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👉Conventional Water Treatment
1. Screening (for the removal of large floating and suspended materials).
- Mostly used at intake site
- Optional unit, and may not be provided if target impurities are not present in water
- Plain (or primary) sedimentation may not be provided, as in most cases, settling units are provided after coagulation and flocculation for chemical assisted settling.
- In many conventional water treatment systems settling unit is combined with flocculation unit, named as clariflocculator.
4. Coagulation and Flocculation
Successive steps intended to overcome the forces stabilizing the fine suspended or colloidal particles, allowing particle collision and growth of floc.
- Destabilization (or Coagulation)
Reduce the forces acting to keep the particles apart after they contact each other (i.e., lower repulsion forces).
Chemical Addition, Rapid Mixing, “Pin‐point” Floc Formation
- Flocculation
Process of bringing destabilized colloidal particles together to allow them to aggregate to a size where they will settle by gravity.
Slow Mixing, Floc Growth, Increased Diameter
Particles in Water
- Dissolved Solids: < 1 nm (10‐6 mm) in size
Electrically charged and can interact with the water, so they are completely stable and will never settle out of the water. Not visible even with microscope.
- Colloidal solids or Non‐settleable solids: 1‐1000 nm in size
Do not dissolve in water although they are electrically charged. Still, the particles are so small that they will not settle in water and cannot be removed by filtration alone. Can be seen only with a high‐powered microscope.
- Suspended or settleable solids: > 1000 nm (10‐3 mm) in size
Larger particle that can be seen through eyes. These are usually supported by buoyant and viscous forces in water and may settle (or float) in non‐flowing water. Also, these can be removed by simple filtration.
Coagulation and Flocculation Steps
Selection of Suitable Coagulant
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Finding Optimum Dose of Coagulant
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Addition of Coagulant and rapid mixing
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Allowing floc formation through slow mixing
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Separation of flocs from water through settling/flotation/filtration
Selection of Coagulant
Required Basic Characteristics:
Nontoxic at the working dosage; High charge density; Insoluble in the neutral pH range
Aluminum and iron salts are the most commonly used coagulants in water treatment:
Aluminium coagulants include: Iron coagulants include:
Aluminium sulfate (Alum) Ferric sulfate
Aluminium chloride Ferrous sulfate
Sodium aluminate Ferric chloride
Polyaluminum Chloride (PAC) Ferric chloride sulfate
Other coagulants:
Organic coagulants, polyelectrolytes, hydrated lime, magnesium carbonate and various polymers etc.
Organic Coagulants vs Inorganic Coagulants
Organic coagulants
Generally used for solids & liquids separation and sludge generation. Polyamines function by charge neutralization alone, and are effective at treating higher turbidity raw water and wastewater. Melamine Formaldehydes and Tannins coagulate the colloidal material in the water, as well as absorb organic materials such as oil and grease. These are particularly well suited to operations that generate hazardous sludge.
Inorganic coagulants
These are mostly Al or Fe based, and are both cost‐effective and applicable for a broad variety of water and wastewater. Inorganic coagulants are particularly effective on raw water with low turbidity and will often treat this type of water when organic coagulants cannot.
Advantages of alum are that it readily dissolves with water, and does not cause the unsightly reddish brown staining of floors, walls and equipment like ferric sulphate. However, it is effective only at certain pH range, and good flocculation may not be possible with alum in some waters. With ferric sulphate, coagulation is possible at pH values as low as 4.0, and the floc formed is heavier than alum floc, as well as does not redissolve at high pH values.
Coagulant Aids
- In some waters, even large doses of primary coagulant fails to produce a satisfactory floc. In such cases, the coagulation process is often enhanced through the use of coagulant aids. Coagulant aids also help to create satisfactory coagulation over a broader pH range.
- Insoluble particulate materials such as clay, sodium silicate, pure precipitated calcium carbonate, diatomite, and activated carbon are common coagulant aids. They are used in waters that have low concentrations of particles (few nucleating sites). Because their density is higher than most floc particles, floc settling velocity is increased by the addition of such coagulant aids.
- Polymeric coagulant aid those help in bridging small floc to agglomerate rapidly into larger and denser floc are also used to reduce the amount of primary coagulant required. These are usually slightly anionic polyacrylamides with very high‐molecular weights. In some studies, non‐ionic or cationic types have also been proven effective. Synthetic organics such as anionic polyelectrolyte, and natural organics such as starch, starch derivatives, proteins, and tannins have been used as coagulant aids.
- The coagulant add dosage must be carefully controlled to avoid lowering the water quality.
Coagulant Doses: Zones of Effectiveness
Zone 1: Low dosage, insufficient coagulant added to produce destabilization.
Zone 2: Dosage sufficient to cause efficient and rapid destabilization
Zone 3: Dosage high enough to cause restabilization (charge reversal or polymer –foldback)
Zone 4: Dosage high enough to get sweep floc which results in good destabilization.
Colloid concentration expressed in terms of surface area S1 < S2 < S3 <S4.
Coagulation Practices based on Colloids and Alkalinity Levels
1. High Colloid, low alkalinity: The strategy here is to add coagulant without worrying about pH. The lower pH is better because destabilizing is by charge neutralization. Generally, there is no concern with overdosing because the colloidal surface area is too large.
2. High colloid concentration, high alkalinity: The choices are to destabilize by adsorption/charge neutralization at neutral pH (a larger dose at higher pH), or add acid to lower pH. Economics dictate choices.
3. Low colloid concentration , high alkalinity: For this case we can either destabilize by high dosage to give sweep floc or we can add coagulant aid such as bentonite (aluminium phyllosilicate clay) to get destabilization at lower dosage.
4. Low colloid concentration, low alkalinity: This is the most difficult case and generally requires added alkalinity or collides. Sweep floc is difficult to form as pH drops and it’s easy to overdose at low pH and low colloid concentration.
Coagulant Dose Optimization in Laboratory: Jar Test
In practice, irrespective of what coagulant or coagulant aid is used, the optimum dose are usually
determined by a Jar Test. A typical Jar Test apparatus consists of four to six beakers of 1‐2 L volume, provided with a variable‐speed stirrer.
Procedure:
Beakers filled with the raw water and varying amounts of coagulant dose are administered. The contents are rapidly mixed for about a minute and then allowed to flocculate at a slower pre‐worked speed (usually 20‐30 rpm) for desired time (usually 15‐30 mins). Thereafter, the contents are allowed to settle for desired time (usually 20‐ 40 mins), and the optimum dose is determined based on the measured turbidity of supernatant water (alternatively, judgement may be made based on visual inspection).
Jar test may be used to optimize:
- pH
- Mixing Speed for Flocculation
- Flocculation time
Importance of Optimum Coagulant Dosing Coagulant over‐dosing may leads to
o Increased treatment costs
o Restabilization of colloids
o Increases sludge mass
o Public health concerns
Coagulant under‐dosing may leads
o Lesser degree of removal
o Failure to meet the water quality targets
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