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Coagulation, Flocculation and Sedimentation in Water Treatment
Coagulation, flocculation and sedimentation are three connected stages in drinking water treatment, used to remove colloidal particles and suspended solids from raw water before filtration.
During coagulation, chemical coagulants destabilize fine particles that cannot settle naturally. Flocculation combines these particles into larger, settleable flocs, while sedimentation separates the formed flocs from clarified water.
Common coagulants include aluminum salts, polymerized aluminum salts and iron salts. Polyacrylamide-based polymers may also be used as flocculation aids.
Effective water treatment instrumentation supports chemical-dosing control, water-quality monitoring and stable clarification performance. Key online measurements include raw-water flow, actual coagulant flow, pH, turbidity, clarified-water flow, sludge-withdrawal flow and sludge-line pressure.
Measurement Overview for Coagulation, Flocculation and Sedimentation
|
Process Stage |
Key Parameters |
Main Instruments |
|---|---|---|
|
Coagulation |
Raw-water flow, coagulant flow, pH and inlet turbidity |
Electromagnetic flow meters, pH analyzer and turbidity analyzer |
|
Flocculation |
Flocculant dosing flow and basin level |
Electromagnetic flow meter, level transmitter |
|
Sedimentation |
Clarified-water turbidity, sludge flow and sludge-line pressure |
Turbidity analyzer, electromagnetic flow meter and pressure transmitter |
1. Coagulation
Process Purpose
The purpose of coagulation is to destabilize colloidal particles that would otherwise remain suspended in water.
Coagulants such as polyaluminum chloride, aluminum sulfate and ferric salts are added to the raw water and dispersed through rapid mixing. The required dose changes with raw-water flow and water quality.
The objective is not simply to increase chemical dosage. It is to maintain the correct chemical-to-water ratio under changing process conditions.
Key Coagulation Parameters
The main parameters are:
|
Online Measurement Parameters |
Operational or Design Parameters |
|---|---|
|
Raw-water flow, Actual coagulant flow, Raw-water pH, Post-dosing pH, Raw-water turbidity, Water temperature |
Target coagulant dose, Coagulant concentration, Alkalinity, Rapid-mixing intensity, Mixing time, Jar-test result, Organic loading |
Inlet flow provides the basic reference for coagulant dosing control, while actual coagulant-flow measurement helps optimize chemical dosage and reduce unnecessary sludge production. pH is also a critical process variable because it affects particle destabilization, adsorption behavior and coagulant chemistry.
Instruments and Their Roles
Electromagnetic Flow Meters for Raw Water and Coagulant Dosing
Raw-water flow provides the main reference for flow-proportional chemical dosing. The flow signal can be transmitted to a PLC or dosing controller to establish the initial coagulant setpoint:
Coagulant mass dosing rate = Raw-water flow × Target dose
The ARTang Aimag-A electromagnetic flow meter can be used to measure raw-water flow and, where the chemical is sufficiently conductive, the actual flow of liquid coagulants such as polyaluminum chloride, aluminum sulfate, ferric chloride and ferric sulfate.
Measuring both water flow and chemical flow allows the control system to compare the required dose with the amount actually delivered. This supports stable dosing, chemical-consumption totalization and early identification of problems such as blocked lines, air ingress or dosing-pump wear.
pH Measurement for Coagulation Control
The ARTang APS1101 industrial pH electrode can monitor pH at the raw-water inlet and after rapid mixing.
Because pH affects coagulant hydrolysis and particle destabilization, these measurements help operators determine whether the water chemistry remains suitable for effective coagulation. Unsuitable pH conditions may increase chemical consumption and reduce subsequent floc formation and clarification performance.
High-Turbidity Measurement at the Raw-Water Inlet
The ARTang ATS11 high-turbidity sensor monitors changes in suspended-particle loading before coagulation.
The turbidity signal can support feed-forward dose adjustment when rainfall, seasonal variation or source-water changes cause rapid fluctuations in raw-water quality. However, turbidity and coagulant demand should not be treated as having a fixed linear relationship. Dose settings should be verified through jar testing and actual plant performance.
