Sedimentation, Coagulation and Flocculation in Water Treatment
Water treatment is essential to ensure the supply of safe and clean water for domestic and industrial use. Among the various treatment methods, sedimentation, coagulation, and flocculation are widely used to remove suspended and colloidal impurities and improve water quality.
Sedimentation in Water Treatment
Sedimentation is one of the most commonly used processes in water treatment. It is a simple and economical method in which suspended impurities are removed by allowing them to settle naturally under the action of gravity.
In this process, water flows slowly through sedimentation basins or tanks, enabling suspended particles to settle at the bottom while relatively clear water is collected from the top.
The efficiency of sedimentation mainly depends on:
- Retention time of water in the tank
- Size and weight of suspended particles
Sedimentation is generally carried out in two stages:
- Plain sedimentation of raw water
- Sedimentation after coagulation and flocculation
Zones in a Sedimentation Basin
A sedimentation basin generally consists of three zones:
- Inlet Zone
- Settling Zone
- Outlet Zone
Raw water continuously enters through the inlet zone and flows horizontally through the tank. Sedimentation tanks may be circular or rectangular in shape. Baffles are provided to regulate and distribute the flow uniformly throughout the tank.
As water moves slowly, suspended particles settle at the bottom as sludge. The sludge is removed by flushing, scraping, or emptying the tank. Clear water is then collected from the outlet zone.
Coagulation and Flocculation in Water Treatment
After sedimentation, many fine suspended particles still remain in water. These particles are generally colloidal in nature and are too small to settle naturally. Most colloidal particles carry a negative electrical charge, which keeps them suspended and causes turbidity.
Coagulation and flocculation are important water treatment processes that convert fine suspended and colloidal particles into larger settleable particles, enabling their removal by gravity.
These particles are removed by adding chemicals known as coagulants, which help combine small particles into larger and heavier particles called flocs.
A coagulant is an electrolyte that releases positively charged ions (cations), which neutralize negatively charged particles and promote floc formation. The higher the positive charge of the cation, the more effective the coagulant.
Therefore, aluminium and ferric compounds are commonly used as coagulants:
- Aluminium (Al³⁺)
- Ferric Iron (Fe³⁺)
Coagulation Process
The coagulation process mainly involves three steps:
- Addition of chemicals
- Formation of flocs
- Settling of flocs
Coagulants are added to raw water to neutralize negatively charged suspended particles.
As a result:
- Small clumps called micro flocs are formed.
- Micro flocs combine to form larger and heavier flocs.
- These heavy flocs settle at the bottom due to gravity.
During coagulation, rapid or flash mixing is carried out for about 2 to 5 minutes. Flash mixing ensures rapid and uniform distribution of coagulants throughout the water.
Flocculation
After coagulation, the small micro flocs are converted into larger and heavier macro flocs. This process is known as flocculation.
Flocculation is achieved through slow mixing for about 15 to 45 minutes. Slow mixing allows the flocs to grow in size without breaking apart, thereby improving settling efficiency.
Types of Coagulants
Commonly used coagulants include:
- Aluminium Sulphate (Filter Alum)
- Activated Alum (contains 9% Sodium Silicate)
- Black Alum (contains activated carbon)
- Ferric Sulphate
- Ferrous Sulphate
- Chlorinated Copperas (mixture of Ferric Sulphate and Ferric Chloride)
- Sodium Aluminate
Coagulant Aids
Coagulant aids improve the efficiency of coagulation by maintaining suitable pH and alkalinity conditions. They act as secondary coagulants.
pH Increasing Chemicals
- Lime
- Sodium Carbonate
- Sodium Bicarbonate
- Sodium Hydroxide
pH Reducing Chemicals
- Hydrochloric Acid (HCl)
- Sulfuric Acid (H₂SO₄)
Polymers
- Poly Aluminium Chloride (PAC)
Jar Test in Water Treatment
A Jar Test is performed to determine the optimum dose of coagulant required for effective water treatment.
Equipment Required
- Six beakers or jars
- Motor with RPM control
- Light source
Procedure
- Fill six jars with one litre of raw water sample.
- Add different doses of coagulant to each jar.
- Carry out rapid mixing at 100–150 RPM for one minute.
- Perform slow mixing at approximately 20 RPM for 20 minutes.
- Observe floc formation using a light source.
- The jar that produces the best floc formation with the minimum coagulant dosage indicates the optimum dose required for treatment.
Common Problems in Coagulation and Flocculation
| Problem | Cause | Solution |
| Poor Floc Formation | Inadequate coagulant dose | Conduct a jar test |
| Poor Flocculation | Improper mixing | Check rapid mixing speed |
| Inadequate Settling | Unsuitable raw water condition | Select an appropriate coagulant |
| Heavy Flocs | Excessive coagulant dose | Conduct a jar test |
Conclusion
Sedimentation, coagulation, and flocculation are essential processes in water and wastewater treatment. These processes help remove suspended and colloidal impurities, reduce turbidity, and improve water quality. Proper selection of coagulants, efficient mixing, and determination of the optimum coagulant dose through a jar test are crucial for achieving effective treatment and ensuring a safe and reliable water supply.
Mr. Prashant Walke
Trainer & Coordinator
Water & Wastewater Management Course
AISSMS College of Engineering, Pune