditions and plant practice.

What to Observe During Rapid Mix

During rapid mixing, confirm:

chemical is dispersed quickly;
all jars receive similar mixing;
no jar is accidentally treated differently.

What to Observe During Flocculation

Operators may compare:

when floc first appears;
floc size;
floc density;
whether floc remains intact.

What to Observe During Settling

After mixing stops, observe:

settling rate;
clarity above the floc;
amount of settled material;
remaining suspended particles.

Best Jar Does Not Always Mean Largest Floc

Very large floc is not automatically the best result.

Operators should evaluate the overall treatment objective, including:

settling;
clarity;
turbidity;
chemical use;
downstream filter performance.

Settled-Water Turbidity

Measuring turbidity after settling can provide a more objective comparison than visual observation alone.

Jar-Test pH

Measure pH when it is important to treatment performance.

A jar with good floc but unacceptable chemical conditions may not represent the best full-scale treatment choice.

From Jar Test to Full-Scale Plant

A jar test is a decision aid, not a perfect duplicate of the plant.

Differences can include:

hydraulics;
mixing equipment;
detention time;
recycle flows;
chemical application points.

Make Controlled Full-Scale Changes

After selecting a promising jar-test condition, operators should make controlled plant adjustments and monitor the response.

Verify Downstream Performance

After changing coagulation conditions, review:

floc formation;
clarifier turbidity;
filter loading;
filtered-water turbidity;
chemical consumption.

Chemical Dose Calculation

A common operator relationship is:

Chemical Feed, lb/day = Flow, MGD × Dose, mg/L × 8.34

Dose Example

A plant treats 3.0 MGD and applies 18 mg/L of coagulant.

Chemical Feed = 3.0 × 18 × 8.34

Chemical Feed = 450.36 lb/day

If the desired dose remains 18 mg/L but flow increases, the required chemical mass per day must also increase.

Solution Strength Matters

The mass of pure or active chemical required is not always the same as the volume of commercial solution that must be pumped.

Operators must account for:

chemical concentration;
solution strength;
specific gravity where relevant;
feed-pump calibration.

Underfeeding Coagulant

Possible signs include:

weak or small floc;
poor settling;
high settled-water turbidity;
short filter runs;
high filtered-water turbidity.

Overfeeding Coagulant

Possible effects can include:

excess chemical use;
increased sludge production;
undesired pH change;
poor treatment under some conditions.

Incorrect pH

If coagulant dose appears adequate but floc remains poor, check whether pH is within an effective operating range for the treatment chemistry.

Low Alkalinity

Low alkalinity can allow a coagulant to depress pH more strongly than expected.

Operators may need to evaluate alkalinity and pH together rather than increasing coagulant dose automatically.

Poor Rapid Mixing

Symptoms can resemble incorrect chemical dose because the coagulant is not distributed properly.

Inspect:

mixer operation;
chemical application point;
flow conditions;
chemical-feed equipment.

Poor Flocculation

If particle destabilization occurs but good floc does not form, review:

mixing intensity;
flocculation time;
water temperature;
hydraulics;
polymer feed where used.

Floc Breakup

Floc breakup can occur because of:

excessive mixer speed;
hydraulic turbulence;
rapid flow changes;
poor basin transitions.

Pin Floc

Pin floc refers to very small floc particles that may remain suspended or carry over.

Possible causes can include:

poor coagulation;
inadequate flocculation;
floc breakup;
changing raw-water conditions.

Floc Carryover

Floc entering filters increases solids loading and can shorten filter runs.

The root cause may be:

coagulation;
flocculation;
clarification;
hydraulic overload.

Storm Response

During a storm, raw-water turbidity may rise rapidly.

Operators should monitor:

raw-water turbidity;
pH;
alkalinity;
jar-test results;
coagulant dose;
clarifier performance;
filter performance.

Seasonal Changes

Seasonal changes can affect:

temperature;
organic matter;
algae;
turbidity;
coagulant demand.

Use Trends

Record and compare:

raw-water quality;
coagulant dose;
pH;
settled-water turbidity;
filtered-water turbidity;
filter run length.

These trends help operators recognize when treatment performance is changing.

Do Not Change Several Variables at Once

If an operator simultaneously changes:

coagulant dose;
pH adjustment;
polymer dose;
mixer speed,

it can become difficult to identify which change improved or worsened treatment.

Confirm Feed-Pump Output

An indicated pump setting does not prove the correct dose is reaching the water.

Verify:

chemical tank level;
pump calibration;
feed line condition;
injection point;
actual feed rate.

Common Coagulation and Flocculation Mistakes

Confusing coagulation with flocculation.
Increasing dose automatically whenever turbidity rises.
Ignoring pH and alkalinity.
Using stale or unrepresentative water for jar testing.
Changing several jar-test variables at once.
Selecting the jar with the largest floc without considering settled-water quality.
Assuming jar-test results will exactly match full-scale plant performance.
Ignoring mixer or chemical-feed equipment problems.
Failing to verify treatment after a full-scale adjustment.
Ignoring filter response when evaluating coagulation performance.

A Practical Coagulation Review

Review raw-water turbidity.
Review raw-water pH and alkalinity.
Verify plant flow.
Verify coagulant dose.
Check rapid-mix operation.
Observe floc formation.
Review clarification.
Review filter performance.
Compare with historical operating trends.

A Practical Jar-Test Review

Collect representative raw water.
Use equal sample volumes.
Select a logical dose range.
Apply chemicals accurately.
Rapid mix consistently.
Flocculate consistently.
Allow equal settling time.
Compare floc, clarity, pH, and turbidity.
Select a reasonable treatment condition.
Verify performance in the full-scale plant.

A Practical Poor-Floc Review

Verify coagulant feed.
Check pH.
Check alkalinity.
Review raw-water changes.
Check rapid mixing.
Check flocculation mixing.
Review temperature.
Perform a jar test.
Make controlled adjustments.

What to Remember for the Exam

Coagulation destabilizes fine suspended and colloidal particles.
Flocculation brings destabilized particles together into larger floc.
Coagulation and flocculation are related but different processes.
Rapid mixing distributes coagulant quickly through the water.
Flocculation uses slower mixing to encourage particle collisions and floc growth.
Coagulant performance depends strongly on dose, pH, alkalinity, mixing, and raw-water quality.
Some coagulants consume alkalinity and can lower pH.
Excessive flocculation energy can break floc apart.
Cold water can change coagulation and flocculation performance.
Jar testing helps compare coagulant dose, pH adjustment, chemical combinations, and settling performance.
Jar-test samples should represent the actual raw water being treated.
Change one major variable at a time when possible during jar testing.
The jar with the largest floc is not automatically the best treatment condition.
Settled-water turbidity can provide an objective comparison among jars.
Jar-test results must be verified in the full-scale treatment plant.
Chemical feed in lb/day can be calculated as MGD × mg/L × 8.34.
Poor coagulation can increase clarification and filter loading.
Poor floc can result from incorrect dose, poor pH, low alkalinity, inadequate mixing, or raw-water changes.
Operators should trend raw-water quality, chemical dose, settled-water turbidity, and filter performance together.
Good coagulation control combines chemistry, mixing, jar testing, process observation, feed verification, and downstream performance.