Common Coagulation Problems
|
Problem |
Possible Causes |
Recommended Action |
|---|---|---|
|
Chemical dosing is inaccurate or unstable |
Incorrect raw-water flow measurement, fixed pump settings, blockage, air ingress or pump wear |
Compare the raw-water flow, dosing setpoint and actual coagulant flow |
|
Chemical consumption is unusually high |
Overdosing, inaccurate flow measurement or unsuitable pH |
Check the dosing ratio, flow measurements and pH conditions |
|
Coagulation performance remains poor |
Unsuitable pH, insufficient alkalinity or inadequate rapid mixing |
Check pH, alkalinity and mixing conditions before increasing the dose |
2. Flocculation
Process Purpose
Flocculation allows destabilized particles to collide and combine into larger, settleable flocs.
Mixing during flocculation is gentler than during coagulation. It must create enough particle contact to promote aggregation without breaking the flocs that have already formed.
A polymer flocculation aid may be added when larger or stronger flocs are required.
Key Flocculation Parameters
The main parameters are:
|
Online Measurement Parameters |
Operational or Design Parameters |
|---|---|
|
Flocculant dosing flow, basin level |
Target flocculant dose, flocculant concentration, mixing intensity, flocculation time and floc condition |
Flocculation performance depends on both chemical and hydraulic conditions. Correct chemical dosage cannot compensate for excessive mixing, insufficient contact time or unsuitable flow distribution.
Instruments and Their Roles
Electromagnetic Flow Meter for Flocculant Dosing
The ARTang Aimag-A electromagnetic flow meter can measure the actual flow of conductive flocculant or polymer solutions where the pipe remains completely filled.
The measurement helps verify chemical delivery, monitor consumption and compare the actual dosing flow with the control setpoint.
Level Transmitter for Flocculation Basin Monitoring
An ARTang level transmitter monitors the water level in the flocculation basin and supports stable hydraulic operation.
The level signal can help identify abnormal inlet or outlet conditions and provide overflow protection.
Common Flocculation Problems
|
Problem |
Possible Causes |
Recommended Action |
|---|---|---|
|
Flocs remain small or weak |
Insufficient flocculant delivery, unsuitable dose or inadequate mixing |
Compare the dosing setpoint with the actual flocculant flow and check mixing conditions |
|
Flocs form slowly |
Low water temperature, insufficient flocculation time or unsuitable polymer dose |
Review the dosing rate, water temperature and flocculation time |
|
Flocs form but break apart |
Excessive mixing or severe hydraulic disturbance |
Reduce mixing intensity and inspect the flow conditions |
3. Sedimentation
Process Purpose
Sedimentation separates formed flocs from clarified water.
Clarified water leaves the settling tank and continues to filtration. Settled solids collect in the lower part of the tank and are removed through the sludge-withdrawal system.
The main objectives are to:
Maintain low clarified-water turbidity
Prevent floc carryover
Control sludge accumulation
Protect downstream filters
Key Sedimentation Parameters
The main parameters are:
|
Online Measurement Parameters |
Operational or Design Parameters |
|---|---|
|
Clarified-water flow, Sludge-withdrawal flow, Sludge-pump discharge pressure, Sedimentation-outlet turbidity |
Sludge accumulation, Sludge-blanket height, Hydraulic loading, Flow distribution Sludge-withdrawal frequency Settling time |
Instruments and Their Roles
Low-Turbidity Sensor at the Sedimentation Outlet
The ARTang ATS12 low-turbidity sensor monitors clarified-water turbidity before the water enters filtration.
An increase in turbidity may indicate inadequate chemical dosing, weak floc formation, hydraulic disturbance or excessive sludge accumulation. The measurement provides early warning of clarification problems and increasing downstream filter load.
Electromagnetic Flow Meter for Clarified Water
The ARTang Aimag-A electromagnetic flow meter measures clarified-water flow leaving the sedimentation stage.
The measurement supports water-balance calculations, hydraulic-loading assessment and downstream filtration control.
Electromagnetic Flow Meter for Sludge Withdrawal
Electromagnetic flow measurement helps verify sludge-withdrawal rates and prevent excessive sludge accumulation in the sedimentation tank.
The Aimag-A is suitable for general sludge-flow applications with relatively low or moderate solids content, while the Aimag-S slurry electromagnetic flow meter is recommended for higher solids concentrations, greater viscosity or more demanding slurry conditions.
Pressure Transmitter for Sludge-Pump Monitoring
The ARTang AE11 or AEA16 pressure transmitter monitors the sludge-pump discharge pressure.
Abnormally high pressure may indicate blockage, sludge accumulation or increased viscosity, while low pressure may indicate pump-performance loss, leakage or insufficient sludge supply.
Common Sedimentation Problems
|
Problem |
Possible Causes |
Recommended Action |
|---|---|---|
|
Clarified-water turbidity increases |
Incorrect chemical dose, unsuitable pH, weak flocs, hydraulic disturbance or sludge accumulation |
Review dosing, pH, upstream flocculation conditions and sludge withdrawal |
|
Sludge accumulates in the tank |
Insufficient, irregular or ineffective sludge withdrawal |
Check the withdrawal schedule, actual sludge flow and pump operation |
|
Sludge-line pressure is abnormal |
Blockage, increased sludge concentration, valve restriction, leakage or pump problems |
Compare sludge flow and discharge pressure, then inspect the pump, valves and transfer line |
|
Turbidity measurement drifts or fluctuates |
Sensor fouling, bubbles or unstable sample flow |
Clean the sensor and inspect the sampling conditions |
Integrated Measurement Strategy
No single measurement can fully evaluate coagulation, flocculation and sedimentation. A practical monitoring strategy combines three types of signals:
Feed-Forward Measurements
- Raw-water flow
- Raw-water turbidity
- Raw-water pH
Process-Confirmation Measurements
- Actual coagulant or flocculant flow
- Post-dosing pH
Performance and Operational Measurements
- Sedimentation-outlet turbidity
- Clarified-water flow
- Sludge-withdrawal flow
- Sludge-pump discharge pressure
The dosing strategy should be developed using jar tests, process knowledge and actual operating data. Turbidity alone is not sufficient because coagulant demand is also affected by pH, alkalinity, temperature, natural organic matter and coagulant type.
Instrument Selection Summary
|
Process Stage |
Measurement Point |
Recommended Instrument |
|---|---|---|
|
Coagulation |
Raw-water inlet |
Aimag-A electromagnetic flow meter APS1101 pH electrode ATS11 high-turbidity sensor |
|
Coagulation |
Coagulant dosing line |
Aimag-A electromagnetic flow meter |
|
Flocculation |
Flocculant dosing line and basin |
Aimag-A electromagnetic flow meter level transmitter |
|
Sedimentation |
Clarified-water outlet |
Aimag-A electromagnetic flow meter |
|
Sedimentation |
Sedimentation outlet |
ATS12 low-turbidity sensor |
|
Sedimentation |
Sludge-withdrawal line |
Aimag-A or Aimag-S electromagnetic flow meter |
|
Sedimentation |
Sludge-pump discharge |
AE11 or AEA16 pressure transmitter |
Conclusion
Coagulation, flocculation and sedimentation should be monitored as one connected treatment process.
Raw-water flow, turbidity and pH establish the incoming conditions. Chemical-flow measurement confirms that the required coagulant or flocculant is actually delivered, while sedimentation-outlet turbidity indicates the overall clarification result. Sludge-flow and pressure measurements support stable sludge withdrawal and solids removal.
By combining online measurement with jar testing and actual plant-operating data, drinking-water treatment plants can improve clarification stability, reduce unnecessary chemical consumption and identify process problems more efficiently.
FAQ
Pump speed represents the command sent to the dosing pump, but it does not confirm the amount of chemical actually delivered. Blockage, crystallization, air ingress, pump wear and pulsation may cause the actual flow to differ from the setpoint.
No. Turbidity is an important input, but coagulant demand is also affected by pH, alkalinity, temperature, natural organic matter, coagulant type and mixing conditions.
Possible causes include overdosing, unsuitable pH, weak flocculation, poor hydraulic distribution or excessive sludge accumulation. The complete coagulation, flocculation and sedimentation process should be reviewed before increasing the dose again.
Need Help Selecting Water Treatment Instruments?
Share your process conditions with ARTang. Our technical team can recommend suitable instruments for flow, pH, turbidity, level and pressure measurement.
